Differences in fine particle chemical composition on clear and cloudy days.

Amy E Christiansen,Annmarie G Carlton,Barron H Henderson

doi:10.5194/acp-20-11607-2020

Abstract

Clouds are prevalent and alter PM2.5 mass and chemical composition. Cloud-affected satellite retrievals are often removed from data products, hindering estimates of tropospheric chemical composition during cloudy times. We examine surface fine particulate matter (PM2.5) chemical constituent concentrations in the Interagency Monitoring of PROtected Visual Environments network during Cloudy and Clear Sky times defined using Moderate Resolution Imaging Spectroradiometer (MODIS) cloud flags from 2010-2014 with a focus on differences in particle hygroscopicity and aerosol liquid water (ALW). Cloudy and Clear Sky periods exhibit significant differences in PM2.5 and chemical composition that vary regionally and seasonally. In the eastern US, relative humidity alone cannot explain differences in ALW, suggesting emissions and in situ chemistry exert determining impacts. An implicit clear sky bias may hinder efforts to quantitatively to understand and improve model representation of aerosol-cloud interactions.

Highlights

At any given time, visible clouds cover over 60 % of the Earth’s surface (King et al, 2013), and a warming climate changes cloud patterns (Norris et al, 2016)
There is evidence that successful retrieval frequency contributes to a clear-sky bias in satellite aerosol optical thickness (AOT) products when compared to surface PM2.5 measurements at Interagency Monitoring of PROtected Visual Environments (IMPROVE) monitoring locations in the eastern US
Across the contiguous US (CONUS), significant differences in PM2.5 mass concentrations measured at IMPROVE monitoring locations are observed between cloudy and clear-sky conditions in the majority (> 60 %) of regions in any given season during 2010–2014 (Fig. 1 and A1; Table S2 in the Supplement), especially during summer

Summary

Introduction

Visible clouds cover over 60 % of the Earth’s surface (King et al, 2013), and a warming climate changes cloud patterns (Norris et al, 2016). Convective cloud droplets act as atmospheric aqueous-phase reactors, and their condensed-phase oxidative chemistry generates particle mass aloft, such as sulfate (Zhou et al, 2019), water-soluble organic carbon (Carlton et al, 2008; Duong et al, 2011), and organosulfur compounds (Pratt et al, 2013). Cloud processing alters physical and chemical parameters of boundary layer aerosol that serve as cloud condensation nuclei (CCN). Aqueous chemistry changes aerosol size distribution (Meng and Seinfeld, 1994), hygroscopicity, and the oxygen to carbon (O : C) ratio (Ervens et al, 2018). Aerosol– cloud interactions and impacts are complex and a critical uncertainty in model projections (Fan et al, 2016)

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