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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 (NOx ). The 2013 mean combustionderived OC concentrations were 0.5 to 1.4 μgm-3 at the five sites operating in that year. Mean annual combustionderived OC concentrations declined from 3.8-0.2 μgmm-3 (68% of total OC) to 1.4±0.1 μgmm-3 (60% of total OC) between 1999 and 2013 at the urban Atlanta, Georgia, site (JST) and from 2.9±0.4 μgmm-3 (39% of total OC) to 0.7±0.1 μgmm-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 μgmm-3. BHM additionally exhibits a decline in OC associated with SO2 from 0.4±0.04 μgmm-3 in 2001 to 0.2±0.03 μgmm-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 μgmm-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 μgmm-3 as SO4 concentrations decreased throughout the SEARCH region. The 2013 mean SO4 concentrations of 1.7 to 2.0 μgmm-3 imply that future decreases in mean SO4-associated OC concentrations would not exceed ∼0.3 to 0.5 μgmm-3. Seasonal OC concentrations, largely identified with ozone (O3), vary from 0.3 to 1.4 μgmm-3 (∼20% of the total OC concentrations). [ABSTRACT FROM AUTHOR]

A yearlong near-real-time characterization of non-refractory submicron aerosol (NR-PM1) was conducted at an urban (Atlanta, Georgia) and rural (Look Rock, Tennessee) site in the southeastern US using the Aerodyne aerosol chemical speciation monitor (ACSM) collocated with established air-monitoring network measurements. Seasonal variations in organic aerosol (OA) and inorganic aerosol species are attributed to meteorological conditions as well as anthropogenic and biogenic emissions in this region. The highest concentrations of NR-PM1 were observed during winter and fall seasons at the urban site and during spring and summer at the rural site. Across all seasons and at both sites, NR-PM1 was composed largely of OA (50-76 %) and inorganic sulfate (12-31 %). Six distinct OA sources were resolved by positive matrix factorization applied to the ACSM organic mass spectral data collected from the two sites over the one year of near-continuous measurements at each site: hydrocarbon-like OA (HOA), biomass burning OA (BBOA), semi-volatile oxygenated OA (SV-OOA), low-volatility oxygenated OA (OOA), isoprene-derived epoxydiol (IEPOX) OA (IEPOX-OA), and 91Fac OA (a factor dominated by a distinct ion at m=z 91 fragment ion previously observed in biogenic influenced areas). LV-OOA was observed throughout the year at both sites and contributed 30-66% of total OA mass. HOA was also observed during the entire year only at the urban site (15-24% of OA mass). BBOA (15-33% of OA mass) was observed during winter and fall, likely dominated by local residential wood burning emission. Although SV-OOA contributes quite significantly (~ 27 %), it was observed only at the urban site during colder seasons. IEPOX-OA was a major component (27-41 %) of OA at both sites, particularly in spring and summer. An ion fragment at m=z 75 is proposed as an additional marker for IEPOX-OA, as it is shown to correlate well with the m=z 82 ion shown to be associated with the aerosol mass spectrum of IEPOX-derived secondary organic aerosol (SOA). The contribution of 91Fac to the total OA mass was quite low at the urban site (~ 9 %), but was significant (21-23 %) at the rural site during warmer months. Comparison of 91Fac OA time series with SOA tracers measured from filter samples collected at Look Rock suggests that isoprene oxidation through a pathway other than IEPOX SOA chemistry may contribute to its formation. Other biogenic sources could also contribute to 91Fac, but there remains a need to resolve the exact source of this factor based on its significant contribution to rural OA mass. [ABSTRACT FROM AUTHOR]

Long-term (1999 to 2013) data from the Southeastern Aerosol Research and Characterization (SEARCH) network are used to characterize the effects of anthropogenic emission reductions on 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 (NOx). 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, site (BHM). 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 associated 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 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 associated with ozone (O3), vary from 0.3 to 1.4 µg m-3 (~ 20 % of the total OC concentrations). [ABSTRACT FROM AUTHOR]

We report ambient measurements of amines and ammonia with a fast response chemical ionization mass spectrometer (CIMS) in a southeastern US forest in Alabama and a moderately polluted Midwestern site during the summer. In the Alabama forest, mostly C3-amines (from pptv to tens of pptv) and ammonia (up to 2 ppbv) were detected on a daily basis. C3-amines and ammonia showed similar diurnal trends and temperature and wind direction dependences, and were not associated with transported CO and SO2 plumes. Consistent with temperature dependences, amine and ammonia in the gas and aerosol phases showed opposite diurnal trends, indicating gas-to-particle partitioning of amines and ammonia. Temperature dependences also imply reversible processes of amines and ammonia evaporation from soil surfaces in daytime and deposition of amines and ammonia to soil surfaces at nighttime. Various amines (C1-C6) at the pptv level were observed in the transported biomass burning plumes, showing that biomass burning can be a substantial source of amines in the Southeast US. At the moderately polluted Kent site, higher concentrations of amines (C1-C6, from pptv to tens of pptv) and ammonia (up to 6 ppbv) were detected. Diurnal variations of C1- to C3-amines and ammonia were correlated with the ambient temperature. C4- to C6-amines showed abrupt increases during the nighttime, suggesting that they were emitted from local sources. These abundant amines and ammonia may in part explain the frequent new particle formation events reported from Kent. Lower amine concentrations at the rural forested site highlight the importance of constraining anthropogenic sources of amines. [ABSTRACT FROM AUTHOR]

Filter-based PM2.5 samples were chemically analyzed to investigate secondary organic aerosol (SOA) formation from isoprene in a rural atmosphere of the southeastern US influenced by both anthropogenic sulfur dioxide (SO2) and ammonia (NH3) emissions. Daytime PM2.5 samples were collected during summer 2010 using conditional sampling approaches based on pre-defined high and low SO2 or NH3 thresholds. Known molecular-level tracers for isoprene SOA formation, including 2-methylglyceric acid, 3-methyltetrahydrofuran-3,4-diols, 2-methyltetrols, C5-alkene triols, dimers, and organosulfate derivatives, were identified and quantified by gas chromatography cou pled to electron ionization mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). Mass concentrations of six isoprene low-NOx SOA tracers contributed to 12-19% of total organic matter (OM) in PM2.5 samples collected during the sampling period, indicating the importance of the hydroxyl radical (OH)-initiated oxidation (so-called photooxidation) of isoprene under low-NOx conditions that leads to SOA formation through reactive uptake of gaseous isoprene epoxydiols (IEPOX) in this region. IEPOX-derived SOA tracers were enhanced under high-SO2 sampling scenarios, suggesting that SO2 oxidation increases aerosol acidity of sulfate aerosols needed for enhancing heterogeneous oxirane ring opening reactions of IEPOX. No clear associations between isoprene SOA formation and high and low NH3 conditional samples were found. Furthermore, weak correlations between aerosol acidity and mass of IEPOX SOA tracers suggests that IEPOX-derived SOA formation might be modulated by other factors as well in addition to aerosol acidity. Positive correlations between sulfate aerosol loadings and IEPOX-derived SOA tracers for samples collected under all conditions indicates that sulfate aerosol could be a surrogate for surface area in the uptake of IEPOX onto preexisting aerosols. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) in the Southeastern US is investigated by analyzing the spatial-temporal distribution of water-soluble organic carbon (WSOC) and other PM2.5 components from 900 archived 24 h Teflon filters collected at 15 urban or rural EPA Federal Reference Method (FRM) network sites throughout 2007. Online measurements of WSOC at an urban/rural-paired site in Georgia in the summer of 2008 are contrasted to the filter data. Based on FRM filters, excluding biomass-burning events (levoglucosan <50 ngm-3), WSOC and sulfate were highly correlated with PM2.5 mass and both comprised a large mass fraction of PM2.5 (13% and 35 %, respectively). Sulfate and WSOC both tracked ambient temperature throughout the year, suggesting the temperature effects were mainly on the photochemical processes that lead to secondary formation. FRM WSOC, and to a lesser extent sulfate, were spatially homogeneous throughout the region, yet WSOC was moderately enhanced (27 %) in locations of greater predicted isoprene emissions in summer. A Positive Matrix Factorization (PMF) analysis identified two major source types for the summer WSOC; 22% of the WSOC were associated with ammonium sulfate, and 56% of the WSOC was associated with brown carbon and oxalate. A small urban excess of FRM WSOC (10 %) was observed in the summer of 2007, however, comparisons of online WSOC measurements at one urban/rural pair (Atlanta/Yorkville) in August 2008 showed substantially greater difference in WSOC (31%) relative to the FRM data, suggesting a low bias for urban filters. The measured Atlanta urban excess, combined with the estimated boundary layer heights, gave an estimated Atlanta daily WSOC production rate in August of 0.55mgCm-2 h-1 between mid-morning and mid-afternoon. This study characterizes the regional nature of fine particles in the Southeastern US, confirming the importance of secondary organic aerosol and the roles of both biogenic and anthropogenic emissions. [ABSTRACT FROM AUTHOR]

Atmospheric deposition represents a major input of mercury to surface environments. The phase of mercury (gas or particle) has important implications for its removal from the atmosphere. We use long-term observations of reactive gaseous mercury (RGM), particle-bound mercury (PBM), fine particulate matter (PM2.5), and temperature at five sites in North America to derive an empirical gas-particle partitioning relationship log10 (K-1) = (10 ± 1) - (2500 ± 300)/T where K = (PBM/PM2.5)/RGM with PBM and RGM in common mixing ratio units, PM2.5 in µg m-3, and T in Kelvin. This relationship is in the range of previous work but is based on far more extensive data from multiple sites. We implement this empirical relationship in the GEOS-Chem global 3-D Hg model to partition divalent mercury (Hg(II)). The resulting gas-phase fraction of Hg(II) ranges from over 90% in warm air with little aerosol to less than 10% in cold air with high aerosol. Hg deposition to high latitudes increases because of more efficient scavenging of particulate Hg(II) by snow. Model comparison to Hg observations at surface sites suggests that subsidence from the free troposphere (warm air, low aerosol) is a major factor driving the seasonality of RGM, while elevated PBM is mostly associated with high aerosol loads. This and other model updates, including the correction of an outstanding algorithm error, to wet deposition improve the simulation of Hg wet deposition fluxes in the US relative to the previous version of the model. The observed wintertime minimum in wet deposition fluxes is attributed to inefficient snow scavenging of gas-phase Hg(II). [ABSTRACT FROM AUTHOR]

Fine particle (PM2.5) light absorption characteristics of aqueous extracts over wavelengths of 250 to 700nm were investigated based on two data sets; 24-h Federal Reference Method (FRM) filter extracts from 15 southeastern US monitoring sites over the year of 2007 (900 filters), and online measurements from a Particle-Into-Liquid Sampler deployed from July to mid-August 2009 in Atlanta, Georgia. Three main sources of soluble chromophores were identified, biomass burning, mobile source emissions, and compounds linked to Secondary Organic Aerosol (SOA) formation. Absorption spectra of aerosol solutions from different sources were similar. Angstrom exponents were ∼7±1 for biomass burning and non-biomass burning-impacted samples (delineated by a levoglucosan concentration of 50 ngm-3) at both rural and urban sites. The absorption coefficient from measurements averaged between wavelength 360 and 370nm (Abs365, in units m-1) was used as a measure of overall brown carbon absorptivity. Biomass-burning-impacted samples were highest during colder months and Abs365 was correlated with levoglucosan at all sites. During periods of little biomass burning in summer, light absorbing compounds were still ubiquitous and correlated with fine particle Water-Soluble Organic Carbon (WSOC), but comprised a much smaller fraction of the WSOC, where Abs365/WSOC (i.e., mass absorption efficiency) was typically ∼3 times higher in biomass burning-impacted samples. Factor analysis attributed 50% of the yearly average Abs365 to biomass burning sources. Brown carbon from primary urban emissions (mobile sources) was also observed and accounted for ∼10% of the regional yearly average Abs365. Summertime diurnal profiles of Abs365 and WSOC showed that morning to midday increases in WSOC from photochemical production were associated with a decrease in Abs365/WSOC. After noon this ratio substantially increased, indicating that either some fraction of the non-light absorbing fresh SOA was rapidly (within hours) converted to chromophores heterogeneously, or that SOA from gas-particle partitioning later in the day was more light-absorbing. Factor analysis associated ∼20 to 30% of Abs365 over 2007 with a secondary source that was highest in summer and also the main source for oxalic acid, suggesting that aqueous phase reactions may account for the light-absorbing fraction of WSOC observed throughout the southeastern US in summer. [ABSTRACT FROM AUTHOR]