In this study we describe the hygroscopic properties of accumulation-and coarse-mode aerosol particles sampled at the Zotino Tall Tower Observatory (ZOTTO) in central Siberia (61 degrees N, 89 degrees E) from 16 to 21 June 2013. The hygroscopic growth measurements were supplemented with chemical analyses of the samples, including inorganic ions and organic/elemental carbon. In addition, the microstructure and chemical compositions of aerosol particles were analyzed by x-ray micro-spectroscopy (STXM-NEXAFS) and transmission electron microscopy (TEM). A mass closure analysis indicates that organic carbon accounted for 61 and 38% of particulate matter (PM) in the accumulation mode and coarse mode, respectively. The water-soluble fraction of organic matter was estimated to be 52 and 8% of PM in these modes. Sulfate, predominantly in the form of ammoniated sulfate, was the dominant inorganic component in both size modes: similar to 34% in the accumulation mode vs. similar to 47% in the coarse mode. The hygroscopic growth measurements were conducted with a filter-based differential hygroscopicity analyzer (FDHA) over the range of 5-99.4% RH in the hydration and dehydration operation modes. The FDHA study indicates that both accumulation and coarse modes exhibit pronounced water uptake approximately at the same relative humidity (RH), starting at similar to 70 %, while efflorescence occurred at different humidities, i.e., at similar to 35% RH for submicron particles vs. similar to 50% RH for supermicron particles. This similar to 15% RH difference was attributed to higher content of organic material in the submicron particles, which suppresses water release in the dehydration experiments. The kappa mass interaction model (KIM) was applied to characterize and parameterize non-ideal solution behavior and concentration-dependent water uptake by atmospheric aerosol samples in the 5-99.4% RH range. Based on KIM, the volume-based hygroscopicity parameter, kappa(v), was calculated. The kappa(v,ws) value related to the water-soluble (ws) fraction was estimated to be similar to 0.15 for the accumulation mode and similar to 0.36 for the coarse mode, respectively. The obtained kappa(v,ws) for the accumulation mode is in good agreement with earlier data reported for remote sites in the Amazon rain forest (kappa(v) approximate to 0.15) and a Colorado mountain forest (kappa(v) approximate to 0.16). We used the Zdanovskii-Stokes-Robinson (ZSR) mixing rule to predict the chemical composition dependent hygroscopicity, kappa(v,p). The obtained kappa(v,p) values overestimate the experimental FDHA-KIM-derived kappa(v,ws) by factors of 1.8 and 1.5 for the accumulation and coarse modes, respectively. This divergence can be explained by incomplete dissolution of the hygroscopic inorganic compounds resulting from kinetic limitations due to a sparingly soluble organic coating. The TEM and STXM-NEXAFS results indicate that aged submicron (> 300 nm) and supermicron aerosol particles possess core-shell structures with an inorganic core, and are en...

Abstract. We present long-term (5-year) measurements of particulate matter with an upper diameter limit of ∼ 10 µm (PM 10 ), elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) in aerosol filter samples collected at the Zotino Tall Tower Observatory in the middle-taiga subzone (Siberia). The data are complemented with carbon monoxide (CO) measurements. Air mass back trajectory analysis and satellite image analysis were used to characterise potential source regions and the transport pathway of haze plumes. Polluted and background periods were selected using a non-parametric statistical approach and analysed separately. In addition, near-pristine air masses were selected based on their EC concentrations being below the detection limit of our thermal-optical instrument. Over the entire sampling campaign, 75 and 48 % of air masses in winter and in summer, respectively, and 42 % in spring and fall are classified as polluted. The observed background concentrations of CO and EC showed a sine-like behaviour with a period of 365 ± 4 days, mostly due to different degrees of dilution and the removal of polluted air masses arriving at the Zotino Tall Tower Observatory (ZOTTO) from remote sources. Our analysis of the near-pristine conditions shows that the longest periods with clean air masses were observed in summer, with a frequency of 17 %, while in wintertime only 1 % can be classified as a clean. Against a background of low concentrations of CO, EC, and OC in the near-pristine summertime, it was possible to identify pollution plumes that most likely came from crude-oil production sites located in the oil-rich regions of Western Siberia. Overall, our analysis indicates that most of the time the Siberian region is impacted by atmospheric pollution arising from biomass burning and anthropogenic emissions. A relatively clean atmosphere can be observed mainly in summer, when polluted species are removed by precipitation and the aerosol burden returns to near-pristine conditions.

<p><strong>Abstract.</strong> We present long-term (5-year) measurements of particulate matter (PM<sub>10</sub>), elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC) in aerosol filter samples with an upper limit of ~ 10 µm collected at the Zotino Tall Tower Observatory in the middle-taiga subzone (Siberia). The data are complemented with carbon monoxide (CO) measurements. Air mass back trajectory analysis and satellite image analysis were used to characterize potential source regions and the transport pathway of haze plumes. Polluted and background periods were selected using a non-parametric statistical approach and analyzed separately. In addition, near-pristine air masses were selected based on their EC concentrations being below the detection limit of our thermal/optical instrument. Over the entire sampling campaign, 75 % and 48 % of air masses in winter and in summer, respectively, and 42 % in spring and fall are classified as polluted. In the winter season, pollution plumes originated mainly from the big industrialized cities to the south and southwest of the site. During the winter pollution events, the pollution concentration enhancements (Δ values) ratios ∆OC / ∆EC and ∆EC / ∆CO are 3.9 ± 0.6 and 5.8 ± 0.7 ng m<sup>−3</sup> ppb<sup>−1</sup>, respectively, suggesting that the contribution of coal and other fossil fuel burning for heating was dominant. In summertime, pollution plumes arrived at the ZOTTO site from nearby large-scale boreal wildfires, which were observed during the three years from 2011 to 2013. As a result, the seasonal concentrations of CO, PM<sub>10</sub>, and OC were as high as 670 ± 710 ppb, 59 ± 53 µg m<sup>−3</sup>, and 26 ± 27 µg m<sup>−3</sup>, respectively, with ∆OC / ∆EC of 26.2 ± 0.1 and ∆EC / ∆CO of 1.3 ± 0.1 ng m<sup>−3</sup> ppb<sup>−1</sup>. Agricultural fires from the steppe zone of southern Siberia and northern Kazakhstan also accounted for elevated concentrations of CO and carbonaceous species. For one extreme pollution episode observed on 28 April 2010 the CO, PM<sub>10</sub>, EC, and OC concentrations were as high as 261 ± 12 ppb, 54.4 ± 3.7, 1.5 ± 0.3, and 18.9 ± 1.2 µg m<sup>−3</sup>, respectively, with ∆OC / ∆EC = 12.7 ± 2.7 and ∆EC / ∆CO = 14.3 ± 4.4 ng m<sup>−3</sup> ppb<sup>−1</sup>. The observed background concentrations of CO and EC showed a sine-like behavior with a period of 365 ± 4 days, with maximum values in winter of 151 ± 20 ppb and 0.08 ± 0.03 µg m<sup>−3</sup> and minimum values in summer of 114 ± 15 ppb and 0.03 ± 0.02 µg m<sup>−3</sup>, respectively. The observed background concentrations are mostly due to different degrees of dilution and removal of polluted air masses arriving at ZOTTO from remote sources. Our analysis of the near-pristine conditions shows that the longest periods with clean air masses were observed in summer, with a frequency of 17 %, while in wintertime only 1 % can be classified as a clean. In summer, variations in the OC / PM ratio during clean periods closely correlated with those in air temperature, which indicates that biogenic sources of OC formation were dominating. Against a background of low concentrations of CO, EC, and OC in the near-pristine summertime it was possible to identify pollution plumes that most likely came from crude oil production sites located in the oil-rich regions of Western Siberia. Overall, our analysis indicates that most of the time the Siberian region is impacted by atmospheric pollution arising from biomass burning and anthropogenic emissions. A relatively clean atmosphere can be observed mainly in summer, when polluted species are removed by precipitation and the aerosol burden returns to near-pristine conditions.</p>

The results of measurements of hygroscopic properties and chemical analysis of atmospheric aerosol samples collected from June 10 to 20 and December 15 to 25, 2011, at the ZOTTO background stations (60.8A degrees N, 89.35A degrees E) in Central Siberia are presented. The sorption properties of aerosols are studied with the help of a differential analyzer of absorbed water mass in the relative humidity range of 5 to 99%. It has been found that the hygroscopic growth factor of aerosol particles collected during the winter campaign is on average 45% higher than that of the aerosol collected in the summer campaign, which leads to a 40% decrease in the critical supersaturation threshold of cloud activation of particles. The measurement data are analyzed and parameterized using a new approach that takes into account the concentration effects in the particle-water vapor system at low humidities. Based on the chemical analysis, the content of water-soluble substances in the winter sample is 2.5 times higher than in the summer sample. Here, the amount of sulfates and nitrates increases 20 and 88 times, respectively. A trajectory analysis of air mass motion shows that the increased content of inorganic ions in aerosols for the winter sample is caused by long-range transport of pollutants from industrial areas of Siberia. This difference in the chemical composition is the main source of the observed difference in hygroscopic and condensation properties of the aerosol particles

Abstract. In this study we describe the hygroscopic properties of accumulation- and coarse-mode aerosol particles sampled at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia (61° N; 89° E) from 16 to 21 June 2013. The hygroscopic growth measurements were supplemented with chemical analyses of the samples, including inorganic ions and organic/elemental carbon. In addition, the microstructure and chemical composition of aerosol particles were analyzed by X-ray micro-spectroscopy (STXM-NEXAFS) and transmission electron microscopy (TEM). A mass closure analysis indicates that organic carbon accounted for 61 and 38% of PM in the accumulation mode and coarse mode, respectively. The water soluble fraction of organic matter was estimated to be 52 and 8% of PM in these modes. Sulfate, predominantly in the form of ammoniated sulfate, was the dominant inorganic component in both size modes: ∼34% in the accumulation vs. ∼47% in the coarse mode. The hygroscopic growth measurements were conducted with a filter-based differential hygroscopicity analyzer (FDHA) over the range of 5–99.4% RH in the hydration and dehydration operation modes. The FDHA study indicates that both accumulation and coarse modes exhibit pronounced water uptake approximately at the same RH, starting at ∼70%, while efflorescence occurred at different humidities, i.e., at ∼35% RH for submicron particles vs. ∼50% RH for supermicron particles. This ∼15% RH difference was attributed to higher content of organic material in the submicron particles, which suppresses water release in the dehydration experiments. The kappa mass interaction model (KIM) was applied to characterize and parameterize non-ideal solution behavior and concentration-dependent water uptake by atmospheric aerosol samples in the 5–99.4% RH range. Based on KIM, the volume-based hygroscopicity parameter, κv, was calculated. The κv, ws value related to the water soluble (ws) fraction was estimated to be ∼0.15 for the accumulation mode and ∼0.36 for the coarse mode, respectively. The obtained κv, ws for the accumulation mode is in good agreement with earlier data reported for remote sites in the Amazon rain forest (κv ≈ 0.15) and a Colorado boreal forest (κv ≈ 0.16). We used the Zdanovskii–Stokes–Robinson (ZSR) mixing rule to predict the chemical composition dependent hygroscopicity, κv, p. The obtained κv, p values overestimate the experimental FDHA-KIM-derived κv, ws by factors of 1.8 and 1.5 for the accumulation and coarse modes, respectively. This divergence can be partly explained by incomplete dissolution of the hygroscopic inorganic compounds resulting from kinetic limitations due to a sparingly soluble organic coating. The TEM and STXM-NEXAFS results indicate that aged submicron (>300 nm) and supermicron aerosol particles possess core-shell structures with an inorganic core, and are enriched in organic carbon at the mixed particle surface. The direct FDHA kinetic studies provide a bulk diffusion coefficient of water of ∼10−12 cm2 s−1 indicating a semi-solid state of the organic-rich phase leading to kinetic limitations of water uptake and release during hydration and dehydration cycles. Overall the present ZOTTO data set, obtained in the growing season, has revealed a strong influence of organic carbon on the hygroscopic properties of the ambient aerosols. The sparingly soluble organic coating controls hygroscopic growth, phase transitions, and microstructural rearrangement processes. The observed kinetic limitations can strongly influence the outcome of experiments performed on multi-second time scales, such as the commonly applied HTDMA (Hygroscopicity Tandem Differential Mobility Analyzer) and CCNC (Cloud Condensation Nuclei Counter) measurements.