Long-term tropospheric formaldehyde concentrations deduced from ground-based fourier transform solar infrared measurements

Abstract

Abstract. Long-term total column measurements of formaldehyde (HCHO) covering a 12 year period from 1992 to 2004 are reported from spectra recorded with a high-resolution Fourier Transform Spectrometer (FTS) using the sun as a light source at a Southern Hemisphere site (Lauder, New Zealand). The ambient HCHO concentrations at this rural location are often at background levels (<250 ppt) typical for remote marine environments. Due to these low values of HCHO, which are often at or below the detection limit of standard techniques, a method of analysis has been developed that successfully produces HCHO columns with sufficient sensitivity throughout the whole season. The HCHO column over Lauder was found to have a strong seasonal cycle (±50%), with a mean column of 4.2×1015 molecules cm−2, the maximum occurring in the summer. A simple box model of CH4 oxidation reproduces the seasonal cycle, but significantly underestimates the maximum HCHO ground concentrations deduced from the column observations, particularly in summer. This implies the existence of a significant source of HCHO that cannot be explained by oxidation of CH4 alone. The ground-based FTS column data compares well with collocated HCHO column measurements from the Global Ozone Monitoring Experiment (GOME) satellite instrument (r2=0.65, mean bias=10%, n=48).