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Abstract:
Formaldehyde (HCHO) is the most abundant aldehyde in the troposphere. While its background-mixing ratio is mostly determined by the oxidation of methane, in many environments, especially in the boundary layer, HCHO can have a large variety of precursors, in particular biogenic and anthropogenic volatile organic compounds (VOCs) and their oxidation products. Here we present shipborne observations of HCHO, hydroxyl radical (OH) and OH reactivity (R(OH)), obtained during the Air Quality and Climate Change in the Arabian Basin (AQABA) campaign in summer 2017. The loss rate of HCHO was inferred from its reaction with OH, measured photolysis rates, and dry deposition. In photo-stationary state, the HCHO loss is balanced by production via OH initiated degradation of volatile organic compounds (VOCs), photolysis of oxygenated volatile organic compounds (OVOCs) and the ozonolysis of alkenes. The slope α from a scatter plot of the HCHO production rate versus the product of R(OH) and OH yields the fraction of R(OH) that contributes to HCHO production. Values of α varied between less than 2 % in rather clean air over the Arabian Sea and the southern Red Sea, and up to 32 % over the polluted Arabian Gulf (also known as the Persian Gulf), signifying that polluted areas harbour a larger variety of HCHO precursors. The HCHO yield from R(OH) depends on the absolute and relative contributions of alkanes, alkenes, oxygenated volatile organic compounds (OVOCs) and aromatics to R(OH), while no significant correlation to NOx mixing ratios was found, indicating that HCHO production was not NOx limited.
