Abstract
Abstract. The sources and atmospheric chemistry of gas-phase organic acids are currently poorly understood, due in part to the limited range of measurement techniques available. In this work, we evaluated the use of SF6- as a sensitive and selective chemical ionization reagent ion for real-time measurements of gas-phase organic acids. Field measurements are made using chemical ionization mass spectrometry (CIMS) at a rural site in Yorkville, Georgia, from September to October 2016 to investigate the capability of this measurement technique. Our measurements demonstrate that SF6- can be used to measure a range of organic acids in the atmosphere. One-hour averaged ambient concentrations of organic acids ranged from a few parts per trillion by volume (ppt) to several parts per billion by volume (ppb). All the organic acids displayed similar strong diurnal behaviors, reaching maximum concentrations between 17:00 and 19:00 EDT. The organic acid concentrations are dependent on ambient temperature, with higher organic acid concentrations being measured during warmer periods.
Highlights
Organic acids are ubiquitous and important species in the troposphere
The detection limits of the organic acids were approximated from 3 times the standard deviation values (3σ ) of the ion signals obtained during background measurements
It should be noted that the SF−6 chemical ionization mass spectrometry (CIMS) method is sensitive to oxalic, propionic and glycolic acids, which are expected to be present at low concentrations in the atmosphere
Summary
Organic acids are ubiquitous and important species in the troposphere. They are major contributors of free acidity in precipitation (Galloway et al, 1982; Keene et al, 1983; Keene and Galloway, 1984) and can affect the formation of secondary organic aerosols (SOA) (Zhang et al, 2004; Carlton et al, 2006; Sorooshian et al, 2010; Yatavelli et al., 2015). Calibration measurements were performed every 5 h during the field study through standard additions of 34SO2 and either formic or acetic acid to the sampled air flow. Formic (m/z 45, 65 and 108) and acetic (m/z 79) acid ion signals took ∼ 1.5 min to reach a steady state after being switched between ambient, calibration and background measurements (Fig. S4a and c). The decays in the formic and acetic acid ion signals and times required for them to reach steady state after the removal of calibration gases during the switch from standard addition calibration to ambient sampling were used to determine the CIMS response time. The m/z 79 ion signal decayed to 1/e2 in 42 ± 2 s (Fig. S4d)
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