[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.
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