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Mitigating ozone pollution involves reducing ozone production and relies on complex air-quality models to design reduction strategies and determine their effectiveness. However, modeled ozone does not always agree with observations. A complementary approach is to measure the ozone production rate directly, leading to the development of the Measurement of Ozone Production Sensor (MOPS). Two improved second-generation MOPSv2s were deployed for NASA's DISCOVER-AQ field campaign in September 2013 at the University of Houston, 5 km south of downtown, and Smith Point, at the mouth of the Houston Ship Channel. Median September P(O 3 ) was low, consistent with the observed ambient ozone. The MOPSv2s provided statistically similar results when they were compared for 8 day sat the University of Houston. October measurements yielded a median ozone production rate of 27 ± 11 ppbv hr −1 , falling within the range of calculated P(O 3 ) from prior Houston field campaigns in 2006 and 2009. Additionally, diurnal patterns are similar to model-derived ozone production from these previous campaigns. An advection analysis for a high ozone event on 25 September 2013 indicates that the Houston site was in a local ozone source region, while Smith Point ozone was likely enhanced by transport from other areas. [ABSTRACT FROM AUTHOR]

Abstract: Concentrations of CO, SO2, NO, NO2, and NOY were measured atop the University of Houston''s Moody Tower supersite during the 2006 TexAQS-II Radical and Aerosol Measurement Project (TRAMP). The lowest concentrations of all primary and secondary species were observed in clean marine air in southerly flow. SO2 concentrations were usually low, but increased dramatically in sporadic midday plumes advected from sources in the Houston Ship Channel (HSC), located NE of the site. Concentrations of CO and NOx displayed large diurnal variations in keeping with their co-emission by mobile sources in the Houston Metropolitan Area (HMA). CO/NOx emission ratios of 5.81 ± 0.94 were observed in the morning rush hour. Nighttime concentrations of NOx (NOx = NO + NO2) and NOY (NOY = NO + NO2 + NO3 + HNO3 + HONO + 2∗N2O5 + HO2NO2 + PANs + RONO2 + p-NO3 − + …) were highest in winds from the NNW-NE due to emission from mobile sources. Median ratios of NOx/NOY were approximately 0.9 overnight, reflecting the persistence and/or generation of NOZ (NOZ = NOY − NOx) species in the nighttime Houston boundary layer, and approached unity in the morning rush hour. Daytime concentrations of NOx and NOY were highest in winds from the HSC. NOx/NOY ratios reached their minimum values (median ca 0.63) from 1300 to 1500 CST, near local solar noon, and air masses often retained enough NOx to sustain additional O3 formation farther downwind. HNO3 and PANs comprised the dominant NOZ species in the HMA, and on a median basis represented 17–20% and 12–15% of NOY, respectively, at midday. Concentrations of HNO3, PANs, and NOZ, and fractional contributions of these species to NOY, were at a maximum in NE flow, reflecting the source strength and reactivity of precursor emissions in the HSC. As a result, daytime O3 concentrations were highest in air masses with HSC influence. Overall, our findings confirm the impact of the HSC as a dominant source region within the HMA. A comparison of total NOY measurements with the sum of measured NOY species (NOYi = NOx + HNO3 + PANs + HONO + p-NO3 −) yielded excellent overall agreement during both day ([NOY](ppb) = ([NOYi](ppb)∗1.03 ± 0.16) − 0.42; r 2 = 0.9933) and night ([NOY](ppb) = ([NOYi](ppb)∗1.01 ± 0.16) + 0.18; r 2 = 0.9975). A similar comparison between NOY–NOx concentrations and the sum of NOZi (NOZi = HNO3 + PANs + HONO + p-NO3 −) yielded good overall agreement during the day ([NOZ](ppb) = ([NOZi](ppb)∗1.01 ± 0.30) + 0.044 ppb; r 2 = 0.8527) and at night ([NOZ](ppb) = ([NOZi](ppb)∗1.12 ± 0.69) + 0.16 ppb; r 2 = 0.6899). Median ratios of NOZ/NOZi were near unity during daylight hours but increased to approximately 1.2 overnight, a difference of 0.15–0.50 ppb. Differences between NOZ and NOZi rarely exceeded combined measurement uncertainties, and variations in NOZ/NOZi ratios may have resulted solely from errors in conversion efficiencies of NOY species and changes in NOY composition. However, nighttime NOZ/NOZi ratios and the magnitude of NOZ − NOZi differences were generally consistent with recent observations of ClNO2 in the nocturnal Houston boundary layer. [Copyright &y& Elsevier]

Photochemical rate constants (j values) are crucial indicators for evaluating the importance of photochemical reactions in environmental systems. While measurement of aqueous j values via chemical actinometry is relatively straightforward under most conditions, problems arise with ambient conditions below freezing, such as at very high latitudes or altitudes. To address this problem, we have developed a new method for low temperature actinometry using solutions of acetonitrile (ACN) and water, which have freezing points down to . In this method we measure the formation of phenol from the photolysis of an-generating chromophore in the presence of benzene. Using results from laboratory tests we correct our phenol field results in to rate constants for chromophore photolysis expected for water–ice (i.e., in the quasi-liquid layer of snow grains) under the same conditions. In part 1 of this study, we use this method at Summit, Greenland on the surface snow and to depths of using hydrogen peroxide (HOOH) and nitrite as the chromophores. While the method works well for determining the rate constant for HOOH photolysis (j(HOOH)), we encountered problems using the technique with nitrite. However, measured PhOH formation rate constants for nitrite in acetonitrile, , still provide an excellent means for calculating snowpack-folding depths for photolysis (i.e., the depth over which the rate constant decreases by a factor of ). Values of j(HOOH) and determined from measurements of actinic flux (above the snow) and irradiance (in snow) suggest that the value of j(HOOH) on the surface snow at midday was in mid-March and increased by 300% by the start of May, while midday surface values were consistently throughout the season. Within the snowpack, average-folding depths determined from chemical actinometry were for j(HOOH) and for ;-folding depths determined from in-snow spectral radiometer measurements of irradiance were similar. The larger-folding depth for nitrite is because this chromophore absorbs at longer wavelengths where there is less light extinction in the snow. [Copyright &y& Elsevier]

From February 7 to 27, 2015, a mobile air quality laboratory was deployed to a location proximate to a major refinery, the Port of Houston, and several neighborhoods to conduct measurements of atmospheric trace gases and particulate matter. Two statistical models were utilized to apportion the sources of pollution impacting this site and the denizens of the nearby neighborhoods. Positive matrix factorization (PMF) was performed on the organic signal of the aerosol mass spectra, resulting in five factors totaling an average of 4.1 μg/m 3 of the organic aerosol: hydrocarbon-like (0.67 μg/m 3 ), cooking (0.35 μg/m 3 ) biomass burning (1.14 μg/m 3 ), low-volatility oxidized (1.15 μg/m 3 ), and semi-volatile oxidized (0.78 μg/m 3 ). Principal component analysis was performed on daytime and nighttime data, including concentrations from PMF output, of other PM 1 components, and of trace gases. This generated five daytime and five nighttime factors that explained 74.5% and 73.0% of the variance, respectively. The most important factors impacting this site were from mobile source exhaust and petrochemical aromatic compound emissions. Together these two factors also constitute most of the observed carcinogens. [ABSTRACT FROM AUTHOR]

The air quality in the outflow from Fort Worth, TX was studied in June 2011 at a location surrounded by oil and gas development in the Barnett Shale. The objectives of this study were to understand the major sources of volatile organic compounds (VOCs) and organic aerosols and explore the potential influence each VOC source had on ozone and secondary organic aerosol formation. Measurements of VOCs were apportioned between six factors using Positive Matrix Factorization (PMF): Natural Gas (25 ± 2%; ±99% CL); Fugitive Emissions (15 ± 2%); Internal Combustion Engines (15 ± 2%); Biogenic Emissions (7 ± 1%); Industrial Emissions/Oxidation 1(8 ± 1%); and Oxidation 2 (18 ± 2%). Reactivity calculations suggest the Biogenic and Oxidation 2 factors were the most likely VOC sources to influence local ozone. However, enough OH reactivity was calculated for factors related to the oil and gas development that they could incrementally increase O 3 . Three organic aerosol (OA) types were identified with PMF applied to high-resolution time-of-flight aerosol mass spectrometry measurements: hydrocarbon-like OA (HOA; 11% of mass) and two classes of oxidized OA (semi- and less-volatile OOA, SV and LV; 45% and 44%, respectively). The HOA correlated with the Internal Combustion Engine VOC factor indicating that a large fraction of the HOA was emitted by gasoline and diesel motors. The SV-OOA correlated with the oxidized VOC factors during most of the study, whereas a correlation between LV-OOA and the oxidized VOC factors was only observed during part of the study. It is hypothesized that SV-OOA and the oxidized VOC factors correlated reasonably well because these factors likely were separated by at most only a few oxidation generations on the oxidation pathway of organic compounds. [ABSTRACT FROM AUTHOR]

Simultaneous high-time-resolution measurements of atmospheric NH3, HNO3, soluble gas-phase chloride, and aerosol species were made in Houston, TX, from August 5, 2010 to August 9, 2010. Gaseous NH3 was measured using a 10.4-μm external cavity quantum cascade laser-based sensor employing conventional photo-acoustic spectroscopy, while gaseous HNO3 and HCl were sampled using a mist chamber–ion chromatograph (IC) system. Particle chemical composition was determined using a particle-into-liquid-sampler–IC system. There was a large amount of variability in the gas phase mixing ratios of NH3 (3.0 ± 2.5 ppb), HNO3 (287.4 ± 291.6 ppt), and HCl (221.3 ± 260.7 ppt). Elevated NH3 levels occurred around mid-day when (0.5 ± 1.0 μg m−3) and (4.5 ± 4.3 μg m−3) also increased considerably, indicating that NH3 likely influenced aerosol particle mass. By contrast, the formation of NH4NO3 and NH4Cl was not observed during the measurements. Point sources (e.g., power plant and chemical plant) might be potential contributors to the enhancements in NH3 at the measurement site under favorable meteorological conditions. Increased particle number concentrations were predicted by the SAM-TOMAS model downwind of a large coal-fired power plant when NH3 emissions (based on these measurements) were included, highlighting the potential importance of NH3 with respect to particle number concentration. Separate measurements also indicate the role of NH3 in new particle formation in Houston under low-sulfur conditions. [ABSTRACT FROM AUTHOR]

Abstract: Nitrous acid (HONO), nitric acid (HNO3), and organic aerosol were measured simultaneously atop an 18-story tower in Houston, TX during August and September of 2006. HONO and HNO3 were measured using a mist chamber/ion chromatographic technique, and aerosol size and chemical composition were determined using an Aerodyne quadrupole aerosol mass spectrometer. Observations indicate the potential for a new HONO formation pathway: heterogeneous conversion of HNO3 on the surface of primary organic aerosol (POA). Significant HONO production was observed, with an average of 0.97ppbvevent−1 and a maximum increase of 2.2ppb in 4h. Nine identified events showed clear HNO3 depletion and well-correlated increases in both HONO concentration and POA-dominated aerosol surface area (SA). Linear regression analysis results in correlation coefficients (r 2) of 0.82 for HONO/SA and 0.92 for HONO/HNO3. After correction for established HONO formation pathways, molar increases in excess HONO (HONOexcess) and decreases in HNO3 were nearly balanced, with an average HONOexcess/HNO3 value of 0.97. Deviations from this mole balance indicate that the residual HNO3 formed aerosol-phase nitrate. Aerosol mass spectral analysis suggests that the composition of POA could influence HONO production. Several previously identified aerosol-phase PAH compounds were enriched during events, suggesting their potential importance for heterogeneous HONO formation. [Copyright &y& Elsevier]

We present data collected from a number of snow-covered environments including two polar locations (Summit, Greenland and the South Pole) and two mid-latitude regions (a remote site in northern Michigan, and Niwot Ridge, Colorado). At each site, concentrations of and were enhanced within the interstitial air near the snow surface, compared to levels in boundary layer air. Fluxes of from surface snow to the atmosphere were observed at each site except Niwot Ridge, where appeared to have a sink. The mid-latitude sites showed significant emissions of , mostly originating at the ground surface and traveling up through the snow, while at the polar locations emissions from firn air were relatively small. In general, methyl halide mixing ratios in firn air were significantly greater at Summit than at the South Pole, with Summit showing a strong diurnal cycle in the production of alkyl halides that was well correlated with actinic radiation and firn temperature. We suggest that the most likely route to alkyl halide formation is through an acid catalyzed nucleophilic substitution of an alcohol type function by a halide, both of which should be preferentially segregated to the quasi-liquid layer at the surface of the snow grains. A series of experiments using a snow-filled quartz chamber irradiated by natural sunlight allowed estimation of emission trends that were hard to measure in the natural snowpack. These static chamber experiments confirmed significant production of the primary alkyl halides, following the order . Our observations at all four locations, including polar and mid-latitude sites, imply that alkyl halide production may be associated with all surface snows. [Copyright &y& Elsevier]

Understanding the drivers for high ozone (O 3) and atmospheric particulate matter (PM) concentrations is a pressing issue in urban air quality, as this understanding informs decisions for control and mitigation of these key pollutants. The Houston, TX metropolitan area is an ideal location for studying the intersection between O 3 and atmospheric secondary organic carbon (SOC) production due to the diversity of source types (urban, industrial, and biogenic) and the on- and off-shore cycling of air masses over Galveston Bay, TX. Detailed characterization of filter-based samples collected during Deriving Information on Surface Conditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ) Houston field experiment in September 2013 were used to investigate sources and composition of organic carbon (OC) and potential relationships between daily maximum 8 h average O 3 and PM. The current study employed a novel combination of chemical mass balance modeling defining primary (i.e. POC) versus secondary (i.e. SOC) organic carbon and radiocarbon (14C) for apportionment of contemporary and fossil carbon. The apportioned sources include contemporary POC (biomass burning [BB], vegetative detritus), fossil POC (motor vehicle exhaust), biogenic SOC and fossil SOC. The filter-based results were then compared with real-time measurements by aerosol mass spectrometry. With these methods, a consistent urban background of contemporary carbon and motor vehicle exhaust was observed in the Houston metropolitan area. Real-time and filter-based characterization both showed that carbonaceous aerosols in Houston was highly impacted by SOC or oxidized OC, with much higher contributions from biogenic than fossil sources. However, fossil SOC concentration and fractional contribution had a stronger correlation with daily maximum 8 h average O 3 , peaking during high PM and O 3 events. The results indicate that point source emissions processed by on- and off-shore wind cycles likely contribute to peak events for both PM and O 3 in the greater Houston metropolitan area. • Contemporary carbon was largest contributor to total organic carbon in Houston. • Secondary organic carbon contributed an average of 67% of total organic carbon. • Secondary fossil carbon was variable ranging from 3 to 27% of total organic carbon. • During peak ozone event, secondary fossil carbon was well correlated with ozone. • Houston was impacted by sea breeze re-circulation during high pollutant day. [ABSTRACT FROM AUTHOR]