Measurements of carbonyl compounds around the Arabian Peninsula indicate large missing sources of acetaldehyde

Abstract

Abstract. Volatile organic compounds (VOCs) were measured around the Arabian Peninsula using a research vessel during the AQABA campaign (Air Quality and Climate Change in the Arabian Basin) from June to August 2017. In this study we examine carbonyl compounds (CxHyO), measured by a proton transfer reaction mass spectrometer (PTR-ToF-MS), and present both a regional concentration distribution and a budget assessment for these key atmospheric species. Among the aliphatic carbonyls, acetone had the highest mixing ratios in most of the regions traversed, varying from 0.43 ppb over the Arabian Sea to 4.5 ppb over the Arabian Gulf, followed by formaldehyde (measured by Hantzsch monitor, 0.82 ppb over the Arabian Sea and 3.8 ppb over the Arabian Gulf) and acetaldehyde (0.16 ppb over the Arabian Sea and 1.7 ppb over the Arabian Gulf). Unsaturated carbonyls (C4–C9) varied from 10 to 700 ppt during the campaign, and followed similar regional mixing ratio dependence as aliphatic carbonyls, which were identified as oxidation products of cycloalkanes over polluted areas. An empirical method based on hydrocarbon ratios was applied to investigate the photochemical source strength of the aliphatic carbonyls. While the distribution and relative concentration enhancements of the C3–C8 aliphatic carbonyls could be explained by this method, that of acetaldehyde could not. A smaller but still significant discrepancy was found when comparing measurements to global chemistry-transport model (EMAC) results, with the model underestimating the measured acetaldehyde mixing ratio up to an order of magnitude. Implementing a photolytically driven marine source of acetaldehyde significantly improved the agreement between measurements and model, particularly over the remote regions (e.g. Arabian Sea). However, the newly introduced acetaldehyde source was still insufficient to describe the observations over the most polluted regions (Arabian Gulf and Suez), where model underestimation of primary emissions and biomass burning events are possible reasons.

A new marine biogenic emission: methane sulfonamide (MSAM), dimethyl sulfide (DMS), and dimethyl sulfone (DMSO<sub>2</sub>) measured in air over the Arabian Sea

“…This study focuses on the two crossings of the Arabian Sea during the AQABA campaign. The Arabian Sea was generally characterized by low values of VOCs, especially VOCs related to anthropogenic activities (Bourtsoukidis et al, 2019;Bourtsoukidis et al, 2020;Pfannerstill et al, 2019;Wang et al, 2020). The three molecules presented in this study are the exception.…”

“…2). Onboard the ship there were a weather station and four laboratory containers equipped with instrumentation for on-and offline measurement of a large suite of trace gases, particles, and radicals (Bourtsoukidis et al, 2019;Bourtsoukidis et al, 2020;Wang et al, 2020).…”

Modelling the impact of gas-phase pyruvic acid on acetaldehyde and peroxy radical formation in the boreal forest

“…Global models have recently revealed discrepancies between simulated and measured acetaldehyde concentrations (Millet et al, 2010;Wang et al, 2019;Wang et al, 2020). Wang et al (2020) reported CH3CHO mixing ratios that were up to a factor of 10 higher than global chemistry-transport model (EMAC) results in the marine boundary layer around the Arabian Peninsula, implying missing sources in remote and polluted regions. Wang et al (2019) also found that models systematically underestimate CH3CHO compared to observations in the remote troposphere, implying a missing source of acetaldehyde.…”