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In this work, an off‐line coupling between the chemistry‐transport model CHIMERE (associated with an aerosol optical module) and the meteorological model Weather Research and Forecasting (WRF) is used to study (1) the direct radiative forcing of pollution aerosols during the heat wave of summer 2003 over western Europe and (2) the possible feedbacks of this direct radiative forcing on the surface‐atmosphere system. Simulations performed for the period 7–15 August 2003 reveal a significant decrease of daily mean solar radiation reaching the surface (ΔFBOA = −(10–30) W/m2) because of back scattering at the top of the atmosphere (ΔFTOA = −(1–12) W/m2) and also absorption of solar radiation by polluted particles (ΔFatm = + (5–23) W/m2). During daytime, the aerosol surface dimming induces a mean reduction of both sensible (16 W/m2) and latent (21 W/m2) heat fluxes emitted by the terrestrial surface, resulting in a radiative cooling of the air near the surface (up to 2.9 K/d at noon). Simultaneously, the absorption of solar energy by aerosols causes an atmospheric radiative heating within the planetary boundary layer reaching 1.20 K/d at noon. As a consequence, the direct radiative effect of aerosols is shown to reduce both the planetary boundary layer height (up to 30%) and the horizontal wind speed (up to 6%); that may have contributed to favor the particulate pollution during the heat wave of summer 2003. Key Points Direct radiative forcing of pollution aerosols Possible feedbacks on the surface‐atmosphere system

In this study, we investigate the shortwave aerosol direct radiative forcing (ADRF) and its feedback on air temperature and atmospheric dynamics during a major fire event that occurred in Russia during August 2010. The methodology is based on an offline coupling between the CHIMERE chemistry-transport and the Weather Research and Forecasting (WRF) models. First, simulations for the period 5–12 August 2010 have been evaluated by using AERONET (AErosol RObotic NETwork) and satellite measurements of the POLarization and Directionality of the Earth's Reflectance (POLDER) and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) sensors. During this period, elevated POLDER aerosol optical thickness (AOT) is found over a large part of eastern Europe, with values above 2 (at 550 nm) in the aerosol plume. According to CALIOP observations, particles remain confined to the first five kilometres of the atmospheric layer. Comparisons with satellite measurements show the ability of CHIMERE to reproduce the regional and vertical distribution of aerosols during their transport from the source region. Over Moscow, AERONET measurements indicate an important increase of AOT (340 nm) from 0.7 on 5 August to 2–4 between 6 and 10 August when the aerosol plume was advected over the city. Particles are mainly observed in the fine size mode (radius in the range 0.2–0.4 μm) and are characterized by elevated single-scattering albedo (SSA) (0.95–0.96 between 440 and 1020 nm). Comparisons of simulations with AERONET measurements show that aerosol physical–optical properties (size distribution, AOT, SSA) have been well simulated over Moscow in terms of intensity and/or spectral dependence. Secondly, modelled aerosol optical properties have been used as input in the radiative transfer code of WRF to evaluate their direct radiative impact. Simulations indicate a significant reduction of solar radiation at the ground (up to 80–150 W m−2 in diurnal averages over a large part of eastern Europe due to the presence of the aerosol plume. This ADRF causes an important reduction of the near-surface air temperature between 0.2 and 2.6° on a regional scale. Moscow has been affected by the aerosol plume, especially between 6 and 10 August. During this period, aerosol causes a significant reduction of surface shortwave radiation (up to 70–84 W m−2 in diurnal averages) with a moderate part (20–30%) due to solar absorption within the aerosol layer. The resulting feedbacks lead to a cooling of the air up to 1.6° at the surface and 0.1° at an altitude of 1500–2000 m (in diurnal averages), that contribute to stabilize the atmospheric boundary layer (ABL). Indeed, a reduction of the ABL height of 13 to 65% has been simulated during daytime in presence of aerosols. This decrease is the result of a lower air entrainment as the vertical wind speed in the ABL is shown to be reduced by 5 to 80% (at midday) when the feedback of the ADRF is taken into account. However, the ADRF is shown to have a lower impact on the horizontal wind speed, suggesting that the dilution of particles would be mainly affected by the weakening of the ABL development and associated vertical entrainment. Indeed, CHIMERE simulations driven by the WRF meteorological fields including this ADRF feedback result in a large increase in the modelled near-surface PM10 concentrations (up to 99%). This is due to their lower vertical dilution in the ABL, which tend to reduce model biases with the ground PM10 values observed over Moscow during this specific period.

In this work, impact of aerosol solar extinction on the photochemistry over eastern Europe during the 2010 wildfires episode is discussed for the period from 5 to 12 August 2010, which coincides to the peak of fire activity. The methodology is based on an online coupling between the chemistry-transport model CHIMERE (extended by an aerosol optical module) and the radiative transfer code TUV. Results of simulations indicate an important influence of the aerosol solar extinction, in terms of intensity and spatial extent, with a reduction of the photolysis rates of NO2 and O3 up to 50 % (in daytime average) along the aerosol plume transport. At a regional scale, these changes in photolysis rates lead to a 3–15 % increase in the NO2 daytime concentration and to an ozone reduction near the surface of 1–12 %. The ozone reduction is shown to occur over the entire boundary layer, where aerosols are located. Also, the total aerosol mass concentration (PM10) is shown to be decreased by 1–2 %, on average during the studied period, caused by a reduced formation of secondary aerosols such as sulfates and secondary organics (4–10 %) when aerosol impact on photolysis rates is included. In terms of model performance, comparisons of simulations with air quality measurements at Moscow indicate that an explicit representation of aerosols interaction with photolysis rates tend to improve the estimation of the near-surface concentration of ozone and nitrogen dioxide as well as the formation of inorganic aerosol species such as ammonium, nitrates and sulfates.

The atmospheric composition measured at the Pic du Midi high-altitude observatory (2875 m MSL) in the French Pyrenees is frequently affected by upward transport of boundary layer air during anabatic circulations at different scales. The Pyrenean Platform for Observation of the Atmosphere (P2OA) includes two observatories located 28 km apart: at the Pic du Midi and at a low-altitude site (580 m MSL) located in the plain north of the mountain chain. From a 10-yr-long data series collected at P2OA, three different methods are used to detect thermally induced circulations. The methods are based on observations collected independently at three key locations in the plain–mountain circulation cell: within the altitude return flow above the plain, close to the surface in the plain, and at the mountaintop. The main aims are 1) to present and compare the three detection methods and 2) to evaluate the impact of thermally driven circulations on in situ air composition measurements at the Pic du Midi. The first method uses radar wind measurements at 3000 and 5000 m above the plain to detect the return flow of the plain–mountain circulation. The second, which is based on surface wind data from the plain site, reveals days during which surface thermally induced winds occur locally. The third method, which is based on surface data at the mountaintop, focuses on diurnal moisture cycles to rank days with decreasing anabatic influence. We then compare the three independent detection methods, discuss possible connections among thermal circulations at different scales and locations, and present an evaluation of their impact on in situ atmospheric composition measurements at Pic du Midi.

The CAPITOUL experiment is a joint experimental effort in urban climate, including the energetic exchanges between the surface and the atmosphere, the dynamics of the boundary layer over the city and its interactions with aerosol chemistry. The campaign took place in the city of Toulouse in southwest France, for one year, from February 2004 to February 2005. This allowed the study of both the day-to-day and seasonal variability of urban climate processes. The observational network included surface stations (meteorology, energy balance, chemistry), profilers and, during intensive observing periods, aircraft and balloons. The urban Surface Energy Balance differs between summer and winter: in summer, the solar heat stored during the previous daytime period is enough to maintain the heat release at night, but in winter, almost all the energy comes from the anthropogenic heat released by space heating. Both processes produce the well known Urban Heat Island (UHI). The city is shown to impact the entire boundary layer on specific days, when an urban breeze is observed. In wintertime, fog is found to be modified due to the vertical structure of the nocturnal boundary layer above the city (which is slightly unstable and not stable). The measurements of aerosol properties in and downwind the city permitted documentation of the urban aerosol as well as the chemical transformation of these aerosols, in particular the ageing of carbonaceous aerosols during transport. The Toulouse aerosol is mainly composed of carbonaceous particles. There is important seasonal variation in the ratio of black carbon to organic carbon, in the concentration of sulfates and nitrates and in the related radiative aerosol impacts. SF6 was released as a tracer in a suburban area of Toulouse during anticyclonic conditions with weak winds. The tracer measurements show dispersion was mainly driven by the surface sensible heat flux, and was highly sensitive to the urban heat island and also to the transport of boundary layer clouds. Modeling was fully integrated into the campaign. Surface energy balance and urban boundary layer processes have already been used to complement the analyses of the physical processes observed during the campaign. Companion papers detail most of these observation or modeling studies.

The aim of this work consists to infer the most probable mixing state of aerosols over the European continent during the heat wave of summer 2003, where large concentrations of biomass burning and anthropogenic aerosols have been observed. The methodology presented here is based on the Single Scattering Albedo (SSA) sensitivity to the mixing state of particles. Three different mixing cases; external mixing, internal mixing, and core‐shell type mixing have been considered. Composite SSA has been computed for this intense pollution event over Europe and are compared with the AErosol RObotic NETwork (AERONET) retrieved SSA values. The most probable mixing state seems to be core‐shell mixing, with secondary aerosols coating over primary soot and mineral dust. This work underlines clearly that this specific representation should be used in modeling exercises for simulating anthropogenic and/or biomass burning direct and semi‐direct aerosol effects and climate impact over the European region.

Intense fires occurred in northwestern Spain on 6 September 2000, filling a valley with smoke haze. Aerosol size distribution measurements were performed during 1 day with a thermal inversion, so the aging process of the smoke aerosol could be closely monitored. In 3.5 h, the fine aerosol increased up to 0.06 μm in the geometric median diameter of the fine mode. This aging process enhanced the scattering ability of aerosols. On the basis of several hypotheses on the data obtained, shortwave radiative forcing at surface level, at top level, and in the atmosphere was estimated: instantaneous surface forcing reached up to between −80.4 and −67.4 W/m2, top of the atmosphere (TOA) instantaneous forcing reached up to between −23.4 and +4.9 W/m2, and instantaneous atmosphere forcing reached up to between +44.2 and +85.3 W/m2. The study reveals not only the absorption of solar radiation in the atmosphere by smoke aerosols but also an aerosol‐induced case study, where TOA cooling forcing shifts to warming for specific aerosol single scattering albedo. The daily mean heating rate of the smoke haze was estimated at 5.9 ± 0.6 K/d.

Black-carbon-containing particles strongly absorb light, causing substantial radiative heating of the atmosphere. The climate-relevant properties of black carbon (BC) are poorly constrained in high-altitude mountain regions, where many complex interactions between BC, radiation, clouds and snow have important climate implications. This study presents 2-year measurements of BC microphysical and optical properties at the Pic du Midi (PDM) research station, a high-altitude observatory located at 2877 m above sea level in the French Pyrenees. Among the long-term monitoring sites in the world, PDM is subject to limited influence from the planetary boundary layer (PBL), making it a suitable site for characterizing the BC in the free troposphere (FT). The classification of the dominant aerosol type using aerosol spectral optical properties indicates that BC is the predominant aerosol absorption component at PDM and controls the variation in single-scattering albedo (SSA) throughout the 2 years. Single-particle soot photometer (SP2) measurements of refractory BC (rBC) show a mean mass concentration (MrBC) of 35 ng m−3 and a relatively constant rBC core mass-equivalent diameter of about 180 nm, which are typical values for remote mountain sites. Combining the MrBC with in situ absorption measurements, a rBC mass absorption cross-section (MACrBC) of 9.2 ± 3.7 m2 g−1 at λ=880 nm has been obtained, which corresponds to an absorption enhancement (Eabs) of ∼2.2 compared to that of bare rBC particles with equal rBC core size distribution. A significant reduction in the ΔMrBC/ΔCO ratio when precipitation occurred along the air mass transport suggests wet removal of rBC. However we found that the wet removal process did not affect the rBC size, resulting in unchanged Eabs. We observed a large seasonal contrast in rBC properties with higher MrBC and Eabs in summer than in winter. In winter a high diurnal variability in MrBC (Eabs) with higher (lower) values in the middle of the day was linked to the injection of rBC originating from the PBL. On the contrary, in summer, MrBC showed no diurnal variation despite more frequent PBL conditions, implying that MrBC fluctuations are rather dominated by regional and long-range transport in the FT. Combining the ΔMrBC/ΔCO ratio with air mass transport analysis, we observed additional sources from biomass burning in summer leading to an increase in MrBC and Eabs. The diurnal pattern of Eabs in summer was opposite to that observed in winter with maximum values of ∼2.9 observed at midday. We suggest that this daily variation may result from a photochemical process driving the rBC mixing state rather than a change in BC emission sources. Such direct 2-year observations of BC properties provide quantitative constraints for both regional and global climate models and have the potential to close the gap between model-predicted and observed effects of BC on the regional radiation budget and climate. The results demonstrate the complex influence of BC emission sources, transport pathways, atmospheric dynamics and chemical reactivity in driving the light absorption of BC.

Atmospheric deposition is an important source of the micronutrient selenium for terrestrial ecosystems and food chains. However, the factors determining the total concentrations and chemical forms (speciation) of selenium in atmospheric deposition remain poorly understood. Here, aerosol samples were collected weekly over 5 years at Pic du Midi Observatory (French Pyrenees), alongside highly temporally resolved samples of aerosols, precipitation, and cloud water taken during a 2-month campaign. Firstly, measurements of selenium, other elements, and water isotopes were combined with sophisticated modelling approaches (aerosol–chemistry–climate SOCOL-AERv2 model and air parcel backward trajectories and Lagrangian moisture source analyses). Aerosol selenium measurements agreed well with SOCOL-AERv2-predicted values, and interestingly, higher fluxes of selenium and other elements were associated with deep convective activity during thunderstorms, highlighting the importance of local cloud dynamics in high deposition fluxes. Our results further indicate the coupling of element and water cycles from source to cloud formation, with decoupling during precipitation due to below-cloud scavenging. Secondly, selenium speciation was investigated in relation to sulfur speciation, organic composition, and moisture sources. While in the 5-year aerosol series, selenite (SeIV) was linked to anthropogenic source factors, in wet deposition it was related to pH and Atlantic moisture sources. We also report an organic selenium fraction, tracing it back to a marine biogenic source in both aerosols and wet deposition. With a comprehensive set of observations and model diagnostics, our study underscores the role of weather system dynamics alongside source contributions in explaining the atmospheric supply of trace elements to surface environments.

High altitude stations are often emphasized as free tropospheric measuring sites but they remain influenced by atmospheric boundary layer (ABL) air masses due to convective transport processes. The local and meso-scale topographical features around the station are involved in the convective boundary layer development and in the formation of thermally induced winds leading to ABL air lifting. The station altitude alone is not a sufficient parameter to characterize the ABL influence. In this study, a topography analysis is performed allowing calculation of a newly defined index called ABL-TopoIndex. The ABL-TopoIndex is constructed in order to correlate with the ABL influence at the high altitude stations and long-term aerosol time series are used to assess its validity. Topography data from the global digital elevation model GTopo30 were used to calculate five parameters for 43 high and 3 middle altitude stations situated on five continents. The geometric mean of these five parameters determines a topography based index called ABL-TopoIndex, which can be used to rank the high altitude stations as a function of the ABL influence. To construct the ABL-TopoIndex, we rely on the criteria that the ABL influence will be low if the station is one of the highest points in the mountainous massif, if there is a large altitude difference between the station and the valleys or high plains, if the slopes around the station are steep, and finally if the inverse drainage basin potentially reflecting the source area for thermally lifted pollutants to reach the site is small. All stations on volcanic islands exhibit a low ABL-TopoIndex, whereas stations in the Himalayas and the Tibetan Plateau have high ABL-TopoIndex values. Spearman's rank correlation between aerosol optical properties and number concentration from 28 stations and the ABL-TopoIndex, the altitude and the latitude are used to validate this topographical approach. Statistically significant (SS) correlations are found between the 5th and 50th percentiles of all aerosol parameters and the ABL-TopoIndex, whereas no SS correlation is found with the station altitude. The diurnal cycles of aerosol parameters seem to be best explained by the station latitude although a SS correlation is found between the amplitude of the diurnal cycles of the absorption coefficient and the ABL-TopoIndex.