27 Sep 2022
27 Sep 2022
Status: this preprint is open for discussion.

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

Zhouxing Zou1, Qianjie Chen1, Men Xia1, Qi Yuan1, Yi Chen2, Yanan Wang1, Enyu Xiong1, Zhe Wang2, and Tao Wang1 Zhouxing Zou et al.
  • 1Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China
  • 2Division of Environment and Sustainability, The Hong Kong University of Science and Technology, Hong Kong, China

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.

Zhouxing Zou et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2022-854', Anonymous Referee #1, 09 Nov 2022 reply

Zhouxing Zou et al.


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Short summary
We present the OH observation and model simulation results at a coastal site in Hong Kong. The model well predicted the OH concentration in high NOx conditions and overpredicted in low NOx conditions. This implies the insufficient understanding of OH chemistry in low NOx conditions. We show evidence of missing OH sinks as a possible cause of the overprediction.