Concurrent Temporal and Spatial Trends in Sulfate and Organic Mass Concentrations Measured in the IMPROVE Monitoring Program

Abstract

Recent modeling and field studies have highlighted a relationship between sulfate concentrations and secondarily formed organic aerosols related to isoprene and other volatile biogenic gaseous emissions. The relationship between these biogenic emissions and sulfate is thought to be primarily associated with the effect of sulfate on aerosol acidity, increased aerosol water at high relative humidities, and aerosol volume. The Interagency Monitoring of Protected Visual Environments (IMPROVE) program provides aerosol concentration levels of sulfate (SO4) and organic carbon (OC) at 136 monitoring sites in rural and remote areas of the United States over time periods of between 15 and 28 years. This data set allows for an examination of relationships between these variables over time and space. The relative decreases in SO4 and OC were similar over most of the eastern United States, even though concentrations varied dramatically from one region to another. The analysis implied that for every unit decrease in SO4 there was about a 0.29 decrease in organic aerosol mass (OA = 1.8 × OC). This translated to a 2 μg/m3 decrease in biogenically derived secondary organic aerosol over 15 years in the southeastern United States. The analysis further implied that 35% and 27% in 2001 and 2015, respectively, of average total OA may be biogenically derived secondary organic aerosols and that there was a small but significant decrease in OA not linked to changes in SO4 concentrations. The analysis yields a constraint on ambient SO4–OC relationships that should help to refine and improve regional‐scale chemical transport models.

Future Fire Impacts on Smoke Concentrations, Visibility, and Health in the Contiguous United States

“…By 2050, total PM 2.5 concentrations are projected to decrease primarily due to expected reductions in anthropogenic emissions in both the RCP4.5 and RCP8.5 scenarios (Figure ). The reductions shown here are most notable in the eastern United States, particularly in the Ohio River Valley, consistent with recently observed downward trends in this region (e.g., Malm et al, ; U.S. EPA, ). In our RCP4.5 scenario simulations, PM 2.5 concentrations will continue to decrease by 2100; however, in the RCP8.5 scenario, several areas in the western United States, northeastern United States, and southeastern United States are projected to have higher concentrations compared to the early 21st century (2000).…”

“…By 2050, total PM 2.5 concentrations are projected to decrease primarily due to expected reductions in anthropogenic emissions in both the RCP4.5 and RCP8.5 scenarios (Figure 3). The reductions shown here are most notable in the eastern United States, particularly in the Ohio River Valley, consistent with recently observed downward trends in this region (e.g., Malm et al, 2017;U.S. EPA, 2012).…”

Assessing the Challenges of Surface‐Level Aerosol Mass Estimates From Remote Sensing During the SEAC4RS and SEARCH Campaigns: Baseline Surface Observations and Remote Sensing in the Southeastern United States

“…The OC term as an indicator species appears to play a greater role at the OLF and UAH sites, and despite its higher mass fraction, a lesser role at the forested CTR site. Again, this suggests that the majority of the organic mass in the region could be due to anthropogenic sources rather than biogenic emissions, and supports recent findings suggesting the formation of OC and SO 4 2− in PM 2.5 are linked (Malm et al, ; Schichtel et al, ). The OC multiplier can also be interpreted as reflecting the amount of oxidation within the POM.…”

“…While the formation of organosulfate SOA can explain some of the correlation, Zheng et al (), suggested that the majority of the OC at the SEARCH sites in the summer months is due to diesel emissions and not biogenic emissions, which would also lead to a connection between SO 4 2− and SOA within the PM 2.5 mass, although the research of Zheng et al () was performed prior to the switch to ultralow sulfur diesel fuel post 2006, which may render their proposed link less viable (https://www.epa.gov/diesel-fuel-standards/diesel-fuel-standards-and-rulemakings; last accessed 15 March 2018). Similarly, Malm et al () found a significant decrease in SOA concentrations corresponding to declines in SO 4 2− since 2001, suggesting a dependence of the SOA formation mechanisms on SO 4 2− PM 2.5 concentrations. Thus, it is not completely correct to assume sulfate and OC as orthogonal components of the PM 2.5 mass.…”

“…The relative amounts of sulfate in the total aerosol mass remained about the same in 2013 and remained the same over the next two years compared to the previous five years. The OC component is slightly more difficult to characterize as changes in aerosol chemistry may change the oxidized mass associated with the OC, but a study of absolute mass shows a drop in the OC component as well, just at a rate slower than the PM 2.5 , with the 2000–2007, and 2008–2013 masses of CTR 3.0 to 2.4 μg/m 3 , OLF 2.5 to 1.8 μg/m 3 , YRK 3.1 to 2.3 μg/m 3 , and UAH 4.1 to 2.5 μg/m 3 , showing either a significant drop in the anthropogenic component of the OC, or a reduced favorability of the production mechanisms of the SOA with the reduction in aerosol acidity, as seen by Malm et al (). This resulted in a significant reduction in the SO 4 :OC ratio, from 1.5 ± 0.7 prior to 2008 to 1.0 ± 0.7 between 2008–2013.…”