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International audience; At northern mid-latitudes the abundance of tropospheric O3 has increased by a factor of approximately 2 since the 1950s. The cause of this increase is generally attributed to increasing anthropogenic precursor emissions, but present chemical and transport models cannot quantitatively reproduce its magnitude. Here we show another manifestation of changes in O3 abundance - a shift of the seasonal cycle at northern mid-latitudes so that the observed peak concentrations now appear earlier in the year than in previous decades. The rate of this shift has been 3 to 6 days per decade since the 1970s. We examine possible reasons to explain this shift and suggest it is due to changes in atmospheric transport patterns combined with spatial and temporal changes in emissions. Detailed modeling is necessary to test these hypotheses; this investigation will provide useful guidance for improving global chemistry-climate models as well as stringent tests of the model results.

International audience; Continuous measurements of concentrations of reactive gases, radiation, and meteorological parameters are carried out at the Meteorological Observatory Hohenpeissenberg (MOHp) as part of the Global Atmosphere Watch (GAW) Program. NO, NO2, O3 and JNO2 data from a four-year period (March 1999-December 2002) are evaluated for consistency with photochemical steady state (PSS, ?=1) conditions. The extent of deviation from PSS reveals a strong dependence on wind direction at the station. Median values of ? in the south sector are in the range of 2.5-5.7 and show a high variability. In contrast, values for the other directions show a relatively low variability around a median level of 2. When taking into account peroxy radical concentrations (?ext=1) PSS was reached in 13-32% of all cases for the years 1999-2002. The differences in wind sectors can be explained by local effects. It is shown that the height of the sample inlet line, its distance to the forest and the surrounding topography has a strong impact on both the absolute and relative deviations from PSS. Global irradiance and thus, photolysis of NO2 is reduced within the dense forest. Since the reaction of NO with O3 is still proceeding under these conditions, increased NO2/NO ratios are produced locally in air which is transported through the forest and advected to the MOHp site. Estimates of the peroxy radical concentration (RO2) inferred from PSS are compared with peroxy radical measurements made at the site in June 2000 during a three-week campaign. The PSS derived RO2 levels were higher than corresponding measured levels by at least a factor of 2-3. This analysis was made for a wind sector with minimal local effects on PSS. Thus the corresponding ? median of 2 can be regarded as an upper limit for a deviation from PSS due to chemical reactions, i.e. by peroxy radicals and possible other oxidants converting additional NO to NO2.

In spring 2010 the Icelandic volcano Eyjafjallajokull erupted high loads of pyroclastic material into the atmosphere. The eruption cloud reached heights between 4 and 7 km. The volcanic ash advisory centre London, the responsible institution for making forecasts of ash coming to Europe, decided the complete shut down of the air traffic over wide parts of Europe for a sequence of days. From April 13th to 20th there was a stable high pressure system over the Atlantic Ocean. Thus, a rather constant air flow from north-west to south-east prevailed over Europe in the upper and middle troposphere. Due to that the volcanic ash was transported from Iceland over the UK to Central Europe. Furthermore, the weather conditions were sunny, dry, and stable, preventing the ash from being mixed throughout the atmosphere or from being washed out by precipitation. These weather conditions led not only to long range ash transport but also to high PM10 concentrations in the boundary layer from other sources. Thus the question arises whether the volcanic ash contributed to the high PM10 levels near ground or if the reason for the high PM10 concentrations measured by the ground based stations has to be found elsewhere. The aim of this paper is to give a review of some of the findings of the ground based measurements, model calculations, and remote sensing in Central Europe during the volcanic event two years ago, with focus on Germany, France, and Great Britain. A large variety of modelling results on the dispersion of the Eyjafjallajokull plume over Europe can be found in the recent literature. They all showed that the ash plume reached the air space over Central Europe and these findings compare rather nicely to satellite pictures, Lidar measurements, and measurements from aircrafts. In the present paper as an example EURAD and Chimere model simulations will be presented. With regard to lidar and satellite measurements, we will concentrate on the findings by the German Weather Service, the Met Office and of the volcanic ash advisory centre in Great Britain. As an example for the ground based measurements, we will show the findings of Germany and France. To figure out whether and to what extend the elevated PM10 concentrations are influenced by the transported volcanic ash the PM10 samples were analysed for heavy metals, chloride, nitrate, sulphate, and ammonium. The composition was compared with the one of the erupted material as measured by Óskarsson (2010). An additional indicator for volcanic material is the occurrence of elevated SO2 concentrations and of unusual particle shapes that can be detected by scanning electron microscope analyses. The results of the ground based observations in Germany and France are quite similar. There was volcanic ash that reached the surface, especially at the Black Forest as well as in the Alsace region (Colette et al., 2011, LUBW, 2010), where part of the sampled particles have the same chemical composition than the erupted material. Thus about 25 µg/m3 of the PM10 can be ascribed to volcanic ash there, in good agreement with long-range transport modelling studies. Nevertheless in other places, for example in North Rhine-Westphalia (LANUV), the volcanic ash fraction contributed merely about 5 µg/m3 to the PM10 burden in the latter regions. This shows that a combination of all existing methods and a close collaboration should be used to predict the impact of future events.

Exceptional patterns in the diurnal course of ozone mixing ratio at a mountain top site (998 m a.s.l.) were observed during a field experiment (September 2005). They manifested themselves as strong and sudden decreases of ozone mixing ratio levels with a subsequent return to previous levels. Considering corresponding long-term time series (2000–2005) it was found, that such events occur mainly during summer, and affect the mountain top site in about 18% of the summer days. Combining (a) surface layer measurements at mountain summit and at the foot of the mountain, (b) in-situ (tethered balloon) and remote sensing (SODAR-RASS) measurements within the atmospheric boundary layer, the origin of these events of sudden ozone decrease could be attributed to free convection, triggered by a rather frequently occurring wind speed minimum around the location of the mountain.

Continuous measurements of reactive gases, radiation, and meteorological parameters are carried out at the Meteorological Observatory Hohenpeissenberg (MOHp) as part of the Global Atmosphere Watch (GAW) Program. NO, NO2, O3 and JNO2 data from a four year period (March 1999?December 2002) are evaluated for consistency with photochemical steady state (PSS, ?=1) conditions. In average PSS was reached in 17%, 13%, 22% and 32% of all cases for the year 1999, 2000, 2001 and 2002, respectively. The extent of deviation from PSS reveals a strong dependence on wind direction at the station. Median values of ? in the south sector are in the range of 2.5?5.7 and show a high variability. In contrast, values for the other directions show a relatively low variability around a median level of 2. These differences can be explained by local effects. It is shown that the height of the sample inlet line, its distance to the forest and the surrounding topography has a strong impact on both the absolute and relative deviations from PSS. Global irradiance and thus, photolysis of NO2 is reduced within the dense forest. Since the reaction of NO with O3 is still proceeding under these conditions, increased NO2/NO ratios are produced locally in air which is transported through the forest and advected to the MOHp site. Estimates of the peroxy radical concentration (RO2) inferred from PSS are compared with peroxy radical measurements made at the site in June 2000 in a three week campaign. The PSS derived RO2 levels were higher than corresponding measured levels by at least a factor of 2?3. This analysis was made for a wind sector with minimal local effects on PSS. Thus the corresponding ? median of 2 can be regarded as an upper limit for a deviation from PSS due to chemical reactions, i.e. by peroxy radicals and possible other oxidants converting additional NO to NO2.

International audience; Ambient aerosol size distributions (>3 nm) and OH, H2SO4, and terpene concentrations were measured from April 1998 to August 2000 at a rural continental site in southern Germany. New particle formation (NPF) events were detected on 18% of all days, typically during midday hours under sunny and dry conditions. The number of newly formed particles correlated significantly with solar irradiance and ambient levels of H2SO4. A pronounced anti-correlatation of NPF events with the pre-existing particle surface area was identified in the cold season, often associated with the advection of dry and relatively clean air masses from southerly directions (Alps). Estimates of the particle formation rate based on observations were around 1 cm-3 s-1, being in agreement with the predictions of ternary homogeneous H2SO4-NH3-H2O nucleation within a few orders of magnitude. The experimentally determined nucleation mode particle growth rates were on average 2.6 nm h-1, with a fraction of 0.7 nm h-1 being attributed to the co-condensation of H2SO4-H2O-NH3. The magnitude of nucleation mode particle growth was neither significantly correlated to H2SO4, nor to the observed particle formation rate. Turn-over rate calculations of measured monoterpenes and aromatic hydrocarbons suggest that especially the oxidation products of monoterpenes have the capacity to contribute to the growth of nucleation mode particles. Although a large number of precursor gases, aerosol and meteorological parameters were measured, the ultimate key factors controlling the occurence of NPF events could not be identified.

PRICE was organised as a formal comparison of different methods to measure peroxy radicals (HO$_{2}$ and RO$_{2}$). lt was conducted in summer 1994 at the TOR$^{1}$ -station Schauinsland in South-West Germany as a task of the EU-project OCTA$^{2}$. The comparison involved matrix isolation ESR spectroscopy (MIESR) and five chemical amplifiers (CA). The campaign lasted 5 weeks, with ambient measurements being performed over a time period of 2 weeks. Between and after the ambient measurements, the different calibration procedures used for the chemical amplifiers were compared. Data delivery was performed after a formal data protocol. At the beginning of the campaign, relatively large differences in the calibration of theLuminol-chemiluminescence instruments used for detection of the NO$_{2}$ from the chemical amplifiers were found for some groups. A meaningful comparison of the radical measurements thus required harmonisation of the NO$_{2}$ standards. This was achieved by tying all NO$_{2}$ measurements to the NO$_{2}$ calibration of the TOR station and by adopting a common procedure for taking the non-linear response of the Luminol detector into account. Thereafter, the different chemical amplifiers and MIESR agreed within 130% for measurements of HO$_{2}$ radicals produced artificially by H$_{2}$O photolysis. Somewhat larger discrepancies were found for a comparison with CH$_{3}$O$_{2}$ radicals produced by photolysis of CH$_{3}$I and for CH$_{3}$COO$_{2}$ radicals produced by thermolysis of PAN. In ambient air, all instruments found similar diurnal variations in the RO$_{2}$ concentrations, with maximum concentrations around or shortly after noon time. The correlation was best under situations with high wind speeds and low precursor concentrations. Overall, the chemical amplifiers seemed to have less dynamic range than the MIESR, with the highest RO$_{2}$ concentrations found by MIESR being underestimated by the chemical amplifiersby up to a factor of 2. Possible explanations are a lower conversion efficiency for large organic peroxy radicals and/or a strong inverse dependence of the chain length on the relative humidity that was found very recently in laboratory experiments and which was not accounted for by the calibration procedures during PRICE. The decrease in chainlength with increasing relative humidity would indeed bring the data from the chemical amplifiers into mach better agreement with MIESR. Given the magnitude of the effect and its sensitivity to fluctuations in ambient humidity and temperature, correction of the data in retrospect, however, will be extremely difficult if not impossible. Since high quality measurements of peroxy radicals ean provide deep insight in the degradation mechanism of organic compounds and photo-oxidant formation, a vital interest remains in the further development of the CA, including the conversion efficiencies for the higher organic peroxy radicals arid, most important, of the infiuence of humidity on the chain length.1

Das durch den Bundesminister für Forschung und Technologie geförderte Vorhaben ist ein Beitrag zu dem EUROTRAC Subprojekt "Tropospheric Ozone Research" (TOR). Der Förderungszeitraum erstreckte sich vom 1. 1.1989 bis zum 31. 12. 1991. Die Bundeszuwendung machte etwa ein Drittel der gesamten Aufwendungen in dem Vorhaben aus, der Rest wurde aus KFA Mitteln bestritten. Folgende wichtige Arbeiten wurden in dem Vorhaben durchgeführt: • Koordinierung von TOR • Entwicklung und Technologietransfer von Meßgeräten (NO$_{x}$, J$_{NO_2}$, H$_{2}$O$_{2}$) • Hilfestellung bei der Durchführung und Qualitätskontrolle von Stickoxidmessungen in TOR $\bullet$ Aufbau der Meßstation Schauinsland und Durchführung kontinuierlicher Messungen von: - O$_{3}$, H$_{2}$O$_{2}$, organischen Peroxiden (Photooxidantien) - NO, NO$_{2}$, NO$_{y}$, C$_{2}$-C$_{5}$-Kohlenwasserstoffen, CO (O$_{3}$-Vorläufer) - Organischen Nitraten (Indikatoren für Radikalchemie) - Meteorologischen Daten - Photolysefrequenz von NO$_{2}$ $\bullet$ Durchführung von Meßkampagnen zur Untersuchung der Radikalchemie - Messungen der RO$_{2}$-Radikale zur Bestimmung der Ozonproduktionsrate - Messung von NO$_{3}$- und RO$_{2}$-Radikalen bei Nacht zur Aufklärung der Nachtchemie Die Entwicklung von Konzepten zur Bewertung der Ozonbilanz, insbesondere bzgl. der katalytischen Effizienz der Stickoxide bei der Ozonerzeugung, erfordert eine möglichst genaue Differenzierung von Luftmassen nach Herkunft, Belastung mit Spurenstoffen und Alter. Es ist also eine Klassifizierung der Messungen sowohl nach meteorologischen als auch nach luftchemischen Gesichtspunkten notwendig. Auf Grund der besonderen geographischen Lage der TOR-Station Schauinsland unmittelbar am Rand des Oberrheingrabens wird die Meßstation durch unterschiedliche Luftmassen beeinflußt. [...]

Vertical ozone soundings were performed during two scientific shipcruises, ANT-V/5 and ANT-VII/1, of the research vessel "Polarstern" over the Atlantic Ocean between 30 °S and 52 °N in March/April 1987 and September/October 1988, respectively. Vertical profiles of ozone and meteorological parameters like pressure, temperature and relative humidity were measured at different latitudes to obtain meridional distributions of tropospheric/lower stratospheric ozone and water vapor during two different seasons. In this volume we will report the results of the individual soundings and the derived meridional distributions of ozone and water vapor measured during the two cruises.

This report describes the development of a new analytical procedure for the measurement of H$_{2}$O$_{2}$ and other soluble oxidants in the atmosphere. The main goal was to develop an instrument that is light-weight and relatively simple to operate so that it can be flown on meteorological balloons. H$_{2}$O$_{2}$ and its organic homologues are formed in the atmosphere from the recombination of peroxy radicals (HO$_{2}$ and RO$_{2}$). It is thus interesting for the understanding of the radical cycles. Because of its high solubility, H$_{2}$O$_{2}$ is considered to play an important role in the oxidation of SO$_{2}$ to H$_{2}$SO$_{4}$ in fog and cloud droplets. The instrument employs iodometry in combination with electrochemical detection and quantification by means of a modified electrochemical concentration cell as is used in commercial ozone sondes. The oxidation of I$^{-}$ to I$_{2}$, which in the case of H$_{2}$O$_{2}$ is extremely slow, can be sufficiently accelerated through the addition of a molybdate complex acting as a catalyst. Separation of the H$_{2}$O$_{2}$ from the more abundant non soluble oxidants, mainly ozone, is achieved by Henrys law, namely by first allowing the ambient air to be in contact with an aqueous stripping solution. While H$_{2}$O$_{2}$ and other well soluble species are quantitavely dissolved in the water, only 0.003 % of the ozone are sampled. Interferences by reducing species, mainly SO$_{2}$, are avoided if necessary by using 0.5 M H$_{2}$SO$_{4}$ as a stripping solution. The instrument was tested in the laboratory and compared to an already established technique (enzyme catalyzed oxidation of p-hydroxy phenylacetic acid followed by fluorimetry) on our field station Schauinsland. The two instruments showed a very good agreement for measurements of ambient air under various conditions and for H$_{2}$O$_{2}$ concentrations ranging from 0.1 to 1 ppb.

Long-term observations of reactive gases in the troposphere are important for understanding trace gas cycles and the oxidation capacity of the atmosphere, assessing impacts of emission changes, verifying numerical model simulations, and quantifying the interactions between short-lived compounds and climate change. The World Meteorological Organization’s (WMO) Global Atmosphere Watch (GAW) program coordinates a global network of surface stations some of which have measured reactive gases for more than 40 years. Gas species included under this umbrella are ozone, carbon monoxide, nitrogen oxides, and volatile organic compounds (VOCs). There are many challenges involved in setting-up and maintaining such a network over many decades and to ensure that data are of high quality, regularly updated and made easily accessible to users. This overview describes the GAW surface station network of reactive gases, its unique quality management framework, and discusses the data that are available from the central archive. Highlights of data use from the published literature are reviewed, and a brief outlook into the future of GAW is given. This manuscript constitutes the overview of a special feature on GAW reactive gases observations with individual papers reporting on research and data analysis of particular substances being covered by the program.

Decesari, S. ... et. al.-- 24 pages, 15 figures, 4 tables, the supplement related to this article is available online https://doi.org/10.5194/acp-14-12109-2014-supplement, The use of co-located multiple spectroscopic techniques can provide detailed information on the atmospheric processes regulating aerosol chemical composition and mixing state. So far, field campaigns heavily equipped with aerosol mass spectrometers have been carried out mainly in large conurbations and in areas directly affected by their outflow, whereas lesser efforts have been dedicated to continental areas characterised by a less dense urbanisation. We present here the results obtained at a background site in the Po Valley, Italy, in summer 2009. For the first time in Europe, six state-of-the-art spectrometric techniques were used in parallel: aerosol time-of-flight mass spectrometer (ATOFMS), two aerosol mass spectrometers (high-resolution time-of-flight aerosol mass spectrometer - HR-ToF-AMS and soot particle aerosol mass spectrometer - SP-AMS), thermal desorption aerosol gas chromatography (TAG), chemical ionisation mass spectrometry (CIMS) and (offline) proton nuclear magnetic resonance (1H-NMR) spectroscopy. The results indicate that, under high-pressure conditions, atmospheric stratification at night and early morning hours led to the accumulation of aerosols produced by anthropogenic sources distributed over the Po Valley plain. Such aerosols include primary components such as black carbon (BC), secondary semivolatile compounds such as ammonium nitrate and amines and a class of monocarboxylic acids which correspond to the AMS cooking organic aerosol (COA) already identified in urban areas. In daytime, the entrainment of aged air masses in the mixing layer is responsible for the accumulation of low-volatility oxygenated organic aerosol (LV-OOA) and also for the recycling of non-volatile primary species such as black carbon. According to organic aerosol source apportionment, anthropogenic aerosols accumulating in the lower layers overnight accounted for 38% of organic aerosol mass on average, another 21% was accounted for by aerosols recirculated in residual layers but still originating in northern Italy, while a substantial fraction (41 %) was due to the most aged aerosols imported from transalpine areas. The different meteorological regimes also affected the BC mixing state: in periods of enhanced stagnation and recirculation of pollutants, the number fraction of the BC-containing particles determined by ATOFMS was 75% of the total, while in the days of enhanced ventilation of the planetary boundary layer (PBL), such fraction was significantly lower (50 %) because of the relative greater influence of non-BC-containing aerosol local sources in the Po Valley. Overall, a full internal mixing between BC and the nonrefractory aerosol chemical components was not observed during the experiment in this environment. © Author(s) 2014, This work was funded by European integrated project on aerosol cloud climate and air quality interactions (no. 036833-2, EUCAARI). The ERA-Interim data were kindly provided by Silvio Davolio (CNR-ISAC). Data analysis was co-funded by the project PEGASOS (EC FP7-ENV-2010-265148) and by the project SUPERSITO of Region Emilia-Romagna. ACCENTC is also gratefully acknowledged. Finally, Emanuela Finessi (CNR-ISAC, now at University of York) is also gratefully acknowledged for the precious work in aerosol filter collection in the field. Manuel Dall’Osto and Roy M. Harrison thank the UK National Centre for Atmospheric Science for financial support