Persistent primary organic tar particles during the regional wintertime hazes inNorth China: insights into their aging and optical changes

Abstract. Primary organic aerosol (POA) is a major component of PM2.5 in the winter polluted air in the North China Plain (NCP), but our understanding on the atmospheric aging process of POA particles and the resulting influences on their optical properties is limited. As part of the Atmospheric Pollution and Human Health in a Chinese Megacity (APHH-Beijing) programme, we collected airborne particles at an urban site (Beijing) and an upwind rural site (Gucheng, Hebei province) in the NCP during 13–27 November 2016 for microscopic analyses. We confirmed that a distinct group of spherical or irregular POA particles with high viscosity, defined as primary organic tar (POT) particles, was emitted from the domestic coal and biomass burning at the rural site and was further transported to the urban site during the regional wintertime hazes. During the heavily polluted period (PM2.5 > 200 μg m−3), more than 60 % of the POT particles were thickly coated with secondary inorganic aerosols (named as core–shell POT-SIA particle) through the aging process, suggesting that POT particles can provide surfaces for the heterogeneous reactions of SO2 and NOx. As a result, their average particle-to-core ratios at the rural and urban sites in the heavily polluted period increased to 1.60 and 1.67, respectively. Interestingly, we found that the aging process did not change the morphology and sizes of the POT cores, indicating that POT particles are quite inert in the atmosphere and can be transported long distances. We using the Mie theory estimated that the light absorption of individual POT particles was enhanced by ~ 1.39 times in the heavily polluted period at the rural and urban sites due to the lensing effect of secondary inorganic coatings. We highlight that the lensing effect on POT particles should be considered in radiative forcing models and the governments should continue to promote clean energy in rural areas to effectively reduce primary emissions.