An observation-based analysis of hydroxyl radicals in Guangdong, China

Hydroxyl radical (OH) is recognized to be one of the most reactive oxidants in the troposphere and plays a critical role in driving the photochemistry of the troposphere. The reactions with OH initiate the oxidation of most trace gases in the troposphere, e.g. NO_x and volatile organic compounds (VOCs), which leads to the formation of secondary oxidation products such as ozone (O₃) and particular matter (e.g. PM_2.5), thus changing the atmospheric composition and affecting the climate. In this study, an observation-based model (OBM) for evaluating the OH concentrations in Guangdong province, China has been developed. We employed the OBM to derive the OH radicals in seven representative districts/cities in Guangdong province from 2015 to 2020. The average OH concentrations in the seven cities gradually increased from an average of (3.3±0.8)×10⁶ cm^–3 at 08:00 to (6.7±0.7)×10⁶ cm^–3 at 13:00 local time. The OH concentrations derived by the OBM were found in reasonable agreement with previous observations as well as modeling studies in Guangdong. In addition, the average OH concentrations derived by the OBM method in the seven cities in Guangdong Province were about five times higher than those in clean background regions such as the Amazonas, Brazil, and pre-industrial Guangdong province. This is a highly significant point in terms of the impacts of the changing OH on the atmospheric capacity and chemical composition, which can have detrimental effects on human health, ecosystem and the climate.