Recent trends in gas-phase ammonia and PM_2.5 ammonium in the Southeast United States

Abstract

Ammonia measurements from the Southeastern Aerosol Research and Characterization (SEARCH) study network were analyzed for trends over 9 yr (2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) of observations. Total ammonia concentrations, defined as the sum of gas-phase ammonia and fine particle ammonium, were found to be decreasing by 1-4% yr −1 and were qualitatively consistent with ammonia emission estimates for the SEARCH states of Alabama, Georgia, Mississippi, and Florida. On the other hand, gas-phase ammonia mixing ratios were found to be slightly rising or steady over the region, leading to the observation that the gas-phase fraction of total ammonia has steadily increased over 2004-2012 as a result of declining emissions of the strong acid precursor species sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ) and consequent reduced partitioning of ammonia to the fine particle phase. Because gas-phase ammonia is removed from the atmosphere more rapidly than fine particle ammonium, an increase in the gas-phase fraction of total ammonia may result in shifted deposition patterns as more ammonia is deposited closer to sources rather than transported downwind in fine particles. Additional long-term measurements and modeling studies are needed to determine if similar transitions of total ammonia to the gas phase are occurring outside of the Southeast and to assess if these changes are impacting plants and ecosystems near major ammonia sources. Unusually high ammonia concentrations observed in 2007 in the SEARCH measurements are hypothesized to be linked to emissions from wildfires that were much more prevalent across the Southeast during that year due to elevated temperatures and widespread drought. Although wildfires are currently estimated to be a relatively small fraction (3-10%) of total ammonia emissions in the Southeast, the projected increased incidence of wildfires in this region as a result of global climate change may lead to this source's increased importance over the rest of the 21st century.Implications: Ammonia concentrations from the Southeastern Aerosol Research and Characterization study (SEARCH) network are analyzed over the 9-yr period 2004-2012. Total ammonia (gaseous ammonia + PM 2.5 ammonium) concentrations declined at a rate of 1-4% yr −1 , consistent with U.S. Environmental Protection Agency (EPA) emission estimates for the Southeast United States, but the fraction of ammonia in the gas phase has risen steadily (+1-3% yr −1 ) over the time period. Declining emissions of SO 2 and NO x resulting from imposed air quality regulations have resulted in decreased atmospheric strong acids and less ammonia partitioning to the particle phase, which may impact the amount and overall pattern of ammonia deposition.

Inconsistency of ammonium–sulfate aerosol ratios with thermodynamic models in the eastern US: a possible role of organic aerosol

“…Sulfate wet deposition fluxes and aerosol concentrations both decrease by 6-8 % yr −1 , consistent with the trend in SO 2 emissions (Hand et al, 2012). There is no significant change in NH emissions during this period (Xing et al, 2013;Saylor et al, 2015). However, aerosol ammonium decreases at a rate similar to sulfate (−8.5 % yr −1 ).…”

“…One would expect from standard sulfate-ammonium thermodynamics that this would result in an increase in the ammonium-sulfate ratio R. However, observations show that the sulfate and ammonium components of the aerosol decreased at similar rates over the period so that R did not increase (Hand et al, 2012;Blanchard et al, 2013;Kim et al, 2015;Saylor et al, 2015;Weber et al, 2016), adding to the thermodynamic puzzle. Weber et al (2016) presented a detailed thermodynamic analysis of 1998-2013 observations of sulfate and ammonium aerosol and gas-phase ammonia at a rural site in the Southeast (Centreville, Alabama).…”

“…Here and elsewhere, mean ratios are calculated as the ratios of the mean quantities. ammonia concentrations at Southeast sites (You et al, 2014;Guo et al, 2015;Saylor et al, 2015;Weber et al, 2016).…”

Trends in PM2.5 transition metals in urban areas across the United States

“…Energy pricing that has shifted substantial electricity generation from coal to natural gas has also contributed to regional and national air quality improvements [35,36]. Many studies have characterized temporal trends in sulfate, nitrate, ammonium, and carbonaceous compounds-the major components of PM 2.5 mass-across different regions of the US [37][38][39][40][41][42][43][44]. However, due to their typically minor contribution to PM mass, atmospheric trends in transition metals have not been closely studied.…”

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

“…Budisulistiorini et al (2015) also report that no strong relationships were found between IEPOX-SOA formation and particle acidity. With SO 4 concentrations decreasing by more than 80% in much of the eastern United States from the early 1990s to current time periods and ammonia concentrations remaining about the same over the same time period (Saylor et al, 2015;Weber et al, 2016; Acknowledgments…”