Seasonal variations of HCN over northern Japan measured by ground-based infrared solar spectroscopy

Ground-based infrared solar spectra were recorded at Rikubetsu (43.5 degrees N) and Moshiri (44.4 degrees N) in Japan using Fourier transform infrared (FTIR) spectrometers from 1995 to 1997. Total column amounts and tropospheric mixing ratios of HCN were derived from these spectra in the 3287.05-3287.40 cm(-1) micro-window using a nonlinear least squares spectral fitting method. The HCN values at these two locations showed significant seasonal variations. The HCN total column reached a maximum value of 6.57+/-0.84x10(15) molecules cm(-2) in summer (June-August) and a minimum value of 3.97+/-0.30x10(15) molecules cm-2 in winter (December-February). The maximum and minimum tropospheric HCN mixing ratios were 333+/-44 (summer) and 195+/-116 (winter) parts per trillion by volume (pptv), respectively. These seasonal variations suggest that the lifetime of HCN is shorter than one year. The enhancement of HCN above its seasonal mean (Delta HCN) was correlated with the enhancement of CO (Delta CO). A constant Delta HCN/Delta CO ratio suggests biomass burning as a source of HCN.