Increased new particle yields with largely decreased probability of survival to CCN size at the summit of Mt. Tai under reduced SO<sub>2</sub> emissions

Abstract

<p><strong>Abstract.</strong> Because anthropogenic sulfur dioxide (SO<sub>2</sub>) emissions have decreased considerably in the last decade, PM<sub>2.5</sub> pollution in China has been alleviated to some extent. However, the effects of reduced SO<sub>2</sub> on the particle number concentrations and subsequent contributions of grown new particles to the cloud condensation nuclei (CCN) populations, particularly at high altitude with low aerosol number loadings, are poorly understood. In this study, we evaluated the campaign-based measurements made at the summit of Mt. Tai (1534 m a.s.l.) from 2007 to 2018. With the decrease in the SO<sub>2</sub> mixing ratios from 15 ± 13 ppb in 2007 to 1.6 ± 1.6 ppb in 2018, the formation rate of new particles (FR) and the net maximum increase in the nucleation-mode particle number concentration (NMINP) increased by 2–3 fold in 2018 against those in 2007. In contrast, the occurrence frequency of new particle formation (NPF) events in which the maximum geometric median diameter of grown new particles (D<sub>pgmax</sub>) was > 50 nm decreased considerably from 43 %–78 % of the NPF events before 2015 to < 12 % in 2017–2018. Assuming > 50 nm as a CCN threshold size at high supersaturations, the observed net CCN production decreased from 3703 cm<sup>−3</sup> (on average) before 2015 to 1026 cm<sup>−3</sup> (on average) in 2017–2018. We argue that the increase in the FR and NMINP is mainly determined by the availability of organic precursors that participate in nucleation and initial growth, whereas the decrease in the growth probability is caused by the reduced emissions of anthropogenic precursors. However, large uncertainties still exist because of a lack of data on the chemical composition of these smaller particles.</p>