Measurements of hydroxyl and hydroperoxy radicals during CalNex‐LA: Model comparisons and radical budgets

Abstract

Abstract 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 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 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 (1 σ ), 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.