Differences in fine particle chemical composition on clear and cloudy days

Abstract

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

Effects of COVID-19 lockdowns on fine particulate matter concentrations

“…The inability to retrieve satellite AOD in the presence of snow or cloud cover introduces sampling limitations when calculating PM 2.5 from satellite AOD ( 75 ). We use GEOS-Chem to address these sampling limitations by scaling the satellite AOD by the ratio of simulated monthly mean AOD to simulated AOD coincident with satellite AOD.…”

Ambient Aerosol Is Physically Larger on Cloudy Days in Bondville, Illinois

“…IMPROVE does not measure ammonium ion concentrations at the Bondville location, and neglecting this species in an agricultural area such as Bondville adds uncertainty to ALW estimates. Ammonia is abundant in agricultural areas and facilitates the formation of particulate nitrate, a hygroscopic species with substantial losses from filter measurements, in particular during summer. , Seasonal cycles and temporal trends of ALW estimates are similar with and without ammonium from 2010 through 2015 for IMPROVE sites across the CONUS . At the Bondville location, seasonal average ALW estimates with weekly aggregated ammonium filter measurements included from a nearby CASTNET site are not significantly different at the 95% confidence level (Figure S2).…”

“…With this, "0" and "1" are "cloudy", "2" as "some clouds", and "3" and "4" as "clear sky". limitations associated with the MODIS Cloud Mask used in a previous analysis, 15 which is only available once or twice daily. We match all AERONET observation days to IMPROVE measurement days.…”

“…The aerosol direct effect (perturbation of solar insolation via light scattering and absorption) is a function of particle shape and size, among other variables such as particle composition and refractive index. Atmospheric liquid water uptake by hygroscopic particle-phase chemical constituents and subsequent aqueous-phase reactions has a determining effect on aerosol size. , Liquid water efficiently scatters visible light, and its presence in the vertical column contributes to aerosol optical depth (AOD) and extinction. − Improved air quality in recent decades over the United States, largely due to decreases in mass concentrations of the hygroscopic sulfate species in fine particulate matter (PM 2.5 particles with an aerodynamic diameter of 2.5 μm), has helped improve visibility and reduce aerosol extinction. − Aerosol indirect effects (perturbation of cloud properties) contribute the largest uncertainties in climate projections. , Christiansen et al (2020) find that PM 2.5 chemical composition, important for overall particle hygroscopicity and water uptake, is significantly different on clear and cloudy days across the contiguous U.S. (CONUS) . This suggests that accurate prediction of aerosol–cloud interactions requires quantitative understanding of aerosol properties during cloudy periods.…”

Evaluating the Impacts of Cloud Processing on Resuspended Aerosol Particles After Cloud Evaporation Using a Particle‐Resolved Model

“…Particles with higher hygroscopicity and larger sizes have smaller critical supersaturation and activate at lower supersaturation level. Aerosol hygroscopicity has been observed to increase significantly because of the increased ammonium sulfate and ammonium nitrate mass fraction after cloud processing (Henning et al, 2014;Christiansen et al, 2020;Saliba et al, 2020).…”