Abstract. In order to improve the current understanding of the dynamics of ammonia (NH 3 ) in a major industrial and urban area, intensive measurements of atmospheric NH 3 were conducted in Houston during two sampling periods (12 February 2010-1 March 2010 and 5 August 2010-25 September 2010). The measurements were performed with a 10.4-µm external cavity quantum cascade laser (EC-QCL)-based sensor employing conventional photo-acoustic spectroscopy. The mixing ratio of NH 3 ranged from 0.1 to 8.7 ppb with a mean of 2.4 ± 1.2 ppb in winter and ranged from 0.2 to 27.1 ppb with a mean of 3.1 ± 2.9 ppb in summer. The larger levels in summer probably are due to higher ambient temperature. A notable morning increase and a mid-day decrease were observed in the diurnal profile of NH 3 mixing ratios. Motor vehicles were found to be major contributors to the elevated levels during morning rush hours in winter. However, changes in vehicular catalytic converter performance and other local or regional emission sources from different wind directions governed the behavior of NH 3 during morning rush hours in summer. There was a large amount of variability, particularly in summer, with several episodes of elevated NH 3 mixing ratios that could be linked to industrial facilities. A considerable discrepancy in NH 3 mixing ratios existed between weekdays and weekends. This study suggests that NH 3 mixing ratios in Houston occasionally exceeded previous modeling predictions when sporadic and substantial enhancements occurred, potentially causing profound effects on particulate matter formation and local air quality.

Although commuting time typically accounts for only 6% of the day for Americans, it has become a significant source of exposure to ultrafine particles (d p < 0.1 µm) from vehicular emissions. Particle deposition onto surfaces, as an important particle loss mechanism, has been studied extensively in the indoor environments. However since air velocities, surface area to volume ratios and other contributing factors differ greatly between indoor and in-cabin environments, conclusions from indoor studies may not be directly applied to in-cabin microenvironments. In this study, ultrafine particle deposition rates were characterized under a range of air velocities and surface areas conditions inside different types of passenger vehicles. A diesel generator was used as a particle source and a Scanning Mobility Particle Sizer (SMPS) was used to measure ultrafine particle size distribution inside the test vehicles. As in-cabin air velocities increased from natural convection (<0.02 m s -1 ) to 0.65 m s -1 , ultrafine particle deposition rates increased with the greatest increases occurred for smaller particles. Other influencing factors, such as the number of passengers inside the vehicle, were also considered and investigated. It was found that ultrafine particle deposition rates are proportional to the surface areas inside vehicles consistent with previous indoor studies. Compared with available ultrafine particle deposition rates reported in the indoor environments, the in-cabin ultrafine particle deposition rates found in this study are about 3 to 20 times greater. This is likely due to higher air velocities and larger surface area to volume ratios in the in-cabin microenvironment.

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We present field observations made in June 2011 downwind of Dallas−Fort Worth, TX, and evaluate the role of stabilized Criegee radicals (sCIs) in gaseous sulfuric acid (H 2 SO 4 ) production. Zerodimensional model calculations show that sCI from biogenic volatile organic compounds composed the majority of the sCIs. The main uncertainty associated with an evaluation of H 2 SO 4 production from the sCI reaction channel is the lack of experimentally determined reaction rates for sCIs formed from isoprene ozonolysis with SO 2 along with systematic discrepancies in experimentally derived reaction rates between other sCIs and SO 2 and water vapor. In general, the maximum of H 2 SO 4 production from the sCI channel is found in the late afternoon as ozone increases toward the late afternoon. The sCI channel, however, contributes minor H 2 SO 4 production compared with the conventional OH channel in the mid-day. Finally, the production and the loss rates of H 2 SO 4 are compared. The application of the recommended mass accommodation coefficient causes significant overestimation of H 2 SO 4 loss rates compared with H 2 SO 4 production rates. However, the application of a lower experimental value for the mass accommodation coefficient provides good agreement between the loss and production rates of H 2 SO 4 . The results suggest that the recommended coefficient for the H 2 O surface may not be suitable for this relatively dry environment. ■ INTRODUCTIONMost sulfur compounds emitted to the atmosphere are in a reduced form (e.g., sulfur dioxide, SO 2 (IV)). Atmospheric gasphase oxidation processes sulfur throughout the troposphere and the stratosphere and transforms these emitted sulfur compounds into the most oxidized form of gas-phase sulfuric acid (H 2 SO 4 ), unless heterogeneous uptake transforms the sulfur into condensed-phase forms. The discussion in this paper will focus exclusively on gas-phase H 2 SO 4 formation from gas-phase SO 2 oxidation. Although sulfur compounds contribute a relatively minor fraction of the chemical composition of the troposphere, 1 the critical role of H 2 SO 4 in determining acidity in precipitation 2 and forming particles that influence regional and global climate has been highlighted. 3−5 Anthropogenic sulfur emission in the form of SO 2 is currently estimated to dominate global sulfur emissions, followed by oceanic dimethylsulfide (CH 3 SCH 3 ). 6 The gas-phase atmospheric oxidation processes of SO 2 were thought previously to be driven mostly by hydroxyl radical (OH), as shown in R1−R3. 7The potential role of stabilized Criegee biradicals (sCIs) 8 in SO 2 oxidation has been discussed since the 1970s. Cox and Penkett 9 reported significant SO 3 formation rates from chamber experiments with various alkene compounds, ozone (O 3 ), and SO 2 . They speculated sCIs prompted SO 2 oxidation because the reaction between SO 2 and O 3 is insignificant under atmospheric conditions. Calvert and Stockwell 2 presented comprehensive zero-dimensional model calculation results examining atmosp...