Global atmospheric budget of acetaldehyde: 3-D model analysis and constraints from in-situ and satellite observations

Abstract

Abstract. We construct a global atmospheric budget for acetaldehyde using a 3-D model of atmospheric chemistry (GEOS-Chem), and use an ensemble of observations to evaluate present understanding of its sources and sinks. Hydrocarbon oxidation provides the largest acetaldehyde source in the model (130 Tg a−1), with alkanes, alkenes, ethanol, and isoprene the main precursors. We use an updated chemical mechanism for GEOS-Chem, and photochemical acetaldehyde yields are consistent with the Master Chemical Mechanism. We apply SeaWiFS satellite observations to define the global distribution of light absorption due to marine dissolved organic matter (DOM), and estimate the corresponding sea-to-air acetaldehyde flux based on measured photoproduction rates from DOM. The resulting net ocean emission is 58 Tg a−1, the second largest global source of acetaldehyde. Quantitative model evaluation over the ocean is complicated by known measurement artifacts in clean air. Simulated concentrations in surface air over the ocean generally agree well with aircraft measurements, though the model tends to overestimate the vertical gradient. PAN:NOx ratios are well-simulated in the marine boundary layer, providing some support for the modeled ocean source. A key uncertainty is the acetaldehyde turnover time in the ocean mixed layer. We introduce the Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) for acetaldehyde and ethanol and use it to quantify their net flux from living terrestrial plants. Including emissions from decaying plants the total direct acetaldehyde source from the land biosphere is 22 Tg a−1. Other terrestrial acetaldehyde sources include biomass burning (3 Tg a−1) and anthropogenic emissions (2 Tg a−1). Simulated concentrations in the continental boundary layer are generally unbiased and capture the spatial gradients seen in observations over North America, Europe, and tropical South America. However, the model underestimates acetaldehyde levels in urban outflow, suggesting a missing source in polluted air. Ubiquitous high measured concentrations in the free troposphere are not captured by the model, and based on present understanding are not consistent with concurrent measurements of PAN and NOx. We find no compelling evidence for a widespread missing acetaldehyde source in the free troposphere. We estimate the current US source of ethanol and acetaldehyde (primary+secondary) at 1.3 Tg a−1 and 7.0 Tg a−1, approximately 60% and 400% of the corresponding increases expected for a national transition from gasoline to ethanol fuel.