OH measurements in the coastal atmosphere of South China: missing OH sinks in aged air masses

Abstract

Abstract. The hydroxyl radical (OH) is the main oxidant responsible for the removal of many reduced trace gases and the formation of secondary air pollutants. However, due to technical difficulties in measuring OH, the existing measurements of atmospheric OH concentrations are limited, and its sources and sinks are not well understood under low NOx conditions. In this study, we observed the OH concentrations using chemical ionization mass spectrometry at a coastal site in Hong Kong from October to November 2020. The average noontime OH concentration over the study period was measured at 4.9 ± 2.1 × 106 cm-3. We found that a box model with comprehensive observational constraints reproduced the observed daytime OH concentrations when air parcels originated from the continental regions. However, this model overpredicted the observed daytime OH concentrations for coastal air parcels by 73 % on average. Missing OH reactivity is proposed to be the cause of this overprediction. High missing OH reactivity was found in the case of low concentrations of nitrogen oxides (NOx) and volatile organic compounds, as well as in aged air, suggesting that there could be unmeasured chemical species that cause the model to overestimate OH in aged coastal air parcels. Further studies are needed to identify these unmeasured chemical species and their contributions to the OH budget, in order to better quantify the formation of secondary air pollutants.

Intensive photochemical oxidation in the marine atmosphere: Evidence from direct radical measurements

“…The heterogeneous uptake of HO 2 is considered to play an important role in the MBL region (Whalley et al, 2010;Zou et al, 2023;Woodward-Massey et al, 2023). In order to assess the impact of HO 2 uptake on HO x radical chemistry, we incorporated the HO 2 uptake reaction into the base model (Reaction R1 and Eqs.…”