Investigation of secondary formation of formic acid: urban environment vs. oil and gas producing region

Abstract. Formic acid (HCOOH) is one of the most abundant carboxylic acids in the atmosphere. However, current photochemical models cannot fully explain observed concentrations and in particular secondary formation of formic acid across various environments. In this work, formic acid measurements made at an urban receptor site in June–July of 2010 during CalNex and a site in an oil and gas producing region in January–February of 2013 during UBWOS 2013 will be discussed. Although the VOC compositions differed dramatically at the two sites, measured formic acid concentrations were comparable: 2.3 ± 1.3 ppb in UBWOS 2013 and 2.0 ± 1.0 ppb in CalNex. We determine that concentrations of formic acid at both sites were dominated by secondary formation (> 8%). A constrained box model using the Master Chemical Mechanism (MCM v3.2) underestimates the measured formic acid concentrations drastically at both sites (by a factor of > 10). Inclusion of recent findings on additional precursors and formation pathways of formic acid in the box model increases modeled formic acid concentrations for UBWOS 2013 and CalNex by a factor of 6.4 and 4.5, respectively. A comparison of measured and modeled HCOOH/acetone ratios is used to evaluate the model performance for formic acid. We conclude that the modified chemical mechanism can explain 21 and 47% of secondary formation of formic acid in UBWOS 2013 and CalNex, respectively. The contributions from aqueous reactions in aerosol and heterogeneous reactions on aerosol surface to formic acid are estimated to be −7 and 0–6% in UBWOS 2013 and CalNex, respectively. We observe that air-snow exchange processes and morning fog events may also contribute to ambient formic acid concentrations during UBWOS 2013 (∼20% in total). In total, 50–57% in UBWOS 2013 and 48–53% in CalNex of secondary formation of formic acid remains unexplained. More work on formic acid formation pathways is needed to reduce the uncertainties in the sources and budget of formic acid and to narrow the gaps between measurements and model results.