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Organic acids are ubiquitous compounds in the troposphere and can affect human health, the climate, air quality, and the linked ecosystems. Depending on their solubility and volatility, they can partition in both gas phase and in the particle phase. In the particle phase, organic acids partly represent about 10% of the water-soluble organic matter. However, their partitioning between different phases is not fully understood yet. Therefore, an upgraded monitor for aerosols and gases in ambient air (MARGA) was applied for one year at the Central European TROPOS research site Melpitz to study the gas- and particle-phase partitioning of formic, acetic, propionic, butyric, glycolic, pyruvic, oxalic, malonic, succinic, malic, and methanesulfonic acid (MSA). Measured gas- and PM10 particle-phase mean concentrations were 12−445 and 7−31 ng m-3 for monocarboxylic acids (MCAs), between 0.6−8 and 4−31 ng m-3 for dicarboxylic acids (DCAs), and 2 and 31 ng m-3 for MSA, respectively. Assuming full dissolution in nonideal aerosol solutions, empirical noneffective Henry’s law constants (Hemp) were calculated and compared with literature values (Hlit). Calculated mean Hemp were 4.5 × 109−2.2 × 1010 mol L−1 atm−1 for MCAs, 3.6 × 1010−7.5 × 1011 mol L−1 atm−1 for DCAs, and 7.5 × 107 mol L−1 atm−1 for MSA and, thus, factors of 5.1 × 103−9.1 × 105 and 2.5−20.3 higher than their corresponding Hlit for MCAs and DCAs, respectively, and 9.0 × 10−5 lower than Hlit,MSA. Data analyses and thermodynamic calculations implicate that the formation of chemical association complexes and organic salts inhibits the partitioning of organic acids toward the gas phase and, thus, at least partly explains higher Hemp values for both MCAs and summertime DCAs. Low Hemp,MSA are also unexpected because of the high MSA solubility and are reported for the first time in this study. Overall, the results of the present study implicate that processes responsible for the observed stronger partitioning of carboxylic acids toward the particle phase need to be further investigated and accounted for in complex multiphase chemistry models as they affect the contribution of organic acids to secondary organic aerosol mass, their chemical processing, and lifetime.

Aerosol particles contribute to the climate forcing through their optical properties. Measuring these optical aerosol particle properties is still challenging, especially considering the hygroscopic growth of aerosol particles, which alters their optical properties. Lidar and in-situ techniques can derive a variety of aerosol optical properties, like aerosol particle light extinction, backscattering, and absorption. But these techniques are subject to some limitations and uncertainties. Within this study, we compared airborne in-situ based and, on Mie-theory based, modeled optical properties at dry state. At ambient state, modeled optical properties were compared with lidar-based estimates. Also, we examined the dependence of the aerosol particle light extinction-to-backscatter ratio, also lidar ratio, to relative humidity. The used model was fed with measured physicochemical aerosol properties and ambient atmospheric conditions. The model considered aerosol particles in an internal core-shell mixing state with constant volume fractions of the aerosol components over the entire observed aerosol particle size-range. The underlying set of measurements was conducted near the measurement site Melpitz, Germany, during two campaigns in summer, 2015, and winter, 2017, and represent Central European background aerosol conditions. Two airborne payloads deployed on a helicopter and a balloon provided measurements of microphysical and optical aerosol particle properties and were complemented by the polarization Raman lidar system PollyXT as well as by a holistic set of microphysical, chemical and optical aerosol measurements derived at ground level. Comparisons of calculated optical aerosol properties with ground-based in-situ measured aerosol optical properties at dry state showed an agreement of the model within 13 % (3 %) in terms of scattering at 450 nm wavelength during the winter (summer) campaign. The model also represented the aerosol particle light absorption at 637 nm within 8 % (18 %) during the winter (summer) campaign and agreed within 13 % with the airborne in-situ aerosol particle light extinction measurements during summer. During winter, in a comparatively clean case with equivalent black carbon mass-concentrations of around 0.2 µg m−3 the modeled airborne measurement-based aerosol particle light absorption, was up to 32–37 % larger than the measured values during a relatively clean period. However, during a high polluted case, with an equivalent black carbon mass concentration of around 4 µg m−3, the modeled aerosol particle light absorption coefficient was, depending on the wavelength, 13–32 % lower than the measured values. Spread and magnitude of the disagreement highlighted the importance of the aerosol mixing state used within the model, the requirement of the inclusion of brown carbon, and a wavelength-dependent complex refractive index of black and brown carbon when such kind of model is used to validate aerosol particle light absorption coefficient estimates of, e.g., lidar systems. Besides dry state comparisons, ambient modeled aerosol particle light extinction, as well as aerosol particle light backscattering, were compared with lidar estimates of these measures. During summer, on average, for four of the twelve conducted measurement flights, the model calculated lower aerosol particle light extinction (up to 29 % lower) as well as backscattering (up to 32 % lower) than derived with the lidar. In winter, the modeled aerosol particle light extinction coefficient was 17 %–41 % lower, the aerosol particle light backscattering coefficient 14 %–42 % lower than the lidar estimates. For both, the winter and summer cases, the Mie-model estimated reasonable extinction-to-backscatter (LR) ratios. Measurement-based Mie-modeling showed evidence of the dependence of the lidar ratio on relative humidity (RH). With this result, we presented a fit for lidar wavelengths of 355, 532, and 1064 nm with an underlying equation of fLR (RH,γ(λ)) = fLR (RH = 0,λ) × (1 − RH)(−γ(λ)) and estimates of γ(355 nm) = 0.29 (±0.01), γ(532 nm) = 0.48 (±0.01), and γ(1064 nm) = 0.31 (±0.01). However, further measurements are required to entangle the behavior of the lidar ratio with respect to different aerosol types, to set up a climatology, and to assess the influence of the aerosol mixing state. This comprehensive study combining airborne and ground-based in-situ and remote sensing measurements, which simulated multiple aerosol optical coefficients in the ambient and dry state, is with its complexity unique of its kind.

The identification of different sources of the carbonaceous aerosol (organics and black carbon) was investigated at a mountain forest site located in central Germany from September to October 2010 to characterize incoming air masses during the Hill Cap Cloud Thuringia 2010 (HCCT-2010) experiment. The near-PM1 chemical composition, as measured by a high-resolution time-offlight aerosol mass spectrometer (HR-ToF-AMS), was dominated by organic aerosol (OA; 41 %) followed by sulfate (19 %) and nitrate (18 %). Source apportionment of the OA fraction was performed using the multilinear engine (ME-2) approach, resulting in the identification of the following five factors: hydrocarbon-like OA (HOA; 3% of OA mass), biomass burning OA (BBOA; 13 %), semi-volatilelike OA (SV-OOA; 19 %), and two oxygenated OA (OOA) factors. The more oxidized OOA (MO-OOA, 28 %) was interpreted as being influenced by aged, polluted continental air masses, whereas the less oxidized OOA (LO-OOA, 37 %) was found to be more linked to aged biogenic sources. Equivalent black carbon (eBC), measured by a multi-angle absorption photometer (MAAP) represented 10% of the total particulate matter (PM). The eBC was clearly associated with HOA, BBOA, and MO-OOA factors (all together R-2=0 :83). Therefore, eBC's contribution to each factor was achieved using a multi-linear regression model. More than half of the eBC (52 %) was associated with long-range transport (i.e., MO-OOA), whereas liquid fuel eBC (35 %) and biomass burning eBC (13 %) were associated with local emissions, leading to a complete apportionment of the carbonaceous aerosol. The separation between local and transported eBC was well supported by the mass size distribution of elemental carbon (EC) from Berner impactor samples. Air masses with the strongest marine influence, based on back trajectory analysis, corresponded with a low particle mass concentration (6.4-7.5 mu gm(-3)/ and organic fraction (approximate to 30 %). However, they also had the largest contribution of primary OA (HOA approximate to 4% and BBOA 15 %-20 %), which was associated with local emissions. Continental air masses had the highest mass concentration (11.4-12.6 mu gm(-3)), and a larger fraction of oxygenated OA (approximate to 45 %) indicated highly processed OA. The present results emphasize the key role played by long-range transport processes not only in the OA fraction but also in the eBC mass concentration and the importance of improving our knowledge on the identification of eBC sources.

The mass concentration of black carbon (BC) particles in the atmosphere has traditionally been quantified with two methods: as elemental carbon (EC) concentrations measured by thermal-optical analysis and as equivalent black carbon (eBC) concentrations when BC mass is derived from particle light absorption coefficient measurements. Over the last decade, ambient measurements of refractory black carbon (rBC) mass concentrations based on laser-induced incandescence (LII) have become more common, mostly due to the development of the Single-Particle Soot Photometer (SP2) instrument. In this work, EC and rBC mass concentration measurements from field campaigns across several background European sites (Paris, Bologna, Cabauw and Melpitz) have been collated and examined to identify the similarities and differences between BC mass concentrations measured by the two techniques. All EC concentration measurements in PM2.5 were performed with the EUSAAR-2 thermal-optical protocol. All rBC concentration measurements were performed with SP2s calibrated with the same calibration material as recommended in the literature. The median ratio between observed rBC and EC mass concentrations was 0.92, when considering all data points from all five campaigns, and the corresponding geometric standard deviation (GSD) was 1.5. The minimal and maximal observed values of median rBC to EC mass concentration ratios on single campaign level were 0.53 and 1.29, respectively. This shows that substantial systematic bias between these two quantities occurred during some campaigns, which also contributes to the large overall GSD. On single campaign level, the relative spread of individual rBC to EC mass concentration ratios was typically between a factor of 1.2 and 1.3 (1 GSD), which indicates fairly good precision of both methods. Despite considerable variability of BC properties and sources across the whole data set, it was not possible to clearly assign reasons for discrepancies to one or the other method, both known to have their own specific limitations and uncertainties. However, differences in the particle size range covered by these two methods were identified as one likely reason for discrepancies. In particular, rBC to EC mass concentration ratios were found to be systematically less than unity, despite applying a correction for small BC cores that remain undetected by the SP2. This was observed when the rBC mass size distribution was shifted towards smaller modal diameter, which occurred during traffic emission dominated episodes. Overall, the high correlation between rBC and EC mass concentrations indicates that both methods essentially quantify the same property of atmospheric aerosols, whereas systematic differences in measured absolute values by up to a factor of 2 can occur. This finding for the level of agreement between two current state-of-the-art techniques has important implications for studies based on BC mass concentration measurements, for example for the interpretation of uncertainties of inferred BC mass absorption coefficient values, which are required for modelling the radiative forcing of BC. Homogeneity between BC mass determination techniques is very important also towards a routine BC mass measurement for air quality or human health regulations.

We have shown in previous work that Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) in combination with the “Chemical Analysis of Aerosol Online" (CHARON) inlet is a powerful tool for direct and online analysis of sub-µm particulate organic matter in the urban atmosphere. Herein, we report on the first CHARON PTR-ToF-MS measurements in the continental background environment, at the TROPOS Research Station in Melpitz near Leipzig (Germany) during a three week period in February 2019.A state-of-the-art CHARON PTR-TOF 6000X2 instrument (IONICON Analytik GmbH, Austria) was used for measuring particulate organic compounds online (i.e., without filter pre-collection) and in real-time (< 1-min time resolution), at sub-ng m-3 mass concentrations, and on an elementary composition level. Periodic switching between the standard PTR-MS gas-phase inlet and the CHARON particle inlet made it possible to comprehensively measure atmospheric organic matter in both the gaseous and particulate state at a time resolution of 10 minutes. In addition, an aerosol mass spectrometer (HR-TOF-AMS) and an aerosol chemical speciation monitor (ACSM; both Aerodyne Inc., USA) were deployed for monitoring the composition of non-refractory particulate matter. A Dual Mobility Particle Size Spectrometer (TROPOS-type T-MPSS) was used for determining the total particle size distribution. In addition, particles were collected on filters once per day and analysed offline in the laboratory.The CHARON PTR-TOF 6000X2 instrument operated stably and reliably over the three week measurement period. Our data show that a single instrument can be used for characterizing both gaseous and particle-bound organic matter in the atmosphere at 10 minute time resolution. The obtained data agree well with ACSM, HR-TOF-AMS and T-MPSS results. A comparison with the offline results obtained from the filter samples confirmed that the CHARON PTR-ToF-MS technique accurately measures the atmospheric concentrations of selected anhydrosugars and polycyclic aromatic hydrocarbons (PAHs). We also show that CHARON PTR-ToF-MS data are useful for improving the source apportionment of particles via positive matrix factorization (PMF).This work is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654109 (ACTRIS-2). F. P. has received funding through the EU's Horizon 2020 programme under grant agreement Nº674911 (IMPACT).

The heterogeneous hydrolysis of N2O5 on the surface of deliquescent aerosol leads to HNO3 formation and acts as a major sink of NOx in the atmosphere during night-time. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition, and the surface area concentration (S). However, these parameters were not comprehensively considered in the parameterization of the heterogeneous hydrolysis of N2O5 in previous mass-based 3-D aerosol modelling studies. In this investigation, we propose a sophisticated parameterization (NewN2O5) of N2O5 heterogeneous hydrolysis with respect to T, RH, aerosol particle compositions, and S based on laboratory experiments. We evaluated closure between NewN2O5 and a state-of-the-art parameterization based on a sectional aerosol treatment. The comparison showed a good linear relationship (R = 0.91) between these two parameterizations. NewN2O5 was incorporated into a 3-D fully online coupled model, COSMO–MUSCAT, with the mass-based aerosol treatment. As a case study, we used the data from the HOPE Melpitz campaign (10–25 September 2013) to validate model performance. Here, we investigated the improvement of nitrate prediction over western and central Europe. The modelled particulate nitrate mass concentrations ([NO3−]) were validated by filter measurements over Germany (Neuglobsow, Schmücke, Zingst, and Melpitz). The modelled [NO3−] was significantly overestimated for this period by a factor of 5–19, with the corrected NH3 emissions (reduced by 50 %) and the original parameterization of N2O5 heterogeneous hydrolysis. The NewN2O5 significantly reduces the overestimation of [NO3−] by ∼ 35 %. Particularly, the overestimation factor was reduced to approximately 1.4 in our case study (12, 17–18 and 25 September 2013) when [NO3−] was dominated by local chemical formations. In our case, the suppression of organic coating was negligible over western and central Europe, with an influence on [NO3−] of less than 2 % on average and 20 % at the most significant moment. To obtain a significant impact of the organic coating effect, N2O5, SOA, and NH3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on the reaction probability of N2O5 may not be as significant as expected over western and central Europe.

The Aerosol Chemical Speciation Monitor (ACSM) is an instrument for identifying and quantifying the influence of air quality mitigations. For this purpose, a European ACSM network has been developed within the research infrastructure project ACTRIS (European Research Infrastructure for the observation of Aerosol, Clouds and Trace Gases). To ensure the uniformity of the dataset, as well as instrumental performance and variability, regular intercomparisons are organized at the Aerosol Chemical Monitoring Calibration Center (ACMCC, part of the European Center for Aerosol Calibration, Paris, France). However, in-situ quality assurance remains a fundamental tracking point of the instrument’s stability. In order to check the robustness of the ACSM over the years and to characterize the seasonality effect, nitrate, sulfate, ammonium, organic, and particle mass concentrations were systematically compared with collocated measurements including daily off-line high-volume PM1 and PM2.5 filter samples. Mass closure analysis was made by comparing the total particle mass (PM) concentration obtained by adding the mass concentration of equivalent black carbon (eBC) from the Multi-Angle Absorption Photometer (MAAP) to the ACSM chemical composition, to that of PM1 and PM2.5 during filter weighting, as well as to the derived mass concentration of particle number size distribution measurements (PNSD). A combination of PM1 and PM2.5 filter samples helps identify the critical importance of the upper size cut-off of the ACSM during such exercises. The ACSM-MAAP-derived mass concentrations systematically deviate from the PM1 samples when the mass concentration of the latter represents less than 60 % of PM2.5, which is linked to the transmission efficiency of the aerodynamic lenses of the ACSM. The best correlations are obtained for sulfate (slope 0.96, R2 = 0.77) and total PM (slope 1.02, R2 = 0.90). Although, sulfate does not exhibit a seasonal dependency, total PM mass concentration shows a small seasonal effect associated with an increase in non-water-soluble fractions. The nitrate suffers from a loss of ammonium nitrate during filter collection, and the contribution of organo-nitrate compounds to the ACSM nitrate signal make it difficult to directly compare the two methods. The contribution of m/z 44 (f44) to the total organic mass concentration was used to convert the ACSM organic mass to OC by using a similar approach as for the AMS. The resulting estimated OC-ACSM was compared with the measured OC-PM1 (slope 0.74, R2 = 0.77), indicating that the f44 signal was relatively free of interferences during this period. The PM2.5 filter samples use for the ACSM data quality might suffer from a systematic bias due to a size cutting effect as well as to the presence of chemical species that cannot be detected by the ACSM in coarse mode (e.g. sodium nitrate and sodium sulfate). This may lead to a systematic underestimation of the ACSM particle mass concentration and/or a positive artefact that artificially decreases the discrepancies between the two methods. Consequently, ACSM data validation using PM2.5 filters has to be interpreted with extreme care. The particle mass closure with the PNSD was satisfying (slope 0.77, R2 = 0.90 over the entire period), with a slightly overestimation of the MPSS derived mass concentration in winter. This seasonal variability was related to a change on the PNSD and a larger contribution of the super-µm particles in winter. This long-term analysis between the ACSM and other collocated instruments confirms the robustness of the ACSM and its suitability for long-term measurements. Particle mass closure with the PNSD is strongly recommended to ensure the stability of the ACSM. A near real-time mass closure procedure within the entire ACTRIS-ACSM network certainly represents an optimal way of both warranting the quality assurance of the ACSM measurements as well as identifying possible deviations in one of the two instruments.

Atmospheric aerosol particles are the precondition for the formation of cloud droplets and therefore have large influence on the microphysical and radiative properties of clouds. In this work, four different methods to derive or measure number concentrations of cloud condensation nuclei (CCN) were analyzed and compared for present-day aerosol conditions: (i) a model parameterization based on simulated particle concentrations, (ii) the same parameterization based on gravimetrical particle measurements, (iii) direct CCN measurements with a CCN counter, and (iv) lidar-derived and in situ measured vertical CCN profiles. In order to allow for sensitivity studies of the anthropogenic impact, a scenario to estimate the maximum CCN concentration under peak aerosol conditions of the mid-1980s in Europe was developed as well. In general, the simulations are in good agreement with the observations. At ground level, average values between 0.7 and 1.5×109 CCN m−3 at a supersaturation of 0.2 % were found with the different methods under present-day conditions. The discrimination of the chemical species revealed an almost equal contribution of ammonium sulfate and ammonium nitrate to the total number of CCN for present-day conditions. This was not the case for the peak aerosol scenario, in which it was assumed that no ammonium nitrate was formed while large amounts of sulfate were present, consuming all available ammonia during ammonium sulfate formation. The CCN number concentration at five different supersaturation values has been compared to the measurements. The discrepancies between model and in situ observations were lowest for the lowest (0.1 %) and highest supersaturations (0.7 %). For supersaturations between 0.3 % and 0.5 %, the model overestimated the potentially activated particle fraction by around 30 %. By comparing the simulation with observed profiles, the vertical distribution of the CCN concentration was found to be overestimated by up to a factor of 2 in the boundary layer. The analysis of the modern (year 2013) and the peak aerosol scenario (expected to be representative of the mid-1980s over Europe) resulted in a scaling factor, which was defined as the quotient of the average vertical profile of the peak aerosol and present-day CCN concentration. This factor was found to be around 2 close to the ground, increasing to around 3.5 between 2 and 5 km and approaching 1 (i.e., no difference between present-day and peak aerosol conditions) with further increasing height.

Natural sea-salt aerosols, when interacting with anthropogenic emissions, can enhance the formation of particulate nitrate. This enhancement has been suggested to increase the direct radiative forcing of nitrate, called the “mass-enhancement effect”. Through a size-resolved dynamic mass transfer modeling approach, we show that interactions with sea salt shift the nitrate from sub- to super-micron-sized particles (“redistribution effect”), and hence this lowers its efficiency for light extinction and reduces its lifetime. The redistribution effect overwhelms the mass-enhancement effect and significantly moderates nitrate cooling; e.g., the nitrate-associated aerosol optical depth can be reduced by 10 %–20 % over European polluted regions during a typical sea-salt event, in contrast to an increase by ∼10 % when only accounting for the mass-enhancement effect. Global model simulations indicate significant redistribution over coastal and offshore regions worldwide. Our study suggests a strong buffering by natural sea-salt aerosols that reduces the climate forcing of anthropogenic nitrate, which had been expected to dominate the aerosol cooling by the end of the century. Comprehensive considerations of this redistribution effect foster better understandings of climate change and nitrogen deposition.

Elemental Carbon (EC) has a significant impact on human health and climate change. In order to evaluate the size segregation of EC emission in the EUCAARI inventory and investigate its influence on the simulation of EC long-range transportation in Europe, we used the fully coupled online Weather Research and Forecasting/Chemistry model (WRF-Chem) at a resolution of 2 km focusing on a region in Germany, in conjunction with a high-resolution EC emission inventory. The ground meteorology conditions, vertical structure and wind pattern were well reproduced by the model. The simulations of particle number and/or mass size distributions were evaluated with observations at the central European background site Melpitz. The fine mode particle concentration was reasonably well simulated, but the coarse mode was substantially overestimated by the model mainly due to the plume with high EC concentration in coarse mode emitted by a nearby point source. The comparisons between simulated EC and Multi-angle Absorption Photometers (MAAP) measurements at Melpitz, Leipzig-TROPOS and Bösel indicated that the coarse mode EC (ECc) emitted from the nearby point sources might be overestimated by a factor of 2–10. The fraction of ECc was overestimated in the emission inventory by about 10–30 % for Russia and 5–10 % for Eastern Europe (e.g., Poland and Belarus). This incorrect size-dependent EC emission results in a shorter atmospheric life time of EC particles and inhibits the long-range transport of EC. A case study showed that this effect caused an underestimation of 20–40 % in the EC mass concentration in Germany under eastern wind pattern.

Source apportionment (SA) of carbonaceous aerosol was performed as part of the EMEP Intensive Measurement Periods (EIMPs), conducted in fall 2008 and winter/spring 2009. Levels of elemental carbon (EC), particulate organic carbon (OCp), particulate total carbon (TCp), levoglucosan and 14C in PM10, observed at nine European rural background sites, were used as input for the SA, whereas Latin Hypercube Sampling (LHS) was used to statistically treat the multitude of possible combinations resulting when ambient concentrations were combined with appropriate emission ratios. Five predefined sources/subcategories of carbonaceous aerosol were apportioned: Elemental and organic carbon from combustion of biomass (ECbb and OCbb) and from fossil fuel (ECff and OCff) sources, as well as remaining non-fossil organic carbon (OCrnf), typically dominated by natural sources. The carbonaceous aerosol concentration decreased from South to North, as did the concentration of the apportioned carbonaceous aerosol. OCrnf was more abundant in fall compared to winter/spring, reflecting the vegetative season, and made a larger contribution to TCp than anthropogenic sources (here: ECbb, OCbb, ECff and OCff) at four of the sites, whereas anthropogenic sources dominated at all but one sites in winter/spring. Levels of OCbb and ECbb were typically higher in winter/spring than in fall, due to larger residential wood burning emissions in the heating season, whereas there was no consistent seasonal pattern for fossil fuel emissions. Biomass burning (OCbb + ECbb) was the major anthropogenic source at the Central European sites in fall, whereas fossil fuel sources dominated at the southernmost and the two northernmost sites. In winter/spring, biomass burning was the major anthropogenic source at all but two sites. Addressing EC in particular, fossil fuel sources dominated at all sites in fall, whereas there was as shift towards biomass burning in winter/spring for the southernmost sites. Influence of residential wood burning emissions was substantial already in the first week of sampling in fall, constituting 30–50 % of TCp at most sites, showing that this source can be dominating even at a time of the year when the ambient temperature in Europe is still rather high. Model calculations were made, attempting to reproduce LHS-derived OCbb and ECbb, using two different residential wood burning emission inventories. Both simulations strongly under-predicted the LHS-derived values at most sites outside Scandinavia. Emissions based on a consistent bottom-up inventory for residential combustion (and including intermediate volatility compounds, IVOC) improved model results at most sites compared to the base-case emissions (based mainly on officially reported national emissions), but at the three southernmost sites the modelled OCbb and ECbb concentrations were still much lower than the LHS source apportioned results. The current study shows that natural sources is a major contributor to the carbonaceous aerosol in Europe even in fall and in winter/spring, and that residential wood burning emissions can be equally large or larger than that of fossil fuel sources, depending on season and region. Our results suggest that residential wood burning emissions are still poorly constrained for large parts of Europe. The need to improve emission inventories is obvious, with harmonization of emission factors between countries likely being the most important step to improve model calculations, not only for biomass burning emissions, but for European PM2.5 concentrations in general.

A method is presented to quantify the low-molecular weight organic acids formic, acetic, propionic, butyric, pyruvic, glycolic, oxalic, malonic, succinic, malic, glutaric, and methanesulfonic acid in the atmospheric gas and particle phase in a two-hourly time resolution, based on a combination of the Monitor for AeRosols and Gases in ambient Air (MARGA) and an additional ion chromatography (IC) instrument. A proper separation of the organic target acids was initially tackled by a laboratory IC optimization study, testing different separation columns, eluent compositions and eluent flow rates both for isocratic and for gradient elution. Satisfactory resolution of all compounds was achieved using a gradient system with two coupled anion exchange separation columns. Online pre-concentration with an enrichment factor of approximately 400 was achieved by solid phase extraction consisting of a methacrylate polymer based sorbent with quaternary ammonium groups. The limits of detection of the method range between 7.1 ng m−3 for methanesulfonate and 150.3 ng m−3 for pyruvate. Precisions are below 1.0 %, except for glycolate (2.9 %) and succinate (1.0 %). Comparisons of inorganic anions measured at the TROPOS research site in Melpitz, Germany, by the original MARGA and the additional organic acid IC systems are in agreement with each other (R2 = 0.95 − 0.99). Organic acid concentrations from May 2017 as an example period are presented. Monocarboxylic acids were dominant in the gas phase with mean concentrations of 553 ng m−3 for acetic acid, followed by formic (286 ng m−3), pyruvic acid (182 ng m−3), propionic (179 ng m−3), butyric (98 ng m−3) and glycolic (71 ng m−3). Particulate glycolate, oxalate and methanesulfonate were quantified with mean concentrations of 63 ng m−3, 74 ng m−3 and 35 ng m−3, respectively. Elevated concentrations in the late afternoon of gas phase formic acid and particulate oxalate indicate a photochemical formation.

Heterogeneous hydrolysis of N 2 O 5 on the surface of deliquescent aerosol particles leads to HNO 3 formation and acts as a major sink of NOx in the atmosphere during nighttime. The reaction constant of this heterogeneous hydrolysis is determined by temperature (T), relative humidity (RH), aerosol particle composition as well as the surface area concentration (S). However, its parameterization in previous 3-D modelling studies did not comprehensively consider these parameters. In this investigation, we propose a sophisticated parameterization of the heterogeneous hydrolysis of N 2 O 5 with respect to T, RH, aerosol particle compositions and S, based on laboratory experiments. This new parameterization was incorporated into a 3-D fully online coupled model: COSMO-MUSCAT. As case study, we used the data from the HOPE-Melpitz campaign (10–25 September 2013). Here, we investigated the improvement of nitrate prediction over the western and central Europe. The modelled particulate nitrate mass concentrations ([NO 3 − ]) were validated by filter measurements over Germany (Neuglobsow, Schmucke, Zingst, and Melpitz). The modelled [NO 3 − ] were significantly overestimated for this period by a factor of 5–19, with the corrected NH3 emissions (reduced by 50 %) and the original parameterization of N 2 O 5 heterogeneous hydrolysis. The proposed new parameterization significantly reduces the overestimation of [NO 3 − ] by ~ 35 %. Particularly, the overestimation factor was reduced to approximately 1.4 within our case study period (September 12, 17–18 and 25, 2013), when [NO 3 − ] was dominated by local chemical formations. Furthermore, the organic coating effect on a suppression of the N 2 O 5 reaction probability may have been also significantly overestimated in previous modelling studies, due to a strong overestimation of the N 2 O 5 reaction probability on coatings. Based on the original parameterization, previous studies reported a decrease of modelled [NO 3 − ] up to 90 %, where both secondary organic aerosol (SOA) and N 2 O 5 were built-up over western and central Europe. For this case study, the suppression of organic coating was negligible over western and central Europe, with influence on [NO 3 − ] less than 2 % on average and 20 % at the most significant moment. As for a significant impact of the organic coating effect, N 2 O 5 , SOA and NH 3 need to be present when RH is high and T is low. However, those conditions were rarely fulfilled simultaneously over western and central Europe. Hence, the organic coating effect on reaction probability of N 2 O 5 over Europe may not be as important as expected in previous studies.

Hygroscopic growth of aerosol particles is of significant importance in quantifying the aerosol radiative effect in the atmosphere. In this study, hygroscopic properties of ambient particles are investigated based on particle chemical composition at a suburban site in the North China Plain during the HaChi campaign (Haze in China) in summer 2009. The size-segregated aerosol particulate mass concentration as well as the particle components such as inorganic ions, organic carbon and water-soluble organic carbon (WSOC) are identified from aerosol particle samples collected with a ten-stage impactor. An iterative algorithm is developed to evaluate the hygroscopicity parameter κ from the measured chemical composition of particles. During the HaChi summer campaign, almost half of the mass concentration of particles between 150 nm and 1 μm is contributed by inorganic species. Organic matter (OM) is abundant in ultrafine particles, and 77% of the particulate mass with diameter (Dp) of around 30 nm is composed of OM. A large fraction of coarse particle mass is undetermined and is assumed to be insoluble mineral dust and liquid water. The campaign's average size distribution of κ values shows three distinct modes: a less hygroscopic mode (Dp < 150 nm) with κ slightly above 0.2, a highly hygroscopic mode (150 nm < Dp < 1 μm) with κ greater than 0.3 and a nearly hydrophobic mode (Dp > 1 μm) with κ of about 0.1. The peak of the κ curve appears around 450 nm with a maximum value of 0.35. The derived κ values are consistent with results measured with a high humidity tandem differential mobility analyzer within the size range of 50–250 nm. Inorganics are the predominant species contributing to particle hygroscopicity, especially for particles between 150 nm and 1 μm. For example, NH4NO3, H2SO4, NH4HSO4 and (NH4)2SO4 account for nearly 90% of κ for particles of around 900 nm. For ultrafine particles, WSOC plays a critical role in particle hygroscopicity due to the predominant mass fraction of OM in ultrafine particles. WSOC for particles of around 30 nm contribute 52% of κ. Aerosol hygroscopicity is related to synoptic transport patterns. When southerly wind dominates, particles are more hygroscopic; when northerly wind dominates, particles are less hygroscopic. Aerosol hygroscopicity also has a diurnal variation, which can be explained by the diurnal evolution of planetary boundary layer, photochemical aging processes during daytime and enhanced black carbon emission at night. κ is highly correlated with mass fractions of SO42−, NO3− and NH4+ for all sampled particles as well as with the mass fraction of WSOC for particles of less than 100 nm. A parameterization scheme for κ is developed using mass fractions of SO42−, NO3−, NH4+ and WSOC due to their high correlations with κ, and κ calculated from the parameterization agrees well with κ derived from the particle's chemical composition. Further analysis shows that the parameterization scheme is applicable to other aerosol studies in China.

The third intensive measurement period (IMP) organised by the European Monitoring and Evaluation Programme (EMEP) under the UNECE CLTRAP took place in summer 2012 and winter 2013, with PM10 filter samples concurrently collected at 20 (16 EMEP) regional background sites across Europe for subsequent analysis of their mineral dust content. All samples were analysed by the same or a comparable methodology. Higher PM10 mineral dust loadings were observed at most sites in summer (0.5–10 µg m−3) compared to winter (0.2–2 µg m−3), with the most elevated concentrations in the southern- and easternmost countries, accounting for 20–40 % of PM10. Saharan dust outbreaks were responsible for the high summer dust loadings at western and central European sites, whereas regional or local sources explained the elevated concentrations observed at eastern sites. The eastern Mediterranean sites experienced elevated levels due to African dust outbreaks during both summer and winter. The mineral dust composition varied more in winter than in summer, with a higher relative contribution of anthropogenic dust during the former period. A relatively high contribution of K from non-mineral and non-sea-salt sources, such as biomass burning, was evident in winter at some of the central and eastern European sites. The spatial distribution of some components and metals reveals the influence of specific anthropogenic sources on a regional scale: shipping emissions (V, Ni, and SO42−) in the Mediterranean region, metallurgy (Cr, Ni, and Mn) in central and eastern Europe, high temperature processes (As, Pb, and SO42−) in eastern countries, and traffic (Cu) at sites affected by emissions from nearby cities.

Sea salt aerosol (SSA) is one of the major components of primary aerosols and has significant impact on the formation of secondary inorganic aerosol particles on a global scale. In this study, the fully online coupled WRF-Chem model was utilized to evaluate the SSA emission scheme and its influence on the nitrate simulation in a case study in Europe during September 10–20, 2013. Meteorological conditions near the surface, wind pattern, and thermal stratification structure were well reproduced by the model. Nonetheless, coarse mode (PM1–10) particle mass concentration was substantially overestimated due to the overestimation of SSA and nitrate. Compared to filter measurements at 4 EMEP stations (coastal stations: Bilthoven, Kollumerwaard and Vredepeel; inland station: Melpitz), the modeled SSA concentrations were overestimated by a factor of 8–20. We found that the overestimation was mainly caused by the overestimated SSA emission over North Sea during September 16–20. Over the coastal regions, the SSA was injected into the continental free troposphere through an “aloft bridge” (about 500 to 1000 meter above the ground), a result of the different thermodynamic properties and planetary boundary layer (PBL) structure between continental and marine regions. The injected SSA was further transported inland and mixed downward to the surface through downdraft and PBL turbulence. This process broadened the influence of SSA to a larger downwind region, for example, leading to an overestimation of SSA at Melpitz, Germany by a factor of ~20. As a result, nitrate partitioning fraction (ratio between particulate nitrate and the summation of particulate nitrate and gas-phase nitric acid) increased by about 0.2 for the coarse mode nitrate due to the overestimation of SSA at Melpitz, but no significant difference in the partitioning fraction for the fine mode nitrate. About 140 % overestimation of the coarse mode nitrate was resulted from the influence of SSA at Melpitz. On the other hand, the overestimation of SSA inhibited the nitrate formation in the fine mode by about 20 %, because of the increased consumption of precursors by coarse mode nitrate formation.

Gaseous sulphuric acid is a key precursor for new particle formation in the atmosphere. Previous experimental studies have confirmed a strong correlation between the number concentrations of freshly formed particles and the ambient concentrations of sulphuric acid. This study evaluates a body of experimental gas phase sulphuric acid concentrations, as measured by Chemical Ionization Mass Spectrometry (CIMS) during six intensive measurement campaigns and one long-term observational period. The campaign datasets were measured in Hyytiälä, Finland, in 2003 and 2007, in San Pietro Capofiume, Italy, in 2009, in Melpitz, Germany, in 2008, in Atlanta, Georgia, USA, in 2002, and in Niwot Ridge, Colorado, USA, in 2007. The long term data were obtained in Hohenpeissenberg, Germany, during 1998 to 2000. The measured time series were used to construct proximity measures ("proxies") for sulphuric acid concentration by using statistical analysis methods. The objective of this study is to find a proxy for sulfuric acid that is valid in as many different atmospheric environments as possible. Our most accurate and universal formulation of the sulphuric acid concentration proxy uses global solar radiation, SO2 concentration, condensation sink and relative humidity as predictor variables, yielding a correlation measure (R) of 0.87 between observed concentration and the proxy predictions. Interestingly, the role of the condensation sink in the proxy was only minor, since similarly accurate proxies could be constructed with global solar radiation and SO2 concentration alone. This could be attributed to SO2 being an indicator for anthropogenic pollution, including particulate and gaseous emissions which represent sinks for the OH radical that, in turn, is needed for the formation of sulphuric acid.

Measurements of aerosol size distribution and different gas and meteorological parameters, made in three polluted sites in Central and Southern Europe: Po Valley, Italy, Melpitz and Hohenpeissenberg in Germany, were analysed for this study to examine which of the meteorological and trace gas variables affect the number concentration of Aitken (Dp= 50 nm) particles. The aim of our study was to predict the number concentration of 50 nm particles by a combination of in-situ meteorological and gas phase parameters. The statistical model needs to describe, amongst others, the factors affecting the growth of newly formed aerosol particles (below 10 nm) to 50 nm size, but also sources of direct particle emissions in that size range. As the analysis method we used multivariate nonlinear mixed effects model. Hourly averages of gas and meteorological parameters measured at the stations were used as predictor variables; the best predictive model was attained with a combination of relative humidity, new particle formation event probability, temperature, condensation sink and concentrations of SO2, NO2 and ozone. The seasonal variation was also taken into account in the mixed model structure. Model simulations with the Global Model of Aerosol Processes (GLOMAP) indicate that the parameterization can be used as a part of a larger atmospheric model to predict the concentration of climatically active particles. As an additional benefit, the introduced model framework is, in theory, applicable for any kind of measured aerosol parameter.

Nucleation experiments starting from the reaction of OH radicals with SO2 have been performed in the IfT-LFT flow tube under atmospheric conditions at 293±0.5 K for a relative humidity of 13–61%. The presence of different additives (H2, CO, 1,3,5-trimethylbenzene) for adjusting the OH radical concentration and resulting OH levels in the range (4–300) ×105 molecule cm−3 did not influence the nucleation process itself. The number of detected particles as well as the threshold H2SO4 concentration needed for nucleation was found to be strongly dependent on the counting efficiency of the used counting devices. High-sensitivity particle counters allowed the measurement of freshly nucleated particles with diameters down to about 1.5 nm. A parameterization of the experimental data was developed using power law equations for H2SO4 and H2O vapour. The exponent for H2SO4 from different measurement series was in the range of 1.7–2.1 being in good agreement with those arising from analysis of nucleation events in the atmosphere. For increasing relative humidity, an increase of the particle number was observed. The exponent for H2O vapour was found to be 3.1 representing an upper limit. Addition of 1.2×1011 molecule cm−3 or 1.2×1012 molecule cm−3 of NH3 (range of atmospheric NH3 peak concentrations) revealed that NH3 has a measureable, promoting effect on the nucleation rate under these conditions. The promoting effect was found to be more pronounced for relatively dry conditions, i.e. a rise of the particle number by 1–2 orders of magnitude at RH = 13% and only by a factor of 2–5 at RH = 47% (NH3 addition: 1.2×1012 molecule cm−3). Using the amine tert-butylamine instead of NH3, the enhancing impact of the base for nucleation and particle growth appears to be stronger. Tert-butylamine addition of about 1010 molecule cm−3 at RH = 13% enhances particle formation by about two orders of magnitude, while for NH3 only a small or negligible effect on nucleation in this range of concentration appeared. This suggests that amines can strongly influence atmospheric H2SO4-H2O nucleation and are probably promising candidates for explaining existing discrepancies between theory and observations.

In anthropogenically influenced atmospheres, sulphur dioxide (SO2) is the main precursor of gaseous sulphuric acid (H2SO4), which in turn is a main precursor for atmospheric particle nucleation. As a result of socio-economic changes, East Germany has seen a dramatic decrease in anthropogenic SO2 emissions between 1989 and present, as documented by routine air quality measurements in many locations. We have attempted to evaluate the influence of changing SO2 concentrations on the frequency and intensity of new particle formation (NPF) using two different data sets (1996–1997; 2003–2006) of experimental particle number size distributions (diameter range 3–750 nm) from the atmospheric research station Melpitz near Leipzig, Germany. Between the two periods SO2 concentrations decreased by 65% on average, while the frequency of NPF events dropped by 45%. Meanwhile, the average formation rate of 3 nm particles decreased by 68% on average. The trends were statistically significant and therefore suggest a connection between the availability of anthropogenic SO2 and freshly formed new particles. In contrast to the decrease in new particle formation, we found an increase in the mean growth rate of freshly nucleated particles (+22%), suggesting that particle nucleation and subsequent growth into larger sizes are delineated with respect to their precursor species. Using three basic parameters, the condensation sink for H2SO4, the SO2 concentration, and the global radiation intensity, we were able to define the characteristic range of atmospheric conditions under which particle formation events take place at the Melpitz site. While the decrease in the concentrations and formation rates of the new particles was rather evident, no similar decrease was found with respect to the generation of cloud condensation nuclei (CCN; particle diameter >100 nm) as a result of atmospheric nucleation events. On the contrary, the production of CCN following nucleation events appears to have increased by tens of percents. Our aerosol dynamics model simulations suggest that such an increase can be caused by the increased particle growth rate.

Atmospheric aerosol particle size distributions at a continental background site in Eastern Germany were examined for a one-year period. Particles were classified using a twin differential mobility particle sizer in a size range between 3 and 800 nm. As a novelty, every second measurement of this experiment involved the removal of volatile chemical compounds in a thermodenuder at 300°C. This concept allowed to quantify the number size distribution of non-volatile particle cores – primarily associated with elemental carbon, and to compare this to the original non-conditioned size distribution. As a byproduct of the volatility analysis, new particles originating from nucleation inside the thermodenuder can be observed, however, overwhelmingly at diameters below 6 nm. Within the measurement uncertainty, every particle down to particle sizes of 15 nm is concluded to contain a non-volatile core. The volume fraction of non-volatile particulate matter (non-conditioned diameter < 800 nm) varied between 10 and 30% and was largely consistent with the experimentally determined mass fraction of elemental carbon. The average size of the non-volatile particle cores was estimated as a function of original non-conditioned size using a summation method, which showed that larger particles (>200 nm) contained more non-volatile compounds than smaller particles (

Elemental Carbon (EC) has significant impact on human health and climate change. In order to evaluate the size segregation of EC emission and investigation of its influence on atmospheric transport processes in Europe, we used the fully coupled online Weather Research and Forecasting/Chemistry model (WRF-Chem) at a resolution of 2 km focusing on a region in Germany, in conjunction with a high-resolution EC emission inventory. The ground meteorology conditions, vertical structure and wind pattern were well reproduced by the model. The simulations of particle number/mass size distributions were evaluated by observations taken at the central European background site Melpitz. The fine mode aerosol was reasonably well simulated, but the coarse mode was substantially overestimated by the model. We found that it was mainly due to the nearby point source plume emitting a high amount of EC in the coarse mode. The comparisons between simulated EC and Multi-angle Absorption Photometers (MAAP) measurements at Melpitz, Leipzig-TROPOS and Bösel indicated that coarse mode EC (ECc) emission in the nearby point sources might be overestimated by a factor of 2–10. The emission fraction of EC in coarse mode was overestimated by about 10–30 % for Russian and 5–10 % for Eastern Europe (e.g.: Poland and Belarus), respectively. This overestimation in ECc emission fraction makes EC particles having less opportunity to accumulate in the atmosphere and participate to the long range transport, due to the shorter lifetime of coarse mode aerosol. The deposition concept model showed that the transported EC mass from Warsaw and Moskva to Melpitz may be reduced by 25–35 and 25–55 % respectively, due to the overestimation of ECc emission fraction. This may partly explain the underestimation of EC concentrations for Germany under eastern wind pattern in some other modelling research.

This study presents a comprehensive and critical assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010). HCCT-2010 was performed in September and October 2010 at Mt. Schmücke in the Thuringian forest, Germany, applying three measurements sites (upwind, in-cloud, downwind) to study physical and chemical aerosol-cloud-interactions. A Lagrangian-type hill cap cloud experiment requires suitable cloud and particularly connected airflow conditions, i.e. representative air masses at the different measurement sites. Therefore, the present study aimed at the identification of time periods during the 6-weeks duration of the campaign, where such conditions were fulfilled and which can be used in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations including the mesoscale flow conditions by means of a classification of advected air masses and calculation of non-dimensional flow parameters (e.g. Froude number), (ii) local meteorological conditions, including synoptic front passages, the presence of orographic or frontal cloudiness, cloud base heights and vertical stratification, and (iii) local flow conditions by means of statistical analyses using the quasi-inert trace gas ozone and selected size bins of particle number size distributions as well as SF6 tracer experiments in the campaign area. A comprehensive analyses using statistical measures such as the COD (Coefficient Of Divergence) and cross-correlation have been carried out for the first time in the context of a Lagrangian-type hill cap cloud experiment. Suitable criteria for the aimed statistical analyses were thus developed and applied in the present study to characterise the local flow connectivity in detail. The comprehensive examination resulted in a total of 14 so-called "Full Cloud Events" (FCE), which are shown to conform to the Lagrange-type experimental philosophy of HCCT-2010. In addition, 15 so-called "Non-Cloud Events" (NCEs) could be established, which can be used as reference cases as they provide similarly suitable flow conditions but no cloud at the summit site. Orographic cloudiness was identified for approx. one third of the FCE periods, while about two thirds were associated to synoptic fronts. The statistical flow analyses indicate the existence of a strong link between the sites during the events, particularly under constant south-westerly flow conditions, high wind speeds and slightly stable stratification. The COD analyses using continuously measured concentrations of ozone and the 49 nm diameter particle bin revealed particularly for COD values below 0.1 very consistent time series, i.e. closely linked air masses between the different sites. The cross-correlation analysis revealed under connected flow conditions typical overflow times of about 15 to 30 min between the two valley sites. Additionally, the performed SF6 tracer experiments during the campaign clearly demonstrate that under appropriate meteorological conditions a Lagrangian-type approach is valid and that the connected flow validation procedure developed in this work is suitable for identifying such conditions. Finally, an overall evaluation of the identified FCEs is presented, which provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010.

In the course of two field campaigns, size-segregated particle samples were collected at nine sites in Germany, including traffic, urban, rural, marine and mountain sites. During the chemical characterisation of the samples some of them were found to contain an unknown substance that was later identified as hydroxymethanesulfonic acid (HMSA). HMSA is known to be formed during the reaction of S(IV) (HSO3− or SO32−) with formaldehyde in the aqueous phase. Due to its stability, HMSA can act as a reservoir species for S(IV) in the atmosphere and is therefore of interest for the understanding of atmospheric sulfur chemistry. However, no HMSA data are available for atmospheric particles from central Europe, and even on a worldwide scale data are scarce. Thus, the present study now provides a representative data set with detailed information on HMSA concentrations in size-segregated central European aerosol particles. HMSA mass concentrations in this data set were highly variable: HMSA was found in 224 out of 738 samples (30%), sometimes in high mass concentrations exceeding those of oxalic acid. On average over all 154 impactor runs, 31.5 ng m−3 HMSA was found in PM10, contributing 0.21% to the total mass. The results show that the particle diameter, the sampling location, the sampling season and the air mass origin impact the HMSA mass concentration. Highest concentrations were found in the particle fraction 0.42–1.2 μm, at urban sites, in winter and with eastern (continental) air masses, respectively. The results suggest that HMSA is formed during aging of pollution plumes. A positive correlation of HMSA with sulfate, oxalate and PM is found (R2 > 0.4). The results furthermore suggest that the fraction of HMSA in PM slightly decreases with increasing pH.

Five years of PM10 and PM2.5 ambient air measurements at a roadside, an urban, and a regional background site in Leipzig (Germany) were analyzed for violations of the legal PM10 limit value (1999/30/EC, 1999). The annual mean PM10 concentrations at the three sites were well below the legal threshold of 40 μg m−3 (32.6, 22.0 and 21.7 μg m−3, respectively). However, at the roadside site, the daily maximum value of 50 μg m−3 was exceeded on 232 days (13 % of all days) in 2005–2009, which corresponds to 57 days more than warranted by the EC directive. We analysed the meteorological and local source factors that eventually led to these surplus exceedences. Not surprisingly, the highest pollutant concentrations and most exceedance days were observed in winter. Average concentrations for exceedance and non exceedance days of 64 and 28 μg m−3 at roadside and 40 and 19 μg m−3 in the regional background were observed, suggesting urban contributions of 24 and 8 μg m−3, respectively. Statistical and back trajectory cluster analysis yielded the essential result that PM10 concentrations were regionally enhanced during high pressure conditions, characterized by very low temperature, dry air masses, very low wind speeds, and stable stratification. The latter factor was instrumental in generating high PM10 concentrations at roadside as well as in the regional background through pollution trapping below the atmospheric inversion. During winter exceedance days, the highest organic and elemental carbon mass concentrations were measured. The fewest exceedance days were observed during fast moving air masses from the west, characterized by slightly unstable stratification and lower air pressure. During wintrily exceedance days, about half of PM10 at roadside was originating from regional transport and half from the urban-related sources. This result indicates that both are equally important in generating exceedance days in case of favourable meteorological conditions and cannot be separately considered. Our conclusion is that a combined effort of local, national and international reduction measures could most likely avoid systematic exceedances of the daily limit value in the future.

Nitrate and several organic compounds such as dicarboxylic acids (e.g. succinic acid, glutaric acid), Polycyclic Aromatic Hydrocarbons (PAHs) or n-alkanes form the group of the most volatile compounds in atmospheric aerosol particles. The transition of these compounds between gas and particulate phase may significantly change the aerosol particles radiative properties, the heterogeneous chemical properties, and, naturally, the total particulate mass concentration. To better assess these time-dependent effects, three intensive field experiments were conducted in 2008–2009 at the Central European EMEP research station Melpitz (Germany) using an Aerodyne Aerosol Mass Spectrometer (AMS). Data coverage from all seasons highlighted organic matter as being the most important particulate fraction during summertime, while the nitrate fraction was more prevalent in winter. The variation in particulate nitrate was inherently linked to the gas-to-particle-phase equilibrium of ammonium nitrate, which depends on ambient temperature and relative humidity. During short episodes immediately after dawn, the particulate nitrate seems to disobey this dependency so that additional local nitrate formation, such as from HONO photolysis is needed as an explanation. During the summer 2008's experiment, a remarkable diurnal evolution in the oxidation state of the organic matter became evident, which could be correlated to hydroxyl radical (OH) and ozone concentrations indicating photochemical transformation process. In summer, the organic particulate matter seems to be heavily influenced by regional secondary formation and transformation processes, facilitated by photochemical production processes as well as a diurnal cycling of the substances between the gas and particulate phase. In winter, these processes were obviously much weaker, so that organic matter apparently originated mainly from aged particles and long range transport.