Acetone variability in the upper troposphere: Analysis of CARIBIC observations and LMDz-INCA chemistry-climate model simulations
This paper investigates the acetone variability in the upper troposphere (UT) as sampled during the CARIBIC airborne experiment and simulated by the LMDz-INCA global chemistry climate model. The aim is to (1) describe spatial distribution and temporal variability of acetone; (2) define observation-based constraints to improve tropospheric modelling of the acetone; and (3) investigate the representativeness of the observational data set. According to the model results, South Asia (including part of the Indian Ocean, all India, China, and Indochinese peninsula) and Europe are net source regions of acetone, where near 25% of North Hemispheric (NH) primary emissions and 40% of the NH chemical production of acetone take place. The impact of these net source regions on continental upper tropospheric acetone is studied by analysing CARIBIC observations of 2006 and 2007 when most flight routes stretch between Frankfurt, Germany, and Manila, Philippines, and by focussing over 3 sub-regions where acetone variability is strong: Europe-Mediterranean, Central South China and South China Sea. Acetone volume mixing ratio (vmr) in UT varies with the season, increasing from winter to summer by a factor 2 to 4. Spatial variability is also important, as acetone vmr may vary in summer by more than 1000 pptv within only 5 latitude-longitude degrees, and standard deviation on measurements acquired during a short flight sequence over a sub-region may reach 40%. 200 pptv difference may also be observed between successive inbound and outbound flights over the same sub-region, due to different flight specifications (trajectory in relation to plume, time for insulation). A satisfactory agreement for the abundance of acetone is found between model results and observations, with e.g. only 30% over-estimation of the annual average over Central-South China and the South China Sea (between 450 and 600 pptv), and an under-estimation by less than 20% over Europe Mediterranean (around 800 pptv). Consequently, annual budget terms could be computed with LMDz-INCA, yielding a global atmospheric burden of 7.2 Tg acetone, and a 127 Tg yr-1 global source/sink strength. Moreover the study shows that LMDz-INCA can reproduce the impact of summer convection over China when boundary layer compounds are lifted to cruise altitude of 10-11 km and higher. The consequent enhancement of acetone vmr during summer is reproduced by LMDz-INCA, to reach agreement on observed maximum of 970 400 pptv (average during each flight sequence over the defined zone standard deviation). The summer enhancement of acetone is characterized by a high spatial and temporal heterogeneity, showing the necessity to increase the airborne measurement frequency over Central-South China and the South China Sea in August and September, when the annual maximum is expected (daily average model values reaching potentially 2000 pptv). In contrary the annual cycle in the UT over Europe-Mediterranean is not reproduced by LMDz-INCA, and in particular the observed summer enhancement of acetone to 1400 400 pptv after long-range transport of free tropospheric air masses over North Atlantic Ocean. Confirmed agreement on the acetone annual cycle at surface level indicates misrepresentation of simulated transport of primary acetone or biased spatial distribution of acetone sinks and secondary sources. 2011 Author(s).