Texas Air Quality Research Articles

WRF/Chem‐MADRID: Incorporation of an aerosol module into WRF/Chem and its initial application to the TexAQS2000 episode

The Model of Aerosol Dynamics, Reaction, Ionization and Dissolution (MADRID) with three improved gas/particle mass transfer approaches (i.e., bulk equilibrium (EQUI), hybrid (HYBR), and kinetic (KINE)) has been incorporated into the Weather Research and Forecast/Chemistry Model (WRF/Chem) (referred to as WRF/Chem‐MADRID) and evaluated with a 5‐day episode from the 2000 Texas Air Quality Study (TexAQS2000). WRF/Chem‐MADRID demonstrates an overall good skill in simulating surface/aloft meteorological parameters and chemical concentrations of O3 and PM2.5, tropospheric O3 residuals, and aerosol optical depths. The discrepancies can be attributed to inaccuracies in meteorological predictions (e.g., overprediction in mid‐day boundary layer height), sensitivity to meteorological schemes used (e.g., boundary layer and land‐surface schemes), inaccurate total emissions or their hourly variations (e.g., HCHO, olefins, other inorganic aerosols) or uncounted wildfire emissions, uncertainties in initial and boundary conditions for some species (e.g., other inorganic aerosols, CO, and O3) at surface and aloft, and some missing/inactivated model treatments for this application (e.g., chlorine chemistry and secondary organic aerosol formation). Major differences in the results among the three gas/particle mass transfer approaches occur over coastal areas, where EQUI predicts higher PM2.5 than HYBR and KINE due to improperly redistributing condensed nitrate from chloride depletion process to fine PM mode. The net direct, semi‐direct, and indirect effects of PM2.5 decrease domainwide shortwave radiation by 11.2–14.4 W m−2 (or 4.1–5.6%) and near‐surface temperature by 0.06–0.14°C (or 0.2–0.4%), lead to 125 to 796 cm−3 cloud condensation nuclei at a supersaturation of 0.1%, produce cloud droplet numbers as high as 2064 cm−3, and reduce domainwide mean precipitation by 0.22–0.59 mm day−1.

Source contributions of volatile organic compounds to ozone formation in southeast Texas

A source‐oriented SAPRC‐99 gas phase photochemical mechanism was incorporated into the Community Multiscale Air Quality (CMAQ) model to determine the contributions of volatile organic compounds (VOCs) to predicted net ozone (O3) formation rates during the Texas Air Quality Study (TexAQS) from 16 August to 7 September 2000. Contributions from biogenic sources, diesel vehicles, highway gasoline vehicles, off‐highway gasoline vehicles, solvent utilization, petroleum industries, other industries and wildfires were determined. Peak column‐averaged O3 formation rate due to industrial sources averaged over this episode was approximately 8.5 ppb hr−1. Contributions of gasoline vehicles and solvent utilization were large in urban areas to the west of the industrial region with highest column‐averaged formation rates of 3.7 and 1.4 ppb hr−1, respectively. Large regional contributions of biogenic sources to O3 formation were predicted with highest O3 formation rate of 11.9 ppb hr−1 in downwind rural areas. Wildfires could contribute to large O3 formation but their influence was generally localized. Analysis of 2400 back‐trajectories from areas with maximum daily 8‐h O3 greater than 90 ppb showed that industrial sources were the largest anthropogenic sources of VOC that contributed to the these high O3 events, followed by gasoline vehicle sources. The median of relative contributions from biogenic and anthropogenic sources from this analysis was approximately 60% and 40%, respectively. Analysis of the back‐trajectories where 1‐h peak O3 concentrations were greater than 120 ppb showed that the median relative contributions due to anthropogenic sources were increased to over 60%. This suggests that high O3 events in the HGB region were driven by anthropogenic VOC emissions from industrial sources.

Characterization of NOx, SO2, ethene, and propene from industrial emission sources in Houston, Texas

The Houston‐Galveston‐Brazoria urban area contains industrial petrochemical sources that emit volatile organic compounds and nitrogen oxides, resulting in rapid and efficient ozone production downwind. During September to October 2006, the NOAA WP‐3D aircraft conducted research flights as part of the second Texas Air Quality Study (TexAQS II). We use measurements of NOx, SO2, and speciated hydrocarbons from industrial sources in Houston to derive source emission ratios and compare these to emission inventories and the first Texas Air Quality Study (TexAQS) in 2000. Between 2000 and 2006, NOx/CO2 emission ratios changed by an average of −29% ± 20%, while a significant trend in SO2/CO2 emission ratios was not observed. We find that high hydrocarbon emissions are routine for the isolated petrochemical facilities. Ethene (C2H4) and propene (C3H6) are the major contributors to ozone formation based on calculations of OH reactivity for organic species including C2–C10 alkanes, C2–C5 alkenes, ethyne, and C2–C5 aldehydes and ketones. Measured ratios of C2H4/NOx and C3H6/NOx exceed emission inventory values by factors of 1.4–20 and 1–24, respectively. We examine trends in C2H4/NOx and C3H6/NOx ratios between 2000 and 2006 for the isolated petrochemical sources and estimate a change of −30% ± 30%, with significant day‐to‐day and within‐plume variability. Median ambient mixing ratios of ethene and propene in Houston show decreases of −52% and −48%, respectively, between 2000 and 2006. The formaldehyde, acetaldehyde, and peroxyacetyl nitrate products produced by alkene oxidation are observed downwind, and their time evolution is consistent with the rapid photochemistry that also produces ozone.

A top‐down analysis of emissions from selected Texas power plants during TexAQS 2000 and 2006

Airborne measurements were taken downwind of eleven Texas power generation facilities in 2000 and 2006 as part of the two Texas Air Quality Study (TexAQS) campaigns. From these measurements, we determine emission ratios of NOx (= NO + NO2), SO2, and CO to coemitted CO2 for each facility. These measurements provide an independent external assessment of reported emission ratios from continuous emission monitoring systems (CEMS). During the TexAQS study years, we find the SO2/CO2 and NOx/CO2 emission ratios derived from measurements aboard the aircraft agree quantitatively with inventory values from CEMS, with standard deviations of less than ±14%. We document significant decreases in atmospheric mixing ratios of NOx as a result of emission reductions due to controls implemented at the W. A. Parish plant after TexAQS 2000. For several of the facilities, CO emissions appear relatively constant in time. Derived CO/CO2 emission ratios agree substantially better with Texas Commission on Environmental Quality inventories in 2006 than in 2000, which we attribute to better inventory data from three facilities that installed CO CEMS between the two study years and not because of any significant change in CO emissions. Other plants appear to have varying CO emissions over time, complicating comparison to annual inventory values. Finally, we use two independent NO2 measurements, along with measurements of O3, NO3, and N2O5, to quantify the fraction of NOx directly emitted as NO2 from the Oklaunion Power Plant, providing the first quantitative estimate of NO2 emissions from a power generation facility using ambient data.

Relationships of coastal nocturnal boundary layer winds and turbulence to Houston ozone concentrations during TexAQS 2006

Measurements made with a ship‐based Doppler wind lidar during the summertime 2006 Texas Air Quality Study are used to study the relationship between lower‐tropospheric vertical structure and winds and ozone (O3) concentrations in Houston, Texas, under two different flow regimes. We observed that strong southerly flow regimes, dominated by the subtropical anticyclone (Bermuda high) off the Atlantic coast of the United States, resulted in strong (i.e., high wind speed) onshore nocturnal low‐level jets (LLJ) and low O3 and oxidant Ox (where Ox = O3 + NO2) concentrations at night and the following afternoon. In contrast, periods dominated by northerly or easterly flow resulted in relatively weak (low wind speed), but still onshore, nocturnal LLJs associated with higher concurrent and next‐day concentrations of O3 and Ox. We present lidar data from 24 h example periods for each of these conditions and demonstrate how each type of flow regime is related to in situ ship‐based ozone measurements. We expanded the study to include all days during the study when the ship was near Houston, to demonstrate how the strength of the meridional winds aloft show a better relationship to concurrent ship‐measured Ox concentrations than the winds near the surface do. We found a strong relationship between a parameterization of the observed nocturnal jets, which reflect the synoptic conditions, to peak hourly O3 measured the next day at the ship and averaged throughout the Houston/Galveston/Brazoria continuous ambient monitoring stations monitoring network, indicating potential applications for planning air quality.

An examination of sensitivity of WRF/Chem predictions to physical parameterizations, horizontal grid spacing, and nesting options

An accurate representation of meteorological processes is important to the accurate predictions of meteorology and air quality. In this study, the Weather Research and Forecasting model with Chemistry (WRF/Chem) is utilized to examine the sensitivity of air quality predictions to two planetary boundary layer (PBL) schemes and three land-surface models (LSMs). Model simulations with different PBL schemes and LSMs are conducted over the Houston–Galveston area for a 5-day summer episode from the 2000 Texas Air Quality Study (TexAQS-2000). Sensitivity to horizontal grid spacing (12 vs. 4 km) and nesting methods (1- or 2-way) is also studied. Model predictions are evaluated with available surface and aircraft observations. Both meteorological and chemical predictions at the surface and aloft show stronger sensitivity to LSMs than to the PBL schemes. The model predictions also show a stronger sensitivity to horizontal grid spacing using 1-way nesting than 2-way nesting and to the nesting method at 4 km than 12 km. The benefits (or disbenefits) of using more complex meteorological schemes, finer horizontal grid spacing, and more sophisticated 2-way nesting may vary and must be evaluated for specific episodes. The results from this study also indicate a need to refine model treatments at a fine grid spacing and the current 2-way nesting method used in WRF/Chem for improvement of model performance.

Quantification of NO2 and SO2 emissions from the Houston Ship Channel and Texas City industrial areas during the 2006 Texas Air Quality Study

In August–September 2006, as part of the Second Texas Air Quality Study, NO2 and SO2 emissions from the Houston Ship Channel and Texas City industrial areas were quantified using mobile mini‐differential optical absorption spectroscopy instruments. The measured NO2 emissions from the Houston Ship Channel and Texas City industrial areas were 2542 and 452 kg h−1, respectively, yielding NOx emissions 70% and 43%, respectively, above the reported inventory values. Quantified SO2 emissions from the Houston Ship Channel area were 1749 kg h−1 and were found to be 34% above the values reported in the inventory. Short‐term variability of NO2 and SO2 emissions was found at the Houston Ship Channel. On 31 August 2006, a plume was detected at the HSC during three consecutive measurements, yielding a HCHO flux of 481 kg h−1. This event has been mainly attributed to photochemical production.

Ensemble‐based simultaneous state and parameter estimation for treatment of mesoscale model error: A real‐data study

This study explores the treatment of model error and uncertainties through simultaneous state and parameter estimation (SSPE) with an ensemble Kalman filter (EnKF) in the simulation of a 2006 air pollution event over the greater Houston area during the Second Texas Air Quality Study (TexAQS‐II). Two parameters in the atmospheric boundary layer parameterization associated with large model sensitivities are combined with standard prognostic variables in an augmented state vector to be continuously updated through assimilation of wind profiler observations. It is found that forecasts of the atmosphere with EnKF/SSPE are markedly improved over experiments with no state and/or parameter estimation. More specifically, the EnKF/SSPE is shown to help alleviate a near‐surface cold bias and to alter the momentum mixing in the boundary layer to produce more realistic wind profiles.

Biogenic emission measurement and inventories determination of biogenic emissions in the eastern United States and Texas and comparison with biogenic emission inventories

During the NOAA Southern Oxidant Study 1999 (SOS1999), Texas Air Quality Study 2000 (TexAQS2000), International Consortium for Atmospheric Research on Transport and Transformation (ICARTT2004), and Texas Air Quality Study 2006 (TexAQS2006) campaigns, airborne measurements of isoprene and monoterpenes were made in the eastern United States and in Texas, and the results are used to evaluate the biogenic emission inventories BEIS3.12, BEIS3.13, MEGAN2, and WM2001. Two methods are used for the evaluation. First, the emissions are directly estimated from the ambient isoprene and monoterpene measurements assuming a well‐mixed boundary layer and are compared with the emissions from the inventories extracted along the flight tracks. Second, BEIS3.12 is incorporated into the detailed transport model FLEXPART, which allows the isoprene and monoterpene mixing ratios to be calculated and compared to the measurements. The overall agreement for all inventories is within a factor of 2 and the two methods give consistent results. MEGAN2 is in most cases higher, and BEIS3.12 and BEIS3.13 lower than the emissions determined from the measurements. Regions with clear discrepancies are identified. For example, an isoprene hot spot to the northwest of Houston, Texas, was expected from BEIS3 but not observed in the measurements. Interannual differences in emissions of about a factor of 2 were observed in Texas between 2000 and 2006.

Comparison of in situ and columnar aerosol spectral measurements during TexAQS-GoMACCS 2006: testing parameterizations for estimating aerosol fine mode properties

Abstract. During the 2006 Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS 2006), the optical, chemical and microphysical properties of atmospheric aerosols were measured on multiple mobile platforms and at ground based stations. In situ measurements of the aerosol light extinction coefficient (σep) were performed by two multi-wavelength cavity ring-down (CRD) instruments, one located on board the NOAA R/V Ronald H. Brown (RHB) and the other located at the University of Houston, Moody Tower (UHMT). An AERONET sunphotometer was also located at the UHMT to measure the columnar aerosol optical depth (AOD). The σep data were used to extract the extinction Ångström exponent (åep), a measure of the wavelength dependence of σep. There was general agreement between the åep (and to a lesser degree σep) measurements by the two spatially separated CRD instruments during multi-day periods, suggesting a regional scale consistency of the sampled aerosols. Two spectral models are applied to the σep and AOD data to extract the fine mode fraction of extinction (η) and the fine mode effective radius (Reff,f). These two parameters are robust measures of the fine mode contribution to total extinction and the fine mode size distribution, respectively. The results of the analysis are compared to Reff,f values extracted using AERONET V2 retrievals and calculated from in situ particle size measurements on the RHB and at UHMT. During a time period when fine mode aerosols dominated the extinction over a large area extending from Houston/Galveston Bay and out into the Gulf of Mexico, the various methods for obtaining Reff,f agree qualitatively (showing the same temporal trend) and quantitatively (pooled standard deviation = 28 nm).

An observational and modeling strategy to investigate the impact of remote sources on local air quality: A Houston, Texas, case study from the Second Texas Air Quality Study (TexAQS II)

Quantifying the impacts of remote sources on individual air quality exceedances remains a significant challenge for air quality forecasting. One goal of the 2006 Second Texas Air Quality Study (TexAQS II) was to assess the impact of distant sources on air quality in east Texas. From 23 to 30 August 2006, retrievals of tropospheric carbon monoxide (CO) from NASA's Atmospheric InfraRed Sounder (AIRS) reveal the transport of CO from fires in the United States Pacific Northwest to Houston, Texas. This transport occurred behind a cold front and contributed to the worst ozone exceedance period of the summer in the Houston area. We present supporting satellite observations from the NASA A‐Train constellation of the vertical distribution of smoke aerosols and CO. Ground‐based in situ CO measurements in Oklahoma and Texas track the CO plume as it moves south and indicate mixing of the aloft plume to the surface by turbulence in the nocturnal boundary layer and convection during the day. Ground‐based aerosol speciation and lidar observations do not find appreciable smoke aerosol transport for this case. However, MODIS aerosol optical depths and model simulations indicate some smoke aerosols were transported from the Pacific Northwest through Texas to the Gulf of Mexico. Chemical transport and forward trajectory models confirm the three major observations: (1) the AIRS envisioned CO transport, (2) the satellite determined smoke plume height, and (3) the timing of the observed surface CO increases. Further, the forward trajectory simulations find two of the largest Pacific Northwest fires likely had the most significant impact.

Ensemble and bias-correction techniques for air quality model forecasts of surface O 3 and PM 2.5 during the TEXAQS-II experiment of 2006

Several air quality forecasting ensembles were created from seven models, running in real-time during the 2006 Texas Air Quality (TEXAQS-II) experiment. These multi-model ensembles incorporated a diverse set of meteorological models, chemical mechanisms, and emission inventories. Evaluation of individual model and ensemble forecasts of surface ozone and particulate matter (PM) was performed using data from 119 EPA AIRNow ozone sites and 38 PM sites during a 50-day period in August and September of 2006. From the original set of models, two new bias-corrected model data sets were built, either by applying a simple running mean average to the past 7 days of data or by a Kalman-Filter approach. From the original and two bias-corrected data sets, three ensembles were created by a simple averaging of the seven models. For further improvements three additional weighted model ensembles were created, where individual model weights were calculated using the singular value decomposition method. All six of the ensembles are compared to the individual models and to each other in terms of root mean square error, correlation, and contingency and probabilistic statistics. In most cases, each of the ensembles show improved skill compared to the best of the individual models. The over all best ensemble technique was found to be the combination of Kalman-Filtering and weighted averaging. PM 2.5 aerosol ensembles demonstrated significant improvement gains, mostly because the original model's skill was very low.

Emissions of NOx, SO2, CO, and HCHO from commercial marine shipping during Texas Air Quality Study (TexAQS) 2006

We report measurements of NOx, SO2, CO, and HCHO mass‐based emission factors from more than 200 commercial vessel encounters in the Gulf of Mexico and the Houston‐Galveston region of Texas during August and September, 2006. For underway ships, bulk freight carriers have the highest average NOx emissions at ∼87 g NOx (kg fuel)−1, followed by tanker ships at ∼79 g NOx (kg fuel)−1, while container carriers, passenger ships, and tugs all emit an average of about ∼60 g NOx (kg fuel)−1. Emission of NOx from stationary vessels was lower, except for container ships and tugs, and likely reflects use of medium‐speed diesel engines. Overall, our mean NOx emission factors are 10–15% lower than published data. Average emission of SO2 was lower for passenger ships and tugs and tows (6–7 g SO2 (kg fuel)−1) than for larger cargo vessels (20–30 g SO2 (kg fuel)−1). Our data for large cargo ships in this region indicate an average residual fuel sulfur content of ∼1.4% which is a factor of two lower than the global average of 2.7%. Emission of CO was low for all categories (7–16 g CO (kg fuel)−1), although our mean overall CO emission factor is about 10% higher than published data. Emission of HCHO was less than 5% that of CO. Despite considerable variability, no functional relationships, such as emissions changes with engine speed or load, could be discerned. Comparison of emission factors from ships to those from other sources suggests ship emissions in this region cannot be ignored.

Deciphering the Role of Radical Precursors during the Second Texas Air Quality Study

The Texas Environmental Research Consortium (TERC) funded significant components of the Second Texas Air Quality Study (TexAQS II), including the TexAQS II Radical and Aerosol Measurement Project (TRAMP) and instrumented flights by a Piper Aztec aircraft. These experiments called attention to the role of short-lived radical sources such as formaldehyde (HCHO) and nitrous acid (HONO) in increasing ozone productivity. TRAMP instruments recorded daytime HCHO pulses as large as 32 parts per billion (ppb) originating from upwind industrial activities in the Houston Ship Channel, where in situ surface monitors detected HCHO peaks as large as 52 ppb. Moreover, Ship Channel petrochemical flares were observed to produce plumes of apparent primary HCHO. In one such combustion plume that was depleted of ozone by large emissions of oxides of nitrogen (NOx), the Piper Aztec measured a ratio of HCHO to carbon monoxide (CO) 3 times that of mobile sources. HCHO from uncounted primary sources or ozonolysis of underestimated olefin emissions could significantly increase ozone productivity in Houston beyond previous expectations. Simulations with the CAMx model show that additional emissions of HCHO from industrial flares or mobile sources can increase peak ozone in Houston by up to 30 ppb. Other findings from TexAQS II include significant concentrations of HONO throughout the day, well in excess of current air quality model predictions, with large nocturnal vertical gradients indicating a surface or near-surface source of HONO, and large concentrations of nighttime radicals (∼30 parts per trillion [ppt] HO2). HONO may be formed heterogeneously on urban canopy or particulate matter surfaces and may be enhanced by organic aerosol of industrial or motor vehicular origin, such as through conversion of nitric acid (HNO3). Additional HONO sources may increase daytime ozone by more than 10 ppb. Improving the representation of primary and secondary HCHO and HONO in air quality models could enhance the simulated effectiveness of control strategies.

Field Measurements of Small Marine Craft Gaseous Emission Factors during NEAQS 2004 and TexAQS 2006

Exhaust emission factors were calculated for a number (n = 116) of small marine craft encountered during the 2004 New England Air Quality Study-International Transport and Chemical Transformation and 2006 Texas Air Quality Study II field campaigns. Emission factors are reported for NO(x), SO(2), and CO in units of grams of pollutant per kilogram of fuel. These factors are compared to emission factors derived from the U.S. Environmental Protection Agency (EPA) NONROAD model, separated into spark-ignition and compression-ignition sources. NO(x) emission factors observed were significantly and substantially higher than predicted by the model by a factor of 2-10. CO emission factors were not significantly different than the model outputs. Because of the correlation between exhaust hydrocarbon and CO for marine craft, it is expected that EPA estimates of hydrocarbon exhaust emission factors are not significantly in error. Small commercial marine craft (e.g., inshore fishing trawlers) are not part of NONROAD, but their measured emission factors were comparable to those of large diesel recreational marine craft in the model.

An evaluation of the interaction of morning residual layer and afternoon mixed layer ozone in Houston using ozonesonde data

The Tropospheric Ozone Pollution Project (TOPP) launched >220 ozonesondes in Houston (July 2004–June 2008) providing examples of pollution transported into, re-circulated within, and exported from the Houston area. Fifty-one launches occurred during the Texas Air Quality Study (TexAQS) II and the summer portion of IONS-06 (INTEX [Intercontinental Transport Experiment] Ozonesonde Network Study). On 11 days during TexAQS II and on 8 other occasions, ozonesondes were launched both at dawn and in the afternoon. Analysis of these “intensive” launch sequences shows that morning residual layer (RL) ozone concentrations ([O 3]) explained 60–70% of the variability found in the afternoon mixed layer (ML). Furthermore, maximum RL [O 3] is nearly identical to the mean ML [O 3] from the previous afternoon (morning minus afternoon = −1.6 ± 8.4 ppbv). During TexAQS II, mean [O 3] below 1.3 km (the mean ML height from ozonesonde data) increased from 37 ± 22 ppbv in the morning to 74 ± 18 ppbv in the afternoon, suggesting an average net local daily O 3 production of ∼500–900 tons over the metropolitan Houston area.

Measurement of Aerosol Organic Compounds Using a Novel Collection/Thermal-Desorption PTR-ITMS Instrument

We report the development and characterization of a proton-transfer-reaction ion trap mass spectrometer for the speciated measurement of organic compounds in atmospheric aerosols and show results from its first field deployment. The instrument uses an aerosol collection inlet to accumulate aerosol mass followed by rapid thermal desorption to volatilize the organic compounds for in situ analysis. We have performed laboratory studies to characterize instrument performance and the instrument was deployed aboard a NOAA research vessel during the Texas Air Quality Study 2006/Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS 2006/GoMACCS) in August–September 2006. The laboratory-determined detection limit for glutaric acid in mixed glutaric acid/NH 4 HSO 4 test aerosols was 0.22 ng collected mass, which corresponds to an estimated detection limit of 12 ng m−3 for a 10 min sample based on the instrument sample flow rate of 1.8 L min−1. During TexAQS 2006/GoMACCS, signals well above the detection limit were observed at a number of mass to charge ratios, mostly occurring during an extended period of active pollution photochemistry, but also including detection of possible primary emissions of aerosol-phase pyridine.

Overview of the Second Texas Air Quality Study (TexAQS II) and the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

The Second Texas Air Quality Study (TexAQS II) was conducted in eastern Texas during 2005 and 2006. This 2‐year study included an intensive field campaign, TexAQS 2006/Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), conducted in August–October 2006. The results reported in this special journal section are based on observations collected on four aircraft, one research vessel, networks of ground‐based air quality and meteorological (surface and radar wind profiler) sites in eastern Texas, a balloon‐borne ozonesonde‐radiosonde network (part of Intercontinental Transport Experiment Ozonesonde Network Study (IONS‐06)), and satellites. This overview paper provides operational and logistical information for those platforms and sites, summarizes the principal findings and conclusions that have thus far been drawn from the results, and directs readers to appropriate papers for the full analysis. Two of these findings deserve particular emphasis. First, despite decreases in actual emissions of highly reactive volatile organic compounds (HRVOC) and some improvements in inventory estimates since the TexAQS 2000 study, the current Houston area emission inventories still underestimate HRVOC emissions by approximately 1 order of magnitude. Second, the background ozone in eastern Texas, which represents the minimum ozone concentration that is likely achievable through only local controls, can approach or exceed the current National Ambient Air Quality Standard of 75 ppbv for an 8‐h average. These findings have broad implications for air quality control strategies in eastern Texas.

Impacts of background ozone production on Houston and Dallas, Texas, air quality during the Second Texas Air Quality Study field mission

A major objective of the 2006 Second Texas Air Quality Study (TexAQS II) focused on understanding the effects of regional processes on Houston and Dallas ozone nonattainment areas. Here we quantify the contributions of background (continental scale) ozone production on Houston and Dallas air quality during TexAQS II using ensemble Lagrangian trajectories to identify remote source regions that impact Houston and Dallas background ozone distributions. Global‐scale chemical analyses, constrained with composition measurements from instruments on the NASA Aura satellite, are used to provide estimates of background composition along ensemble back trajectories. Lagrangian averaged O3 net photochemical production (production minus loss, P‐L) rates along the back trajectories are used as a metric to classify back trajectories. Results show that the majority (6 out of 9 or 66%) of the periods of high ozone in Houston were associated with periods of enhanced background ozone production. Slightly less than 50% (7 out of 15) of the days with high ozone in the Dallas Metropolitan Statistical Area (MSA) show enhanced background ozone production. Source apportionment studies show that 5‐day Lagrangian averaged O3 P‐L in excess of 15 ppbv/d can occur during continental‐scale transport to Houston owing to NOy enhancements from emissions within the Southern Great Lakes as well as recirculation of the Houston emissions. Dallas background O3 P‐L is associated with NOy enhancements from emissions within Chicago and Houston.

Aerosol optical and hygroscopic properties during TexAQS‐GoMACCS 2006 and their impact on aerosol direct radiative forcing

In situ measurements of aerosol optical and hygroscopic properties were made on board the National Oceanic and Atmospheric Administration R/V Ronald H. Brown during the Texas Air Quality Study–Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS‐GoMACCS). The aerosol light extinction coefficient (σep) was measured at 355, 532, and 1064 nm at 25%, 60%, and 85% relative humidity (RH) for both sub‐1‐ and sub‐10‐μm‐diameter particles with a cavity ring–down aerosol extinction spectrometer. The 532‐nm σep was coupled with the 532‐nm light absorption coefficient (σap) measured with a photoacoustic absorption spectrometer to calculate the aerosol single scattering albedo (ω) with absolute uncertainty <0.01. The σep dependence on RH was expressed in terms of gamma (γ). The sampled aerosols covered a broad spectrum of γ and ω values; aerosols from traffic emissions were hydrophobic and highly light‐absorbing with γ ∼ 0.4 and ω ∼ 0.6, whereas the regional aerosols exhibited variable values of both γ and ω. Aerosols with the highest sulfate content also had the highest γ and ω values (>0.65 and >0.9, respectively). The optical data were used to estimate local, top of atmosphere aerosol‐induced climate forcing (ΔFR). The ΔFR calculations were performed using both ω values measured at 25% RH and ω values converted to ambient RH. The calculated ambient ΔFR ranged from −7 to −40 W/m2 with absolute uncertainty between 0.7 and 2.5 W/m2. The results show that including aerosol hygroscopic properties in climate calculations is critical for improving estimates of aerosol forcing on climate.