Evaluation of the contribution of new particle formation to cloud droplet number concentration in the urban atmosphere

Yele Sun,Jieyao Liu,Sihui Jiang,Lu Chen,Jingye Ren,Fang Zhang,Zhanqing Li,Xing Yan

doi:10.5194/acp-21-14293-2021

Abstract

Abstract. 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.

Highlights

In the global climate system, aerosols, cloud condensation nuclei (CCN) and cloud droplets are very important components
The variation trend of number concentration (NCN) is more correlated with that of NCCN than Nd. This is because the NCCN was calculated based on a constant S rather than referring to the availability of water vapor, while the calculation of Nd is based on the Smax that can be reached in the real atmosphere at a given updraft velocity
We quantified the contribution of new particle formation (NPF) to Nd at typical updraft velocities in clouds using field measurements of aerosol number size distributions and chemical composition in urban Beijing

Summary

Introduction

In the global climate system, aerosols, cloud condensation nuclei (CCN) and cloud droplets are very important components. S. Jiang et al.: The contribution of new particle formation to cloud droplet number concentration et al, 2017; Kerminen et al, 2018; Bousiotis et al, 2019; Zimmerman et al, 2020). It has been found that NPF contributed about 76 % of the total fine particle number concentration in urban Beijing (Wu et al, 2011). These nucleated particles subsequently grow through coagulation or condensation processes to CCN-relevant sizes or act as CCN in convective clouds (Fan et al, 2013; Li et al, 2010). It was estimated that up to 80 % of CCN number concentration (NCCN) is from the nucleation process in urban Beijing (Wiedensohler et al, 2008)

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Conclusion