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Abstract:
Siberia is one of few continental regions in the Northern Hemisphere where the atmosphere may sometimes
approach pristine background conditions. We present the
time series of aerosol and carbon monoxide (CO) measurements
between September 2006 and December 2011 at the
Zotino Tall Tower Observatory (ZOTTO) in Central Siberia
(61 N; 89 E).We investigate the seasonal, weekly and diurnal
variations of aerosol properties (including absorption and
scattering coefficients and derived parameters, such as equivalent
black carbon (BCe), Ångström exponent, single scattering
albedo, and backscattering ratio) and the CO mixing
ratios. Criteria were established to distinguish polluted from
near-pristine air masses, providing quantitative characteristics
for each type. Depending on the season, 23–36% of the
sampling time at ZOTTO was found to be representative of a
clean atmosphere. The summer pristine data indicate that primary
biogenic and secondary organic aerosol formation are
quite strong particle sources in the Siberian taiga. The summer
seasons 2007–2008 were dominated by an Aitken mode
around 80 nm size, whereas the summer 2009 with prevailing
easterly winds produced particles in the accumulation mode
around 200 nm size.We found these differences to be mainly
related to air temperature, through its effect on the production
rates of biogenic volatile organic compounds (VOC) precursor
gases. In winter, the particle size distribution peaked
at 160 nm, and the footprint of clean background air was
characteristic for aged particles from anthropogenic sources
at great distances from ZOTTO and diluted biofuel burning
emissions from domestic heating. The wintertime polluted
air originates mainly from large cities south and southwest
of the site; these particles have a dominant mode around
100 nm, and the 1BCe /1CO ratio of 7–11 ngm−3 ppb−1
suggests dominant contributions from coal and biofuel burning
for heating. During summer, anthropogenic emissions
are the dominant contributor to the pollution particles 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 1BCe /1CO ratios from different fire types, with
the agricultural fires in April 2008 yielding a very high ratio
of 21 ngm−3 ppb−1. Overall, we find that anthropogenic
sources dominate the aerosol population at ZOTTO most of
the time, even during nominally clean episodes in winter, and that near-pristine conditions are encountered only in the growing season and then only episodically
