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

Abstract. Because anthropogenic sulfur dioxide (SO2) emissions have decreased considerably in the last decade, PM2.5 pollution in China has been alleviated to some extent. However, the effects of reduced SO2 on the particle number concentrations and subsequent contributions of grown new particles to cloud condensation nuclei (CCN) populations, particularly at high altitudes with low aerosol number loadings, are poorly understood. In contrast, the increase in provincial forest areas in China with rapid afforestation over the last few decades expectedly increases the biogenic emissions of volatile organic compounds and their oxidized products as nucleating precursors therein. 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 SO2 mixing ratios from 15 ± 13 ppb in 2007 to 1.6 ± 1.6 ppb in 2018, the apparent formation rate (FR) of new particles and the net maximum increase in the nucleation-mode particle number concentration (NMINP) in the spring campaign of 2018 was 2- to 3-fold higher than those in the spring campaign of 2007 with almost the same occurrence frequency of new particle formation (NPF) events. In contrast, the campaign-based comparison showed that the occurrence frequency, in which the maximum geometric median diameter of the grown new particles (Dpgmax) 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 3.7 × 103 cm−3 (on average) in the five campaigns before 2015 to 1.0 × 103 cm−3 (on average) in the two campaigns in 2017–2018. We argue that the increases in the apparent FR and NMINP are 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.

Investigation of Particle Number Concentrations and New Particle Formation With Largely Reduced Air Pollutant Emissions at a Coastal Semi‐Urban Site in Northern China

“…Figure 7c shows a strong linear correlation between the apparent FR and NMINP (r = 0.78, p < 0.01). The fitted equation is highly consistent with our previous results derived from urban, marine, and mountain atmospheres (L. Ma et al, 2021;Man et al, 2015;Zhu et al, 2017Zhu et al, , 2019Zhu et al, , 2021. As mentioned, sulfuric acid formed by the oxidization of SO 2 is commonly considered as the key precursor for ambient nucleation in the continental atmosphere.…”

“…Increased accumulation‐mode PNCs over the last decade were also reported for a mountainous location in the North China Plain, based on wide‐range particle spectrometer (WPS, Model 1000XP, MSP Corporation) measurements, implying that the increase in accumulation‐mode PNCs may occur regionally. However, the increase in accumulation‐mode particles is inconsistent with the decrease in PM 2.5 mass concentrations over a large regional scale (Zhu et al., 2021).…”

“…H. Lee et al, 2019;X. Li et al, 2020;McMurry et al, 2005;Nieminen et al, 2009;Zhu et al, 2021). We proposed that sulfuric acid vapor was sufficient for nucleation and that the NPF intensity was probably determined by the abundance of organic vapors available for participating in nucleation.…”

“…We used seven parameters to evaluate the NPF characteristics: NPF frequency; apparent new particle formation rate (FR); growth rate (GR); condensation sink (CS); net maximum increase in the nucleation-mode particle number concentration (NMINP); maximum size of the geometric median diameter of the new particles (D pgmax ); and the NPF duration (Kulmala et al, 2012;Sihto et al, 2006;Zhu et al, 2019Zhu et al, , 2021. FR and NMINP represent the intensity of an NPF event and are calculated based on the nucleation-mode particles in sizes of 5.6-25 nm.…”

“…CS is the loss rate of condensable vapor molecules onto pre-existing particles and is calculated based on 10-857 nm particles. Details of the calculation of these parameters can be found in numerous studies (e.g., Kulmala et al, 2012;Sihto et al, 2006;Zhu et al, 2019Zhu et al, , 2021 and in the Supporting Information S1.…”