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Measurement and calculation of OH reactivity at a United Kingdom coastal site

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JournalJournal of Atmospheric Chemistry
DatePublished - Sep 2009
Issue number1
Volume64
Number of pages24
Pages (from-to)53-76
Original languageEnglish

Abstract

Measurements of OH reactivity were made at the Weybourne Atmospheric Observatory on the North Norfolk coast, UK in May 2004. A wide range of supporting species was also measured concurrently as part of the TORCH-2 field campaign, allowing a detailed study of the OH oxidation chemistry to be carried out. Measurements were made in a variety of air masses, with the 3 most prevalent being air from the Atlantic that arrived at the site from over mainland UK in a South Westerly direction, and much cleaner Northerly air that originated over the far North Sea or Arctic, passed over the North Sea and arrived at the site from a North/North Easterly direction. Direct OH reactivity measurements were made on 6 days during the campaign and with influence of 2 of the 3 air masses prevalent during the study period. The average, minimum and maximum measured OH reactivity are: 4.9, 1.3 and 9.7 respectively. The measured OH reactivity was compared to key OH sinks such as NO2 and CO and a general positive correlation was observed. OH reactivity (k') was then calculated using the full range of OH sinks species that were measured (including > 30 NMHCs) and their pseudo first order rate constants for reaction with OH. For much of the measurement period there is a significant difference between the measured and calculated k', with an average value of k'(meas)- k'(calc) = 1.9 s(-1), indicative of unmeasured OH sinks. A zero-dimensional box model containing a subset of the Master Chemical Mechanism was used to calculate the OH reactivity more accurately. The simultaneously measured trace species were used as inputs to the model and their oxidative degradation was described by a chemical mechanism containing similar to 5,000 species. The extra OH sinks species produced by the model, resulted in an improvement in the agreement between k'(meas) and k'(calc), however the averaged missing OH reactivity across the entire measurement period remained at 1.4 s(-1). Speculation is made as to the source of this missing reactivity, including reference to studies showing that a potentially large number of high molecular weight aromatic species could be unmeasured by standard instrumentation.

    Research areas

  • Hydroxyl, Reactivity, Troposphere, Weybourne, TORCH, VOLATILE ORGANIC-COMPOUNDS, MASTER CHEMICAL MECHANISM, MARINE BOUNDARY-LAYER, LASER-INDUCED PUMP, MCM V3 PART, TROPOSPHERIC DEGRADATION, NONMETHANE HYDROCARBONS, ATMOSPHERIC OXIDATION, URBAN ATMOSPHERE, OZONE PRODUCTION

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