PNNL

Abstract

New particle formation (NPF) and its subsequent growth are important sources of condensation nuclei (CN) and cloud condensation nuclei (CCN). A number of observed evidence has shown the positive contributions of NPF on CCN under low supersaturation (SS), nevertheless, traditional numerical studies reveal opposite findings with decrease of CCN along with NPF events at low SS, leaving the contradiction unresolved. Here, based on observations in a typical coastal city of Qingdao, we thoroughly evaluate the numbers of CN and CCN using the NPF-explicit WRF-Chem. In terms of CN, the initial simulation shows large biases of particle number concentrations at 10–40 nm (CN10–40) and 40–100 nm (CN40–100). By adjusting the process of gas particle partitioning, including mass accommodation coefficient of sulfuric acid, the emission phase of primary organic aerosol and the condensation amount of nitrate acid, the concomitant improvement of particle growth process yields substantial reduction of overestimates of CN10–40 and CN40–100. Regarding CCN, SOA formed from semivolatile and intermediate volatility organic vapors (SI-SOA) yield is an important contributor. In the default WRF-Chem model setting, the yield of SI-SOA is too high without considering the differences in oxidation rates of the precursors. Through lowering the SI-SOA yield, CCN become much closer to observations at least over the coastal city of Qingdao. Contradictory to the high SI-SOA yield in the default model setting, which has been previously shown the negative effect of NPF on CCN at low SS towards the real atmospheric environment, we find substantial positive contribution of NPF to CCN over the broad areas of China, primarily due to the increase of particle hygroscopicity surpassing the effect of particle size decrease. This study highlights the underestimated role of NPF on CCN previously, calling for urgent attention to re-examine the effect across the globe.