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 (31.9degreesN, 111.6degreesE, 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 nu(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.