Enhancement of nanoparticle formation and growth during the COVID-19 lockdown period in urban Beijing

Abstract

Abstract. Influenced by the spread of the global 2019 novel coronavirus (COVID-19) pandemic, primary emissions of particles and precursors associated with anthropogenic activities decreased significantly in China during the Chinese New Year of 2020 and the lockdown period (24 January–16 February 2020). The 2-month measurements of the number size distribution of neutral particles and charged ions showed that during the lockdown (LCD) period, the number concentration of particles smaller than 100 nm decreased by approximately 40 % compared to the pre-LCD period in January. However, the accumulation mode particles increased by approximately 20 % as several polluted episodes contributed to secondary aerosol formation. In this study, new particle formation (NPF) events were found to be enhanced in the nucleation and growth processes during the LCD period, as indicated by the higher formation rate of 2 nm particles (J2) and the subsequent growth rate (GR). The relevant precursors, e.g., SO2 and NO2, showed a clear reduction, and O3 increased by 80 % during LCD period, as compared with pre-LCD. The volatile organic vapors showed different trends due to their sources. The proxy sulfuric acid during the LCD period increased by approximately 26 %, as compared with pre-LCD. The major oxidants (O3, OH, and NO3) of VOCs were also found to be elevated during LCD. That indicated higher J2 and GR (especially below 5 nm) during the LCD period were favored by the increased concentration level of condensing vapors and decreased condensation sink. Several heavy haze episodes have been reported by other studies during the LCD period; however, the increase in nanoparticle number concentration should also be considered. Some typical NPF events produced a high number concentration of nanoparticles that intensified in the following days to create severe aerosol pollution under unfavorable meteorological conditions. Our study confirms a significant enhancement of the nucleation and growth process of nanoparticles during the COVID-19 LCD in Beijing and highlights the necessity of controlling nanoparticles in current and future air quality management.

Generalized Additive Models Analysis of the Atmospheric Pollutants Response to the Emission Reduction and Meteorology During the COVID-19 Lockdown in the North of Africa (Morocco)

“…Figure 1b shows spatial distribution for NO 2 mass concentrations before and during LCD, we clearly observed that the highest concentrations of NO 2 are found in the north and the center (Casablanca, Rabat, and Marrakech), however we show that the level of NO 2 does not exceed limit value (40µg/m 3 ). During the LCD, NO 2 dropped dramatically in all the zone which is in agreement with all that is found by other studies in Europe (chauhan and Singh et al,2021), Asia (Shen et al, 2021) or America (Cazorla et al, 2021), the decrease in NO 2 during LCD is due to the reduction in road tra c which represents the main source of NO 2 . Moreover, we show also that the O 3 and NO 2 spatial distributions are inversely correlated which prove that the increase in O 3 during LCD is due to the reduction in its titration by NO 2 .…”

“…Besides, the industrial activities especially phosphate production has not been completely suspended. Some study also indicate an increase in PM accumulation mode by 20%, this was explained by improvement of nucleation and growth process during the LCD period, due to the higher formation rate of 2nm particles and the subsequent growth rate (Shen et al, 2021).…”

The effect of COVID-19 restrictions on atmospheric new particle formation in Beijing

“…Enhanced NPF during the lockdown period has been reported (Shen et al, 2021b), but without a detailed explanation due to the lack of simultaneous measurements of both particles at the size where NPF starts (e.g., 1.5 nm) and key vapors for NPF, such as SA and OOMs. We fill that gap with comprehensive measurements from urban Beijing covering the lockdown period, enabling the investigation on how NPF responded to the emission reductions during lockdown on molecular and process levels.…”

“…Restrictions on population movement (lockdowns) worldwide led to arguably the most significant reduction of primary anthropogenic emissions in recent history. NOx concentrations declined on average by about 50 -60 % in several European, South American, Indian, and Chinese cities (Sicard et al, 2020;Krecl et al, 2020;Shi and Brasseur, 2020;Agarwal et al, 2020), and mixing ratios of other primary pollutants, such as black carbon (BC), carbon monoxide (CO), sulfur dioxide (SO2), and volatile organic compounds (VOCs) were also reduced in varying degrees (Bao and Zhang, 2020;Chu et al, 2021;Shen et al, 2021b;Xing et al, 2020;Pei et al, 2020).…”

“…This persistent particulate pollution has been attributed to both unfavorable meteorology, such as stagnant meteorological conditions and high relative humidity (RH) (Le et al, 2020;Wang et al, 2020b) and to enhanced atmospheric oxidative capacity caused by increased O3 and NO3 radical formation (Huang et al, 2021;Le et al, 2020). To date, few studies have focused on either atmospheric new particle formation or the overall particle number size distribution (Shen et al, 2021a;Shen et al, 2021b) during the lockdown period, although NPF has been shown to enhance haze formation (Guo et al, 2014;, and the particle number size distribution is known to influence the health effect of particles (Harrison et al, 2010). NPF contains two consecutive stages: formation of particles via molecular clustering followed by particle growth (Kulmala et al, 2014).…”