Texas Air Quality Study Research Articles

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

During the 2000 Texas Air Quality Study (TexAQS) and 2006 Texas Air Quality Study (TexAQS II) field experiments, aircraft measured ozone concentrations upwind, across, and downwind of the Houston and Dallas urban areas. Background ozone transported into Houston contributed, on average, approximately 50% and 66% of the total ozone on 8‐h ozone exceedance days investigated by aircraft flights during TexAQS and TexAQS II, respectively. Analysis of a flight over Dallas on one exceedance day showed that transported ozone constituted 72% of the total ozone concentration. The aircraft measurements show that these two major metropolitan areas can be brought close to exceeding the 1997 8‐h National Ambient Air Quality Standard for ozone of 0.08 ppm solely by the ozone contribution of regional transport before additional contribution from local sources. Large local contributions were also observed, particularly in Houston. Transport contributions to Dallas area ozone were quantified using the Comprehensive Air Quality Model with Extensions (CAMx) photochemical grid model and source apportionment methods. Model‐predicted ozone concentrations were compared to ozone measurements from the aircraft and the surface monitoring network, and showed agreement on the importance of regional transport and local ozone formation. These results emphasize the benefits of regional control strategies, and suggest that local controls alone may not be sufficient to ensure attainment of the 8‐h ozone standard in Houston and Dallas.

Unique isoprene oxidation products demonstrate chlorine atom chemistry occurs in the Houston, Texas urban area

As part of the 2000 Texas Air Quality Study (TexAQS), we studied the isoprene oxidation process under ambient conditions to discern the presence of chlorine atom (Cl) chemistry in the Houston, Texas urban area. By measuring chloromethylbutenone (CMBO) and an isomer of chloromethylbutenal (CMBA), we clearly observed sixteen episodes of active Cl chemistry during the 24-day experiment. Estimated median Cl concentration during each of these episodes was between the detection limit of ~102 atoms cm−3 and $$ 50_{ - 30}^{ + 70} \times {10^4} $$ atoms cm−3. Cl concentration during all the episodes averaged $$ 7.6_{ - 2.0}^{ + 4.7} \times {10^4} $$ atoms cm−3 and thus amounted to less than 3% of the OH concentration during the same periods. During the episodes, the fraction of oxidation chemistry initiated by Cl ranged from 3–43% and was strongly dependent on the quantity and type of hydrocarbons present in the atmosphere. Because of its intermittent presence and low concentration, Cl is not a broadly influential oxidant in the Houston, Texas urban area.

Empirical correlations between black carbon aerosol and carbon monoxide in the lower and middle troposphere

Single‐particle measurements of black carbon (BC) aerosol and simultaneous measurements of carbon monoxide (CO) were acquired aboard the NOAA WP‐3D aircraft during the 2006 Texas Air Quality Study (TexAQS). Observed average BC mass loadings, estimated to account for ∼90% of the ambient BC mass, decreased by more than 2 orders of magnitude from the polluted boundary layer to the clean middle troposphere (6 km). A strong positive, but non‐linear, correlation was observed between simultaneous measurements of BC and CO. Based on an analysis of all the data below 1 km, we report a compact relationship between BC and CO with a slope of 5.8 ± 1.0 ng BC (kg dry air)−1 (ppb CO)−1 that is representative of regional urban and industrial emissions from Houston and Dallas. The BC/CO emission ratio for a fresh biomass‐burning plume was estimated at 9 ± 2 ng kg−1 ppb−1.

Boundary layer aerosol chemistry during TexAQS/GoMACCS 2006: Insights into aerosol sources and transformation processes

The air quality and climate forcing impacts of atmospheric aerosols in a metropolitan region depend on the amount, composition, and size of the aerosol transported into the region; the input and removal of aerosols and aerosol precursors within the region; and the subsequent chemical processing in the atmosphere. These factors were studied in the Houston‐Galveston‐Gulf of Mexico region, aboard the NOAA R/V Ronald H. Brown during the Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS/GoMACCS 2006). The aerosol measured in the Gulf of Mexico during onshore flow (low radon concentrations indicating no contact with land for several days) was highly impacted by Saharan dust and what appear to be ship emissions (acidic sulfate and nitrate). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 6.5 (4.6) μg m−3 and 17.2 (8.7) μg m−3, respectively. These mass loadings of “background” aerosol are much higher than typically observed in the marine atmosphere and thus have a substantial impact on the radiative energy balance over the Gulf of Mexico and particulate matter (PM) loadings (air quality) in the Houston‐Galveston area. As this background aerosol moved onshore, local urban and industrial sources added an organic rich submicrometer component (66% particulate organic matter (POM), 20% sulfate, 14% elemental carbon) but no significant supermicrometer aerosol. The resulting aerosol had mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol of 10.0 (9.1) μg m−3 and 16.8 (11.2) μg m−3, respectively. These air masses, with minimal processing of urban emissions contained the highest SO2/(SO2 + SO4=) ratios and the highest hydrocarbon‐like organic aerosol to total organic aerosol ratios (HOA/POM). In contrast, during periods of offshore flow, the aerosol was more processed and, therefore, much richer in oxygenated organic aerosol (OOA). Mean (median) mass concentrations of the total submicrometer and supermicrometer aerosol were 20.8 (18.6) μg m−3 and 7.4 (5.0) μg m−3, respectively. Sorting air masses based on their trajectories and time over land provides a means to examine the effects of transport and subsequent chemical processing. Understanding and parameterizing these processes is critical for the chemical transport modeling that forms the basis for air quality forecasts and radiative forcing calculations.

Comparisons of modeled and observed isoprene concentrations in southeast Texas

Biogenic emissions of hydrocarbons, primarily isoprene, dominate the VOC emission inventory in eastern Texas. Air quality model predictions of isoprene in southeast Texas were evaluated using ground and aircraft measurements collected during the Texas Air Quality Study 2000. The effects of two different vertical mixing schemes on model predictions of isoprene concentrations were also evaluated. The photochemical and biogenic emission estimation models used were the Comprehensive Air Quality Model with Extensions and the Global Biosphere Emissions and Interactions System. Ground level isoprene concentrations predicted by the models showed markedly good agreement with measured diurnal isoprene patterns. The vertical mixing schemes were most influential on surface concentrations, resulting in differences of as much as 270% in modeled isoprene concentrations. The model over predicted observations from airborne canister samples by as much as a factor of two over rural areas, but under predicted observations over urban areas. Modeled isoprene concentrations were also compared with measurements from an airborne Proton Transfer Reaction Mass Spectrometer, and the results indicated under prediction of isoprene by the model over urban areas, but better agreement in rural areas. The impacts of the vertical mixing schemes on isoprene concentrations were less direct aloft than at the surface. The resulting vertical redistribution of isoprene affected transport rates, chemistry, and the accumulation of mass. As a result, differences in concentrations aloft ranged from none to as much as 30%. This study reinforces the challenges of air quality model validation for highly reactive species such as isoprene, and the need for carefully controlled studies of biogenic emissions and chemical processing during different meteorological conditions in regions with spatially heterogeneous land use.

Improving correlations between MODIS aerosol optical thickness and ground-based PM 2.5 observations through 3D spatial analyses

The Center for Space Research (CSR) continues to focus on developing methods to improve correlations between satellite-based aerosol optical thickness (AOT) values and ground-based, air pollution observations made at continuous ambient monitoring sites (CAMS) operated by the Texas commission on environmental quality (TCEQ). Strong correlations and improved understanding of the relationships between satellite and ground observations are needed to formulate reliable real-time predictions of air quality using data accessed from the moderate resolution imaging spectroradiometer (MODIS) at the CSR direct-broadcast ground station. In this paper, improvements in these correlations are demonstrated first as a result of the evolution in the MODIS retrieval algorithms. Further improvement is then shown using procedures that compensate for differences in horizontal spatial scales between the nominal 10-km MODIS AOT products and CAMS point measurements. Finally, airborne light detection and ranging (lidar) observations, collected during the Texas Air Quality Study of 2000, are used to examine aerosol profile concentrations, which may vary greatly between aerosol classes as a result of the sources, chemical composition, and meteorological conditions that govern transport processes. Further improvement in correlations is demonstrated with this limited dataset using insights into aerosol profile information inferred from the vertical motion vectors in a trajectory-based forecast model. Analyses are ongoing to verify these procedures on a variety of aerosol classes using data collected by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite (Calipso) lidar.

Impacts of biogenic emissions on photochemical ozone production in Houston, Texas

A regional chemical transport model (CTM) has been employed to assess the influence of biogenic emissions on ozone (O3) formation in Houston, Texas during an eight‐day episode of 24–31 August 2000 in association with the Texas Air Quality Study (TexAQS) 2000. The effects of isoprene and monoterpene emissions on O3 photochemistry in this region were investigated. Isoprene emissions played an important role in O3 formation when the O3 plume occurred in the afternoon over the urban Houston area. When the isoprene emissions were decreased or increased by 50%, the O3 concentration was decreased or increased by 5–25 ppb over the urban Houston area, respectively, but the change in the O3 concentration was less than 5–10 ppb over the industrial Ship Channel. Additional sensitivity studies showed that the surface O3 change resulted primarily from local isoprene emissions, although transport of isoprene from the north of the urban Houston area was found to be nonnegligible in the isoprene budget on several days. The contribution of monoterpene emissions to O3 formation was insignificant due to low emission rate and relatively slow reaction rate.

Correcting photolysis rates on the basis of satellite observed clouds

Clouds can significantly affect photochemical activities in the boundary layer by altering radiation intensity, and therefore their correct specification in the air quality models is of outmost importance. In this study we introduce a technique for using the satellite observed clouds to correct photolysis rates in photochemical models. This technique was implemented in EPA's Community Multiscale Air Quality modeling system (CMAQ) and was tested over a 10 day period in August 2000 that coincided with the Texas Air Quality Study (TexAQS). The simulations were performed at 4 and 12 km grid size domains over Texas, extending east to Mississippi, for the period of 24 to 31 August 2000. The results clearly indicate that inaccurate cloud prediction in the model can significantly alter the predicted atmospheric chemical composition within the boundary layer and exaggerate or underpredict ozone concentration. Cloud impact is acute and more pronounced over the emission source regions and can lead to large errors in the model predictions of ozone and its by‐products. At some locations the errors in ozone concentration reached as high as 60 ppb which was mostly corrected by the use of our technique. Clouds also increased the lifetime of ozone precursors leading to their transport out of the source regions and causing further ozone production down‐wind. Longer lifetime for nitrogen oxides (NOx = NO + NO2) and its transport over regions high in biogenic hydrocarbon emissions (in the eastern part of the domain) led to increased ozone production that was missing in the control simulation. Over Houston‐Galveston Bay area, the presence of clouds altered the chemical composition of the atmosphere and reduced the net surface removal of reactive nitrogen compounds. Use of satellite observed clouds significantly improved model predictions in areas impacted by clouds. Errors arising from an inconsistency in the cloud fields can impact the performance of photochemical models used for case studies as well as for air quality forecasting. Air quality forecast models often use the model results from the previous forecast (or some adjusted form of it) for the initialization of the new forecast. Therefore such errors can propagate into the future forecasts, and the use of observed clouds in the preparation of initial concentrations for air quality forecasting could be beneficial.

Impacts of meteorological uncertainties on ozone pollution predictability estimated through meteorological and photochemical ensemble forecasts

This study explores the sensitivity of ozone predictions from photochemical grid point simulations to small meteorological initial perturbations that are realistic in structure and evolution. Through both meteorological and photochemical ensemble forecasts with the Penn State/NCAR mesoscale model MM5 and the EPA Community Multiscale Air Quality (CMAQ) Model‐3, the 24‐hour ensemble mean of meteorological conditions and the ozone concentrations compared fairly well against the observations for a high‐ozone event that occurred on 30 August during the Texas Air Quality Study of 2000 (TexAQS2000). Moreover, it was also found that there were dramatic uncertainties in the ozone prediction in Houston and surrounding areas due to initial meteorological uncertainties for this event. The high uncertainties in the ozone prediction in Houston and surrounding areas due to small initial wind and temperature uncertainties clearly demonstrated the importance of accurate representation of meteorological conditions for the Houston ozone prediction and the need for probabilistic evaluation and forecasting for air pollution, especially those supported by regulating agencies.

Evolution of ozone, particulates, and aerosol direct radiative forcing in the vicinity of Houston using a fully coupled meteorology‐chemistry‐aerosol model

A new fully coupled meteorology‐chemistry‐aerosol model is used to simulate the urban‐ to regional‐scale variations in trace gases, particulates, and aerosol direct radiative forcing in the vicinity of Houston over a 5 day summer period. Model performance is evaluated using a wide range of meteorological, chemistry, and particulate measurements obtained during the 2000 Texas Air Quality Study. The predicted trace gas and particulate distributions were qualitatively similar to the surface and aircraft measurements with considerable spatial variations resulting from urban, power plant, and industrial sources of primary pollutants. Sulfate, organic carbon, and other inorganics were the largest constituents of the predicted particulates. The predicted shortwave radiation was 30 to 40 W m−2 closer to the observations when the aerosol optical properties were incorporated into the shortwave radiation scheme; however, the predicted hourly aerosol radiative forcing was still underestimated by 10 to 50 W m−2. The predicted aerosol radiative forcing was larger over Houston and the industrial ship channel than over the rural areas, consistent with surface measurements. The differences between the observed and simulated aerosol radiative forcing resulted from transport errors, relative humidity errors in the upper convective boundary layer that affect aerosol water content, secondary organic aerosols that were not yet included in the model, and uncertainties in the primary particulate emission rates. The current model was run in a predictive mode and demonstrates the challenges of accurately simulating all of the meteorological, chemical, and aerosol parameters over urban to regional scales that can affect aerosol radiative forcing.

Implementing a ‘bottom-up,’ multi-sector research collaboration: The case of the Texas air quality study

The vast majority of research collaboration among firms is informal. Unfortunately, little research has focused on informal, multi-institutional research collaboration, partly because by their very nature these collaborations are difficult to study systematically. In this study, we employ case study methodology to examine a large-scale research collaboration, the 2000 Texas Air Quality Study, which could be labeled ‘multi-sector, multi-institution’ and ‘informal.’ We develop the case based on a contingency model of research collaboration effectiveness, our chief objective being to assess the impact of various characteristics of the collaboration on the project’s outcomes. We find the case to align with the terms of the model, thereby distilling some implications for a theory of research collaboration effectiveness, at least within the domain of large-scale, multi-institutional, multi-sector research collaborations.

Cluster Analysis of Surface Winds in Houston, Texas, and the Impact of Wind Patterns on Ozone

Abstract The city of Houston, Texas, is near a complex coastline and numerous petrochemical plants, the combination of which plays a large role in Houston’s air pollution events. It has long been known that the thermally driven afternoon onshore flow (sea breeze or gulf breeze) transports ozone-rich air inland. As a way of quantifying the role of the gulf breeze in Houston’s high-ozone events, cluster analysis of hourly averaged surface winds from a regional network of meteorological sensors was performed for 27 summer days of 2000, with the dates coinciding with the Texas Air Quality Study 2000 (TexAQS 2000). Hourly averaged winds were partitioned into 16 independent clusters, or wind patterns, while simultaneously keeping track of the maximum ozone in the network for each hour. Clusters emerged that represented various wind patterns, including thermally driven flows, stagnant winds, and a thunderstorm outflow. All clusters were used to assess which wind patterns were most likely to be coincident with the maximum ozone of the day. High ozone was most likely to occur with clusters representing the gulf breeze. Clusters occurring before the ozone maximum of the day were analyzed to determine which sequences of wind patterns were most likely to precede high ozone. A transition from offshore flow to onshore flow, with at least 1 h of stagnant winds in between, routinely occurred in the 6 h preceding ozone measurements reaching ≥ 120 parts per billion by volume (ppbv). On nontransition days with high ozone, ozone maxima ≥ 120 ppbv often occurred the hour after a wind direction shift of greater than about 45°.

Simulations of fine particulate matter (PM2.5) in Houston, Texas

Simulations of fine particulate matter (PM2.5) during an eight‐day episode (24 to 31 August 2000) is conducted in association with the 2000 Texas Air Quality Study (TexAQS 2000) and the Houston Supersite Project using the EPA's Models‐3 Community Multiscale Air Quality model (CMAQ). The mass concentrations of PM2.5 and major chemical constituents during the episode are calculated and compared with available field measurements. The predicted daily PM2.5 mass concentrations are about 8.5–13.0 μg/m3, consistent with the observed values. The diurnal patterns of PM2.5 mass concentrations are similar throughout the region, with a strong morning peak and a weak peak in the late afternoon to the early evening. High primary emissions, high formation rates of the secondary fine particulate matter, and low planetary boundary layer (PBL) heights contribute to the morning peak. The major components of the fine particulate matter in this region are sulfate, organic carbon, elemental carbon and ammonium. The model predicts about 30% sulfate, 32% organics (including elemental carbon (EC)), and 10% ammonium of the total PM2.5 mass. The balance of the primary cations and anions indicates that fine particulate matter in this region is acidic. Comparison with field observation reveals that CMAQ produces good simulations of averaged daily mass concentrations of major components such as sulfate, organic carbon, elemental carbon and ammonium with normalized mean biases (NMB) of less than ±25%. Uncertainties in the aerosol precursor emissions, the aerosol chemistry especially about secondary organic aerosol (SOA) formation and aqueous reactions, and the calculated PBL heights are likely responsible for the differences.

Chemical evolution of an isolated power plant plume during the TexAQS 2000 study

Abstract Stack emissions from a coal-burning power plant were measured during a research flight of the DOE G-1 during the Texas Air Quality Study (TexAQS 2000) on 10 September 2000. Clean upstream air and an isolated location allowed the plume to be unambiguously sampled during 12 successive downwind transects to a distance of 63 km—corresponding to a processing time of 4.6 h. The chemical transformation rates of sulfur and nitrogen primary pollutants into aerosol SO 4 2 - and HNO 3 yield independent values of OH concentration (8.0 and 11 × 10 6 cm - 3 , respectively) that are consistent within experimental uncertainty and qualitatively agree with constrained steady-state (CSS) box model calculations. Ozone production efficiency increases with plume age as expected. Primary aerosol emissions with D p > 5 μ m were sampled near the stack. As the plume ages, aerosol size distributions adjusted for dilution show constant number concentrations of aerosols D p > 10 nm and a marked increase in accumulation-mode particles ( D p > 0.1 μ m) as gas-to-particle-conversion causes smaller particles to grow.

A Bad Air Day in Houston

Abstract A case study from the Texas Air Quality Study 2000 field campaign illustrates the complex interaction of meteorological and chemical processes that produced a high-pollution event in the Houston area on 30 August 2000. High 1-h ozone concentrations of nearly 200 ppb were measured near the surface, and vertical profile data from an airborne differential-absorption lidar (DIAL) system showed that these high-ozone concentrations penetrated to heights approaching 2 km into the atmospheric boundary layer. This deep layer of pollution was transported over the surrounding countryside at night, where it then mixed out the next day to become part of the rural background levels. These background levels thus increased during the course of the multiday pollution episode. The case study illustrates many processes that numerical forecast models must faithfully represent to produce accurate quantitative predictions of peak pollutant concentrations in coastal locations such as Houston. Such accurate predictions ...

Nitric acid loss rates measured in power plant plumes

Measurements of HNO3, NOx, NOy, and CO2 in the plumes from electric utility power plants located in eastern Texas are used to determine the HNO3 loss rates in the planetary boundary layer during plume transport. These 1 Hz measurements were obtained from the National Center for Atmospheric Research Electra aircraft flying on clear afternoons in a well‐mixed planetary boundary layer during the Texas Air Quality Study in August and September of 2000. The analysis uses data obtained in 48 crosswind transects of plumes from seven different power plants on six flight days. The NOx lifetime was found to vary from 1.3 to 3.1 hours in these plumes, indicating that the measurements were obtained during periods of rapid photochemical activity. The HNO3 loss rate was 0.45 ± 0.21 hour−1 in power plant plumes that were advected over rural regions. Particle size distributions measured in the plumes indicated that gas phase HNO3 was not lost to particle formation. Meteorological effects and land use were also investigated as possible causes for the observed HNO3 loss rates, which were insensitive to the land cover and measured wind speeds and boundary layer heights. The HNO3 loss rate measured in most plumes was more than 4 times greater than what is calculated from deposition velocities and the measured boundary layer height.

Modeling the effects of VOC and NOX emission sources on ozone formation in Houston during the TexAQS 2000 field campaign

Abstract A meteorological and chemical modeling system is used to determine the effect of ethene and propene point source emission rates on the magnitude and distribution of ozone in the vicinity of Houston. The model performance is evaluated using surface and airborne meteorological and chemical measurements made as part of the 2000 Texas Air Quality Study. A simulation that employed the reported mobile, area, biogenic, and point source emissions produced ozone mixing ratios as high as 120 ppb and distributions of nitrogen oxides that were similar to measurements at most locations, but the model underestimated ozone mixing ratios greater than 140 ppb that were located just downwind of petrochemical facilities. When the point source emission rates of ethene and propene were increased by a factor of 10, the simulated peak ozone levels were in better agreement with surface, aircraft, and lidar observations. The magnitude of the simulated ethene and olefin concentrations were in better agreement with canister samples aloft as well; however, there was still a large amount of scatter in the results. While the highest ozone mixing ratios were produced just downwind of large point source emissions of VOCs, sensitivity simulations also showed that reductions in anthropogenic emissions of NO x would be needed to reduce ozone mixing ratios over a larger area.

Correction to “Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 1. Observations during the Texas Air Quality Study 2000”

Correction to “Vertical profiles of NO₃, N₂O₅, O₃, and NO_x in the nocturnal boundary layer: 1. Observations during the Texas Air Quality Study 2000”

Correction to “Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 2. Model studies on the altitude dependence of composition and chemistry”

[1] In the paper “Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 2. Model studies on the altitude dependence of composition and chemistry” by Andeas Geyer and Jochen Stutz (Journal of Geophysical Research, 109, D12307, doi:10.1029/2003JD004211, 2004), the citation number given for the Stutz et al. [2004] reference is incorrect. The correct reference is given here. [0] Stutz, J., B. Alicke, R. Ackermann, A. Geyer, A. White, and E. Williams (2004), Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 1. Observations during the Texas Air Quality Study 2000, J. Geophys. Res., 109, D12306, doi:10.1029/2003JD004209.

Vertical profiles of NO3, N2O5, O3, and NOx in the nocturnal boundary layer: 1. Observations during the Texas Air Quality Study 2000

Nocturnal chemistry in urban areas can considerably influence the composition of the boundary layer by removing nitrogen oxides and hydrocarbons, as well as changing the size and composition of aerosol particles. Although these processes can have a severe impact on pollution levels at night and during the following day, little quantitative information is available. In particular, the vertical variation of trace gas concentrations and chemistry at night has received little attention and is thus poorly understood. Here we present differential optical absorption spectroscopy (DOAS) measurements of the vertical distributions of O3, NO2, and NO3 during the Texas Air Quality Study 2000 near Houston, TX. Distinct vertical profiles, with lower mixing ratios of O3 and NO3 near the ground than above 100 m altitude, were observed. Mixing ratios of NO3 aloft reached 50 ppt and above, and steady state N2O5 levels were calculated to be 100–300 ppt. A one‐dimensional chemical transport model reveals that the formation of the vertical trace gas distributions is driven by deposition, surface emissions of NO, reactions of O3 and NO3, and vertical mixing. The removal of O3 in Houston is found to proceed by dry deposition, while NOx is primarily lost above 10 m altitude by N2O5 chemistry. The study shows that chemistry in polluted areas is strongly altitude dependent in the lowest 100 m of the nocturnal atmosphere. This altitude dependence should be considered in future field and model studies of urban air pollution.