Exploring wintertime regional haze in northeast China: role of coal and biomass burning

As one of the intense anthropogenic emission regions across the relatively high-latitude ( > 40° N) areas on Earth, northeast China faces the serious problem of regional haze during the heating period of the year. Aerosols in polluted haze in northeast China are poorly understood compared with the haze in other regions of China such as the North China Plain. Here, we integrated bulk chemical measurements with single-particle analysis from transmission electron microscopy (TEM), nanoscale secondary ion mass spectrometry (NanoSIMS), and atomic force microscopy (AFM) to obtain morphology, size, composition, aging process, and sources of aerosol particles collected during two contrasting regional haze events (Haze-I and Haze-II) at an urban site and a mountain site in northeast China and further investigated the causes of regional haze formation. Haze-I evolved from moderate (average PM_2.5: 76-108 μgm^-3) to heavy pollution (151-154 μgm^-3), with the dominant PM2:5 component changing from organic matter (OM) (39-45 μgm^-3) to secondary inorganic ions (94-101 μgm^-3). Similarly, TEM observations showed that Srich particles internally mixed with OM (named S-OM) increased from 29% to 60% by number at an urban site and 64% to 74% at a mountain site from the moderate Haze-I to heavy Haze-I events, and 75 %-96% of Haze-I particles included primary OM. We found that change of wind direction caused Haze-I to rapidly turn into Haze-II (185-223 μgm^-3) with predominantly OM (98-133 μgm^-3) and unexpectedly high KC (3.8 μgm^-3). TEM also showed that K-rich particles internally mixed with OM (named K-OM) increased from 4 %-5% by number to 50 %-52 %. The results indicate that there were different sources of aerosol particles causing the Haze-I and Haze-II formation: Haze-I was mainly induced by accumulation of primary OM emitted from residential coal burning and further deteriorated by secondary aerosol formation via heterogeneous reactions; Haze-II was caused by long-range transport of agricultural biomass burning emissions. Moreover, abundant primary OM particles emitted from coal and biomass burning were considered to be one typical brown carbon, i.e., tar balls. Our study highlights that large numbers of light-absorbing tar balls significantly contribute to winter haze formation in northeast China and they should be further considered in climate models.