Measurement report: Observation-based formaldehyde production rates and their relation to OH reactivity around the Arabian Peninsula

Dirk Dienhart,Achim Edtbauer,Jean-Daniel Paris,Hartwig Harder,Eva Y Pfannerstill,Christof Stönner,Jan Schuladen,Lisa Ernle,Bettina Hottmann,Efstratios Bourtsoukidis,Roland Rohloff,Nijing Wang,Jonathan Williams,Uwe Parchatka,Sebastian Tauer,Philipp Eger ,J Lelieveld ,John N Crowley,Ivan Tadić ,Mònica Martínez ,H Fischer

doi:10.5194/acp-21-17373-2021

Abstract

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)), which were 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 photochemical steady state, the HCHO loss is balanced by production via OH-initiated degradation of VOCs, photolysis of oxygenated VOCs (OVOCs) and the ozonolysis of alkenes. The slope αeff from a scatter plot of the HCHO production rate versus the product of OH and R(OH)eff (excluding inorganic contribution) yields the fraction of OH reactivity that contributes to HCHO production. Values of αeff varied between less than 2 % in relatively clean air over the Arabian Sea and the southern Red Sea and up to 32 % over the polluted Arabian Gulf (also known as Persian Gulf), signifying that polluted areas harbor a larger variety of HCHO precursors. The separation of R(OH)eff into individual compound classes revealed that elevated values of αeff coincided with increased contribution of alkanes and OVOCs, with the highest reactivity of all VOCs over the Arabian Gulf.

Highlights

Formaldehyde (HCHO) is a ubiquitous trace gas that can help provide insight into the dynamical and chemical processes controlling atmospheric composition as an important source of hydroperoxyl radicals (HO2) (Volkamer et al, 2010; Whalley et al, 2010; Anderson et al, 2017)
Median HCHO concentrations between 1 and 2 ppbv were measured in most of the other regions (Mediterranean Sea (MS); Red Sea north (RN); Gulf of Aden (GA); Arabian Sea leg 2 (AS Leg 2); Gulf of Oman (GO)), while pollution events occurred in the area around the Suez Canal (SU)
We found a similar trend, as the increased reactivity of alkenes in the southern Red Sea at rather low NOx was not accompanied by enhanced HCHO mixing ratios, high values of αeff coincided with elevated NOx

Summary

Introduction

Formaldehyde (HCHO) is a ubiquitous trace gas that can help provide insight into the dynamical and chemical processes controlling atmospheric composition as an important source of hydroperoxyl radicals (HO2) (Volkamer et al, 2010; Whalley et al, 2010; Anderson et al, 2017). The global atmospheric distribution of HCHO is dominated by in situ production during the oxidation of volatile organic compounds (VOCs) (Fortems-Cheiney et al, 2012; Anderson et al, 2017), primary emissions from biomass burning (Akagi et al, 2011; Coggon et al, 2019; Kluge et al, 2020), vegetation (DiGangi et al, 2011), the industry sector (Parrish et al, 2012), shipping (Marbach et al, 2009; Celik et al, 2020) and agriculture (Kaiser et al, 2015) can contribute significantly to the local HCHO abundance. If HCHO loss rates are well defined, estimation of the concentrations of OH from HCHO mixing ratios is feasible since HCHO production is dominated by the oxidation of VOCs via OH

Results

Discussion

Conclusion