Observations of ozone formation in power plant plumes and implications for ozone control strategies

Control strategy optimization for attainment and exposure mitigation: case study for ozone in Macon, Georgia

Implementation of more stringent 8-hour ozone standards has led the U.S. Environmental Protection Agency to designate nonattainment status to 474 counties nationwide, many of which had never previously violated air quality standards. As states select emission control measures to achieve attainment in these regions, their choices pose significant implications to local economies and the health of their citizens. Considering a case study of one such nonattainment region, Macon, Georgia, we develop a menu of potential controls that could be implemented locally or in neighboring parts of the state. The control menu offers the potential to control about 20-35% of ozone precursor emissions in most Georgia regions, but marginal costs increase rapidly beyond 15-20%. We link high-order ozone sensitivities with the control menu to identify cost-optimized strategies for achieving attainment and for alternative goals such as reducing spatially averaged or population-weighted ozone concentrations. Strategies targeted toward attainment of Macon ozone would prioritize local reductions of nitrogen oxides, whereas controls in the more densely populated Atlanta region are shown to be more effective for reducing statewide potential population exposure to ozone. A U.S. EPA-sanctioned approach for demonstrating ozone attainment with photochemical models is shown to be highly dependent on the choice of a baseline period and may not foster optimal strategies for assuring attainment and protecting human health.

Adjoint sensitivity analysis of ozone nonattainment over the continental United States

An application of the adjoint method in air quality management is demonstrated. We use a continental scale chemical transport model (STEM) to calculate the sensitivities of a nationwide U.S. ozone national ambient air quality standard (NAAQS) nonattainment metric to precursor emissions for the period July 1 to August 15, 2004. The model shows low bias and error (-4 and 24%, respectively), particularly for areas with high ozone concentrations. The nonattainment metric accounts for both 1-h and 8-h ozone standards, but is dominated by the 8-h exceedances (97% of the combined metric). Largest values of sensitivities are found to be with respect to emissions in the south and southeast U.S., Ohio River Valley, and California. When nonattainment sensitivities are integrated over the entire U.S., NOx emissions account for the largest contribution (62% of the total), followed by biogenic and anthropogenic VOCs (24% and 14%, respectively). For NOx emissions, point/area and mobile sources account for 54% and 46% of the total sensitivities, respectively. We also provide a state-by-state comparison for the nonattainment magnitude, nonattainment sensitivity, and emission magnitudes to explore the influence of interstate transport of ozone and its precursors, and policy implications of the results. Our analysis of the nationwide ozone nonattainment metric suggests that simple cap-and-trade programs may prove inadequate in achieving sought-after air quality objectives.

Ambient Sulfate Trends and the Influence of Meteorology

Data on atmospheric levels of sulfur dioxide (SO2) and sulfate were examined to quantify changes since 1989. Changes in sulfur species were adjusted to account for meteorological variability. Adjustments were made using meteorological variables expressed in terms of their principal components that were used as predictors in statistical models. Several models were tested. A generalized additive model (GAM)—based in part on nonparametric, locally smoothed predictor functions—computed the greatest association between sulfate and the meteorological predictors. Sulfate trends estimated after a GAM-based adjustment for weather-related influences were found to be primarily downward across the eastern United States by as much as 6.7% per year (average of −2.6% per year), but large spatial variability was noted. The most conspicuous characteristic in the trends was over portions of the Appalachian Mountains where very small (average = −1.6% per year) and often insignificant sulfate changes were found. The Appalachian region also experienced a tendency, after removing meteorological influences, for increases in the ratio RS of sulfate sulfur to total sulfur. Before 1991, this ratio averaged 0.33 across all sites. Appalachian increases in RS were equivalent to 0.07 during 1989–2001 (significant for most sites at the 0.05 level), or nearly 2 times the average change at the other sites. This suggests that conditions over the Appalachians became notably more efficient at oxidizing SO2 into sulfate. Alternatively, subtle changes in local deposition patterns occurred, preferentially in and near mountainous monitoring sites, that changed the SO2–sulfate balance.

Gene expression patterns of trembling aspen trees following long-term exposure to interacting elevated CO2 and tropospheric O3

Expression of 4600 poplar expressed sequence tags (ESTs) was studied over the 2001-2002 growing seasons using trees of the moderately ozone (O(3))-tolerant trembling aspen (Populus tremuloides) clone 216 exposed to elevated CO(2) and/or O(3) for their entire 5-yr life history. Based on replication of the experiment in years 2001 and 2002, 238 genes showed qualitatively similar expression in at least one treatment and were retained for analysis. Of these 238 genes, 185 were significantly regulated (1.5-fold) from one year to the other in at least one treatment studied. Less than 1% of the genes were regulated 2-fold or more. In the elevated CO(2) treatment, relatively small numbers of genes were up-regulated, whereas in the O(3) treatment, higher expression of many signaling and defense-related genes and lower expression of several photosynthesis and energy-related genes were observed. Senescence-associated genes (SAGs) and genes involved in the flavonoid pathway were also up-regulated under O(3), with or without CO(2) treatment. Interestingly, the combined treatment of CO(2) plus O(3) resulted in the differential expression of genes that were not up-regulated with individual gas treatments. This study represents the first investigation into gene expression following long-term exposure of trees to the interacting effects of elevated CO(2) and O(3) under field conditions. Patterns of gene-specific regulation described in this study correlated with previously published physiological responses of aspen clone 216.

Inter-comparison of laser photoacoustic spectroscopy and gas chromatography techniques for measurements of ethene in the atmosphere

Laser photoacoustic spectroscopy (LPAS) is highly suitable for the detection of ethene in air due to the overlap between its strongest absorption lines and the wavelengths accessible by high-powered CO2 lasers. Here, we test the ability of LPAS to measure ethene in ambient air by comparing the measurements in urban air with those from a gas chromatography flame-ionization detection (GC-FID) instrument. Over the course of several days, we obtained quantitative agreement between the two measurements. Over this period, the LPAS instrument had a positive offset of 330 +/- 140 pptv (parts-per-trillion by volume) relative to the GC-FID instrument, possibly caused by interference from other species. The detection limit of the LPAS instrument is currently estimated around 1 ppbv and is limited by this offset and the statistical noise in the data. We conclude that LPAS has the potential to provide fast-response measurements of ethene in the atmosphere, with significant advantages over existing techniques when measuring from moving platforms and in the vicinity of emission sources.

Missing OH reactivity in a forest: evidence for unknown reactive biogenic VOCs

Forest emissions of biogenic volatile organic compounds (BVOCs), such as isoprene and other terpenes, play a role in the production of tropospheric ozone and aerosols. In a northern Michigan forest, the direct measurement of total OH reactivity, which is the inverse of the OH lifetime, was significantly greater than expected. The difference between measured and expected OH reactivity, called the missing OH reactivity, increased with temperature, as did emission rates for terpenes and other BVOCs. These measurements are consistent with the hypothesis that unknown reactive BVOCs, perhaps terpenes, provide the missing OH reactivity.

Industrial emissions cause extreme urban ozone diurnal variability

Simulations with a regional chemical transport model show that anthropogenic emissions of volatile organic compounds and nitrogen oxides (NO(x) = NO + NO(2)) lead to a dramatic diurnal variation of surface ozone (O(3)) in Houston, Texas. During the daytime, photochemical oxidation of volatile organic compounds catalyzed by NO(x) results in episodes of elevated ambient O(3) levels significantly exceeding the National Ambient Air Quality Standard. The O(3) production rate in Houston is significantly higher than those found in other cities over the United States. At night, a surface NO(x) maximum occurs because of continuous NO emission from industrial sources, and, consequently, an extensive urban-scale "hole" of surface ozone (<10 parts per billion by volume in the entire Houston area) is formed as a result of O(3) removal by NO. The results suggest that consideration of regulatory control of O(3) precursor emissions from the industrial sources is essential to formulate ozone abatement strategies in this region.

Validation of a laboratory-constructed automated gas chromatograph for the measurement of ozone precursors through comparison with a commercial analogy

An automated gas chromatographic (auto-GC) system aiming at performing unattended hourly measurement of ozone precursors was developed in the laboratory. To encompass volatile organic compounds (VOCs) of a wide range of volatility within each analysis, the system uses dual-traps and dual-columns to simultaneously analyze both low and high-boiling compounds with each injection. Since sorbents with sufficient retention of C2 compounds at room temperature, namely ethane, ethene, and ethyne are not yet available, cooling with a thermoelectrical device was built around the low-boiling trap to facilitate quantitative enrichment of C2 compounds. The effectiveness of using micro-trap with low dead volume plumbing was manifested in reducing peak width and increasing peak height for particularly the lower-boiling compounds. The increase in sensitivity allowed sufficient detector response with a small amount of air sample, e.g. 200 ml in our routine operation, which in term eliminate the need for remove water prior to sampling trapping. The performance and applicability of this laboratory-built auto-GC system was validated by comparison with a commercial analog, i.e. the ATD-400 system made by Perkin-Elmer, in the field sharing a common air intake. During more than 3 weeks of synchronized monitoring of ambient volatile organic compounds both systems showed highly consistent results on almost every monitored compound, clearly demonstrating the robustness of this self-built system.

Modeling the oxidative capacity of the atmosphere of the south coast air basin of California. 1. Ozone formation metrics

Metrics associated with ozone (O3) formation are investigated using the California Institute of Technology (CIT) three-dimensional air-quality model. Variables investigated include the O3 production rate (P(O3)), O3 production efficiency (OPE), and total reactivity (the sum of the reactivity of carbon monoxide (CO) and all organic gases that react with the hydroxyl radical). Calculations are spatially and temporally resolved; surface-level and vertically averaged results are shown for September 9, 1993 for three Southern California locations: Central Los Angeles, Azusa, and Riverside. Predictions indicate increasing surface-level O3 concentrations with distance downwind, in line with observations. Surface-level and vertically averaged P(O3) values peak during midday and are highest downwind; surface P(O3) values are greater than vertically averaged values. Surface OPEs generally are highest downwind and peak during midday in downwind locations. In contrast, peaks occur in early morning and late afternoon in the vertically averaged case. Vertically averaged OPEs tend to be greater than those for the surface. Total reactivities are highest in upwind surface locations and peak during rush hours; vertically averaged reactivities are smaller and tend to be more uniform temporally and spatially. Total reactivity has large contributions from CO, alkanes, alkenes, aldehydes, unsubstituted monoaromatics, and secondary organics. Calculations using estimated emissions for 2010 result in decreases in P(O3) values and reactivities but increases in OPEs.

Intercomparison of volatile organic carbon measurement techniques and data at La Porte during the TexAQS2000 Air Quality Study

The Texas Air Quality Study 2000 (TexAQS2000) investigated the photochemical production of ozone and the chemistry of related precursors and reaction products in the vicinity of Houston, TX. The colocation of four instruments for the measurement of volatile organic carbon compounds (VOCs) allowed a unique opportunity for the intercomparison of the different in-situ measuring techniques. The instruments included three gas chromatographs, each with a different type of detector, and a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) with each system designed to measure a different suite of VOCs. Correlation plots and correlation statistics are presented for species measured by more than one of these instruments. The GC instruments were all in agreement to within 10-20% (slope) with coefficients of variation (r2) of > or = 0.85. The PTR-MS agreement with other instruments was more dependent on species with some very good agreements (r2 values of approximately 0.95 for some aromatics), but isoprene, acetaldehyde and propene were substantially less highly correlated (0.55 < r2 < 0.80). At least part of these differences were undoubtedly due to the timing of sample acquisition in an environment in which VOC levels changed very rapidly on both quantitative and temporal scales.

A regression-based approach for estimating primary and secondary particulate matter intake fractions

One of the common challenges for life cycle impact assessment and risk assessment is the need to estimate the population exposures associated with emissions. The concept of intake fraction (a unitless term representing the fraction of material or its precursor released from a source that is eventually inhaled or ingested) can be used when limited site data are available or the number of sources to model is large. Although studies have estimated intake fractions for some pollutant-source combinations, there is a need to quickly and accurately estimate intake fractions for sources and settings not previously evaluated. It would be expected that limited source or site information could be used to yield intake fraction estimates with reasonable accuracy. To test this theory, we developed regression models to predict intake fractions previously estimated for primary fine particles (PM2.5) and secondary sulfate and nitrate particles from power plants and mobile sources in the United States. Our regression models were able to predict pollutant-specific intake fractions with R2 between 0.53 and 0.86 and equations that reflected expected relationships (e.g., intake fraction increased with population density, stack height influenced the intake fraction of primary but not secondary particles). Further analysis would be needed to generalize beyond this case study and construct models applicable across source categories and settings, but our analysis demonstrates that inclusion of a limited number of parameters can significantly reduce the uncertainty in population-average exposure estimates.

Protection by isoprene against singlet oxygen in leaves

Isoprene (2-methyl-1,3-butadiene) protection against effects of singlet oxygen was investigated in Myrtus communis and Rhamnus alaternus. In M. communis, singlet oxygen produced in the leaves by Rose Bengal (RB) led to a 65% decrease in net assimilation rates within 3 h, whereas isoprene emission rates showed either a 30% decrease at ambient CO2 concentrations or a 70% increase under high CO2. In both cases, these changes led to an increase in calculated internal isoprene concentrations. The isoprene protection effect was directly demonstrated by fumigation of young (non-emitting) leaves, treated with RB or bromoxynil (simulating photoinhibition). There was 42% and 29% reduction in the damage to net assimilation compared with non-fumigated leaves for RB or bromoxynil, respectively. In R. alaternus, similar effects of RB on net assimilation were observed, and additional fluorescence measurements showed a significantly smaller decrease in Fv/Fm in isoprene-fumigated young leaves treated with RB (from 0.78 to 0.52), compared with non-fumigated leaves (from 0.77 to 0.27). The internal isoprene concentrations used in this study and possible rate of 1O2 production in leaves indicate that the protective effects observed should be beneficial also under natural conditions.

Growth responses of Populus tremuloides clones to interacting elevated carbon dioxide and tropospheric ozone

The Intergovernmental Panel of Climate Change (IPCC) has concluded that the greenhouse gases carbon dioxide (CO2) and tropospheric ozone (O3) are increasing concomitantly globally. Little is known about the effect of these interacting gases on growth, survival, and productivity of forest ecosystems. In this study we assess the effects of three successive years of exposure to combinations of elevated CO2 and O3 on growth responses in a five trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45 degrees N 89 degrees W) and employs free air carbon dioxide and ozone enrichment (FACE) technology. The aspen stand was exposed to a factorial combination of four treatments consisting of elevated CO2 (560 ppm), elevated O3 (episodic exposure-90 microl l(-1) hour(-1)), a combination of elevated CO2 and O3, and ambient control in 30 m treatment rings with three replications. Our overall results showed that our three growth parameters including height, diameter and volume were increased by elevated CO2, decreased by elevated O3, and were not significantly different from the ambient control under elevated CO2 + O3. However, there were significant clonal differences in the responses; all five clones exhibited increased growth with elevated CO2, one clone showed an increase with elevated O3, and two clones showed an increase over the control with elevated CO2 + O3, two clones showed a decrease, and one was not significantly different from the control. Notably. there was a significant increase in current terminal shoot dieback with elevated CO2 during the 1999-2000 dormant season. Dieback was especially prominent in two of the five clones, and was attributed to those clones growing longer into the autumnal season where they were subject to frost. Our results show that elevated O3 negates expected positive growth effects of elevated CO2 in Populus tremuloides in the field, and suggest that future climate model predictions should take into account the offsetting effects of elevated O3 on CO2 enrichment when estimating future growth of trembling aspen stands.