Significant missing OH reactivity measured at a remote tropical marine site Samuel Seldon 1 , Lisa Whalley 1 , Graham Boustead 1 , Rachel Lade 1 , Dwayne Heard 1 , Katie Read 2 , Anna Callaghan 2 , Shalini Punjabi 2 , James Lee 2 , Lucy Carpenter 2 , Luis Neves 3 1 School of Chemistry, University of Leeds, UK, 2 Wolfson Atmospheric Chemistry Laboratories, University of York, UK, 3 Instituto Nacional de Meteorologia e Geofísica, Cabo Verde In the atmosphere primary emissions from both biogenic and anthropogenic sources are oxidised into secondary products both of which can contribute to poor air quality and negatively impact human health. The OH radical is the dominant daytime tropospheric oxidant, reacting with almost all Volatile Organic Compounds (VOCs). The majority of global methane, an important greenhouse gas that contributes to climate change, is removed in the tropical troposphere by OH. The oxidation of VOCs by OH forms peroxy radicals, HO 2 and RO 2 , with formaldehyde (HCHO) often formed as an intermediate. In the remote marine boundary layer ozone (O 3 ), a harmful secondary product which is also a greenhouse gas, is destroyed during the day by reaction with OH or HO 2 . Ground-based measurements of OH, HO 2 , RO 2 , OH reactivity and HCHO, using techniques based on Laser- induced Fluorescence (LIF), together with a comprehensive suite of supporting measurements, were made at the Cape Verde Atmospheric Observatory (CVAO), situated on the island of São Vicente located in the Tropical North Atlantic Ocean, during February 2023. With no nearby emissions and prevailing winds from over the ocean, the clean marine air sampled was representative of the open ocean. In this work, time series and diurnal variations of the measured species are presented, and production and destruction rates of OH, HO 2 , CH 3 O 2 (the simplest RO 2 ) and HCHO have been calculated. Reactions of halogen oxides were shown to be important for understanding the chemistry of OH, HO 2 , CH 3 O 2 and HCHO. The CH 3 O 2 reaction with halogen oxides was an important sink for CH 3 O 2 and source for HCHO that likely enhances O 3 destruction in remote marine environments. Heterogeneous losses were shown to be important for HO 2 . HO 2 production and destruction rates were only balanced when the reactions between CH 3 O 2 and halogen oxides were considered. These reactions were also needed for a 0-D box model to reproduce both the concentrations and variation of the measured HCHO. Measured OH concentrations and calculated OH production rates were used to determine an OH reactivity comparable to modelled OH reactivity but lower than the measured OH reactivity indicating a missing OH reactivity of ~0.5 s -1 . Using the measured OH reactivity, the calculated OH destruction rate was a factor of ~1.5 times greater than the calculated production rate. This suggests either the presence of a missing OH source or that the missing reactivity is a species that recycles OH such as an alkene.
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© The Author(s), 2023
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