Global distribution and variability of formic acid as observed by MIPAS-ENVISAT

Formic acid (HCOOH) vertical profiles have been retrieved from MIPAS-ENVISAT satellite data in the upper troposphere for the first time. Based on new spectroscopic line-strength measurements by Vander Auwera et al. (2007) of HCOOH, a retrieval setup was developed and optimized to study its global distribution between 2002 and 2008. A strong seasonality, directly associated to plant growth and corresponding biogenic emissions, is observed and dominates in the middle latitudes of the Northern Hemisphere. Here, the mean monthly volume mixing ratios (VMR) at 8 km altitude typically reach 100-110 parts per trillion in volume (pptv) during the summer and decrease to about 45 pptv in the early winter. At 16 km and higher altitudes, the VMRs remain under the 20 pptv level and have a much smaller amplitude (<10 pptv). In the Southern Hemisphere, strong signals (up to 1 ppbv at 10 km in a single measurement) are detected from biomass burning during the August-October time period and can enhance the monthly mean background levels above specific tropical and midlatitudinal regions by a factor of 2 or more. In-plume production of HCOOH through photochemical processes has been identified during an extreme event in September 2006, although it is not likely to contribute significantly to the overall upper tropospheric abundances of formic acid.