Chemical climatology of the southeastern United States, 1999–2013

Abstract

Abstract. A series of experiments (the Southern Oxidant and Aerosol Study – SOAS) took place in central Alabama in June–July, 2013 as part of the broader Southern Atmosphere Study (SAS). These projects were aimed at studying oxidant photochemistry and formation and impacts of aerosols at a detailed process level in a location where high biogenic organic vapor emissions interact with anthropogenic emissions, and the atmospheric chemistry occurs in a subtropical climate in North America. The majority of the ground-based experiments were located at the Southeastern Aerosol Research and Characterization (SEARCH) Centreville (CTR) site near Brent, Alabama, where extensive, unique aerometric measurements of trace gases and particles and meteorology were made beginning in the early 1990s through 2013. The SEARCH network data permits a characterization of the temporal and spatial context of the SOAS findings. Our earlier analyses of emissions and air quality trends are extended through 2013 to provide a perspective for continued decline in ambient concentrations, and the implications of these changes to regional sulfur oxide, nitrogen–ozone, and carbon chemistry. The narrative supports the SAS program in terms of long-term average chemistry (chemical climatology) and short-term comparisons of early summer average spatial variability across the southeastern US at high temporal (hourly) resolution. The long-term measurements show that the SOAS experiments took place during the second wettest and coolest year in the 2000–2013 period, with lower than average solar radiation. The pollution levels at CTR and other SEARCH sites were the lowest since full measurements began in 1999. Changes in anthropogenic gas and particle emissions between 1999 and 2013 account for the decline in pollutant concentrations at the monitoring sites in the region. The data provide an opportunity to contrast SOAS results with temporally and spatially variable conditions in support of the development of tests for the robustness of SOAS findings.

The underappreciated role of nonvolatile cations on aerosol ammonium-sulfate molar ratios

“…In (a), based on regression slope, the observed RSO4 trend was -0.021 ± 0.007 at CTR versus a predicted value of -0.017 ± 0.006 unit yr -1 for ISORROPIA run with Na + from the charge balance, and -0.003 ± 0.001 unit yr -1 10 for a constant Na + of 0.03 μg m -3 , used by Weber et al (2016). These results are consistent with the reported RSO4 trend of -0.01 to -0.03 yr -1 reported by Hidy et al (2014) for SEARCH data set. In (b), the pH predictions with inferred Na + or with limited Na + of 0.03 μg m -3 shows a fairly stable PM2.5 pH in the last 15 years.…”

“…A negative RSO4 can occur for conditions of high NO3 -and low NH4 + , SO4 2-concentrations (e.g., NaNO3), but rare for ambient fine particles. R or RSO4 is typically observed in the range of 1 and 2 in the southeastern US (i.e., between NH4HSO4 and (NH4)2SO4) (Hidy et al, 2014;15 Guo et al, 2015;Weber et al, 2016). In cases where NO3 -levels are low relative to SO4 2-, the two ratios, RSO4 and R, are equivalent, as is observed in the summertime southeastern US, where NO3 -is typically ~0.2 µg m -3 , NH4 + ~1 µg m -3 , and SO4…”

“…The molar ratio discrepancy has led to the hypothesis that thermodynamic predictions are incorrect, and particles are coated by organic films that inhibit the condensation of NH3 from the gas phase and give rise to the molar ratio discrepancy 25 (Silvern et al, 2017). Such kinetic limitations, if prevalent, opposes the validity of aerosol thermodynamic equilibrium and could significantly impact aerosol chemistry and acidity-mediated processes, given the large organic aerosol mass fractions worldwide (Zhang et al, 2007) and expected increasing organic mass fractions in the future due to changing emission, such as SO2 emission reduction in the eastern US (Hand et al, 2012;Attwood et al, 2014;Hidy et al, 2014). The hypothesis of organic films, however, is in stark contrast to established literature showing that NH3, water vapor, and HNO3 equilibrate with organic-rich 30 aerosols (Fountoukis et al, 2009;Guo et al, 2015;Guo et al, 2016;Guo et al, 2017a).…”

“…In the 20 southeastern US, where total ammonium (NHx = NH3 + NH4 + ) is observed to be in large excess of particle sulfate and observed NH4 + /SO4 2-molar ratios are in the range of 1-2 (Hidy et al, 2014;Guo et al, 2015;Kim et al, 2015), thermodynamic models often predict very low pH (0.5 to 2) (Guo et al, 2015) and molar ratios close to 2 (Kim et al, 2015;Weber et al, 2016;Silvern et al, 2017). The molar ratio discrepancy has led to the hypothesis that thermodynamic predictions are incorrect, and particles are coated by organic films that inhibit the condensation of NH3 from the gas phase and give rise to the molar ratio discrepancy 25 (Silvern et al, 2017).…”

Effects of temperature-dependent NO<sub>x</sub> emissions on continental ozone production

“…Measurements during 5 SOAS have been described in detail elsewhere (e.g. Hidy et al, 2014;Romer et al, 2016;Feiner et al, 2016) and are summarized below. The primary ground site for SOAS was co-located with the CTR site of the SEARCH network (32.90289 …”