Investigating the Tropospheric Chemistry of Acetic Acid Using the Global 3‐D Chemistry Transport Model, STOCHEM‐CRI

M Anwar H Khan,Carl J Percival,Kyle Lyons,Rebecca L Caravan,Dudley E Shallcross,Andrew J Orr‐Ewing,Craig A Taatjes,Rabi Chhantyal‐Pun,Max R Mcgillen

doi:10.1029/2018jd028529

Abstract

AbstractAcetic acid (CH3COOH) is one of the most abundant carboxylic acids in the troposphere. In the study, the tropospheric chemistry of CH3COOH is investigated using the 3‐D global chemistry transport model, STOCHEM‐CRI. The highest mixing ratios of surface CH3COOH are found in the tropics by as much as 1.6 ppb in South America. The model predicts the seasonality of CH3COOH reasonably well and correlates with some surface and flight measurement sites, but the model drastically underpredicts levels in urban and midlatitudinal regions. The possible reasons for the underprediction are discussed. The simulations show that the lifetime and global burden of CH3COOH are 1.6–1.8 days and 0.45–0.61 Tg, respectively. The reactions of the peroxyacetyl radical (CH3CO3) with the hydroperoxyl radical (HO2) and other organic peroxy radicals (RO2) are found to be the principal sources of tropospheric CH3COOH in the model, but the model‐measurement discrepancies suggest the possible unknown or underestimated sources which can contribute large fractions of the CH3COOH burden. The major sinks of CH3COOH in the troposphere are wet deposition, dry deposition, and OH loss. However, the reaction of CH3COOH with Criegee intermediates is proposed to be a potentially significant chemical loss process of tropospheric CH3COOH that has not been previously accounted for in global modeling studies. Inclusion of this loss process reduces the tropospheric CH3COOH level significantly which can give even larger discrepancies between model and measurement data, suggesting that the emissions inventory and the chemical production sources of CH3COOH are underpredicted even more so in current global models.

Highlights

Acetic acid (CH3COOH) plays a significant role in the acidity of precipitation and cloud water in the troposphere (Galloway et al, 1982; Keene et al, 1995; Khare et al, 1999; Vet et al, 2014; Zhang et al, 2011). Keene and Galloway (1986), and Andreae et al (1988) proposed that carboxylic acids could account for as much as 80–90% of precipitation acidity in remote regions of the world
The Criegee chemistry was integrated in STOCH-IM to calculate the steady state concentration of stabilized Criegee Intermediates by including the production of the Criegee intermediate (CI) from the ozonolysis reactions of six alkenes using the rate coefficients taken from the Master Chemical Mechanism and their individual loss through unimolecular, water, water dimer, and CH3COOH reactions
Peroxyacetyl radicals are abundant in the atmosphere over South America due to the large quantities of biogenic volatile organic compound precursors that are released from the forest

Summary

Introduction

Acetic acid (CH3COOH) plays a significant role in the acidity of precipitation and cloud water in the troposphere (Galloway et al, 1982; Keene et al, 1995; Khare et al, 1999; Vet et al, 2014; Zhang et al, 2011). Keene and Galloway (1986), and Andreae et al (1988) proposed that carboxylic acids could account for as much as 80–90% of precipitation acidity in remote regions of the world. The most detailed global budget of CH3COOH is presented by Paulot et al (2011), where they produced bottom-up estimates of the global sources of CH3COOH following the construction of an updated emissions inventory and chemical scheme and evaluated the model results against an extensive array of ground, ship, satellite, and flight campaign data. The simulations are performed with updates to the chemical mechanism (e.g., water complexation of peroxy radicals and their subsequent reactions with CH3CO3, SOA oxidation, loss of CH3COOH by Criegee) and emissions inventory of the species (e.g., acetaldehyde and isoprene) of the model, and comparisons of the results made with other modeling studies as well as surface stations and flight campaigns measurement data. The relevance of the reaction of gas phase CH3COOH with Criegee intermediates as a potential loss process of CH3COOH has been explored and is unaccounted for in the previous modeling studies

Model Description

Surface Distribution

Model-Measurement Comparison

Global Budget

Conclusion