Abstract
The goals of this study were to determine whether formic and acetic acid could be quantified from the Deep Convective Clouds and Chemistry Experiment (DC3) through post-mission calibration and analysis and to optimize a reagent gas mix with CH3I, CO2, and O2 that allows quantitative cluster ion formation with hydroperoxides and organic acids suitable for use in future field measurements. There is a scarcity of organic acid measurements in the upper troposphere under stormy conditions with which to compare and assess photochemical and transport theory. DC3 observations made in May and June 2012 extending from the surface to 13 km over the central United States during convective storm conditions would be a significant addition to our knowledge base. Organic acid measurements were made with a chemical ionization mass spectrometer (CIMS) aboard the NCAR Gulfstream-V platform in DC3. The primary objective of the CIMS was to observe hydrogen peroxide and methyl hydroperoxide though it recorded signals attributed to iodide clusters of formic and acetic acid at 173 and 187 townsend, respectively. Both organic acids were targets of opportunity as the reagent ion (I-) needed for clustering was not specifically used in the field. However an iodide source gas (iodomethane, CH3I), was used during pre-mission experiments and a residual amount adsorbed onto the inlet surfaces was found to be bleeding off the plumbing in the field. Post-mission laboratory experiments were performed to determine the CIMS instrument’s sensitivity to these organic acids under iodomethane outgassing and the varying water vapor and sample flow conditions encountered during DC3 flights. Laboratory calibration experiments with varying iodomethane concentrations, inlet pressures, and water vapor mixing ratios were performed. Calibrations for hydrogen peroxide and methyl hydroperoxide were used to tune the reagent gas mixture composed of CH3I, CO2, and O2 to best match the observed sensitivities for hydrogen peroxide and methyl hydroperoxide in DC3. Formic and acetic calibration functions were fit to the water vapor and sample flow results. Formic acid displays both a water vapor and inlet pressure dependence, whereas, acetic acid sensitivity was found to be independent of water vapor and inlet pressure within the range of conditions encountered in DC3 and the laboratory. These laboratory calibrations were further evaluated by comparison to in-flight calibrations from the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) to confirm the reliability of the fits and laboratory methodology. The laboratory and field calibration work showed very good correspondence for
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