[1] Tropospheric O 3 concentrations are functions of the chain lengths of NO x (NO x NO + NO 2 ) and HO x (HO x OH + HO 2 + RO 2 ) radical catalytic cycles. For a fixed HO x source at low NO x concentrations, kinetic models indicate the rate of O 3 production increases linearly with increases in NO x concentrations (NO x limited). At higher NO x concentrations, kinetic models predict ozone production rates decrease with increasing NO x (NO x saturated). We present observations of NO, NO 2 , O 3 , OH, HO 2 , H 2 CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (P O3 ) as a function of NO abundances and the primary HO x production rate (P HOx ). These observations provide quantitative evidence for the response of P O3 to NO x . For high P HOx (0.5 < P HOx < 0.7 ppt/s), O 3 production at this site increases linearly with NO to $500 ppt. P O3 levels out in the range 500-1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently P O3 , have a large error in the rate or product yield of the RO 2 + HO 2 reaction for the classes of RO 2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO 2 + HO 2 ! ROOH + O 2 , is reduced by a factor of 3-12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HO x reservoir. This result implies that O 3 production in or near urban areas with similar hydrocarbon reactivity and HO x production rates may be NO x saturated more often than current models suggest.

[1] Airborne formaldehyde (CH 2 O) measurements were made by tunable diode laser absorption spectroscopy (TDLAS) at high time resolution (1 and 10 s) and precision (±400 and ±120 parts per trillion by volume (pptv) (2s), respectively) during the Texas Air Quality Study (TexAQS) 2000. Measurement accuracy was corroborated by in-flight calibrations and zeros and by overflight comparison with a ground-based differential optical absorption spectroscopy (DOAS) system. Throughout the campaign, the highest levels of CH 2 O precursors and volatile organic compound (VOC) reactivity were measured in petrochemical plumes. Correspondingly, CH 2 O and ozone production was greatly enhanced in petrochemical plumes compared with plumes dominated by power plant and mobile source emissions. The photochemistry of several isolated petrochemical facility plumes was accurately modeled using three nonmethane hydrocarbons (NMHCs) (ethene (C 2 H 4 ), propene (C 3 H 6 ) (both anthropogenic), and isoprene (C 5 H 8 ) (biogenic)) and was in accord with standard hydroxyl radical (OH)-initiated chemistry. Measurement-inferred facility emissions of ethene and propene were far larger than reported by inventories. Substantial direct CH 2 O emissions were not detected from petrochemical facilities. The rapid production of CH 2 O and ozone observed in a highly polluted plume (30+ parts per billion by volume (ppbv) CH 2 O and 200+ ppbv ozone) originating over Houston was well replicated by a model employing only two NMHCs, ethene and propene.

[1] Extensive chemical characterization of ozone (O 3 ) depletion events in the Arctic boundary layer during the TOPSE aircraft mission in March-May 2000 enables analysis of the coupled chemical evolution of bromine (BrO x ), chlorine (ClO x ), hydrogen oxide (HO x ) and nitrogen oxide (NO x ) radicals during these events. We project the TOPSE observations onto an O 3 chemical coordinate to construct a chronology of radical chemistry during O 3 depletion events, and we compare this chronology to results from a photochemical model simulation. Comparison of observed trends in ethyne (oxidized by Br) and ethane (oxidized by Cl) indicates that ClO x chemistry is only active during the early stage of O 3 depletion (O 3 > 10 ppbv). We attribute this result to the suppression of BrCl regeneration as O 3 decreases. Formaldehyde and peroxy radical concentrations decline by factors of 4 and 2 respectively during O 3 depletion and we explain both trends on the basis of the reaction of CH 2 O with Br. Observed NO x concentrations decline abruptly in the early stages of O 3 depletion and recover as O 3 drops below 10 ppbv. We attribute the initial decline to BrNO 3 hydrolysis in aerosol, and the subsequent recovery to suppression of BrNO 3 formation as O 3 drops. Under halogen-free conditions we find that HNO 4 heterogeneous chemistry could provide a major NO x sink not included in standard models. Halogen radical chemistry in the model can produce under realistic conditions an oscillatory system with a period of 3 days, which we believe is the fastest oscillation ever reported for a chemical system in the atmosphere.

Emissions of volatile chemicals control the hydroxyl radical (OH), the atmosphere's main cleansing agent, and thus the production of secondary pollutants. Accounting for all of these chemicals can be difficult, especially in environments with mixed urban and forest emissions. The first direct measurements of the atmospheric OH reactivity, the inverse of the OH lifetime, were made as part of the Southern Oxidant Study (SOS) at Cornelia Fort Airpark in Nashville, TN in summer 1999. Measured OH reactivity was typically 11 s(-1). Measured OH reactivity was 1.4 times larger than OH reactivity calculated from the sum of the products of measured chemical concentrations and their OH reaction rate coefficients. This difference is statistically significant at the 1sigma uncertainty level of both the measurements and the calculations but not the 2sigma uncertainty level. Measured OH reactivity was 1.3 times larger than the OH reactivity from a model that uses measured ambient concentrations of volatile organic compounds (VOCs), NO, NO2, SO2, and CO. However, it was within approximately 10% of the OH reactivity from a model that includes hydrocarbon measurements made in a Nashville tunnel and scaled to the ambient CO at Cornelia Fort Airpark. These comparisons indicate that 30% of the OH reactivity in Nashville may come from short-lived highly reactive VOCs that are not usually measured in field intensive studies or by US EPA's Photochemical Assessment Monitoring Stations.

[1] OH and HO 2 mixing ratios and total OH reactivity were measured together with photolysis frequencies, NO x , O 3 , many VOCs, and other trace gases during the midsummer 1999 SOS campaign in Nashville, Tennessee. These measurements provided an excellent opportunity to study OH and HO 2 (collectively called HO x ), and their sources and sinks in a polluted metropolitan environment. HO x generally showed the expected diurnal evolution, with maxima around noon of up to about 0.8 pptv of OH and 80 pptv of HO 2 during sunny days. Overall, daytime observed OH and HO 2 were a factor of 1.33 and 1.56 times modeled values, within the combined 2s instrument and model uncertainties. The chain length of HO x , which is determined from the ratio of the measured total OH reactivity that cycles OH to the total HO x loss, was on average 3-8 during daytime and up to 3 during nighttime, in general agreement with expectations. However, differences occurred between observed HO x behavior and expectations from theory and models. First, HO 2 was greater than expected during daytime when NO mixing ratios were high; ozone production did not decrease as expected when NO was greater than 2 ppbv. Ozone production determined by the imbalance of the NO x photostationary state, which was almost twice that from HO 2 , also shows this dependence on NO. Second, the calculated OH production rate, which should equal the measured OH loss rate because OH is in steady state, is instead less than the measured OH loss rate by (1-2) Â 10 7 molecules cm -3 s -1 , with low statistical significance during the day and high statistical significance at night. Third, surprisingly high OH and HO 2 mixing ratios were often observed during nighttime. The nighttime OH mixing ratio and the HO 2 /OH ratio cannot be explained by known reaction mechanisms, even those involving O 3 and alkenes. Because instrument tests have failed to reveal any instrument artifacts, more exotic chemicals or chemistry, such as OH adducts or other radicals that fall apart into OH inside the instrument, are suspected.

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[1] Airborne measurements of CH 2 O were acquired employing tunable diode laser absorption spectroscopy during the 2001 Transport and Chemical Evolution Over the Pacific (TRACE-P) study onboard NASA's DC-8 aircraft. Above $2.5 km, away from the most extreme pollution influences and heavy aerosol loadings, comprehensive comparisons with a steady state box model revealed agreement to within ±37 pptv in the measurement and model medians binned according to altitude and longitude. Likewise, a near unity slope (0.98 ± 0.03) was obtained from a bivariate fit of the measurements, averaged into 25 pptv model bins, versus the modeled concentrations for values up to $450 pptv. Both observations suggest that there are no systematic biases on average between CH 2 O measurements and box model results out to model values $450 pptv. However, the model results progressively underpredict the observations at higher concentrations, possibly due to transport effects unaccounted for in the steady state model approach. The assumption of steady state also appears to contribute to the scatter observed in the point-by-point comparisons. The measurement-model variance was further studied employing horizontal flight legs. For background legs screened using a variety of nonmethane hydrocarbon (NMHC) tracers, measurement and model variance agreed to within 15%. By contrast, measurement variance was $60% to 80% higher than the model variance, even with small to modest elevations in the NMHC tracers. Measurement-model comparisons of CH 2 O in clouds and in the lower marine troposphere in the presence of marine aerosols suggest rather significant CH 2 O uptake by as much as 85% in one extreme case compared to expectations based on modeled gas phase processes.

[1] During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) aircraft program, ozone depletion events (ODEs) in the high latitude surface layer were investigated using lidar and in situ instruments. Flight legs of 100 km or longer distance were flown 32 times at 30 m altitude over a variety of regions north of 58°between early February and late May 2000. ODEs were found on each flight over the Arctic Ocean but their occurrence was rare at more southern latitudes. However, large area events with depletion to over 2 km altitude in one case were found as far south as Baffin Bay and Hudson Bay and as late as 22 May. There is good evidence that these more southern events did not form in situ but were the result of export of ozone-depleted air from the surface layer of the Arctic Ocean. Surprisingly, relatively intact transport of ODEs occurred over distances of 900-2000 km and in some cases over rough terrain. Accumulation of constituents in the frozen surface over the dark winter period cannot be a strong prerequisite of ozone depletion since latitudes south of the Arctic Ocean would also experience a long dark period. Some process unique to the Arctic Ocean surface or its coastal regions remains unidentified for the release of ozone-depleting halogens. There was no correspondence between coarse surface features such as solid ice/snow, open leads, or polynyas with the occurrence of or intensity of ozone depletion over the Arctic or subarctic regions. Depletion events also occurred in the absence of long-range transport of relatively fresh ''pollution'' within the high latitude surface layer, at least in spring 2000. Direct measurements of halogen radicals were not made. However, the flights do provide detailed information on the vertical structure of the surface layer and, during the constant 30 m altitude legs, measurements of a variety of constituents including hydroxyl and peroxy radicals. A summary of the behavior of these constituents is made. The measurements were consistent with a source of formaldehyde from the snow/ice surface. Median NO x in the surface layer was 15 pptv or less, suggesting that surface emissions were substantially converted to reservoir constituents by 30 m altitude and that ozone production rates were small (0.15-1.5 ppbv/d) at this altitude. Peroxyacetylnitrate (PAN) was by far the major constituent of NO y in the surface layer independent of the ozone mixing ratio.