Is there an aerosol signature of chemical cloud processing?

Abstract

Abstract. The formation of sulfate and secondary organic aerosol mass in the aqueous phase (aqSOA) of cloud and fog droplets can significantly contribute to ambient aerosol mass. While tracer compounds give evidence that aqueous-phase processing occurred, they do not reveal the extent to which particle properties have been modified in terms of mass, chemical composition, hygroscopicity, and oxidation state. We analyze data from several field experiments and model studies for six air mass types (urban, biogenic, marine, wild fire biomass burning, agricultural biomass burning, and background air) using aerosol size and composition measurements for particles 13–850 nm in diameter. We focus on the trends of changes in mass, hygroscopicity parameter κ, and oxygen-to-carbon (O ∕ C) ratio due to chemical cloud processing. We find that the modification of these parameters upon cloud processing is most evident in urban, marine, and biogenic air masses, i.e., air masses that are more polluted than very clean air (background air) but cleaner than heavily polluted plumes as encountered during biomass burning. Based on these trends, we suggest that the mass ratio (Rtot) of the potential aerosol sulfate and aqSOA mass to the initial aerosol mass can be used to predict whether chemical cloud processing will be detectable. Scenarios in which this ratio exceeds Rtot∼0.5 are the most likely ones in which clouds can significantly change aerosol parameters. It should be noted that the absolute value of Rtot depends on the considered size range of particles. Rtot is dominated by the addition of sulfate (Rsulf) in all scenarios due to the more efficient conversion of SO2 to sulfate compared to aqSOA formation from organic gases. As the formation processes of aqSOA are still poorly understood, the estimate of RaqSOA is likely associated with large uncertainties. Comparison to Rtot values as calculated for ambient data at different locations validates the applicability of the concept to predict a chemical cloud-processing signature in selected air masses.

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…Schum et al () showed that the oxygen‐to‐carbon (O:C) ratio for wildfire‐influenced aerosols (~0.45–0.48) transported over 7–10 days in the free troposphere to the PICO Mountain Observatory in the North Atlantic was lower compared to that of North American outflow aerosols transported mainly through the MBL over only three days (~0.57). While the O:C ratio difference was attributed to the free tropospheric particles being more viscous and thus less vulnerable to oxidation than the MBL aerosols, it is worth considering in future model‐measurement studies how enhanced humidity and cloudiness in the WNAO MBL could enhance the O:C ratio via aqueous oxidation pathways (Ervens et al, ; Ervens et al, ). Further, it is unclear as to what the vertical and spatial profile is of brown carbon over the WNAO and how those profiles are impacted by humidity and clouds.…”

Impact of Aerosol-Cloud Cycling on Aqueous Secondary Organic Aerosol Formation

“…Due to a lack of observations of IEPOX SOA tracers in cloudwater and the high variability of liquid water content and pH in both the aerosol and cloud phases, the percentage of IEPOX SOA produced in cloudwater and aqueous aerosol remains unclear [11,38,56]. The observed lack of correlation between SOA mass and several aerosol properties, including liquid water content and pH, for ambient data may also be clarified by considering production in clouds [57].…”

“…Although the composition of aqueous aerosols can vary considerably, liquid water and aqueous organic material are a significant portion of atmospheric aerosol mass [7][8][9], and much of the aqueous-phase organic material that comprises aerosols is known to be secondary. The cycling between aqueous aerosols and cloudwater provides two different aqueous environments in which aqueous secondary organic aerosol material (aqSOA) can form [10,11]. Since both environments are aqueous, the same chemical processes occur in both environments.…”

Long-range aerosol transport and impacts on size-resolved aerosol composition in Metro Manila, Philippines

“…2a and S2), the overall signal of organic aerosol is much stronger in these two sets, such that the absolute concentration of oxalate (MO7: 0.47 µg m −3 ; MO8: 0.42 µg m −3 ) is still greater than in MO12 (0.19 µg m −3 ) and MO14 (0.37 µg m −3 ). However, biomass burning is a welldocumented source of both oxalate and longer-chain dicarboxylic acids (e.g., Falkovich et al, 2005;Nirmalkar et al, 2015;Cheng et al, 2017;Deshmukh et al, 2018;Thepnuan et al, 2019).…”

“…Succinate has been linked to biomass burning emissions (Wang and Shooter, 2004;Falkovich et al, 2005;Zhao et al, 2014;Balla et al, 2018), vehicular emissions (Kawamura andKaplan, 1987;Kawamura et al, 1996;Yao et al, 2004), and secondary production via photochemical reactions of precursor organic compounds (Kawamura and Ikushima, 1993;Kawamura et al, 1996;Kawamura and Sakaguchi, 1999). The two MO MOUDI sets thought to have the most influence from biomass burning emissions (MO7 and MO8) had the highest organic aerosol mass concentrations and the highest mass percent contributions of succinate to the organic aerosol (MO7: 14.3 % and MO8: 17.5 %).…”