Airborne measurements of HCHO and HCOOH during the New England Air Quality Study 2004 using a pulsed quantum cascade laser spectrometer

Abstract

Atmospheric mixing ratios of formaldehyde and formic acid have been measured from the NOAA WP‐3 aircraft during the New England Air Quality Study (NEAQS) of July and August 2004 using a newly developed quantum cascade laser (QCL) spectrometer operating at a wavelength of 5.6 μm. The laser operates in pulsed mode with thermoelectric cooling. The detection is based on direct absorption in a compact 76‐m multiple pass absorption cell. The laser is swept over a 0.5 cm−1 spectral region containing multiple lines of both HCHO and HCOOH. Absolute concentrations are retrieved by simultaneous spectral fitting routines with a detection limit (2σ) for HCHO of 0.3 parts in 109 (ppbv) with an averaging time of 60 s under stable flight conditions. HCHO mixing ratios in the range from 0.3 to 5 ppb were encountered during flight conditions. Some of the highest mixing ratios of HCHO were observed over heavily vegetated areas of Florida during the test flights where the ratios of HCHO to methacrolein and methyl vinyl ketone, measured by proton transfer mass spectroscopy, are consistent with formaldehyde production by isoprene oxidation. The highest mixing ratios of HCOOH, up to 10 ppbv, were observed in an aged forest fire plume encountered over northern Canada, in which ratios of HCOOH/CO are greater than previous observations, while the ratios of HCHO/CO are less than previous reports from forest fire plumes. Observations of HCHO/CO and HCOOH/CO in urban plumes are indicative of a mixture of sources from direct emissions and secondary oxidation of anthropogenic and biogenic hydrocarbons. The ability to measure both HCHO and HCOOH simultaneously is of value in assessing the oxidation mechanisms of atmospheric hydrocarbons and secondary organic aerosol formation and oxidation.