A Laboratory Study to Constrain Gas-Phase Formic Acid and Acetic Acid Sources

Organic acids contribute to cloudwater and aerosol acidity and are therefore consequential for aqueous phase chemistry in the atmosphere. However, the sources and sinks of the two most abundant organic acids in the atmosphere, formic acid (FA) and acetic acid (AA), are not well characterized. Recent field measurements performed in a range of environments have demonstrated that atmospheric boundary layer gas-phase FA and AA levels exceed modeled levels by up to a factor of 3 (Millet et al., 2015). The gap between modeled and observed FA and AA suggest a substantial and unknown source. What is unclear is whether this source results from poor modeling of volatile organic compounds (VOCs) oxidation chemistry (secondary source), from the oxidation of condensed-phase organic material (secondary source), or an unaccounted flux to the atmosphere (primary source).

We present a series of laboratory studies to assess gas-phase yields of FA and AA from different primary and secondary sources to provide insight in constraining potential contributions from a range of source types and aging processes. We employ a custom built combustion and emission chamber for primary source generation. In addition, a Potential Aerosol Mass (PAM) reactor (Kang et al., 2007) is used to generate secondary sources (i.e., gas-phase photochemical production and production from the oxidative aging of primary particles). The PAM reactor can achieve oxidant exposures equivalent to several days in the atmosphere, thus allowing us to track the gas-phase FA and AA production as a function of precursor type, precursor concentration, oxidative exposure, and oxidant type (e.g., OH vs. O₃).

Detection of gas-phase FA and AA is achieved via a proton-transfer reaction mass spectrometer (PTR-MS; Ionicon Analytik). A custom calibration system is integrated with the PTR-MS to eliminate interfering compounds at selected m/z's, as well as quantifying the FA and AA mixing ratios (Baasandorj et al., 2015). Experiments on single-component precursor VOC oxidation in the PAM reactor allows a determination of acid yields from selected compounds that are found in large abundance in the atmosphere (e.g., isoprene and monoterpenes in forested regions, toluene and xylene in urban regions, etc.). Direct primary FA and AA emissions from biomass combustion sources are also characterized. Here, the vapor-phase fraction of biomass combustion emissions are removed using a denuder, and the particle-phase fraction undergoes further aging in the PAM reactor, releasing gas-phase organic acids. A scanning mobility particle sizer (SMPS; TSI Inc.) and an aerosol mass spectrometer (AMS; Aerodyne Research Inc.) are used to quantify the total particulate matter and organic aerosol mass concentrations. Gas-phase acid yields from the oxidative aging of biomass combustion aerosol will be presented. Results from this study can be used to improve our modeling of the sources and sinks of these abundant acids.

Baasandorj., M. et al., Measuring acetic and formic acid by proton-transfer-reaction mass spectrometry: sensitivity, humidity dependence, and quantifying intereferences, Atmos. Meas. Tech., 8, 2015

Kang, E. et al., Introducing the concept of Potential Aerosol Mass (PAM), Atmos. Chem. Phys., 7, 2007

Millet, D. B. et al., A large and ubiquitous source of atmospheric formic acid, Atmos. Chem. Phys., 15, 2015