The variability of the relative sensitivity of photochemical ozone formation to volatile organic compounds (VOC) and NO_x emissions, the chemical regime, over Europe during summers 2001 to 2003 is simulated with a regional scale transport-chemistry model. The robustness and variability of chemical regimes is shown. A VOC sensitive regime over North-Western Europe and a mainly NO_x sensitive regime over the Mediterranean basin and Eastern Europe are found, confirming earlier published results. The chemical regime time variability, its robustness with respect to several environmental factors (seasonality, interannual variability) and with respect to model uncertainty are thoroughly analysed. For the regions with well pronounced chemical regimes over North-Western Europe and the Mediterranean, the chemical regime occurrence only slightly depends on the ozone target considered – daily ozone or O_x (= O₃ + NO₂) maximum or mean, AOT's, SOMO35, .... For these regions, differences between particular years and summer months are weak, day to day variability is significant but does not change the occurrence of one or another chemical regime. On the contrary, over North-Eastern Germany, the chemical regime changes form one day to another and is also dependent on the ozone target chosen. Expected decreases in anthropogenic NO_x emissions over Europe since the last and for the next few decades have shifted and will shift chemical regimes to more NO_x sensitive. The predictive skill of chemical regime indicator species is made evident at continental scale, extending their spatial range of applicability with respect to earlier studies. Several sensitivity tests were performed in order to account for major sources of model uncertainty. With the exception of regions near ship tracks over the Mediterranean basin, the spatial pattern of chemical regimes appears to be robust with respect to model uncertainty for all cases tested.

Air pollution in Chinese megacities has reached extremely hazardous levels, and human activities are responsible for the emission or production of large amounts of particulate matter (PM). In addition to PM from anthropogenic sources, natural phenomena, such as dust storms over Asian deserts, may also emit large amounts of PM, which lead episodically to poor air quality over Chinese megacities. In this paper, we quantify the degradation of air quality by dust over Beijing, Chengdu and Shanghai megacities using the three dimensions (3D) chemistry transport model CHIMERE, which simulates dust emission and transport online. In the first part of our work, we evaluate dust emissions using Moderate Resolution Imaging Spectroradiometer (MODIS) and Infrared Atmospheric Sounding Interferometer (IASI) satellite observations of aerosol optical depth, respectively, in the visible and the thermal infrared over source areas. PM simulations were also evaluated compared to surface monitoring stations. Then, mineral dust emissions and their impacts on particle composition of several Chinese megacities were analyzed. Dust emissions and transport over China were simulated during three years (2011, 2013 and 2015). Annual dust contributions to the PM ${}_{10}$ budget over Beijing, Chengdu and Shanghai were evaluated respectively as 6.6%, 9.5% and 9.3%. Dust outbreaks largely contribute to poor air quality events during springtime. Indeed it was found that dust significantly contribute for 22%, 52% and 43% of spring PM ${}_{10}$ events (for Beijing, Chengdu and Shanghai respectively).

The balance between the cooling and warming effects of aerosol originating from open biomass burning (BB) critically depends on the ratio of its major absorbing and scattering components, such as elemental carbon (EC) and organic carbon (OC), but available direct measurements of this ratio in remote regions are limited and rather uncertain. Here, we propose a method to estimate the EC/OC mass ratio in BB aerosol using continuous observations of aerosol optical properties by the Aerosol Robotic Network (AERONET) and apply it to the data from two AERONET sites situated in Siberia. Our method exploits a robust experimental finding (that was reported recently based on laboratory analysis of aerosol from the combustion of wildland fuels) that the single scattering albedo of BB aerosol particles depends linearly on the EC/(EC + OC) mass ratio. We estimated that the mean value of the EC/OC ratio in BB aerosol observed in summer 2012 was 0.036 (±0.009), which is less than the corresponding value (0.061) predicted in our simulations with a chemistry transport model using the emission factors from the Global Fire Emissions Database 4 (GFED4) fire emission inventory. Based on results of our analysis, we propose a parameterization that allows constraining the EC/OC ratio in BB aerosol with available satellite observations of the absorption and extinction aerosol optical depths.

The content and evolution of brown carbon (BrC) in smokes from Siberian forest fires were analyzed using measurements of the aerosol absorption optical depth (AAOD) at three Russian AERONET stations located in Tomsk, Zvenigorod, and Yekaterinburg. Estimates are obtained of the relative contribution of BrC in fine aerosol particles to the absorption of solar radiation at a wavelength of 440 nm (η_BrC) and, in particular, for an anomalous episode of long-range transport of smokes from Siberia to the European part of Russia in summer 2016. A considerable BrC content is found in smokes in Tomsk and Zvenigorod (where η_BrC is estimated to be 15 and 18% on average). It is noteworthy that no significant η_BrC values were revealed during passage of smokes from Siberian fires over Yekaterinburg. The η_BrC values were found to decrease with aerosol aging under sunlit conditions on the characteristic timescale of about 30 h. At the same time, the measurements in Zvenigorod indicate that the absorption properties of the organic component of smoke aerosol increase during a much longer evolution.

Satellite measurements of the fire radiation power, measurements of atmospheric pollution in the network of GPU Mosekomonitoring stations, and the modern CHIMERE chemical transport model (CHIMERE CTM) are used for estimating the influence that forest fires have on the air pollution level in the Moscow megalopolis region during the summer of 2007. The method by which the radiation power caused by natural fires determined from satellite measurements is converted into emissions of individual model species is described. General problems related to the optimization of estimates of fire emission and the effects caused by them based on the combined use of measurement data on the composition of the atmosphere and the CTM are considered using a concrete example. It is shown, in particular, that the use of the standard least squares method for the optimization of fire emissions from leads in the general case to obtaining biased (underestimated) estimates. The results of calculations consistent with measurements show that forest fires near Moscow can occasionally be responsible for a considerable part of the air pollution observed in Moscow and its vicinities, and they can be the main reason for the high level of atmospheric pollution in some neighboring regions.