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Siberia is one of few continental regions in the Northern Hemisphere wherethe atmosphere may sometimes approach pristine background conditions. Wepresent the time series of aerosol and carbon monoxide (CO) measurementsbetween September 2006 and December 2011 at the Zotino Tall Tower Observatory(ZOTTO) in Central Siberia (61° N; 89° E). We investigatethe seasonal, weekly and diurnal variations of aerosol properties (includingabsorption and scattering coefficients and derived parameters, such asequivalent black carbon (BCe), Ångström exponent, singlescattering albedo, and backscattering ratio) and the CO mixing ratios.Criteria were established to distinguish polluted from near-pristine airmasses, providing quantitative characteristics for each type. Depending onthe season, 23–36% of the sampling time at ZOTTO was found to berepresentative of a clean atmosphere. The summer pristine data indicate thatprimary biogenic and secondary organic aerosol formation are quite strongparticle sources in the Siberian taiga. The summer seasons 2007–2008 weredominated by an Aitken mode around 80 nm size, whereas the summer 2009 withprevailing easterly winds produced particles in the accumulation mode around200 nm size. We found these differences to be mainly related to airtemperature, through its effect on the production rates of biogenic volatileorganic compounds (VOC) precursor gases. In winter, the particle sizedistribution peaked at 160 nm, and the footprint of clean background air wascharacteristic for aged particles from anthropogenic sources at greatdistances from ZOTTO and diluted biofuel burning emissions from domesticheating. The wintertime polluted air originates mainly from large citiessouth and southwest of the site; these particles have a dominant mode around100 nm, and the ΔBCe / ΔCO ratio of7–11 ng m−3 ppb−1 suggests dominant contributions from coal andbiofuel burning for heating. During summer, anthropogenic emissions are thedominant contributor to the pollution particles at ZOTTO, while only 12%of the polluted events are classified as biomass-burning-dominated, but thenoften associated with extremely high CO concentrations and aerosol absorptioncoefficients. Two biomass-burning case studies revealed different ΔBCe / ΔCO ratios from different fire types, with theagricultural fires in April~2008 yielding a very high ratio of21 ng m−3 ppb−1. Overall, we find that anthropogenic sourcesdominate the aerosol population at ZOTTO most of the time, even duringnominally clean episodes in winter, and that near-pristine conditions areencountered only in the growing season and then only episodically

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