Abstract
Relationships between oceanic emissions and air chemistry are intricate and still not fully understood. For regional air chemistry, a better understanding of marine halogen emission on hydroxyl (OH) radical is crucial. OH radical is a key species in atmospheric chemistry because it can oxidize almost all trace species in the atmosphere. In the marine atmosphere, OH level could be significantly affected by the halogen species emitted from the ocean. However, due to the complicated interactions of halogens with OH through different pathways, it is not well understood how halogens influence OH and even great uncertain in the signs of net effect. Therefore, in this study, we aim to quantify the impact of marine-emitted halogens (including Cl, Br, and I) through different pathways on OH in the high OH season by using WRF-CMAQ model with process analysis and state-of-the-art halogen chemistry in the East Asia Seas. Results show a very complicated response of OH production rate (POH) to marine halogen emissions. The monthly POH is generally decreased over the ocean with maxima of about 10–15 % in the Philippine Sea, but is increased in many nearshore areas with maxima of about 7–9 % in the Bohai Sea. In the coastal areas of southern China, the monthly POH could also decrease 3–5 % in the Greater Bay Area, but with a daytime hourly maximum decrease over 30 %. Analysis to the individual reactions using integrated reaction rate (IRR) show that the net change of POH is controlled by the competitions of three main pathways through different halogen species. Sea spray aerosols (SSA) and inorganic iodine gases are the main species to influence the strengths of these three pathways and therefore have the most significant impacts on POH. Both of these two types of species decrease POH through physical processes, while generally increase POH through chemical processes. In the ocean atmosphere, the controlling species are inorganic iodine gases and the complicated iodine chemistry determines the basic pattern of ΔPOH, while over the continent, SSA is the controlling species and the SSA extinction effect leads to the negative ΔPOH in the southern China. Our results indicate that marine-emitted halogen species have notable impacts over the ocean and have potential impact on the coastal atmospheric oxidation. The identified main (previously known or unknown) pathways and their controlling factors from different halogen species to OH radical explains the halogen-induced change of POH East Asia and also can be applied in other circumstances (e.g., different domains, regions, and emission rates).
Highlights
Hydroxyl radical is the most important daytime oxidant in the troposphere
We evaluate stations in the three major polluted areas near the seas in mainland China, namely, the North China Plain (NCP), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD) (Fig. S1)
To examine the mass interaction between ocean and atmosphere on the regional air oxidation capacity, we explore the impact of marine-emitted halogen species on atmospheric OH in East Asia in summer
Summary
Hydroxyl radical is the most important daytime oxidant in the troposphere. It can oxidize almost all directly emitted gases such as CO, CH4 and other VOCs, while produce some secondary species such as O3 and secondary aerosols at the same time. The main sinks of OH are CO and VOCs. Recently, studies showed that in the low-NOx conditions, OH modeled by current chemical mechanisms are usually underestimated (Tan et al, 2019;Rohrer et al, 2014;Lelieveld et al, 2008;Hofzumahaus et al, 2009;Lu et al, 2019a;Fuchs et al, 2013;Stone et al, 2012;Fittschen et al, 2019;Whalley et al, 40 2021), and missing sources of OH is a common hypothesis. Recent studies in understanding the impacts of halogen chemistry on OH usually focused on the two pre-described pathways (i.e., enhanced HO2 conversion by XO and O3 consumption by X atoms) which are not directly quantified.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
