Characterization of non-methane hydrocarbons in Asian summer monsoon outflow observed by the CARIBIC aircraft

Abstract

Abstract. Between April and December 2008 the CARIBIC commercial aircraft conducted monthly measurement flights between Frankfurt, Germany and Chennai, India. These flights covered the period of the Asian summer monsoon (June-September), during which enhancements in a number of atmospheric species were observed in the upper troposphere over southwestern Asia. In addition to in situ measurements of trace gases and aerosols, whole air samples were collected during the flights, and these were subsequently analyzed for a suite of trace gases that included a number of C 2 -C 8 non-methane hydrocarbons. Non-methane hydrocarbons are relatively short-lived compounds and the large enhancements in their mixing ratios in the upper troposphere over southwestern Asia during the monsoon, sometimes more than double their spring and fall means, provides qualitative evidence for the influence of convectively uplifted boundary layer air. The particularly large enhancements of the combustion tracers benzene and ethyne, along with the similarity of their ratios with carbon monoxide and emission ratios from the burning of household biofuels, indicate a strong influence of biofuel burning to NMHC emissions in this region. Conversely, the ratios of ethane and propane to carbon monoxide, along with the ratio between i-butane and n-butane, indicate a significant source of these compounds from the use of fossil fuels, and comparison to previous campaigns suggests that this source could be increasing. Photochemical aging patterns of NMHCs showed that the CARIBIC samples were collected in two distinctly different regions of the monsoon circulation: a southern region where air masses had been recently influenced by low level contact and a northern region, where air parcels had Correspondence to: A. K. Baker (angela.baker@mpic.de) spent substantial time in transit in the upper troposphere before being probed. Estimates of age using ratios of individual NMHCs have ranges of 3-6 days in the south and 9-12 days in the north.

Acetone variability in the upper troposphere: analysis of CARIBIC observations and LMDz-INCA chemistry-climate model simulations

“…Schuck et al (2010) showed that air masses sampled north of 30 • N generally travel for more than a week, in contrast with air masses south of 30 • N which had ground contact within the last four days prior to sampling. Baker et al (2011) confirmed that in the southern monsoon region, sampled air masses travelled 3 to 6 days before sampling, and 9 to 12 days in northern monsoon region. This is consistent with the summer BL air mass fingerprint mostly encountered over CSChi andSCSea (in 2006 and, but rarely encountered over Northern Asia.…”

“…The summer monsoon affects the composition of the UT by dynamical and meteorological processes, basically strong and rapid convection lifting of boundary layer air masses combined with high precipitation affecting emissions from certain sources. Consistent enhancements of CO, H 2 O, CH 4 , N 2 O and nonmethane hydrocarbons were observed with CARIBIC over South Asia in summer 2008 Baker et al, 2011). A concomitant decrease of ozone was also observed by CARIBIC .…”

Dynamics and composition of the Asian summer monsoon anticyclone

“…8c). These types of mixing lines in the troposphere could result from one or a combination of the following: (i) mixing between stratospherically and tropospherically influenced air masses, (ii) mixing between photochemically aged and freshly uplifted lower-tropospheric air and (iii) an O 3 -depleting photochemical regime (Baker et al, 2011). While the latter is unlikely in the ASMA (Fig.…”

Dynamic variability of the Asian monsoon anticyclone observed in potential vorticity and correlations with tracer distributions

“…The strong circulation of the anticyclone acts to isolate air, and because this isolated air is tied to the outflow of deep convection, the anticyclone often has a distinct chemical signature [Dethof et al, 1999;Randel and Park, 2006;Park et al, 2007Park et al, , 2008Baker et al, 2011]. The UTLS anticyclone is a seasonal feature, beginning in June and ending in September.…”