Long-term measurements of aerosols and carbon monoxide at the ZOTTO tall tower to characterize polluted and pristine air in the Siberian Taiga

Abstract

Abstract. Siberia is one of few background regions in the Northern Hemisphere where the atmosphere may sometimes approach pristine conditions. We present the time series of aerosol and carbon monoxide (CO) measurements between September~2006 and December 2010 at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia (61° N; 90° E). We investigate the seasonal, weekly and diurnal variations of aerosol properties (including absorption and scattering coefficients and derived parameters, like equivalent black carbon (BCe), Ångström exponent, single scattering albedo, and backscattering ratio) and the CO mixing ratios. Criteria were established to distinguish polluted and near-pristine air masses and characterize them separately. Depending on the season, 15–47% of the sampling time at ZOTTO was representative of a clean atmosphere. The summer pristine data indicates that primary biogenic and/or secondary organic aerosol formation are quite strong particle sources in the Siberian Taiga. The summer seasons 2007–2008 are dominated by an Aitken mode of 80 nm size, whereas the summer 2009 with prevailing easterly winds produced aerosols in the accumulation mode around 200 nm size. We found these differences mainly related to air temperature, in parallel with production rates of biogenic volatile organic compounds (VOC). In winter, the footprint and aerosol size distribution (with a peak at 160 nm) of the clean background air are characteristic for aged aerosols from anthropogenic sources at great distances from ZOTTO and diluted biofuel burning emissions from heating. The wintertime polluted air originates from the large cities to the south and southwest of the site; these aerosols have a dominant mode around 100 nm, and the Δ BCe/Δ CO ratio of 7–11 ng m−3 ppb−1 suggests dominant contributions from coal and biofuel burning for heating. During summer, anthropogenic emissions are the dominant contributor to the pollution aerosols at ZOTTO, while only 12% of the polluted events are classified as biomass burning dominated, but then often associated with extremely high CO concentrations and aerosol absorption coefficients. Two biomass-burning case studies revealed different Δ BCe/Δ CO ratios from different fire types, with the agricultural fires in April 2008 yielding a very high ratio of 21 ng m−3 ppb−1. Overall, we find that anthropogenic sources dominate the aerosol population at our site most of the time, even during nominally clean episodes in winter, and that near-pristine conditions are encountered only episodically in the growing season.