The Southeastern Aerosol Research and Characterization (SEARCH) study: Temporal trends in gas and PM concentrations and composition, 1999–2010

Abstract

The SEARCH study began in mid 1998 with a focus on particulate matter and gases in the southeastern United States. Eight monitoring sites, comprising four urban/nonurban pairs, are located inland and along the coast of the Gulf of Mexico. Downward trends in ambient carbon monoxide (CO), sulfur dioxide (SO 2 ), and oxidized nitrogen species (NO y ) concentrations averaged 1.2 AE 0.4 to 9.7 AE 1.8% per year from 1999 to 2010, qualitatively proportional to decreases of 4.7 to 7.9% per year in anthropogenic emissions of CO, SO 2 , and oxides of nitrogen (NO x ) in the SEARCH region. Downward trends in mean annual sulfate (SO 4 ) concentrations ranged from 3.7 AE 1.1 to 6.2 AE 1.1% per year, approximately linear with, but not 1:1 proportional to, the 7.9 AE 1.1% per year reduction in SO 2 emissions from 1999 to 2010. The 95th percentile of the March-October peak daily 8-hr ozone (O 3 ) concentrations decreased by 1.1 AE 0.4 to 2.4 AE 0.6 ppbv per year (1.5 AE 0.6 to 3.1 AE 0.8% per year); O 3 precursor emissions of NO x and volatile organic compounds (VOC) decreased at rates of 4.7 and 3.3% per year, respectively. Ambient particulate nitrate (NO 3 ) concentrations decreased by 0.6 AE 1.2 to 5.8 AE 0.9% per year, modulated in comparison with mean annual ambient NO y concentration decreases ranging from 6.0 AE 0.9 to 9.0 AE 1.3% per year. Mean annual organic matter (OM) and elemental carbon (EC) concentrations declined by 3.3 AE 0.8 to 6.5 AE 0.3 and 3.2 AE 1.4 to 7.8 AE 0.7% per year. The analysis demonstrates major improvements in air quality in the Southeast from 1999 to 2010. Meteorological variations and incompletely quantified uncertainties for emission changes create difficulty in establishing unambiguous quantitative relationships between emission reductions and ambient air quality.Implications: Emissions and secondary pollutants show complex relationships that depend on year-to-year variations in dispersion and atmospheric chemistry. The observed response of O 3 to NO x and VOC emissions in the Southeast implies that continuing reductions of precursor emissions, probably achieved through vehicle fleet turnover and emission control measures, will be needed to attain the National Ambient Air Quality Standard for O 3 . Reductions in fine particle concentrations have resulted from reductions of primary PM, especially EC and a portion of OM, and from reduction of gas precursors known to form particles, especially SO 4 from SO 2 . Continued reduction of PM 2.5 mass concentrations will require attention to organic constituents, which may be complicated by potentially unmanageable biogenic species present in the Southeast.

Long Term Trends in New York: PM2.5 Mass and Particle Components

“…These trends were attributed to pollution abatement strategies, downturn in the economy (which had a greater impact in Europe compared to the US) and unusual meteorological conditions. Ambient measurements in the southeastern US (Blanchard et al, 2013) show reductions in PM 2.5 of around 4.5 µg m -3 and SO 4 of 2.3 µg m -3 from 1999 to 2010 which is similar to that reported here. Controls on EGU and vehicle emissions is largely responsible for these decreases.…”

“…There is a linear relationship between SO 2 emissions and ambient SO 4 with coefficients of determination above 0.89 for all sites. However, there is not quite a 1:1 proportional response in SO 4 to the SO 2 emissions reductions as previously observed in the Southeastern US (Blanchard et al, 2013). A change in the rate of SO 4 decrease is evident around [2005][2006][2007], with a general levelling off in concentrations in recent years.…”

“…Controls on EGU and vehicle emissions is largely responsible for these decreases. However, Blanchard et al (2013) also report large reductions in carbonaceous species, OM (2.2 µg m -3 ) which is not observed in New York. An analysis of trends in PM 2.5 OC and EC across the US (Hand et al, 2013) found the largest downward trends in the western US with generally small or insignificant changes at eastern US locations.…”

“…A recent review (Lurmann et al, 2015) reported reductions in ambient air pollutants over a 20 yr period in Southern California. Blanchard et al (2013) found major improvements in air quality since 1999 in the southeastern US. Reductions urban as well as urban-rural contrasts.…”

A critical review of filter transmittance measurements for aerosol light absorption, and de novo calibration for a decade of monitoring on PTFE membranes

“…In a subsequent examination of the method's internal optical data, Chen et al (2012) concluded that the transition's effects on longterm EC trends were generally minor. Comparable EC declines between 1999 and 2010 were reported by Blanchard et al (2013) for the SEARCH network in the southeast U.S.…”