Effects of emission reductions on organic aerosol in the southeastern United States

Abstract. Long-term (1999 to 2013) data from the Southeastern Aerosol Research and Characterization (SEARCH) network are used to show that anthropogenic emission reductions led to important decreases in fine-particle organic aerosol (OA) concentrations in the southeastern US On average, 45 % (range 25 to 63 %) of the 1999 to 2013 mean organic carbon (OC) concentrations are attributed to combustion processes, including fossil fuel use and biomass burning, through associations of measured OC with combustion products such as elemental carbon (EC), carbon monoxide (CO), and nitrogen oxides (NO_x). The 2013 mean combustion-derived OC concentrations were 0.5 to 1.4 µg m⁻³ at the five sites operating in that year. Mean annual combustion-derived OC concentrations declined from 3.8 ± 0.2 µg m⁻³ (68 % of total OC) to 1.4 ± 0.1 µg m⁻³ (60 % of total OC) between 1999 and 2013 at the urban Atlanta, Georgia, site (JST) and from 2.9 ± 0.4 µg m⁻³ (39 % of total OC) to 0.7 ± 0.1 µg m⁻³ (30 % of total OC) between 2001 and 2013 at the urban Birmingham, Alabama (BHM), site. The urban OC declines coincide with reductions of motor vehicle emissions between 2006 and 2010, which may have decreased mean OC concentrations at the urban SEARCH sites by > 2 µg m⁻³. BHM additionally exhibits a decline in OC associated with SO₂ from 0.4 ± 0.04 µg m⁻³ in 2001 to 0.2 ± 0.03 µg m⁻³ in 2013, interpreted as the result of reduced emissions from industrial sources within the city. Analyses using non-soil potassium as a biomass burning tracer indicate that biomass burning OC occurs throughout the year at all sites. All eight SEARCH sites show an association of OC with sulfate (SO₄) ranging from 0.3 to 1.0 µg m⁻³ on average, representing  ∼  25 % of the 1999 to 2013 mean OC concentrations. Because the mass of OC identified with SO₄ averages 20 to 30 % of the SO₄ concentrations, the mean SO₄-associated OC declined by  ∼  0.5 to 1 µg m⁻³ as SO₄ concentrations decreased throughout the SEARCH region. The 2013 mean SO₄ concentrations of 1.7 to 2.0 µg m⁻³ imply that future decreases in mean SO₄-associated OC concentrations would not exceed  ∼  0.3 to 0.5 µg m⁻³. Seasonal OC concentrations, largely identified with ozone (O₃), vary from 0.3 to 1.4 µg m⁻³ ( ∼  20 % of the total OC concentrations).