Free tropospheric measurements of formic acid (HCOOH) from infrared ground‐based solar absorption spectra: Retrieval approach, evidence for a seasonal cycle, and comparison with model calculations

The seasonal variation of the free tropospheric volume mixing ratio of formic acid (HCOOH) has been derived from high‐spectral‐resolution solar absorption spectra recorded with the Fourier transform spectrometer in the U.S. National Solar Observatory facility on Kitt Peak (31.9°N, 111.6°E, 2.09 km altitude) at a typical spectral resolution of 0.005 cm −1 . The spectra have been analyzed with the SFIT2 algorithm, which is based on a semiempirical application of the optimal estimation method. Absorption by HCOOH is weak in these solar spectra, but successful retrievals have been obtained with a new procedure that fits the HCOOH ν 6 band Q branch at 1105 cm −1 simultaneously with a window to account for a temperature‐sensitive HDO line, which overlaps the HCOOH Q branch. After retaining only the best measurements from a database extending from June 1980 to October 2002 the retrievals show a seasonal variation, with a summer maximum and a winter minimum. Average 2.09–10 km volume mixing ratios binned in 3 month intervals range from a maximum of 792 ± 323 parts per trillion by volume (pptv), or 10 −12 , in July–September to a minimum of 313 ± 175 pptv in October–December, with the uncertainties corresponding to statistical means from daily averages. The results are compared with previously reported measurements and model calculations.