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Placitas, NM, United States

Hidy G.M.,Envair Aerochem | Blanchard C.L.,Envair
Journal of the Air and Waste Management Association | Year: 2015

Numerous papers analyze ground-level ozone (O3) trends since the 1980s, but few have linked O3 trends with observed changes in nitrogen oxide (NOx) and volatile organic compound (VOC) emissions and ambient concentrations. This analysis of emissions and ambient measurements examines this linkage across the United States on multiple spatial scales from continental to urban. O3 concentrations follow the general decreases in both NOx and VOC emissions and ambient concentrations of precursors (nitrogen dioxide, NO2; nonmethane organic compounds, NMOCs). Annual fourth-highest daily peak 8-hr average ozone and annual average or 98th percentile daily maximum hourly NO2 concentrations show a statistically significant (p < 0.05) linear fit whose slope is less than 1:1 and intercept is in the 30 to >50 ppbv range. This empirical relationship is consistent with current understanding of O3 photochemistry. The linear O3–NO2 relationships found from our multispatial scale analysis can be used to extrapolate the rate of change of O3 with projected NOx emission reductions, which suggests that future declines in annual fourth-highest daily average 8-hr maximum O3 concentrations are unlikely to reach 65 ppbv or lower everywhere in the next decade. Measurements do not indicate increased annual reduction rates in (high) O3 concentrations beyond the multidecadal precursor proportionality, since aggressive measures for NOx and VOC reduction are in place and have not produced an accelerated O3 reduction rate beyond that prior to the mid-2000s. Empirically estimated changes in O3 with emissions suggest that O3 is less sensitive to precursor reductions than is found by the CAMx (v. 6.1) photochemical model. Options for increasing the rate of O3 change are limited by photochemical factors, including the increase in NOx sensitivity with time (NMOC/NOx ratio increase), increase in O3 production efficiency at lower NOx concentrations (higher O3/NOy ratio), and the presence of natural NOx and NMOC precursors and background O3. Implications: This analysis demonstrates empirical relations between O3 and precursors based on long term trends in U.S. locations. The results indicate that ground-level O3 concentrations have responded predictably to reductions in VOC and NOx since the 1980s. The analysis reveals linear relations between the highest O3 and NO2 concentrations. Extrapolation of the historic trends to the future with expected continued precursor reductions suggest that achieving the 2014 proposed reduction in the U.S. National Ambient Air Quality Standard to a level between 65 and 70 ppbv is unlikely within the next decade. Comparison of measurements with national results from a regulatory photochemical model, CAMx, v. 6.1, suggests that model predictions are more sensitive to emissions changes than the observations would support. Copyright © 2015 A&WMA.

Blanchard C.L.,Envair | Hidy G.M.,Envair Aerochem | Tanenbaum S.,Envair | Edgerton E.S.,Atmospheric Research and Analysis Inc. | Hartsell B.E.,Atmospheric Research and Analysis Inc.
Journal of the Air and Waste Management Association | Year: 2013

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 (SO2), and oxidized nitrogen species (NOy) concentrations averaged 1.2 ± 0.4 to 9.7 ± 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, SO2, and oxides of nitrogen (NOx) in the SEARCH region. Downward trends in mean annual sulfate (SO4) concentrations ranged from 3.7 ± 1.1 to 6.2 ± 1.1% per year, approximately linear with, but not 1:1 proportional to, the 7.9 ± 1.1% per year reduction in SO2 emissions from 1999 to 2010. The 95th percentile of the March-October peak daily 8-hr ozone (O3) concentrations decreased by 1.1 ± 0.4 to 2.4 ± 0.6 ppbv per year (1.5 ± 0.6 to 3.1 ± 0.8% per year); O3 precursor emissions of NOx and volatile organic compounds (VOC) decreased at rates of 4.7 and 3.3% per year, respectively. Ambient particulate nitrate (NO3) concentrations decreased by 0.6 ± 1.2 to 5.8 ± 0.9% per year, modulated in comparison with mean annual ambient NOy concentration decreases ranging from 6.0 ± 0.9 to 9.0 ± 1.3% per year. Mean annual organic matter (OM) and elemental carbon (EC) concentrations declined by 3.3 ± 0.8 to 6.5 ± 0.3 and 3.2 ± 1.4 to 7.8 ± 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.Emissions and secondary pollutants show complex relationships that depend on year-to-year variations in dispersion and atmospheric chemistry. The observed response of O3 to NOx 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 O3. 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 SO4 from SO2. Continued reduction of PM2.5 mass concentrations will require attention to organic constituents, which may be complicated by potentially unmanageable biogenic species present in the Southeast. © 2013 Copyright 2013 A&WMA.

Blanchard C.L.,Envair | Hidy G.M.,Envair Aerochem | Tanenbaum S.,Envair | Edgerton E.S.,Atmospheric Research and Analysis Inc. | Hartsell B.E.,Atmospheric Research and Analysis Inc.
Journal of the Air and Waste Management Association | Year: 2013

The Southeastern Aerosol Research and Characterization (SEARCH) study, which has been in continuous operation from 1999 to 2012, was implemented to investigate regional and urban air pollution in the southeastern United States. With complementary data from other networks, the SEARCH measurements provide key knowledge about long-term urban/nonurban pollution contrasts and regional climatology affecting inland locations and sites along the Gulf of Mexico coastline. Analytical approaches ranging from comparisons of mean concentrations to the application of air mass trajectories and principal component analysis provide insight into local and area-wide pollution. Gases (carbon monoxide, sulfur dioxide, nitrogen oxides, ozone, and ammonia), fine particle mass concentration, and fine particle species concentrations (including sulfate, elementary carbon, and organic carbon) are affected by a combination of regional conditions and local emission sources. Urban concentrations in excess of regional baselines and intraurban variations of concentrations depend on source proximity, topography, and local meteorological processes. Regional-scale pollution events (95th percentile concentrations) involving more than 6 of the 8 SEARCH sites are rare (< 2% of days), while subregional events affecting 4-6 sites occur on ~10% of days. Regional and subregional events are characterized by widely coincident elevated concentrations of ozone, sulfate, and particulate organic carbon, driven by persistent synoptic-scale air mass stagnation and higher temperatures that favor formation of secondary species, mainly in the summer months. The meteorological conditions associated with regional stagnation do not favor long-range transport of polluted air masses during episodes. Regional and subregional pollution events frequently terminate with southward and eastward penetration of frontal systems, which may initially reduce air pollutant concentrations more inland than along the Gulf Coast.Regional distribution of emission sources and synoptic-scale meteorological influences favoring stagnation lead to high regionwide pollution levels. The regional influence is greatest with secondary species, including ozone (O3) particulate sulfate (SO4), and particulate organic matter, some of which is produced by atmospheric oxidation of volatile organic compounds (VOCs) from vegetation and anthropogenic sources. Other species, many of which are from primary emissions, are more influenced by local sources, especially within the Atlanta, GA, and Birmingham, AL, metropolitan areas. Limited measurements of modern and fossil total carbon point to the importance of biological and biogenic emissions in the Southeast. © 2013 Copyright 2013 A&WMA.

Blanchard C.L.,Envair | Tanenbaum S.,Envair | Hidy G.M.,Envair Aerochem
Atmospheric Environment | Year: 2014

Quantification of the spatial and temporal variations of outdoor air pollutant concentrations provides important information for epidemiological and other air-pollution studies, many of which have relied in the past on data from a single, centrally-located air pollution monitoring site. A method is developed for combining air pollution measurements from multiple monitors and monitoring networks to generate daily air pollution concentration fields representing spatial variations over distances of approximately 1-10km. Meteorological and co-pollutant data are used to estimate missing site measurements, yielding more realistic concentration fields as the number of monitoring locations with available data increases. Monitoring data are interpolated with weights computed from intersite pollutant correlations, which decay with distance, so distances between interpolation points and monitoring sites are factored into the interpolation weights. The approach minimizes the influence of source-oriented sites that represent limited areas, because data from such sites exhibit low intersite correlations and yield interpolation weights that decay rapidly to zero. Interpolated values represent pollutant concentrations averaged over spatial scales that depend on intersite distances and the interpolation grid, and do not delineate sharp spatial gradients associated with roadside or near-source conditions. The approach yields quantified interpolation errors the values of which depend on measurement uncertainties, intersite distances, and the representativeness of monitoring site locations. The method is illustrated using an 11-year period of measurements of ozone, PM2.5, and PM10 concentrations from Jefferson County, Alabama. The principal city is Birmingham, which is influenced by regional-scale air pollution and by local emissions from mobile sources, industrial facilities, and residential communities. Emission sources are not distributed uniformly throughout Birmingham, the ridge-and-valley topography complicates dispersion of local emissions, and monitoring data indicate that air pollutant concentrations vary spatially as well as temporally. No single monitor represents air quality across the entire study area. © 2014 Elsevier Ltd.

Blanchard C.L.,Envair | Hidy G.M.,Envair Aerochem | Tanenbaum S.,Envair
Atmospheric Environment | Year: 2014

A generalized additive model (GAM) is used to examine the influence of meteorological factors, nitrogen oxides (NOx=NO+NO2), and non-methane hydrocarbons (NMOC) on daily peak 8-hozone (O3) concentrations. Application to 2002-2011 monitoring data from the Southeastern Aerosol Research and Characterization (SEARCH) program showed sensitivity of peak 8-hO3 to morning concentrations of nitric oxide (NO) and nitrogen dioxide (NO2) and to afternoon concentrations of NO2 reaction products (NOz). Peak O3 decreased with increasing NO and increased with increasing NO2 concentrations, consistent with reactions involving O3, NO, and NO2. Ozone production efficiency (OPE), estimated from the modeled relation between peak 8-hO3 and afternoon NOz, was ~40-100 percent higher at rural compared to urban sites. OPE was nonlinear at all sites, decreasing with increasing NOz concentration. The mean ratio of NOz/NOy showed a two-fold increase from urban to rural sites, associated with chemical aging in stagnant air masses from one day (urban sites) to two or more days (non-urban sites). Peak 8-hO3 concentrations in Atlanta were sensitive to concentrations of both non-biogenic NMOC and NOz. Non-urban Yorkville, Georgia, peak 8-hO3 concentrations were sensitive to NOz but not to non-biogenic NMOC concentrations. The results are consistent with expected NMOC and NOx sensitivity in urban and non-urban locales. © 2014 Elsevier Ltd.

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