Abstract
Abstract. Secondary aerosols (SAs, including secondary organic and inorganic aerosols, SOAs and SIAs) are predominant components of aerosol particles in the North China Plain (NCP), and their formation has significant impacts on the evolution of particle size distribution (PNSD) and hygroscopicity. Previous studies have shown that distinct SA formation mechanisms can dominate under different relative humidity (RH). This would lead to different influences of SA formation on the aerosol hygroscopicity and PNSD under different RH conditions. Based on the measurements of size-resolved particle activation ratio (SPAR), hygroscopicity distribution (GF-PDF), PM2.5 chemical composition, PNSD, meteorology and gaseous pollutants in a recent field campaign, McFAN (Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain), conducted during the autumn–winter transition period in 2018 at a polluted rural site in the NCP, the influences of SA formation on cloud condensation nuclei (CCN) activity and CCN number concentration (NCCN) calculation under different RH conditions were studied. Results suggest that during daytime, SA formation could lead to a significant increase in NCCN and a strong diurnal variation in SPAR at supersaturations lower than 0.07 %. During periods with daytime minimum RH exceeding 50 % (high RH conditions), SA formation significantly contributed to the particle mass and size changes in a broad size range of 150 to 1000 nm, leading to NCCN (0.05 %) increases within the size range of 200 to 500 nm and mass concentration growth mainly for particles larger than 300 nm. During periods with daytime minimum RH below 30 % (low RH conditions), SA formation mainly contributed to the particle mass and size and NCCN changes for particles smaller than 300 nm. As a result, under the same amount of mass increase induced by SA formation, the increase of NCCN (0.05 %) was stronger under low RH conditions and weaker under high RH conditions. Moreover, the diurnal variations of the SPAR parameter (inferred from CCN measurements) due to SA formation varied with RH conditions, which was one of the largest uncertainties within NCCN predictions. After considering the SPAR parameter (estimated through the number fraction of hygroscopic particles or mass fraction of SA), the relative deviation of NCCN (0.05 %) predictions was reduced to within 30 %. This study highlights the impact of SA formation on CCN activity and NCCN calculation and provides guidance for future improvements of CCN predictions in chemical-transport models and climate models.
Highlights
The cloud condensation nuclei (CCN) activity of aerosol particles describes the ability to activate and grow into cloud droplets at given supersaturations and has important impacts on cloud microphysics and the aerosol indirect effect on climate
Where S represents the saturation ratio, ρw is the density of water, Mw is the molecular weight of water, σs/a is the surface tension of the solution–air interface, R is the universal gas constant, T is the temperature, Dd is the diameter of dry particle and Dwet is the diameter of the humidified particle
We focus on the variations of κ values derived from hygroscopocity-tandem differential mobility analyzer (HTDMA) and CCN measurement during the Secondary aerosol (SA) formation events, rather than the closure between κ values derived using different techniques, which will be addressed in an upcoming study
Summary
The cloud condensation nuclei (CCN) activity of aerosol particles describes the ability to activate and grow into cloud droplets at given supersaturations and has important impacts on cloud microphysics and the aerosol indirect effect on climate. SA formation on existing particles, especially under polluted conditions, significantly adds mass to and changes the chemical composition of accumulationmode particles (Farmer et al, 2015), affecting CCN at lower SSs (< 0.2 %) (Wiedensohler et al, 2009; Mei et al, 2013; Yue et al, 2016; Thalman et al, 2017; Duan et al, 2018). At a specific particle size, the CCN activity is determined both by the chemical composition of particles which originally were and stayed this size and that of particles which grew into this size via added SA mass These two groups of particles can exert different variations to CCN activity at the same particle size (Wiedensohler et al, 2009, and references therein). We will study the influence of SA formation on the size-resolved particle activation ratio (SPAR) of accumulation-mode particles in the NCP under different RH conditions, which fills a gap of knowledge within CCN studies in the NCP and may provide guidance for the improvement of current CCN parameterization schemes in chemical-transport and climate models
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