Process Analysis of Elevated Concentrations of Organic Acids at Whiteface Mountain, New York

Abstract. Organic acids represent an important class of compounds in the atmosphere but there are many uncertainties in understanding their formation; in particular, few investigations have been carried out as to their sources in the Northeast U.S. Associated with a heat wave and pollution event on 1–2 July, 2018, unusually high concentrations of formic (HCOOH), acetic (CH₃COOH), and oxalic (OxAc) acid in cloud water were measured at the summit of Whiteface Mountain (WFM) in upstate New York. To investigate the gas phase production of organic acids for this pollution event, this work uses a combination of the regional transport model WRF-Chem which gives information on transport and environmental factors affecting air parcels reaching WFM, the Lagrangian chemical box model BOXMOX, which allows analysis analysi of chemistry with different chemical mechanisms. Two chemical mechanisms are used in BOXMOX: 1) the Model for Ozone and Related chemical Tracers (MOZART T1), and 2) the Master Chemical Mechanism version 3.3.1 (MCM). The WRF-Chem results show that air parcels sampled during the pollution event at WFM originated in central Missouri, which has strong biogenic emissions of isoprene. Many air parcels were influenced by emissions of nitrogen oxides (NO_x) from the Chicago Metropolitan Area. Ozonolysis of isoprene and related oxidation products were the major sources of HCOOH in both mechanisms. CH₃COOH was produced from acetyl peroxy radical (CH₃CO₃) reacting with the hydroperoxy (HO₂) radical, with MCM producing up to 40 % more CH3COOH under conditions of high isoprene and low NO_x compared to MOZART T1. Both mechanisms underpredicted HCOOH and and CH₃COOH by an order of magnitude compared to measurements at WFM. A simple gas+aqueous box model was used to determine if cloud water chemistry could have had an appreciable impact on organic acid formation. Aqueous chemistry exacerbated the discrepancies of HCOOH by leading to a net depletion within cloud water. There were large disagreements in the production of glyoxal (a key precursor of OxAc) between the two gas-phase mechanisms, with MOZART T1 showing stronger daytime production under high NO_x conditions, while MCM showed strong nocturnal production via ozonolysis chemistry. The gas + aqueous model exhibited strong production of OxAc within cloud droplets, with glyoxal serving as an important precursor. The substantial differences between chemical mechanisms and between observations and models indicates that further studies are required to better constrain gas and aqueous production of low molecular weight organic acids.