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The effect of new particle formation (NPF) on cloud condensation nuclei (CCN) varies widely in diverse environments. CCN or cloud droplets from NPF sources remain highly uncertain in the urban atmosphere; they are greatly affected by the high background aerosols and frequent local emissions. In this study, we quantified the effect of NPF on cloud droplet number concentration (CDNC, or Nd) at typical updraft velocities (V) in clouds based on field observations on 25 May–18 June 2017 in urban Beijing. We show that NPF increases the Nd by 32 %–40 % at V=0.3–3 m s−1 during the studied period. The Nd is reduced by 11.8 ± 5.0 % at V=3 m s−1 and 19.0 ± 4.5 % at V=0.3 m s−1 compared to that calculated from constant supersaturations due to the water vapor competition effect, which suppresses the cloud droplet formation by decreasing the environmental maximum supersaturation (Smax). The effect of water vapor competition becomes smaller at larger V that can provide more sufficient water vapor. However, under extremely high aerosol particle number concentrations, the effect of water vapor competition becomes more pronounced. As a result, although a larger increase of CCN-sized particles by NPF events is derived on clean NPF days when the number concentration of preexisting background aerosol particles is very low, no large discrepancy is presented in the enhancement of Nd by NPF between clean and polluted NPF days. We finally reveal a considerable impact of the primary sources on the evaluation of the contribution of NPF to CCN number concentration (NCCN) and Nd based on a case study. Our study highlights the importance of full consideration of both the environmental meteorological conditions and multiple sources (i.e., secondary and primary) to evaluate the effect of NPF on clouds and the associated climate effects in polluted regions.

The aerosol hygroscopic growth describes the interaction between aerosols and water vapor, which varies largely, depending on the chemical composition, types, and emissions of gas precursors under diverse environments. In this study, we analyzed size-resolved hygroscopic growth measured at five field sites of China by a hygroscopic tandem differential mobility analyzer (H-TDMA). Results show that the probability density function of hygroscopic growth factor (GF-PDF) at the megacity sites of Guangzhou (GZ), Shanghai (SH), and Beijing (BG) was generally with bimodal hydrophobic and hydrophilic modes, while a unimodal hydrophilic mode was dominated at the suburb sites of Xinzhou (XZ) and Xingtai (XT) throughout the measured particle size of 40–200 nm. As a result, the more hygroscopic (MH) mode accounts for a number fraction of >80 % at the suburb sites, compared to only 20 %–40 % for 40 nm particles at the megacity sites. Further analysis shows that the GF value increases with the aggravated PM2.5 pollution at the sites (BG, XZ, and XT) in northern China, but that is not the case for GZ and SH, which are located in the southern regions. The distinct dependence of GF on the variations in PM2.5 concentrations among the sites suggests the spatial variability in particle composition with the evolution of pollution events in different regions of China. Moreover, different particle hygroscopic behaviors during new particle formation (NPF) events were observed at the five sites, reflecting the distinct mechanisms of NPF in diverse atmospheric environments. By including results from more sites, we find that the aerosol particles observed at those suburb sites are basically more hygroscopic than those in megacities. However, a large variability in the hygroscopic parameter κ at a given particle size among different sites is also observed, suggesting a complex impact from local sources and atmospheric processes. The measured dataset is helpful for improving the understanding of the formation of fine particles and the regional environmental and climate change.

The aerosol hygroscopic growth describes the interaction between aerosols and water vapor, which varies largely depending on the chemical composition, types and emissions of gas precursors under diverse environments. In this study, we analysed size-resolved hygroscopic growth measured at five field sites of China by a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA). Results show that the probability density function of hygroscopic growth factor (GF-PDF) at the megacity sites of Guangzhou (GZ), Shanghai (SH) and Beijing (BJ) was generally with bimodal hydrophobic and hydrophilic modes; while a unimodal hydrophilic mode was dominated at the suburb sites of Xinzhou (XZ) and Xingtai (XT) throughout the measured particle size of 40–200 nm. As a result, the more hygroscopic (MH) mode accounts for a number fraction of > 80 % at the suburb sites, compared to only 20–40 % at the megacity sites. Further analysis shows that the GF value increases with the aggravated PM2.5 pollution at the sites (BJ, XZ and XT) of north China, but that is not the case for GZ and SH which are located in the southern regions. The distinct dependence of GF on the variations of PM2.5 concentrations among the sites suggests the spatial variability in particle composition with the evolution of pollution events in different regions of China. Moreover, different particle hygroscopic behaviors during new particle formation (NPF) events were observed at the five sites, reflecting the distinct mechanisms of NPF in diverse atmospheric environments. By including results from more sites, we find that the aerosol particles observed at those suburb sites are basically more hygroscopic than those in megacities. However, a large variability in the hygroscopic parameter κ at a given particle size among different sites is also observed, suggesting a complex impact from local sources and atmospheric processes. The measured dataset is helpful to improve understanding of the formation of fine particles and the regional environmental and climate change.

Atmospheric aging of black carbon (BC) leads to changes in its physiochemical properties, exerting complex effects on environment and climate. In this study, we have conducted laboratory chamber experiments to investigate the effects of BC aging on its morphology, hygroscopicity and optical properties by exposing monodisperse fresh BC particles to ambient ubiquitous species of nitrogen dioxide (NO

Understanding the volatility and mixing state of atmospheric aerosols is important for elucidating the formation of fine particles and to help determining their effect on environment and climate. In this study, the volatility of the fine particles is characterized by the size-dependent volatility shrink factor (VSF) for summer and winter in the urban area of Beijing using measurements of a volatility tandem differential mobility analyzer (VTDMA). We show the volatility of aerosols is always with one high-volatile and one less- or non-volatile mode both in the summer and winter. On average, the particles are more volatile in the summer (with mean VSF of 0.3) than in the winter (with mean VSF of 0.6). The outstanding high-volatile mode around noontime illustrates the role of nucleation in producing more volatile particles in the summer. We further retrieve the mixing state of the ambient fine particles from the size-resolved VSF and find that the non-black carbon (BC) particles that formed from nucleation processes accounted for 52–69 % of the total number concentration in the summer. While, particles containing a refractory core that is thought to be BC-containing particles dominate and contribute 67–77 % toward the total number concentration in the winter. The diurnal cycles of the retrieved aerosol mixing state for the summer further supports the conclusion that nucleation process is the main contributors to non-BC particles. In addition, the extent of aging of BC particles was characterized as the ratio of the BC diameter before and after heating at 300 °C (Dp/Dc), showing that the average ratio of ~2.2 in the winter is higher than the average of ~1.5 in the summer, which indicates that BC aging is more efficient in wintertime, with resulting differences in light absorption enhancement between cold and warm seasons.

New particle formation (NPF) is a large source of cloud condensation nuclei (CCN) and cloud droplet in the troposphere. In this study, we quantified the contribution of NPF to cloud droplet number concentration (CDNC, or Nd) at typical updraft velocities (V) in clouds using a field campaign data of aerosol number size distribution and chemical composition observed on May 25–June 18, 2017 in urban Beijing. We show that the NPF drives the variations of CCN and cloud droplet and increases Nd by 30–33 % at V = 0.3–3 m s−1 in urban atmosphere. A markedly reduction in Nd is observed due to water vapor competition with consideration of actual environmental updraft velocity, decreasing by 11.8 ± 5.0 % at V = 3 m s−1 and 19.0 ± 4.5 % at V = 0.3 m s−1 compared to that from a prescribed supersaturation. The effect of water vapor competition becomes smaller at larger V that can provide more sufficient water vapor. Essentially, water vapor competition led to the reduction in Nd by decreasing the environmental maximum supersaturation (Smax) for the activation of aerosol particles. It is shown that Smax was decreased by 14.5–11.7 % for V = 0.3–3 m s−1. Particularly, the largest suppression of cloud droplet formation due to the water vapor competition is presented at extremely high aerosol particle number concentrations. As a result, although a larger increase of CCN-size particles by NPF event is derived on clean NPF day when pre-existing background aerosol particles are very low, there is no large discrepancy in the enhancement of Nd by NPF between the clean and polluted NPF day. We finally show a considerable impact of the primary sources when evaluating the NPF contribution to cloud droplet based on a case study. Our study highlights the importance of fully consideration of both the environmental meteorological conditions and multiple sources (i.e. secondary and primary) to evaluate the NPF effect on clouds and the associated climate effects in polluted regions.

A variety of legislative actions for air quality improvement have been conducted in China since 2013, and the emission control measures have achieved remarkable reduction in severe haze frequency. The composition of the fine particles may change along with the improved air quality, and thus may induce distinct environmental and climate effects in future. In this study, a very recent long-term dataset (2018–2019) of non-refractory chemical composition measured by a quadrupole aerosol chemical speciation monitor (Q-ACSM) observed in urban Beijing is applied to investigate the changes in ratio of nitrate (NO3−) to sulfate (SO42−) in PM2.5 (particulate matter with diameters of less than 2.5 μm). We show that the ratio of NO3− to SO42− varies seasonally, with a maximum in winter (1.6 ± 1.2) and a minimum in summer (0.7 ± 1.0). Compared with results from earlier studies showing a continuous increase in the ratio of NO3− to SO42− since 1999, a decline in the ratio is found during the period of 2018–2019. This is partially associated with an attenuated nitrate formation likely due to reduced nitrogen oxides emissions since 2016 in China. Our results suggest that the strict reduction control measures in place serve only to improve SO42− pollution in winter but not in summer when high SO42− levels are still observed. SO42− and NO3− concentrations during study periods together comprise 37–53% of PM2.5, presenting significant role in dominating the levels of PM2.5. In addition, we show that the ratio of NO3−/SO42− in warm seasons generally increases with increasing relative humidity (RH) due to enhanced NO3− hydrolysis formation, but with a maximum value of only ~1.0 that is pulled down by the high levels of sulfate in summer, while the dependence of the ratios on RH is more pronounced in cold seasons when multiple factors (regional transportation, planetary boundary layer, PBL and sources of sulfate, etc.) can impact nitrate levels. We finally characterize two typical processes that lead to the rapid accumulation of nitrate in the atmosphere over urban Beijing: the regional transportation and PBL variations, which is found driving heavy haze in cold seasons, and the hydrolysis formation and partitioning of NO3− that tends to impact the diurnal patterns of nitrate in warm seasons.