Synergy of multiple drivers leading to severe winter haze pollution in a megacity in Northeast China

Severe haze repeatedly shrouded Harbin during the winter of 2019–2020, making it one of the 10 most polluted cities in China for January of 2020. Here we explored the major drivers for the extreme atmospheric pollution events. Enhanced formation of secondary organic and inorganic aerosols was evident at higher pollution levels, accompanied with a sharp growth of aerosol water content (AWC) and a persistent increase of aerosol pH. Non-biomass burning (non-BB) combustion sources were inferred to be relatively stable during the measurement period. The elemental carbon mass apportioned to the non-BB sources exhibited an apparent increasing trend as aerosol concentrations became higher, pointing to the influence of unfavorable dispersion conditions. In addition, biomass burning (BB) emissions were found to be strengthened at higher aerosol concentrations. This trend was associated with a large drop of temperature, presumably indicating that the strengthened BB emissions were due to more intensive use of biomass for heating. In brief, the winter haze events in Harbin were caused by a synergy of three factors, i.e., enhanced secondary aerosol formation, unfavorable dispersion conditions and strengthened BB emissions. A semi-quantitative approach was developed to further estimate the relative importance of these drivers. Strengthened BB emissions were always less responsible for haze events compared to other factors, whereas the less severe and heavily polluted haze events were caused mainly by unfavorable dispersion conditions and enhanced secondary aerosol formation, respectively. This study provides observational evidences for efficient production of secondary species in aerosol water despite the frigid winter in Harbin, and thus highlights the need for effective control of gaseous precursors, both organic and inorganic.