Publication Date:
2016-04-03
Description:
Measurements of hydroxyl (OH) and hydroperoxy (HO 2 *) radical concentrations were made at the Pasadena ground site during the CalNex-LA 2010 campaign using the Laser-Induced Fluorescence - Fluorescence Assay by Gas Expansion (LIF-FAGE) technique. The measured concentrations of OH and HO 2 * exhibited a distinct weekend effect, with higher radical concentrations observed on the weekends corresponding to lower levels of nitrogen oxides (NO x ). The radical measurements were compared to results from a zero-dimensional model using the Regional Atmospheric Chemical Mechanism-2 (RACM2) constrained by NO x and other measured trace gases. The chemical model overpredicted measured OH concentrations during the weekends by a factor of approximately 1.4 ± 0.3 (1σ), but the agreement was better during the weekdays (ratio of 1.0 ± 0.2). Model predicted HO 2 * concentrations underpredicted by a factor of 1.3 ± 0.2 on the weekends, while measured weekday concentrations were underpredicted by a factor of 3.0 ± 0.5. However, increasing the modeled OH reactivity to match the measured total OH reactivity improved the overall agreement for both OH and HO 2 * on all days. A radical budget analysis suggests that photolysis of carbonyls and formaldehyde together accounted for approximately 40% of radical initiation with photolysis of nitrous acid accounting for 30% at the measurement height and ozone photolysis contributing less than 20%. An analysis of the ozone production sensitivity reveals that during the week, ozone production was limited by volatile organic compounds throughout the day during the campaign, but NO x -limited during the afternoon on the weekends.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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