Significant contribution of organics to aerosol liquid water content in winter in Beijing, China

Xiaoai Jin,Weiqi Xu,Guangyu Chen,Qiuyan Wang,Xinxin Fan,Yuying Wang,Yele Sun,Fang Zhang,Maureen Cribb,Hao Wu,Jingye Ren,Zhanqing Li

doi:10.5194/acp-20-901-2020

Abstract

Abstract. The aerosol liquid water (ALW) content (ALWC), an important component of atmospheric particles, has a significant effect on atmospheric optical properties, visibility and multiphase chemical reactions. In this study, ALWC is determined from aerosol hygroscopic growth factor (GF) and particle number size distribution (PNSD) measurements and is also simulated by ISORROPIA II, a thermodynamic equilibrium model, with measured aerosol chemical composition data taken at an urban site in Beijing from 8 November to 15 December 2017. Rich measurements made during the experiment concerning virtually all aerosol properties allow us not only to derive the ALWC but also to study the contributions by various species for which little has been done in this region. The simulated ALWC including the contribution of organics and the calculated ALWC are highly correlated (coefficient of determination R2=0.92). The ALWC contributed by organics (ALWCOrg) accounts for 30 %±22 % of the total ALWC during the sampling period. These results suggest a significant contribution of organics to ALWC, which is rather different from previous studies that showed negligible contributions by organics. Our results also show that ALWC correlates well with the mass concentrations of sulfate, nitrate, and secondary organic aerosols (SOAs) (R2=0.66, 0.56 and 0.60, respectively). We further noted that accumulation mode particles play a key role in determining ALWC, dominating among all the aerosol modes. ALWC is an exponential function of ambient relative humidity (RH), whose strong diurnal variation influence the diurnal variation of ALWC. However, there is a 3 h lag between the extremes of ALWC and RH values, due to the diurnal variations in PNSD and aerosol chemical composition. Finally, a case study reveals that ALWCOrg plays an important role in the formation of secondary aerosols through multiphase reactions at the initial stage of a heavy-haze episode.

Highlights

China has experienced rapid economic developments during the past few decades, resulting in frequent heavy-haze events
For related humidity (RH) below the simulated DRH (∼ 60 %), particles may still be dry in the ISORROPIA II model, but they may have been hydrated in the real atmosphere
The aerosol liquid water content was calculated using the size-resolved aerosol hygroscopic growth factor and the particle number size distribution measured at a Beijing urban site during the APHH winter campaign (8 November to 15 December 2017)

Summary

Introduction

China has experienced rapid economic developments during the past few decades, resulting in frequent heavy-haze events. Many studies highlight the significance of aerosol liquid water content (ALWC) in the formation of SA through chemical reactions (e.g., Arellanes et al, 2006; Wang et al, 2016; Cheng et al, 2016). This is because ALW can dilute the absolute concentration of solutes, adjust aerosol acidity and serve as a reactant, resulting in increases in trace gas (e.g., N2O5 and HO2) uptake coefficients (Wahner et al, 1998; Bertram et al, 2009; Abbatt et al, 2012). Overall, investigating the formation of SA and haze in northern China requires an examination of ALWC and its factors including aerosol particle number size distribution (PNSD), aerosol chemical composition and ambient related humidity (RH) in this region

Methods

Results

Conclusion