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Abstract:
We present long-term (5-year) measurements of particulate matter (PM10), 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−3 ppb−1, 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, PM10, and OC were as high as 670 ± 710 ppb, 59 ± 53 µg m−3, and 26 ± 27 µg m−3, respectively, with ∆OC / ∆EC of 26.2 ± 0.1 and ∆EC / ∆CO of 1.3 ± 0.1 ng m−3 ppb−1. 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, PM10, 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−3, respectively, with ∆OC / ∆EC = 12.7 ± 2.7 and ∆EC / ∆CO = 14.3 ± 4.4 ng m−3 ppb−1. 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−3 and minimum values in summer of 114 ± 15 ppb and 0.03 ± 0.02 µg m−3, 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.
