Greenhouse gas relationships in the Indian summer monsoon plume measured by the CARIBIC passenger aircraft

Abstract. During the Indian summer monsoon period 2008, regular measurement flights were performed by the CARIBIC aircraft between Germany and India. Measurements included whole air sampling and subsequent analysis for greenhouse gases (CH₄, CO₂, N₂O, SF₆) and in-situ measurements of CO, O₃, and H₂O. For all these tracers a distinct monsoon signature was observed in the longitude range 50–80° E south of 40° N at flight altitudes 8–12.5 km. The formation of a monsoon plume with enhanced mixing ratios was observed for CH₄, N₂O, SF₆, CO and H₂O. The plume began to develop in May and persisted through September, and maximum mixing ratios and maximum spatial extension of the plume were observed in August. For CO₂ and O₃ a minimum was observed. The amplitude of the CO₂ seasonal cycle was larger than at comparable latitudes outside the monsoon region, and the CO₂ spring maximum was observed with a delay of one month. Different tracer relationships show strong spatial variations across the plume. The comparison with NOAA ground station data shows a clear enhancement for CH₄, N₂O, SF₆ and CO and a decrease in CO₂ mixing ratios. Assuming seasonally invariant SF₆ emissions based on the EDGAR 4.0 inventory, monthly emissions were estimated for the period June–September for CH₄, N₂O, and CO, and the CO₂ uptake was estimated. While for N₂O an emission decrease during the monsoon period was derived, emissions of CH₄ were highest in September indicating a different seasonal cycle of emissions. Net CO₂ uptake by the vegetation was largest in September.