Formic Acid Production From Photochemical Oxidation in condensed and Non-condensed Mechanisms

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Chen, Hongyu
Kaiser, Jennifer
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Organic acids are secondary components generated from the oxidation of volatile organic compounds in the atmosphere, affecting particulate matter composition, aerosol, and rain acidity. Among them, formic acid is the most abundant gas-phase organic acid, which has been observed over 2.5 ppbV in concentrations in rural areas. However, current atmospheric models’ predictions of formic acid are typically biased low, potentially because of the underestimation of direct emissions and chemistry. In our work, we first applied the Framework for 0-D Atmospheric Modeling version4 (F0AMv4) to simulate gas-phase formic acid using four current mechanisms (Master Chemical Mechanism (MCM) v3.3.1, GEOS-Chem Mechanism in version v12-08, SAPRC-07B mechanism and Carbon Bond 6 (CB6) mechanism). The observation data were collected in Yorkville, Georgia, lasted from September to October in 2016 during an intensive campaign. Like earlier studies, the modeled diurnal trend of formic acid was lower than the observation, showing a dissimilar diurnal profile. Then we used the box model combined with the observation data to understand the production of formic acid from isoprene oxidation. Our box model simulations revealed a large difference in formic acid production pathways among four isoprene oxidation mechanisms under different NOx levels. We discovered that MCM v3.3.1 was likely to underestimate formic acid production from isoprene oxidation because it only considered a small contribution from isoprene epoxydiol (IEPOX) peroxy radicals reacting with HO2 pathway. However, CB6 mechanism revealed the suppression of formic acid formation under a high NOx condition as proposed by recent studies, which was not included in all other mechanisms. This mechanism also greatly underestimated the formic acid production from isoprene reacting with ozone and overlooked the pathway from glycolaldehyde to formic. To better improve the formic acid prediction in the model, additional gas-phase reactions of isoprene and monoterpenes suggested by recent studies were added to our chemical mechanisms. After the modification, we could see a small increase in predicted formic acid concentration. However, these added reactions had little impact on the diel profile of formic acid. According to Gao et al. (2021), the model-measurement discrepancy in formic acid at Yorkville persists, despite modifications to the chemical mechanisms. Our work elaborated the formic acid pathways among each mechanism under different NOx levels and revealed the role of isoprene and monoterpenes in formic acid production. This shows that the missing monoterpene and isoprene photooxidation reactions discussed in recent studies might not be the only major missing components for formic acid predictions. Models should also consider incorporating other processes like emission.
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