Airborne observations of formic acid using a chemical ionization mass spectrometer

[摘要] Thefirst airborne measurements of formic acid mixing ratios over the UnitedKingdom were measured on the FAAM BAe-146 research aircraft on 16 March 2010with a chemical ionization mass spectrometer using I⁻reagent ions. The I⁻ ionization scheme was able to measure formic acidmixing ratios at 1 Hz in the boundary layer.

In-flight standard addition calibrations from a formic acid source were usedto determine the instrument sensitivity of 35 ± 6 ion countspptv⁻¹ s⁻¹ and a limit of detection of 25 pptv. Routinemeasurements were made through a scrubbed inlet to determine theinstrumental background. Three plumes of formic acid were observed over theUK, originating from London, Humberside and Tyneside. The London plume hadthe highest formic acid mixing ratio throughout the flight, peaking at 358 pptv.No significant correlations of formic acid with NO_x and ozonewere found, but a positive correlation was observed between CO and HCOOHwithin the two plumes where coincident data were recorded.

A trajectory model was employed to determine the sources of the plumes andcompare modelled mixing ratios with measured values. The modelunderestimated formic acid concentrations by up to a factor of 2. This isexplained by missing sources in the model, which were considered to be bothprimary emissions of formic acid of mainly anthropogenic origin and a lackof precursor emissions, such as isoprene, from biogenic sources, whoseoxidation in situ would lead to formic acid formation.

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