Secondary aerosol formation alters CCN activity in the North China Plain

Abstract

Abstract. Secondary aerosols (SAs, including secondary organic and inorganic aerosols,\nSOAs and SIAs) are predominant components of aerosol particles in the North\nChina Plain (NCP), and their formation has significant impacts on the evolution\nof particle size distribution (PNSD) and hygroscopicity. Previous studies\nhave shown that distinct SA formation mechanisms can dominate under\ndifferent relative humidity (RH). This would lead to different influences of\nSA formation on the aerosol hygroscopicity and PNSD under different RH\nconditions. Based on the measurements of size-resolved particle activation\nratio (SPAR), hygroscopicity distribution (GF-PDF), PM 2.5 chemical\ncomposition, PNSD, meteorology and gaseous pollutants in a recent field\ncampaign, McFAN (Multiphase chemistry experiment in Fogs and Aerosols in the\nNorth China Plain), conducted during the autumn–winter transition period in\n2018 at a polluted rural site in the NCP, the influences of SA formation on\ncloud condensation nuclei (CCN) activity and CCN number concentration\n( NCCN ) calculation under different RH conditions were studied. Results\nsuggest that during daytime, SA formation could lead to a significant\nincrease in NCCN and a strong diurnal variation in SPAR at\nsupersaturations lower than 0.07\u2009%. During periods with daytime minimum\nRH exceeding 50\u2009% (high RH conditions), SA formation significantly\ncontributed to the particle mass and size changes in a broad size range of 150 to 1000\u2009nm, leading to NCCN (0.05\u2009%) increases within the size range of\n200 to 500\u2009nm and mass concentration growth mainly for particles larger\nthan 300\u2009nm. During periods with daytime minimum RH below 30\u2009% (low RH\nconditions), SA formation mainly contributed to the particle mass and size and\n NCCN changes for particles smaller than 300\u2009nm. As a result, under the\nsame amount of mass increase induced by SA formation, the increase of\n NCCN (0.05\u2009%) was stronger under low RH conditions and weaker under\nhigh RH conditions. Moreover, the diurnal variations of the SPAR parameter\n(inferred from CCN measurements) due to SA formation varied with RH\nconditions, which was one of the largest uncertainties within NCCN \npredictions. After considering the SPAR parameter (estimated through the\nnumber fraction of hygroscopic particles or mass fraction of SA), the\nrelative deviation of NCCN (0.05\u2009%) predictions was reduced to within\n30\u2009%. This study highlights the impact of SA formation on CCN activity and\n NCCN calculation and provides guidance for future improvements of\nCCN predictions in chemical-transport models and climate models.