Article pubs.acs.org/est
Magnitude, Decadal Changes, and Impact of Regional Background Ozone Transported into the Greater Houston, Texas, Area Shaena R. Berlin,†,‡ Andrew O. Langford,† Mark Estes,§ Melody Dong,†,∥ and David D. Parrish*,† †
Chemical Sciences Division, NOAA ESRL , 325 Broadway R/CSD7, Boulder, Colorado 80305, United States Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Texas Commission on Environmental Quality, 12100 Park 35 Circle, Austin, Texas 78753, United States ‡
S Supporting Information *
ABSTRACT: Two independent analyses of the daily maximum 8 h average ozone concentrations measured during the high ozone season (May through October) at Continuous Ambient Monitoring Stations are used to quantify the regional background ozone transported into the Houston−Galveston−Brazoria (HGB) area. The dependence on wind direction is examined, and long-term trends are determined using measurements made between 1998 and 2012. Both analyses show that the regional background ozone has declined during periods of continental outflow: i.e., the conditions associated with most high ozone episodes in HGB. The changes in regional background ozone found for northeasterly and southeasterly flow are −0.50 ± 0.54 and −0.79 ± 0.65 (95% confidence limit) ppbv yr−1, respectively, which correspond to decreases of ∼7−11 ppbv between 1998 and 2012. This finding is consistent with the summertime downward trend of −0.45 ppbv yr−1 (range of sites: −0.87 to +0.07 ppbv yr−1) for ozone in the eastern U.S. between 1990 and 2010 reported by Cooper et al. and shows that changing background concentrations are at least partially responsible for the decreased surface ozone in the HGB area over the past decade. Baseline ozone concentrations in air flowing into Texas from the Gulf of Mexico have not changed significantly over this period.
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reclassified the HGB area as a “marginal” nonattainment zone under the 2008 standard.8 The apparent success of the control strategies adopted by the State of Texas provides a potential model for other areas in O3 nonattainment. However, it is well established that regional background O3 also makes substantial contributions to exceedances in Texas,9−11 and changes in the regional background originating from upwind emissions or hemispheric scale O3 concentrations could be partially responsible for the reported decreases in measured O3. The primary goal of this study is to quantify the changes in regional background O3 between 1998 and 2012 and determine if these changes contribute significantly to the observed O3 decreases in the HGB area. Regional Background Ozone. Surface O3 measured at any given location can be considered, at least conceptually, to be the sum of that produced locally added to a “regional background” transported into the area by the large-scale winds.10,11 On any particular day, this is the concentration that would be present if no O3 were produced from NOx and VOC precursors emitted locally on that day, or emitted on preceding
INTRODUCTION
Because of its detrimental effects on human health and agriculture, the U.S. Environmental Protection Agency (EPA) regulates ground-level ozone (O3) through the National Ambient Air Quality Standards (NAAQS) promulgated under the Clean Air Act.2 In 2008, the EPA classified the Houston− Galveston−Brazoria (HGB) area of southeast Texas as a “severe” nonattainment area for O3. This region experienced some of the highest O3 concentrations reported in the United States during the 1990s and early 2000s3 and was the subject of two major air quality studies: TexAQS I in 2000 and TexAQS II in 2006. 4 These studies showed that the highest O 3 concentrations in the Houston area were associated with emissions of highly reactive volatile organic compounds (HRVOCs) from petrochemical facilities along Galveston Bay and the Houston Ship Channel, which react with NOx from urban and industrial sources.4 Furthermore, the proximity of Houston to the Gulf of Mexico often results in stagnant afternoon air due to stalled sea breeze fronts, which allow for pollution accumulation.5 Using the findings from the two TexAQS studies as guidance, the State of Texas adopted emission caps and reductions in 2007 to reach attainment of the 2008 8 h NAAQS of 75 parts per billion (ppbv) by 2019.6 Since the implementation of these measures, O3 concentrations in Houston have declined significantly7 and in 2012 the EPA © 2013 American Chemical Society
Received: Revised: Accepted: Published: 13985
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modifications; in the other, the PCA analysis of Langford et al.10 was applied to six selected CAMS stations. The two methods analyzed data from different sites; the TCEQ method used the best sites available each year, while the PCA method used a smaller set of six sites that had data continuously available over a 14 year period. The results of the two methods are compared, and our conclusions are based upon both regional background O3 determinations.
days and recirculated locally by mesoscale circulations such as the land−sea breeze. The regional background O3 defined above and considered in this work is distinguished from terms used elsewhere: preindustrial or “natural background O3”, “policy relevant background (PRB) O3”, and “baseline O3”. Natural background O3 is that which would be present without any human emissions, whatsoever.12 Since anthropogenic emissions impact O3 concentrations at all locations in the northern midlatitudes, we will not further consider this concept. The term baseline O3 is used to represent O3 concentrations measured at times when local emission influences are determined to be negligible and is frequently used to describe the concentrations found in marine air masses entering the west coast of the U.S. from the Pacific Ocean.13,14 We use this term in a similar fashion here to describe concentrations in clean marine O3 transported into Texas from the Gulf of Mexico. Regional background O3 and baseline O3 would be identical when synoptic winds bring undisturbed marine air into HGB. Policy relevant background (PRB) O3 is determined from model calculated concentrations with all North American anthropogenic emissions removed (e.g., Zhang et al.15); McDonald-Buller et al.16 have presented a recent review. PRB O3 is purely a model concept and is thus distinct from both measurement-based regional background O3 and baseline O3. We will not further consider PRB O3 in this work. Several methods have been used to quantify the regional background O3 concentrations in the HGB area. The Texas Commission on Environmental Quality (TCEQ) manually chooses the daily 8 h average (MDA8) O3 concentration at an appropriate rural site upwind of Houston emission sources each day.11 A similar but more automated approach used by NielsenGammon et al.11 chose the lowest MDA8 O3 concentration measured at one of five continuous ambient monitoring stations (CAMS) maintained by the TCEQ that surround Houston on the urban perimeter. This method assumed that at least one monitor was not affected by local emissions and thus would always measure the “true” background. Kemball-Cook et al.9 and Senff et al.17 used airborne sampling during both TexAQS I and II to investigate O3 plumes transported downwind from the HGB and Dallas− Fort Worth urban areas, studies that included a detailed determination of regional background O3 outside the edges of the transported plumes on the specific days of the aircraft flights. Kemball-Cook et al.9 found that their detailed aircraft measurements gave determinations of the regional background O3 concentrations generally consistent with those derived from the method of Nielsen-Gammon et al.11 However, these approaches are limited by the availability of aircraft measurements and cannot be applied to historical records. Langford et al.10 applied principal component analysis (PCA) to the MDA8 O3 measurements from 30 CAMS stations during TexAQS II to determine the O3 contribution that uniformly affected all stations, both urban and rural, in the HGB area. They argued that this contribution represented the regional background O3 and found that the PCA results were usually in excellent agreement with the method of NielsenGammon et al.,11 except the latter method did not always isolate the regional background when the sea breeze was weak. The NOAA and TCEQ authors of this paper, working independently and using different methods, made two determinations of the regional background O3 in the HGB area. In one, the original TCEQ method was applied with some
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METHOD AND DATA SETS Regional Background Determination by TCEQ Method. Monitoring sites capable of measuring background O3 were selected on the basis of their location at or near the periphery of the urbanized and industrialized areas and their distance from local emission sources. Each of these sites is expected to receive air with regional background O3 when it is upwind of the urban area. The selected sites changed from year to year as sites were added to or removed from the monitoring network (see Figure S1 and Table S1 in the Supporting Information for the location of the sites chosen for each year of background determination). Background O3 was estimated as the lowest MDA8 O3 value observed at the selected background sites for each O3 season day (May−October) from 2000 to 2012. This method has been used by the TCEQ for many years.10,11,18−20 Inaccurate background O3 estimates may result if (a) Houston emissions from a previous day have recirculated and re-entered the area, (b) there was a large spatial gradient in background O3, or (c) the Gulf breeze was weak and penetrated only partially inland, affecting only one or two coastal sites rather than the entire urban area. For the first case, any method of estimating background O3 would fail to identify an air mass unaffected by Houston’s emissions on days with multiday recirculation and/or stagnation; it must be recognized that all background O3 estimates on such days are likely to be high. For the second case, the TCEQ method will choose the lowest end of the observed background O3 gradient and therefore may yield lower estimates of background O3 than other methods. Additional analysis was performed to reduce the problem of the Gulf breeze not penetrating the entire urban area. If seabreeze-influenced sites are erroneously identified as representative of background O3 in HGB, then there are particularly large differences between the area-wide peak O3 and the background O3. Days with such large differences were examined further. For each month of the O3 season, a least-squares linear regression was calculated, with the peak O3 as the dependent variable and the estimated background O3 as the independent variable. Residuals were calculated, and for days with residual values greater than 2 standard deviations, the 5 min O3 data for all sites in the Houston area (Table S1 in the Supporting Information) were reviewed to determine whether coastal O3 concentrations (i.e., from Galveston (C34), Galveston 99th Street (C1034), Lake Jackson (C1016), Clute (C11), and Danciger (C618)) were notably different from those of all of the other sites from 0900 to 1800 LST. If they were, a noncoastal site was then chosen to represent background O3. This revision of background O3 was applied sparingly: 63 days out of 2392 (2.6%) were revised; on average these revisions increased the background O3 value by 17.5 ppbv. To some extent, the number and location of monitors, which have varied over time (see Table S1 in the Supporting Information) may affect the results of this method. The lowest MDA8 among all stations would tend to be lower when more 13986
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Since the much larger number and wider distribution of sites used in the PCA30 analysis10 may be expected to yield more robust values for the regional background in comparison with the present work with only 6 sites, the adequacy of the reduced sample size is evaluated by comparing the background concentrations derived from the PCA6 analysis to those obtained from the PCA30 analysis for 76 days during TexAQS II (August 1 to October 15, 2006). The overall correlation is excellent (r2 = 0.98), with the PCA6 background values averaging 3.1 ± 2.4 ppbv (9.4 ± 7%) higher than the PCA30 results. The correlation between the TCEQ and PCA6 results shows larger differences and lower correlation (r2 = 0.80) for 2000− 2011. The PCA6 background values average 7.3 ± 7.8 ppbv (26% with 1σ from −2 to +60%) higher than the TCEQ results. In the following analysis we assume the TCEQ method provides a lower limit and the PCA6 method an upper limit for the actual regional background O3 transported into HGB. The results of the analysis are interpreted in light of this assumption. Determination of Baseline Marine Ozone Concentrations. Additional surface measurements were used to determine baseline marine O3 concentrations in the Gulf of Mexico. These include (1) Caribbean sites, which represent the air flowing into the Gulf, (2) Gulf coast sites in Texas, Louisiana, and Florida, which represent the receptor sites for this inflow, and (3) some relatively remote, interior sites in Texas for comparison (see Figure S3 and Table S3 in the Supporting Information for the location and the time periods of the measurements). For comparison we also examined O3 data from two midlatitude marine sites: Bermuda (in the Atlantic approximately 1000 km southeast of Cape Hatteras, NC) and Trinidad Head (on the northern California Pacific coast, approximately 400 km north-northwest of San Francisco).
monitors are considered. The magnitude of this effect is difficult to judge, but since background O3 is expected to vary only slowly over the HGB region, we do not expect it to be large. Regional Background Determination by PCA Method. Principal component analysis mathematically transforms a large number of variables, in this case time series of MDA8 O3 concentrations from a number of surface sampling sites, into a new set of uncorrelated, orthogonal principal components (PCs). Although the total number of PCs is equal to the number of original variables, it is usually the case that the first few PCs explain most of the variance in the data set. These dominant PCs can often be linked to specific physical or chemical processes by comparing them with ancillary information such as meteorological data.10,21 Ancillary measurements were not included in the PCA itself to simplify the extraction of the regional background concentrations. In their analysis of CAMS measurements made during the TexAQS II campaign, Langford et al.10 determined that the first principal component, PC1, which is positively correlated with higher O3 at all stations regardless of proximity to emission sources, represented the regional background O3 while the next two most significant components, PC2 and PC3, represented changes in O3 due to local photochemistry and transport.10 Equations 1 and 2 of Langford et al.10 allow calculation of the magnitude of the regional background O3 from the PCA results. Although Langford et al.10 used 30 CAMS stations in their analysis, many of the stations in the CAMS network were operational for only a few years during the past decade and thus not available for long-term trend analysis. The present analysis is based on the MDA8 measurements from six CAMS stations that have nearly continuous measurements since 1998. The analysis was restricted to the high ozone season (June 1 to October 15) for each year from 1998 to 2011. In the following discussion, we will refer to the 6-station PCA analysis as PCA6 and the 30-station analysis as PCA30 (see Figure S1 and Table S2 in the Supporting Information for the location and dates of operation of the six sites as well as the 30 sites of Langford et al.10). Table S4 in the Supporting Information gives the principal component magnitudes and the fractional variance explained for both PCA analyses for 2006. Considering the conditions likely to lead to inaccurate background O3 estimates, the PCA6 method may also give an overestimate if Houston emissions have recirculated and reentered the area or if there are stagnant conditions when all HGB area sites are impacted by local photochemical O3 production. Under stagnant conditions the PCA6 method may overestimate background O3 in comparison to the TCEQ method, because all 6 sites are located within or relatively close to the HGB urban area. Both the PCA30 and TCEQ methods are less sensitive to this effect, since they included some stations farther removed from the urban area (see Figure S1 in the Supporting Information). Comparison of Regional Background Determinations. Background O3 determinations from the PCA (both 6- and 30station results) and TCEQ methods have been compared (see Figure S2 in the Supporting Information). Generally the three determinations show similar temporal variability but have significant differences in the magnitude of the results. Since it is expected that the TCEQ method may be biased low and the PCA6 method may be biased high, it is important to quantify the differences in the results.
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RESULTS AND DISCUSSION The TCEQ and the PCA6 methods both find that the transported regional background contributes half or more to the maximum HGB O3 concentrations, not only on average but also on O3 exceedance days: i.e., on days when MDA8 O3 is greater than 75 ppbv. Figure 1 shows that HGB MDA8 O3 correlates well with the regional background O3 determined by both methods, and the correlation coefficients suggest that the regional background accounts for 63% (TCEQ background) to 86% (PCA6 background) of the variance in HGB MDA8 O3 concentrations. For all days the ratios of average regional background O3 to average MDA8 O3 are 0.61 and 0.51 for the background determinations by the PCA and TCEQ methods, respectively. On exceedance days these ratios are slightly larger: 0.63 and 0.52. To put these fractions in perspective, on exceedance days the PCA6 method finds an average background of 59 ppbv and an average local contribution (the difference between MDA8 O3 and regional background O3) of 34 ppbv for a total average MDA8 O3 of 93 ppbv. The corresponding averages for the TCEQ method are a background of 48 ppbv and a local contribution of 44 ppbv for a total of 92 ppbv. Both methods agree that the transported regional background contributes more than half of the O3 in HGB on exceedance days. The local contribution accounts for a substantial but somewhat smaller fraction. The general decline in maximum surface O3 concentrations within HGB over the past decade is shown by the downward trend in the highest MDA8 O3 reported in each year. Figure 2a compares the trend in this quantity for the 6 CAMS stations 13987
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Figure 1. Correlation between MDA8 ozone observed in HGB versus the regional background O3 determined by the PCA6 (upper panel) and TCEQ (lower panel) methods. The solid lines indicate the standard linear regressions whose slopes (±95% confidence intervals) are annotated. The 75 ppbv NAAQS concentrations are indicated on each axis.
used in the PCA analysis (−4.6 ± 1.7 ppbv yr−1 between 1998 and 2011) with that for all available CAMS sites in Houston (−4.3 ± 1.3 ppbv yr−1 between 1998 and 2012). Since the number of available CAMS sites changed significantly with time, the similarity between these two measures of maximum MDA8 provides further evidence that the 6 stations adequately represent the general systematics of O3 concentration in the HGB area. The right axis of Figure 2a shows the number of days each year when any one of the operational CAMS stations exceeded the current NAAQS of 75 ppbv. Each metric shows the marked improvement in ozone air quality in the Houston area since 1998. When the means of the maximum MDA8 O3 concentrations observed at any of the CAMS stations on all days during the respective O3 seasons (black circles in Figure 2b,c) are considered, the downward trend is a factor of 4−5 smaller (−1.15 ± 0.52 and −0.89 ± 0.66 ppbv yr−1 for the 6 sites and all available sites, respectively), but still statistically significant. A contribution to the downward trend (−0.33 ± 0.39 and −0.21 ± 0.39 ppbv yr−1, respectively) and much of the year-to-year variability in mean ozone is due to the regional background O3 concentrations (Figure 2b,c). These background trends are downward but only marginally statistically significant (the probabilities that the trends are negative are approximately 0.96 and 0.87, respectively, as determined from a single-tailed Student’s t test). If the 2011 measurements are excluded from the analysis, the trends are larger (−0.49 ± 0.40 and −0.31 ± 0.41 ppbv yr−1, respectively) and of higher statistical significance (the probabilities that the trends are negative are
Figure 2. (a) Trends in highest MDA8 O3 reported by one of the 6 CAMS stations used in the PCA analysis (Max 6 CAMS) and by all HGB CAMS (Max HGB). Also shown is the total number of days that MDA8 O3 exceeded the 2008 NAAQS at any of the 6 CAMS stations multiplied by the number of those stations that exceeded that standard on each day. (b, c) Trends in mean MDA8 ozone and background and local contributions from the PCA6 and the TCEQ methods, respectively. The solid lines indicate linear regressions whose slopes (±95% confidence intervals) are annotated.
approximately 0.988 and 0.94, respectively); 2011 was an exceptional year, as Texas experienced its worst drought since weather records began in 1895.22 The drought conditions led to abundant sunlight as well as numerous wildfires that emitted copious amounts of NOx and other ozone-forming compounds; such conditions might be expected to lead to increased regional background O3 transported into HGB.23 The trends in the mean local contribution from the HGB area given by the difference between the mean and background values also show linear downward trends of −0.82 ± 0.26 ppbv yr−1 (R2 = 0.80) and −0.68 ± 0.33 ppbv yr−1 (R2 = 0.65) for the PCA6 and TCEQ methods, respectively. These analyses indicate that the majority of the decline in average maximum surface O3 concentrations within HGB is due to a decline in the local contribution but that the decline in the transported regional background O3 concentration makes a significant additional contribution. 13988
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Houston is located on the Texas coast of the Gulf of Mexico, where wind patterns generally import background O3 from two distinct origins. The prevailing southerly winds from the Gulf of Mexico (see Figure S4 in the Supporting Information) bring subtropical marine air that has passed through the Caribbean before entering the Gulf. Background O3 concentrations are relatively low in this air, as we quantitatively document in the next section. The prevailing southerly winds are intermittently interrupted by northerly and easterly flow (see e.g. Bomar24) associated with frontal passages.20 These winds generally bring air of continental origin with significantly higher O 3 concentrations into the area. The strong day-to-day variability in regional background O3 reflects this wind dependence. It is useful to determine the trends in the transported regional background O3 under different wind regimes. The dependence of the regional background O3 on wind direction is shown quantitatively in Figure 3, which resolves the mean PCA and TCEQ background concentrations from Figure 2 into the mean values measured when the prevailing winds originate from the NW, NE, SE, and SW wind quadrants. The selection was based on the 18UT (12 CST) 850 hPa wind components averaged over a 2.5° × 2.5° box centered at 30.25° N/95.75° W from the National Centers for Environmental Prediction (NCEP) reanalysis.25 Winds with components less than 0.35 m s−1 along each axis were excluded to minimize the influence of uncertain wind directions. The spatially averaged 850 hPa winds were used to reduce the influence of the land− sea breeze and other local and mesoscale circulations and better represent the synoptic scale flow. The wind-selected background data were averaged to give the mean (ozone season) background for each wind quadrant for each year. The plots in Figure 3 show that the lowest background ozone concentrations (∼25−29 ppbv) are associated with southwesterly winds, which occur nearly half the time. These baseline conditions will be discussed in more detail in the next section. The trends found for the background O3 from this wind sector are small and not significantly different from 0. The background concentrations are higher when the winds are northwesterly (∼14−18 ppbv higher, ∼9% frequency), northeasterly (∼20 ppbv higher, ∼14% frequency), or southeasterly (∼9−13 ppbv higher, ∼18% frequency), indicating significant continental influences in these quadrants. (The total average frequency sums to 88%, with the excluded low wind speeds accounting for the remaining 12%.) The highest mean background concentrations (∼43 ppbv) are associated with northeasterly winds; most high ozone episodes in the HGB area are generally associated with this synoptic wind direction (e.g., Rappenglück et al.20). Synoptic-scale northwesterly winds are uncommon in the HGB during summer, and the small upward trend in background ozone seen in the PCA6 results (Figure 3a) is not statistically significant. The background ozone during northeasterly and southeasterly winds exhibits decreases of −0.50 ± 0.54 and −0.79 ± 0.65 ppbv yr−1, respectively, where these values are the averages of the PCA and TCEQ analyses with the confidence limit taken from the smaller values from the TCEQ analysis. These decreases are statistically significant in both the PCA and TCEQ analyses (the probabilities that the trends are negative are approximately 0.97 and 0.99, respectively). The decreases of background regional O3 during northeasterly and southeasterly flow are consistent with the 1990−2010 average summertime decrease of −0.45 ppbv yr−1 (range of sites: −0.87 to +0.07 ppbv yr−1) in the eastern U.S. reported in the recent analysis of background ozone by Cooper et al.1 This
Figure 3. Temporal trends of background O3 during periods with prevailing winds originating from the indicated wind quadrants. The closed and open symbols give the results from the PCA and TCEQ analyses, respectively. The solid and dashed straight lines indicate the linear regressions to the respective points; the slopes with 95% confidence intervals are annotated along with the averages and standard deviations of the backgrounds. Thin dashed lines show the fraction of days during the ozone season that the wind was from each quadrant, and the average fraction of days with standard deviation is annotated for each wind sector in parentheses.
consistency provides support for the hypothesis that O3 in the eastern U.S. constitutes the background transported into the HGB area during high ozone episodes. To put these trends in perspective, decreases of −0.50 to −0.79 ppbv yr−1 correspond to mean decreases of 7−11 ppbv in background ozone concentrations between 1998 and 2012. This decrease in transported background O3 has contributed to the decrease in the number of O3 exceedance days in the HGB area over the study period. Marine Baseline O3. The prevailing southwesterly winds from the Gulf of Mexico bring subtropical marine air that has passed through the Caribbean area before entering the Gulf. This flow also contributes significantly to baseline ozone over the central and eastern U.S. during summer. Although Figure 13989
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The marine baseline O3 in Figure 4 is quite low in comparison to continental concentrations, with the lowest baseline concentrations generally falling in the 18−25 ppbv range. Such low concentrations are not surprising because concentrations of this order are characteristic of the tropical marine boundary layer (MBL) in the western north Atlantic,26 and even lower concentrations have been observed over the tropical Pacific.27 These concentrations are comparable to the lowest regional O3 concentrations observed in HGB (Figure 1) and are consistent with the background O3 in HGB during southwesterly flow (Figure 3). However, it is also clear from Figure 4 that the O3 coming ashore along most of the U.S. Gulf coast is enhanced in comparison to that entering the Gulf of Mexico from the Caribbean. The enhancement is smaller in Florida (Everglades data) and on the southwest Texas coast (Brownsville and Isla Blanca data) than further east on the Texas coast (e.g., Galveston and Sabine Pass data). Whether this enhancement is from net O3 production within the Gulf of Mexico (e.g., from precursors emitted from oil and gas development activities or shipping), from recirculated North American influences, or from some other source is undetermined. Trend analysis of the marine data shows no statistically significant systematic changes in baseline O3 over more than two decades of measurements. The combined data set from the Caribbean sites (Barbados and Virgin Islands) provide the longest data record. The long but discontinuous measurement record from Barbados is consistent with the few years of measurements in the Virgin Islands made during the gap in the Barbados measurements. The linear regression to the combined Caribbean record (black dashed line in Figure 4) limits with 95% confidence any systematic baseline O3 change to −2.3 to +1.4 ppbv over that 22-year period. Thus, changes in the marine baseline concentrations cannot account for the interannual variability or downward trend in the HGB regional background seen in Figure 2b,c and are consistent with the broader conclusion of Cooper et al.1 that decreases in surface O3 across the eastern U.S. between 1990 and 2010 are due to reductions in precursor emissions and not to large-scale transport. The marine baseline O3 concentrations transported into Texas from the Gulf of Mexico are smaller than marine baseline O3 concentrations transported ashore to the U.S. west coast from the Pacific. Figure 4 compares the temporal evolution of baseline O3 concentrations within the MBL determined in the Caribbean to those at the U.S. west coast.13 Baseline O3 concentrations at the California coast are significantly higher than those at the Gulf Coast and have increased, apparently as a result of increased emissions of NOx and VOC precursors from East Asia.14 The U.S. west coast faces the additional challenge of stronger vertical mixing caused by the more complex topography characterizing the Pacific coast in comparison to the Gulf of Mexico coastal plain. This topography causes transport of air from higher elevations with higher O3 concentrations to the surface downwind of topographical barriers.28 Marine air flowing into Texas from the Gulf of Mexico provides relatively favorable conditions of low O3 mixing ratios, and these concentrations are not increasing significantly on decadal scales.
3d suggests that there have been no significant changes in the baseline ozone measured in Houston during southwesterly flow, it is desirable to investigate trends over a larger area. Here we implement a four-step process. First, we select surface O3 measurements collected during the five month May− September period at a range of sites (upwind, on the Texas Coast, and within the state) that may represent transported marine air. Second, we examine seasonal and diurnal variations of O3 concentrations at these sites in order to identify measurements likely to represent marine baseline O3. Third, we identify meteorological conditions (i.e., high onshore winds) that bring relatively undisturbed marine baseline O3 to these sites. Finally, we select these wind conditions to quantify marine baseline O3, both the average concentrations in the transported marine air and any temporal changes. The systematic changes derived from this analysis are believed to reflect changes in baseline O3 concentrations representative of marine air flowing into Texas. To quantify baseline O3 concentrations from data collected at surface sites, measurements during relatively high wind speeds (>4 m/s) from the southeast (120° ≤ wind direction ≤ 180°) at coastal sites are expected to represent air least disturbed by sources on the North American continent and within the Gulf of Mexico. Appendices A−C of the Supporting Information describe the rationale for and indications of the success of this selection process. Figure 4 presents these approximate determinations of baseline O3 transported into Texas from the Gulf of Mexico.
Figure 4. (upper panel) Temporal evolution of Gulf of Mexico baseline ozone averaged over the five month (May−September) ozone season. Linear regressions are given with slopes with 95% confidence intervals annotated. (lower panel) Comparison of Gulf of Mexico baseline ozone at the Caribbean sites with that determined in the spring and in the summer at the U.S. Pacific coast. 13990
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(6) Texas Commission on Environmental Quality (TCEQ). (2010). Revision to the state implementation plan for the control of ozone air pollution; 2009-017-SIP-NR. (7) Texas Commission on Environmental Quality (TCEQ). (2012a). Air quality successes-Criteria pollutants; retrieved on December 16, 2012 from http://www.tceq.texas.gov/airquality/airsuccess (8) Texas Commission on Environmental Quality (TCEQ). (2012b). Houston-Galveston-Brazoria: Current attainment status; retrieved on December 16, 2012 from http://www.tceq.texas.gov/airquality/sip/ hgb/hgb-status. (9) Kemball-Cook, S.; Parrish, D.; Ryerson, T.; Nopmongcol, U.; Johnson, J.; Tai, E.; Yarwood, G. Contributions of regional transport and local sources to ozone exceedances in Houston and Dallas: Comparison of results from a photochemical grid model to aircraft and surface measurements. J. Geophys. Res. 2009, 114, D00F02 DOI: 10.1029/2008JD010248. (10) Langford, A. O.; Senff, C.; Banta, R.; Hardesty, M.; Alvarez, R. J., II; Sandberg, S.P.; Darby, L.S. Regional and local background ozone in Houston during Texas Air Quality Study 2006. J. Geophys. Res. 2009, 114, D00F12 DOI: 10.1029/2008JD011687. (11) Nielsen-Gammon, J., Tobin, J.; McNeel, A.; Li, G. A Conceptual Model for Eight-Hour Exceedences in Houston, Texas Part I: Background Ozone Levels in Eastern Texas; Houston Advanced Research Center: Houston, TX, 2005. (12) Volz, A.; Kley, D. Evaluation of the Montsouris series of ozone measurements made in the nineteenth century. Nature 1988, 332, 240−242. (13) Parrish, D. D.; Millet, D. B.; Goldstein, A. H. Increasing ozone in marine boundary layer air inflow at the west coasts of North America and Europe. Atmos. Chem. Phys. 2009, 9, 1303−1323. (14) Cooper, O. R.; Parrish, D. D.; et al. Increasing springtime ozone mixing ratios in the free troposphere over western North America. Nature 2010, 463, 344−348. (15) Zhang, L.; Jacob, D. J.; Downey, N. V.; Wood, D. A.; Blewitt, D.; Carouge, C. C.; van Donkelaar, A.; Jones, D. B. A.; Murray, L. T.; Wang, Y. Improved estimate of the policy-relevant background ozone in the United States using the GEOS-Chem global model with 1/2° x 2/3° horizontal resolution over North America. Atmos. Environ. 2011, 45 (37), 6769−6776. (16) McDonald-Buller, E. C.; Allen, D. T.; Brown, N.; Jacob, D. J.; Jaffe, D.; Kolb, C.E.; Lefohn, A.S.; Oltmans, S.; Parrish, D. D.; Yarwood, G.; Zhang, L. Establishing Policy Relevant Background (PRB) Ozone Concentrations in the United States. Environ. Sci. Technol. 2011, 45, 9484−9497. (17) Senff, C. J.; Alvarez, R. J., II; Hardesty, R. M.; Banta, R. M.; Langford, A. O. Airborne lidar measurements of ozone flux downwind of Houston and Dallas. J. Geophys. Res. 2010, 115, D20307 DOI: 10.1029/2009JD013689. (18) Nielsen-Gammon, J., Tobin, J.; McNeel, A. A Conceptual Model for Eight-Hour Exceedences in Houston, Texas Part II: Eight-Hour Ozone Exceedances in the Houston-Galveston Metropolitan Area; Houston Advanced Research Center: Houston, TX, 2005. (19) Sullivan, D. Effects of Meteorology on Pollutant Trends, Final Report to TCEQ, Grant 582-5-86245-FY08-01, March 16, 2009; http://www.tceq.state.tx.us/assets/public/implementation/air/am/ contracts/reports/da/5820586245FY0801-20090316-ut-met_effects_ on_pollutant_trends.pdf (20) Rappenglück, B.; Perna, R.; Zhong, S.; Morris, G. A. An analysis of the vertical structure of the atmosphere and the upper-level meteorology and their impact on surface ozone levels in Houston, Texas. J. Geophys. Res. 2008, 113, D17315 DOI: 10.1029/ 2007JD009745. (21) Langford, A. O.; Brioude, J.; Cooper, O. R.; Senff, C. J.; Alvarez, R. J., II; Hardesty, R. M.; Johnson, B. J.; Oltmans, S. J. Stratospheric influence on surface ozone in the Los Angeles area during late spring and early summer of 2010. J. Geophys. Res. 2012, 117, D00V06 DOI: 10.1029/2011JD016766. (22) Nielsen-Gammon, J. W. The 2011 Texas Drought. Texas Water J. 2012, 3 (1), 59−95.
ASSOCIATED CONTENT
S Supporting Information *
Tables, figures, and text giving maps with details of the sites and data sets utilized in this study, comparisons of the ozone background determinations and description of the mean winds impacting the Texas region in the summer, and three appendices describing the analysis of marine baseline air. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*D.D.P.: tel, 303-497-5274; fax, 303-497-5126; e-mail, david.d.
[email protected]. Present Address ∥
University of California, San Diego, CA.
Notes
Although this article has been reviewed by the TCEQ, it does not necessarily reflect the policies of the agency. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.R.B. and M.D. gratefully acknowledge support from the NOAA Office of Education Hollings Scholarship Program and the NCAR High School Internship & Research Opportunities (HIRO) Program, respectively. The authors are grateful to the Texas Commission on Environmental Quality (TCEQ) for providing data from Texas monitoring sites, the U.S. Environmental Protection Agency for providing data from monitoring sites in other states and Puerto Rico, NOAA Earth System Research Laboratory, Global Monitoring Division, for providing data from Barbados, Bermuda, and Trinidad Head, and the U.S. National Park Service for providing data from multiple sites in Texas, the Everglades, and the Virgin Islands. This work was supported by the NOAA Air Quality and Atmospheric Chemistry and Climate Programs.
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REFERENCES
(1) Cooper, O. R.; Gao, R.-S.; Tarasick, D.; Leblanc, T.; Sweeney, C. Long-term ozone trends at rural ozone monitoring sites across the United States, 1990−2010. J. Geophys. Res. 2012, 117, D22307 DOI: 10.1029/2012JD018261. (2) U.S. Environmental Protection Agency (EPA) (2012), National Ambient Air Quality Standards (NAAQS), Retrieved on December 16, 2012 from http://www.epa.gov/air/criteria.html. (3) Kleinman, L. I.; Daum, P. H.; Lee, Y.-N.; Nunnermacker, L. J.; Springston, S. R.; Weinstein-Lloyd, J.; Rudolph, J. Ozone production efficiency in an urban area. J. Geophys. Res. 2002, 107 (D23), 4733 DOI: 10.1029/2002JD002529. (4) Parrish, D. D.; Allen, D. T.; Bates, T. S.; Estes, M.; Fehsenfeld, F. C.; Feingold, G.; Ferrare, R.; Hardesty, R. M.; Meagher, J. F.; NielsenGammon, J. W.; Pierce, R. B.; Ryerson, T. B.; Seinfeld, J. H.; Williams, E. J. Overview of the Second Texas Air Quality Study (TexAQS II) and the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). J. Geophys. Res. 2009, 114, D00F13 DOI: 10.1029/ 2009JD011842. (5) Banta, R. M., Senff, C. J., Alvarez, R. J., Parrish, D. D., Trainer, M. K., Ryerson, T. B., Darby, L. S., Hardesty, R. M., Lambeth, B., Neuman, J. A., Angevine, W. M., Nielsen-Gammon, J. W., Sandberg, S. P., Langford, A. O., White, A. B. Dependence of peak daily ozone concentrations in Houston, Texas on the sea breeze and meteorological variables. In Proceedings of the 8th Symposium on the Urban Environment; Phoenix, AZ, USA, January 2009; J13.3. 13991
dx.doi.org/10.1021/es4037644 | Environ. Sci. Technol. 2013, 47, 13985−13992
Environmental Science & Technology
Article
(23) Langmann, B.; Duncan, B.; Textor, C.; Trentmann, J.; van der Werf, G. R. Vegetation fire emissions and their impact on air pollution and climate. Atmos. Environ. 2009, 43, 107−116. (24) Bomar, G. W. Texas Weather; University of Texas Press: Austin, TX, 1995; pp 178−198. (25) Kalnay, E.; et al. The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc. 1996, 77, 437−471. (26) Lelieveld, J.; van Aardenne, J.; Fischer, H.; de Reus, M.; Williams, J.; Winkler, P. Increasing Ozone over the Atlantic Ocean. Science 2004, 304, 1483−1487. (27) Kley, D.; Crutzen, P. J.; Smit, H. G. J.; Vömel, H.; Oltmans, S. J.; Grassl, H.; Ramanathan, V. Observations of near-zero ozone levels over the convective Pacific: Effects on air chemistry. Science 1996, 274, 230−233. (28) Parrish, D. D.; Aikin, K. C.; Oltmans, S. J.; Johnson, B. J.; Ives, M.; Sweeny, C. Impact of transported background ozone inflow on summertime air quality in a California ozone exceedance area. Atmos. Chem. Phys. 2010, 10, 10093−10109.
13992
dx.doi.org/10.1021/es4037644 | Environ. Sci. Technol. 2013, 47, 13985−13992
