Abstract

Formic acid (HFo) and acetic acid (HAc) have both natural and anthropogenic sources and a role in the atmospheric processing of carbon. These organic acids also have an increasing importance in setting the acidity of precipitation as nitrate and sulfate concentrations have decreased. This dissertation examines HFo and HAc tropospheric formation and transport in the continental United States using observations and models. Observational data from two field campaigns were collected with the peroxide chemical ionization mass spectrometer (PCIMS) using iodide clusters for both HFo and HAc recorded at mass-to-charge ratios of 173 and 187. The first campaign, the Deep Convective Clouds and Chemistry Experiment (DC3), was in May and June 2012 and observations extended from the surface to 13 km over the central and eastern United States. The second campaign, the Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ), was in July and August 2014 with measurements from the surface to 7 km over the Colorado Front Range. Post-mission calibration work determined glycolaldehyde (GA) is a significant isobaric interference to HAc with the HAc:GA sensitivity ranging from 1:1 to 1:10. PCIMS HAc data from both campaigns are reported as the acetic acid equivalent sum (AAES). Based on DC3 model work and estimates of secondary production during FRAPPÉ the instrumental sensitivity was closer to a 1:1. Manuscripts 1 and 2 focus on the DC3 May 21st airmass storm case study at the Alabama/Tennessee border. During this flight a 700 ppt HFo plume at 8 km was observed, approximately 300 ppt in excess of boundary layer air. Different potential reasons for this increase including aqueous production and a pH dependent scavenging were evaluated with the Weather Research and Forecasting model version 3.7 coupled with chemistry (WRF-Chem). Manuscript 1 evaluated the WRF-Chem meteorological reproduction of the airmass storm and the applicability of the Model for Ozone And Related chemical Tracers version 4 and Model for Simulating Aerosol Interactions and Chemistry (MOZART-MOSAIC) compatible microphysics schemes, Morrison and Purdue Lin, in conjunction with a lightning data assimilation (LDA) method. The Morrison microphysics scheme with an LDA temperature range of 261 – 291 K best represented the case study storm. Manuscript 2 showed that there was no difference in WRF-Chem scavenging between a convective complex and isolated convection. It is possible to have cloud top HFo greater than cloud base in a more acidic cloud, pH of 3.5, with multiple HFo aqueous sources, and assuming there

Highlights

  • Formic acid (HFo) and acetic acid (HAc) have both natural and anthropogenic sources and play a significant role in atmospheric chemical processes – in particular volatile organic compound (VOC) and oxygenated volatile organic compound (OVOC) processing in the troposphere and precipitation chemistry

  • What HFo potential sources are we not accounting for in models? What does this tell us about the differences in production pathways between HFo and HFo and acetic acid (HAc)? 4

  • The May 21st case study was chosen because there was higher than expected HFo by a few hundred parts per trillion above background levels in a region dominated by convective outflow

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Summary

Introduction

Formic acid (HFo) and acetic acid (HAc) have both natural and anthropogenic sources and play a significant role in atmospheric chemical processes – in particular volatile organic compound (VOC) and oxygenated volatile organic compound (OVOC) processing in the troposphere and precipitation chemistry. Secondary production is a significant source for both acids especially from biogenic precursors, biomass burning, secondary organic aerosols, and photochemical production from VOCs and OVOCs (Khare et al, 1999; Paulot et al, 2011; Yuan et al, 2015). Both organic acids have been studied for decades a great deal of uncertainty remains concerning the extent and pathways of their secondary production. If chemicals are lofted to the UT their lifetimes extend substantially and can travel thousands of kilometers impacting chemistry downwind This could have a large impact during summer when there is a substantial amount of convection across the United States. Measurements from the Intercontinental Chemical Transport Experiment–North America 2004 campaign over the eastern United States and Canada found that 54% of the sampled air between 7.5 and 11.5 km was influenced by convection in the previous 2 days (Bertram et al, 2007)

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