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Accurate measurement of black carbon (BC) mass concentrations in snow and ice is crucial for the assessment of climatic impacts. However, it is difficult to compare methods used to assess BC levels in the literature as they are not the same. The single particle soot photometer (SP2) method appears to be one of the most suitable to measure low concentrations of BC in snow and ice. However, deriving BC concentrations with SP2 is not straightforward and different measurement options may lead to different results. In this paper, we propose an optimized method for the quantification of refractory BC (rBC) in snow and ice samples using SP2. The paper reviews all the steps of rBC determination including SP2 calibration, correction for rBC particle aerosolization, and treatment of the samples. In addition, we compare the SP2 method and the thermal-optical method (Sunset organic carbon (OC)-elemental carbon (EC) aerosol analyzer with EUSAAR-2 protocol), using snow and firn samples with different characteristics from the Greenland Summit, the French Alps, the Caucasus, and the Himalayas. The EC : rBC ratio was 1.8 ± 1.2 for the Greenland site, 0.4 ± 0.2 for the Alpine site, 0.9 ± 0.3 for the Caucasus site, and 3.0 ± 1.2 for the Himalayan site. Careful investigation was undertaken of analytical uncertainties in both methods, concerning the analytical range of detection of BC, aerosolization correction for rBC, filtration efficiency of quartz fiber filter before EC analysis, the impact of dust, and pyrolyzed organic carbon artifacts during EC analysis. We conclude that the complexity of artifacts can lead to inaccurate rBC or EC determination. In particular, we observed significant under-estimation of EC due to incomplete filtration together with positive artifacts caused by OC. These results underline the need for careful assessment of the analytical technique and procedure for correct data interpretation.

A shallow ice core of the southern flank of Nepalese Himalaya range was extracted from the summit of Mera Peak at 6376 m a.s.l. in Nepal. From this core, we have reconstructed the seasonal deposition fluxes of dust and refractory black carbon (rBC) since 1999. This archive presents well preserved seasonal cycles based on monsoonal precipitation pattern. According to the seasonal precipitation regime, 80% of the annual precipitation between June and September, we estimated the surface snow concentrations evolution for these aerosols. The analyzes reveals that mass fluxes are a few orders of magnitude higher for dust (10.2±2.5 g m−2 yr−1) that for rBC (3.2±1.2 mg m−2 yr−1).These data were used to simulate the surface snow albedo changes with time and the induced potential melting related to these impurities. The potential melting associated to joint dust and rBC can reach 660 kg m−2 yr−1, and 220 kg m−2 yr−1 for rBC only under some assumptions. Compared to the melting rate measured by mass and energy balance at 5400 m a.s.l. on Mera glacier, close to the equilibrium altitude, the impact of rBC represents less than 7% of annual potential melting while the joint contribution of dust and rBC of the surface melting represents a maximum 18%. Furthermore, over this 10 yr time span, the fluxes variability in the ice core signal is rather reflecting the variability of the monsoon signal than that of emission intensity.

We applied a climate-chemistry model to evaluate the impact of black carbon (BC) deposition on the Himalayan snow cover from 1998 to 2008. Using a stretched grid with a resolution of 50 km over this complex topography, the model reproduces reasonably well the observations of both the snow cover duration and the seasonal cycle of the atmospheric BC concentration including a maximum in atmospheric BC during the pre-monsoon period. Comparing the simulated BC concentrations in the snow with observations is challenging because of the high spatial variability and the complex vertical distribution of BC in the snow. We estimate that both wet and dry BC depositions affect the Himalayan snow cover reducing its annual duration by one to eight days. The resulting increase of the net shortwave radiation at the surface reaches an annual mean of 1 to 3 W m−2, leading to a localised warming of 0.05 to 0.3 °C.

Cluster particles (0.8–1.9 nm) are key entities involved in nucleation and new particle formation processes in the atmosphere. Cluster ions were characterized in clear sky conditions at the Puy de Dôme station (1465 m a.s.l). The studied dataset spread over five years (February 2007–February 2012), which provided a unique chance to catch seasonal variations of cluster ion properties at high altitude. Statistical values of the cluster ion concentration and diameter are reported for both positive and negative polarities. Cluster ions were found to be ubiquitous at the Puy de Dôme and displayed an annual variation with lower concentrations in spring. Positive cluster ions were less numerous than negative ones but were larger in diameters. Negative cluster ion properties seemed insensitive to the occurrence of a new particle formation (NPF) event while positive cluster ions appeared to be significantly more numerous and larger on event days. The parameters of the balance equation for the positive cluster concentration are reported, separately for the different seasons and for the NPF event days and non-event days. The steady state assumption suggests that the ionization rate is balanced with two sinks which are the ion recombination and the attachment on aerosol particles, referred as "aerosol ion sink". The aerosol ion sink was found to be higher during the warm season and dominated the loss of ions. The positive ionization rates derived from the balance equation were well correlated with the ionization rates obtained from radon measurement, and they were on average higher in summer and fall compared to winter and spring. Neither the aerosol ion sink nor the ionization rate were found to be significantly different on event days compared to non-event days, and thus they were not able to explain the different positive cluster concentrations between event and non-event days. Hence, the excess of positive small ions on event days may derive from an additional source of ions coupled with the fact that the steady state was not verified on event days.

1 Forschungszentrum Julich GmbH, Institut fur Energieund Klimaforschung IEK-8, 52425 Julich, Germany 2 NOAA/ESRL Global Monitoring Division, Boulder, CO 80305, USA 3 Norwegian Institute for Air Research (NILU), N-2027 Kjeller, Norway 4 Laboratoire de Glaciologie et Geophysique de l'Environnement, Universite de Grenoble I CNRS, 38402 Saint Martin d'Heres cedex, France 5 Environment Canada, Processes Research Section, Toronto, ON M3H 5T4, Canada 6 Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland 7 Deutsches Fernerkundungsdatenzentrum, DLR, 82234 Oberpfaffenhofen, Germany 8 Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany 9 Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA), Potenza, I-85050, Italy 10 National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan 11 Physikalisch-Meteorologisches Observatorium Davos (PMOD/WRC), 7260 Davos, Switzerland 12 Leibniz Institute for Tropospheric Research, Permoserstr. 15, 04318 Leipzig, Germany 13 Chinese Academy of Meteorological Sciences, 46 Zhong-Guan-Cun S. Av., Beijing 100081, China

Within the frame of the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project the Météo-France aircraft ATR-42 performed 22 research flights, over central Europe and the North Sea during the intensive observation period in May 2008. For the campaign, the ATR-42 was equipped in order to study aerosol physical, chemical and optical properties, as well as cloud microphysics. During the campaign, continental air masses from Eastern and Western Europe were encountered, along with polar and Scandinavian air masses. For the 22 research flights, retroplume analyses along the flight tracks were performed with FLEXPART in order to classify air masses into five sectors of origin which allows for a qualitative evaluation of emission influence on the respective air parcel. In the polluted boundary layer (BL), typical concentrations of particles with diameters larger than 10 nm (N10) are of the order of 5000–6000 cm−3, whereas N10 concentrations of clean air masses were lower than 1300 cm−3. The detection of the largest particle number concentrations occurred in air masses coming from Polar and Scandinavian regions for which an elevated number of nucleation mode (25–28 nm) particles was observed and attributed to new particle formation over open sea. In the free troposphere (FT), typical observed N10 are of the order of 900 cm−3 in polluted air masses and 400–600 cm−3 in clean air masses, respectively. In both layers, the chemical composition of submicron aerosol particles is dominated by organic matter and nitrate in polluted air masses, while, sulphate and ammonium followed by organics dominate the submicron aerosols in clean air masses. The highest CCN/CN ratios were observed within the polar air masses while the CCN concentration values are the highest within the polluted air masses. Within the five air mass sectors defined and the two layers (BL and FT), observations have been distinguished into anticyclonic (first half of May 2008) and cyclonic conditions (second half of May 2008). Strong relationships between meteorological conditions and physical, chemical as well as optical properties are found.

We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time-series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many Northern Hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long timeseries of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.

Within the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, the Meteo France research aircraft ATR-42 was operated from Rotterdam airport during May 2008, to perform scientific flights dedicated to the investigation of aerosol-cloud interactions. Therein, the objective of this study is to illustrate the impact of cloud processing on the aerosol particles physical and chemical properties. The presented results are retrieved from measurements during a double-flight mission from Rotterdam (Netherlands) to Newcastle (UK) and back using data measured with compact Time of Flight Aerosol Mass Spectrometer (cToF-AMS) and Scanning Mobility Particle Sizer (SMPS). Cloud-related measurements during these flights were performed over the North Sea within as well as in close vicinity of a marine stratocumulus cloud layer. Particle physical and chemical properties observed in the close vicinity (V), below and above the stratocumulus cloud show strong differences. Firstly, measurements at constant altitude above the cloud layer show decreasing mass concentrations with decreasing horizontal distance (210–0 km) to the cloud layer by a factor up to 7, whereas below the cloud and by same means of distance, the mass concentrations merely decrease by a factor of 2 on average. Secondly, the averaged aerosol size distributions, observed above and below the cloud layer, are of bimodal character with pronounced minima between Aitken and accumulation mode which is potentially the consequence of cloud processing. Finally, the chemical composition of aerosol particles is strongly dependent on the location relative to the cloud layer (vicinity or below/above cloud). In general, the nitrate and organic fractions decrease with decreasing distance to the cloud, in the transit from cloud–free conditions towards the cloud boundaries. The decrease of nitrate and organic compounds ranges at a factor of three to ten, affecting sulfate and ammonium compounds to be increasingly abundant in the aerosol chemical composition while approaching the cloud layer. Finally, the chemical composition of non-refractory evaporated cloud droplets measured within the cloud shows increased fractions of nitrate and organics (with respect to concentrations found below clouds), but also large amounts of sulfate, thus, related to activation of particles, made up of soluble compounds.

Detailed investigations of the chemical and microphysical properties of atmospheric aerosol particles were performed at the puy-de-Dôme (pdD) research station (1465 m) in autumn (September and October 2008), winter (February and March 2009), and summer (June 2010) using a Time-of-Flight Aerosol Mass Spectrometer. Over the three campaigns, the average mass concentrations of the non-refractory submicron particles ranged from 10 μg m−3 up to 27 μg m−3. Highest nitrate and ammonium mass concentrations were measured during the winter and during periods when marine modified airmasses were arriving at the site, whereas highest concentrations of organic particles were measured during the summer and during periods when continental airmasses arrived at the site. The measurements reported in this paper show that atmospheric particle composition is strongly influenced by both the season and the origin of the airmass. The total organic mass spectra were analysed using positive matrix factorisation to separate individual organic components contributing to the overall organic particle mass concentrations. These organic components include a low volatility oxygenated organic aerosol particle (LV-OOA) and a semi-volatile organic aerosol particle (SV-OOA). Correlations of the LV-OOA components with fragments of m/z 60 and m/z 73 (mass spectral markers of wood burning) during the winter campaign suggest that wintertime LV-OOA are related to aged biomass burning emissions, whereas organic aerosol particles measured during the summer are likely linked to biogenic sources. Equivalent potential temperature calculations, gas-phase, and LIDAR measurements define whether the research site is in the planetary boundary layer (PBL) or in the free troposphere (FT)/residual layer (RL). We observe that SV-OOA and nitrate particles are associated with air masses arriving from the PBL where as particle composition measured from RL/FT airmasses contain high mass fractions of sulphate and LV-OOA. This study provides unique insights into the effects of season and airmass variability on regional aerosol particles measured at an elevated site.

Particle mobility size spectrometers often referred to as DMPS (Differential Mobility Particle Sizers) or SMPS (Scanning Mobility Particle Sizers) have found a wide application in atmospheric aerosol research. However, comparability of measurements conducted world-wide is hampered by lack of generally accepted technical standards with respect to the instrumental set-up, measurement mode, data evaluation as well as quality control. This article results from several instrument intercomparison workshops conducted within the European infrastructure project EUSAAR (European Supersites for Atmospheric Aerosol Research). Under controlled laboratory conditions, the number size distribution from 20 to 200 nm determined by mobility size spectrometers of different design are within an uncertainty range of ±10% after correcting internal particle losses, while below and above this size range the discrepancies increased. Instruments with identical design agreed within ±3% in the peak number concentration when all settings were done carefully. Technical standards were developed for a minimum requirement of mobility size spectrometry for atmospheric aerosol measurements. Technical recommendations are given for atmospheric measurements including continuous monitoring of flow rates, temperature, pressure, and relative humidity for the sheath and sample air in the differential mobility analyser. In cooperation with EMEP (European Monitoring and Evaluation Program), a new uniform data structure was introduced for saving and disseminating the data within EMEP. This structure contains three levels: raw data, processed data, and final particle size distributions. Importantly, we recommend reporting raw measurements including all relevant instrument parameters as well as a complete documentation on all data transformation and correction steps. These technical and data structure standards aim to enhance the quality of long-term size distribution measurements, their comparability between different networks and sites, and their transparency and traceability back to raw data.

The atmospheric concentrations of ¹³⁷Cs, ²¹⁰Pb, and ⁷Be were measured over a three-year period at two research stations located less than 12 km apart and at different altitudes (puy de Dôme, 1465 m a.s.l. and Opme, 660 m a.s.l., France). Seasonal trends in all radionuclides were observed at both stations, with high concentration measured during the summer and low concentrations during the winter. The ²¹⁰Pb concentrations at both stations were similar to each other. Higher concentrations of both ⁷Be and ¹³⁷Cs were measured at puy de Dôme than at Opme. These observations can be explained by the stratospheric and upper tropospheric sources of ⁷Be and the long-range transportation of ¹³⁷Cs at high altitudes. Air mass origins during sampling periods were classified into several groups by their route to the stations (marine, marine modified, continental and mediterranean). We observed that ⁷Be concentrations were constant regardless of the air mass origins, unlike ¹³⁷Cs and ²¹⁰Pb concentrations that increased when influenced by continental air masses. Higher ⁷Be concentrations were observed when air masses were arriving from the upper troposphere than from the boundary layer, the opposite was observed for ¹³⁷Cs. The temporal trend in concentrations of ⁷Be shows good agreement with previous modelling studies suggesting that there is a good understanding of its sources and the atmospheric vertical mixing of this radionuclide. The sources and mixing of ²¹⁰Pb, however, seem to be more complex than it appeared to be in previous modelling studies.

The possible minimal range of reduction in snow surface albedo due to dry deposition of black carbon (BC) in the pre-monsoon period (March–May) was estimated as a lower bound together with the estimation of its accuracy, based on atmospheric observations at the Nepal Climate Observatory-Pyramid (NCO-P) sited at 5079 m a.s.l. in the Himalayan region. We estimated a total BC deposition rate of 2.89 μg m−2 day−1 providing a total deposition of 266 μg m−2 for March–May at the site, based on a calculation with a minimal deposition velocity of 1.0×10−4 m s−1 with atmospheric data of equivalent BC concentration. Main BC size at NCO-P site was determined as 103.1–669.8 nm by correlation analysis between equivalent BC concentration and particulate size distribution in the atmosphere. We also estimated BC deposition from the size distribution data and found that 8.7% of the estimated dry deposition corresponds to the estimated BC deposition from equivalent BC concentration data. If all the BC is deposited uniformly on the top 2-cm pure snow, the corresponding BC concentration is 26.0–68.2 μg kg−1 assuming snow density variations of 195–512 kg m−3 of Yala Glacier close to NCO-P site. Such a concentration of BC in snow could result in 2.0–5.2% albedo reductions. From a simple numerical calculations and if assuming these albedo reductions continue throughout the year, this would lead to a runoff increases of 70–204 mm of water drainage equivalent of 11.6–33.9% of the annual discharge of a typical Tibetan glacier. Our estimates of BC concentration in snow surface for pre-monsoon season can be considered comparable to those at similar altitude in the Himalayan region, where glaciers and perpetual snow region starts in the vicinity of NCO-P. Our estimates from only BC are likely to represent a lower bound for snow albedo reductions, since a fixed slower deposition velocity was used and atmospheric wind and turbulence effects, snow aging, dust deposition, and snow albedo feedbacks were not considered. This study represents the first investigation about BC deposition on snow from atmospheric aerosol data in Himalayas and related albedo effect is especially the first track at the southern slope of Himalayas.

Aerosol mass and the absorbing fraction are important variables, needed to constrain the role of atmospheric particles in the Earth radiation budget, both directly and indirectly through CCN activation. In particular, their monitoring in remote areas and mountain sites is essential for determining source regions, elucidating the mechanisms of long range transport of anthropogenic pollutants, and validating regional and global models. Since March 2006, aerosol mass and black carbon concentration have been monitored at the Nepal Climate Observatory-Pyramid, a permanent high-altitude research station located in the Khumbu valley at 5079 m a.s.l. below Mt. Everest. The first two-year averages of PM1 and PM1-10 mass were 1.94 μg m−3 and 1.88 μg m−3, with standard deviations of 3.90 μg m−3 and 4.45 μg m−3, respectively, while the black carbon concentration average is 160.5 ng m−3, with a standard deviation of 296.1 ng m−3. Both aerosol mass and black carbon show well defined annual cycles, with a maximum during the pre-monsoon season and a minimum during the monsoon. They also display a typical diurnal cycle during all the seasons, with the lowest particle concentration recorded during the night, and a considerable increase during the afternoon, revealing the major role played by thermal winds in influencing the behaviour of atmospheric compounds over the high Himalayas. The aerosol concentration is subject to high variability: in fact, as well as frequent "background conditions" (55% of the time) when BC concentrations are mainly below 100 ng m−3, concentrations up to 5 μg m−3 are reached during some episodes (a few days every year) in the pre-monsoon seasons. The variability of PM and BC is the result of both short-term changes due to thermal wind development in the valley, and long-range transport/synoptic circulation. At NCO-P, higher concentrations of PM1 and BC are mostly associated with regional circulation and westerly air masses from the Middle East, while the strongest contributions of mineral dust arrive from the Middle East and regional circulation, with a special contribution from North Africa and South-West Arabian Peninsula in post-monsoon and winter season.

Intense anthropogenic emissions over the Indian sub-continent lead to the formation of layers of particulate pollution that can be transported to the high altitude regions of the Himalaya-Hindu-Kush (HKH). Aerosol particles contain a substantial fraction of strongly absorbing material, including black carbon (BC), organic compounds (OC), and dust all of which can contribute to atmospheric warming, in addition to greenhouse gases. Using a 3-year record of continuous measurements of aerosol optical properties, we present a time series of key climate relevant aerosol properties including the aerosol absorption (σap) and scattering (σsp) coefficients as well as the single-scattering albedo (w). Results of this investigation show substantial seasonal variability of these properties, with long range transport during the pre- and post-monsoon seasons and efficient precipitation scavenging of aerosol particles during the monsoon season. The monthly averaged scattering coefficients range from 0.1 Mm−1 (monsoon) to 20 Mm−1 while the average absorption coefficients range from 0.5 Mm−1 to 3.5 Mm−1. Both have their maximum values during the pre-monsoon period (April) and reach a minimum during Monsoon (July–August). This leads to w values from 0.86 (pre-monsoon) to 0.79 (monsoon) seasons. Significant diurnal variability due to valley wind circulation is also reported. Using typical air mass trajectories encountered at the station, and aerosol optical depth (aod) measurements, we calculated the resulting direct local radiative forcing due to aerosols. We found that the presence of absorbing particulate material can locally induce an additional top of the atmosphere (TOA) forcing of 10 to 20 W m−2 for the first atmospheric layer (500 m above surface). The TOA positive forcing depends on the presence of snow at the surface, and takes place preferentially during episodes of regional pollution occurring on a very regular basis in the Himalayan valleys. Warming of the first atmospheric layer is paralleled by a substantial decrease of the amount of radiation reaching the surface. The surface forcing is estimated to range from −4 to −20 W m−2 for small-scale regional pollution events and large-scale pollution events, respectively. The calculated surface forcing is also very dependent on surface albedo, with maximum values occurring over a snow-covered surface. Overall, this work presents the first estimates of aerosol direct radiative forcing over the high Himalaya based on in-situ aerosol measurements, and results suggest a TOA forcing significantly greater than the IPCC reported values for green house gases.

While West Africa is recognized as being one of the global hot-spots of atmospheric aerosols, the presence of West African Monsoon is expected to create significant spatial and temporal variations in the regional aerosol properties through mixing particles from various sources (mineral dust, biomass burning, sulfates, sea salt). To improve our understanding of the complexity of the aerosol-cloud system in that region, the African Monsoon Multidisciplinary Analysis (AMMA) project has been launched, providing valuable data sets of in-situ and remote sensing measurements including satellites for extended modeling. The French ATR-42 research aircraft was deployed in Niamey, Niger (13°30' N, 02°05' E) in summer 2006, during the three special observation periods (SOPs) of AMMA. These three SOPs covered both dry and wet periods before and after the onset of the Western African Monsoon. State of the art physico-chemical aerosol measurements on the ATR-42 showed a notable seasonal transition in averaged number size distributions where (i) the Aitken mode is dominating over the accumulation mode during the dry season preceding the monsoon arrival and (ii) the accumulation mode increasingly gained importance after the onset of the West African monsoon and even dominated the Aitken mode after the monsoon had fully developed. An extended analysis of the vertical dependence of size spectra, comparing the three observation periods, revealed that the decreasing concentration of the Aitken mode particles, as we move from SOP1 (June) to SOP2a1 (July), and SOP2a2 (August), was less pronounced in the monsoon layer as compared to the overlying Saharan dust layer and free troposphere. In order to facilitate to all partners within the AMMA community radiative transfer calculations, validation of satellite remote sensors, and detailed transport modeling, the parameters describing the mean log-normally fitted number size distributions as a function of altitude and special observation periods were summarized and subsequently related to simultaneously performed measurements of major aerosol particle chemical composition. Extended TEM-EDX analysis of the chemical composition of single aerosol particles revealed dominance of mineral dust (aluminosilicate) even in the submicron particle size range during the dry period, gradually replaced by prevailing biomass burning and sulfate particles, after the onset the monsoon period. The spatial and temporal evolution from SOP1 to SOP2a1 and SOP2a2 of the particle physical and chemical properties and associated aerosol hygroscopic properties are remarkably consistent.

The paper presents the first 2-years of continuous surface ozone (O3) observations and systematic assessment of the influence of stratospheric intrusions (SI) at the Nepal Climate Observatory at Pyramid (NCO-P; 27°57' N, 86°48' E), located in the Southern Himalayas at 5079 m a.s.l. Continuous O3 monitoring has been carried out at this GAW-WMO station in the framework of the Ev-K2-CNR SHARE and UNEP ABC projects since March 2006. Over the period March 2006–February 2008, an average O3 value of 49±12 ppbv (±1δ) was recorded, with a large annual cycle characterized by a maximum during the pre-monsoon (61±9 ppbv) and a minimum during the monsoon (39±10 ppbv). In general, the average O3 diurnal cycles had different shapes in the different seasons, suggesting an important interaction between the synoptic-scale circulation and the local mountain wind regime. Short-term O3 behaviour in the middle/lower troposphere (e.g. at the altitude level of NCO-P) can be significantly affected by deep SI which, representing the most important natural input for tropospheric O3, can also influence the regional atmosphere radiative forcing. To identify days possibly influenced by SI at the NCO-P, analyses were performed on in-situ observations (O3 and meteorological parameters), total column O3 data from OMI satellite and air-mass potential vorticity provided by the LAGRANTO back-trajectory model. In particular, a specially designed statistical methodology was applied to the time series of the observed and modelled stratospheric tracers. On this basis, during the 2-year investigation, 14.1% of analysed days were found to be affected by SI. The SI frequency showed a clear seasonal cycle, with minimum during the summer monsoon (1.2%) and higher values during the rest of the year (21.5%). As suggested by the LAGRANTO analysis, the position of the subtropical jet stream could play an important role in determining the occurrence of deep SI transport on the Southern Himalayas. In order to estimate the fraction of O3 due to air-mass transport from the stratosphere at the NCO-P, the 30 min O3 concentrations recorded during the detected SI days were analysed. In particular, in-situ relative humidity and black carbon observations were used to exclude influence from wet and polluted air-masses transported by up-valley breezes. This analysis led to the conclusion that during SI O3 significantly increased by 27.1% (+13 ppbv) with respect to periods not affected by such events. Moreover, the integral contribution of SI (O3S) to O3 at the NCO-P was also calculated, showing that 13.7% of O3 recorded at the measurement site could be attributed to SI. On a seasonal basis, the lowest SI contributions were found during the summer monsoon (less than 0.1%), while the highest were found during the winter period (24.2%). These results indicated that, during non-monsoon periods, high O3 levels could affect NCO-P during SI, thus influencing the variability of tropospheric O3 over the Southern Himalayas. Being a powerful regional greenhouse gas, these results indicate that the evaluation of the current and future regional climate cannot be assessed without properly taking into account the influence of SI to tropospheric O3 in this important area.

In order to gain insights into the characteristics of the mineral dust fraction which actually serves as cloud condensation nuclei (CCN) including the related cloud processing, this study proceeded to directly collect CCN and compare their mixing states with that of the clear-sky aerosol particles. To pursue this goal, the French ATR-42 research aircraft equipped both with a counterflow virtual impactor (CVI) and community aerosol inlet was deployed in Niamey, Niger (13°30´ N, 02°30´ E) in August 2006 during one of the special observation periods (SOP) of the African Monsoon Multidisciplinary Analysis (AMMA) project. Both cloud residual and clear-sky particles were collected separately and later analyzed individually using transmission electron microscope (TEM) and scanning electron microscope coupled with energy dispersive X-ray spectroscopy (SEM-EDX). The analysis revealed interesting characteristics on the coarse dust particles (Dp>1 μm), particularly those which likely had acted as CCN. Traces of heterogeneously formed secondary sulfate, chloride and nitrate were found on many dust particles. These secondary species were particularly enhanced in clouds (i.e. cloud processing). The study illustrates that carbonates (Calcite, Dolomite) contained the secondary species in significantly larger frequency and amount than the silicates (Quartz, Feldspar, Mica, Clay), confirming that carbonates represent the most reactive fraction of the mineral dust. Surprisingly large fraction of the carbonate particles were already found in deliquesced form even in clear-sky conditions, most probably reflecting their extreme hygroscopicity following the reaction with HNO3 gas. There were also some indications that the large carbonate particles may be acting primarily as CCN under very low supersaturations, unless there is sufficient hygroscopic coatings on the silicates particles.

We report on an intercomparison of six different hygroscopicity tandem differential mobility analysers (HTDMAs). These HTDMAs are used worldwide in laboratory experiments and field campaigns to measure the water uptake of aerosol particles and were never intercompared. After an investigation of the different design of the instruments with their advantages and inconveniencies, the methods for calibration, validation and data analysis are presented. Measurements of nebulised ammonium sulphate as well as of secondary organic aerosol generated from a smog chamber were performed. Agreement and discrepancies between the instrument and to the theory are discussed, and final recommendations for a standard instrument are given, as a benchmark for laboratory or field experiments to ensure a high quality of HTDMA data.

Particle number concentration and size distribution are amongst the most important variables needed to constrain the role of the atmospheric particles in the Earth radiative budget. They are also linked to regulated variables such as particle mass (PM) and therefore of interest to air quality studies. However, data on their long-term variability are scarce, in particular at high altitudes where the occurrence of aerosol in elevated layers cannot be resolved from most instruments in space. Therefore it is crucial to provide ground based measurements of suited aerosol variables to obtain closure between all independent information sources. In this paper, we investigate diurnal and seasonal variability of aerosol number concentration and size distribution at the Puy de Dôme research station (France, 1465 m a.s.l.). We report variability of aerosol particle total number concentration measured over a five years (2003–2007) period and aerosol size distributions over a one year period (January to December 2006). Concentrations show a strong seasonality with maxima during summer and minima during winter. A diurnal variation is also observed with maxima between 12:00 and 18:00 UTC. At night (00:00–06:00 UTC), the median hourly total concentration varies from 600 to 800 cm−3 during winter and from 1700 to 2200 cm−3 during summer. During the day (08:00–18:00 UTC), the concentration is in the range of 700 to 1400 cm−3 during winter and from 2500 to 3500 cm−3 during summer. An averaged size distribution of particles (10–500 nm) was calculated for each season. A variability in the size of aerosols sampled at the Puy de Dôme is also observed on the seasonal and diurnal basis. Because the site lies in the free troposphere only a fraction of the time, in particular at night and during the winter season, we have subsequently analyzed the variability for free tropospheric conditions only. We show that the variability is due to both seasonal changes in air mass origin from winter to summer and enhanced concentration of the free troposphere in summer. The later observation can be explained by higher emission intensity in the boundary layer, stronger exchange between the boundary layer and the free troposphere as well as enhanced photochemical processes. Finally, aerosol mean size distributions are calculated for a given air mass type (marine/continental/regional) according to the season, for the specific conditions of the free troposphere. These results are of regional relevance and can be used to constrain chemical-transport models over Western Europe.