Seasonal variability of surface and column carbon monoxide over the megacity Paris, high-altitude Jungfraujoch and Southern Hemispheric Wollongong stations

Abstract : This paper studies the seasonal variation of sur- face and column CO at three different sites (Paris, Jungfrau- joch and Wollongong), with an emphasis on establishing a link between the CO vertical distribution and the nature of CO emission sources. We find the first evidence of a time lag between surface and free tropospheric CO seasonal varia- tions in the Northern Hemisphere. The CO seasonal variabil- ity obtained from the total columns and free tropospheric par- tial columns shows a maximum around March–April and a minimum around September–October in the Northern Hemi- sphere (Paris and Jungfraujoch). In the Southern Hemisphere (Wollongong) this seasonal variability is shifted by about 6 months. Satellite observations by the IASI–MetOp (In- frared Atmospheric Sounding Interferometer) and MOPITT (Measurements Of Pollution In The Troposphere) instru- ments confirm this seasonality. Ground-based FTIR (Fourier transform infrared) measurements provide useful comple- mentary information due to good sensitivity in the bound- ary layer. In situ surface measurements of CO volume mix- ing ratios at the Paris and Jungfraujoch sites reveal a time lag of the near-surface seasonal variability of about 2 months with respect to the total column variability at the same sites. The chemical transport model GEOS-Chem (Goddard Earth Observing System chemical transport model) is employed to interpret our observations. GEOS-Chem sensitivity runs identify the emission sources influencing the seasonal varia- tion of CO. At both Paris and Jungfraujoch, the surface sea- sonality is mainly driven by anthropogenic emissions, while the total column seasonality is also controlled by air masses transported from distant sources. At Wollongong, where the CO seasonality is mainly affected by biomass burning, no time shift is observed between surface measurements and to- tal column data.