Decreasing Aerosol Water Is Consistent with OC Trends in the Southeast U.S.

Abstract

Water is a ubiquitous and abundant component of atmospheric aerosols. It influences light scattering, the hydrological cycle, atmospheric chemistry, and secondary particulate matter (PM) formation. Despite the critical importance of aerosol liquid water, mass concentrations are not well-known. Using speciated ion and meteorological data from the Southeastern Aerosol Research and Characterization network, we employ the thermodynamic model ISORROPIAv2.1 to estimate water mass concentrations and evaluate trends from 2001 to 2012 in urban and rural locations. The purpose of this study is to better understand the historical trends of aerosol liquid water in the southeast U.S. in the context of improved air quality and recently noted reductions in particulate organic carbon (OC). Aerosol water mass concentrations decrease by ∼79% from 2001 to 2012 in the region. Decreases are more prominent in rural than in urban areas. Fractional contribution of water to PM also decreases during the same time period, and this is consistent with recently noted improvements in visibility. These findings agree with the hypotheses that aerosol liquid water facilitates formation of biogenic secondary organic aerosol (SOA) and that biogenically derived SOA is modulated in the presence of anthropogenic perturbations.

Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic Aerosol Production During the 2013 Southern Oxidant and Aerosol Study (SOAS)

“…Furthermore, SO x reductions have also been observed to be correlated with organic aerosol in ambient measurements in the SE US. 44,45 In this study, we find that a 25% sulfate reduction results in a ∼70% decrease in the predicted tetrol concentration and ∼75% decrease in IEPOXOS (Figure 4). Due to the decrease of sulfate, the proton activity (a H + ) decreases up to 64%.…”

Reconciling satellite aerosol optical thickness and surface fine particle mass through aerosol liquid water

“…PM 2.5 (supporting information Figure S12), constituent compounds (Figures c and S9), and ALW (Figure e) mass concentrations in the southeastern regions of the U.S. have been decreasing. Recent studies find decreases of ~50% for sulfate [ Attwood et al ., ; Blanchard et al ., , ; Nguyen et al ., ], ~70% for ALW [ Nguyen et al ., ], and 45% decrease in PM 2.5 mass concentrations (Figure S12e) since 2005. Aerosol extinction at IMPROVE sites decreased by 67% (Figure S10e).…”

“…The impact is difficult to quantify because the effects of individual organic compounds on ALW are complex [ Decesari et al ., ; Shimmo et al ., ] and routine networks provide only limited classification of organic material. While neglect of OM also implies a lower bound for ALW mass concentrations and introduces uncertainty, inorganic aerosol water is found to dominate over organic contributions [ Choi and Chan , ; Nguyen et al ., ; Nguyen et al ., ] and large‐scale trends in space and time are valid (i.e., larger than the uncertainty amount) in a relative sense [ Nguyen et al ., , ].…”

Introductory lecture: atmospheric chemistry in the Anthropocene

“…One mechanism suggested for SOA production is the reaction of smaller, soluble organics in the aqueous phase of particles to generate larger, low volatility products that remain in the condensed phase when the water evaporates. 306,432,433,435,437,438,[596][597][598][599][600] In this case, SOA will be impacted by the availability of water in particles 601 and the composition of the aqueous phase, especially the pH, which is largely determined by the presence of hygroscopic anthropogenically derived species such as sulfate and nitrate. [601][602][603] This is an example of how the interplay between biogenic and anthropogenic species can introduce new, unexpected chemical mechanisms that would occur only in the Anthropocene era.…”