Comparison of free tropospheric western Pacific air mass classification schemes for the PEM‐West A experiment

Abstract

Abstract. During September/October 1991, NASA's Global Tropospheric Experiment (GTE) conducted an airborne field measurement program (PEM-West A) in the troposphere over the western Pacific Ocean. In this paper we describe and use the relative abundance of the combustion products C2H 2 and CO to classify air masses encountered during PEM-West A based on the degree that these tracers were processed by the combined effects of photochemical reactions and dynamical mixing (termed the degree of atmospheric processing). A large number of trace compounds (e.g., C2H6, C3H8, C6H6, NOy, and 03) are found to be well correlated with the degree of atmospheric processing that is reflected by changes in the ratio of C2H2/CO over the range of values from --0.3 to 2.0 (parts per trillion volume) C2H2/(parts per billion volume) CO. This C2H2/CObased classification scheme is compared to model simulations and to two independent classification schemes based on air mass back-trajectory analyses and lidar profiles of 03 and aerosols. In general, these schemes agree well, and in combination they suggest that the functional dependence that other observed species exhibit with respect to the C2H2/CO atmospheric processing scale can be used to study the origin, sources, and sinks of trace species and to derive several important findings. First, the degree of atmospheric processing is found to be dominated by dilution associated with atmospheric mixing, which is found to primarily occur through the vertical mixing of relatively recent emissions of surface layer trace species. Photochemical reactions play their major role by influencing the background concentrations of trace species that are entrained during the mixing (i.e., dilution) process. Second, a significant noncontinental source(s) of NO (and NOx) in the free troposphere is evident. In particular, the enhanced NO mixing ratios that were observed in convected air masses are attributed to either emissions from lightning or the rapid recycling of NOy compounds. Third, nonsoluble trace species emitted in the continental boundary layer, such as CO and hydrocarbons, are vertically transported to the upper troposphere as efficiently as they are to the midtroposphere. In addition, the mixing ratios of CO and hydrocarbons in the upper troposphere over the western Pacific may reflect a significant contribution from northern hemisphere land areas other than Asia. Finally, we believe that these results can be valuable for the quantitative evaluation of the vertical transport processes that are usually parameterized in models.

Are biogenic emissions a significant source of summertime atmospheric toluene in the rural Northeastern United States?

“…C 2 H 2 also has a significantly faster rate of reaction with OH, its major sink, than CO making the ratio of the two compounds an indicator of air mass photochemical and mixing processes (Smyth et al, 1996). The C 2 H 2 /CO ratios observed at Thompson Farm reached their minimum values in summer reflecting higher levels of OH and increased air mass photochemical processing (Fig.…”

TRACE A trajectory intercomparison: 2. Isentropic and kinematic methods

“…Therefore large values of the ratio indicate relatively recent passage over regions of combustion, while small values denote longer transit times from source regions. Air mass classifications based on this ratio have agreed favorably with those based on backward trajectory analyses [Smyth et al, 1996]. We examined this chemical signature to determine whether it provides guidance as to the relative quality of our two types of trajectories.…”

“…The ratio of acetylene to carbon monoxide (C2H2/CO) has been identified as an excellent indicator of the length of time that an air mass has traveled since encountering combustion [Smyth et al, 1996]. That is, the relative abundance of these two combustion products indicates the degree of atmospheric processing (photochemical reactions and mixing) that the air mass has undergone during transport.…”

Observed distributions of nitrogen oxides in the remote free troposphere from the Nasa Global Tropospheric Experiment Programs

“…Generally, the ratio of these compounds is in the range of 3-6, where C2H 2 is given in parts per trillion by volume and CO is in parts per billion by volume, for boundary layer air residing in the source region (i.e., "relatively fresh" emissions). On the basis of a comparison of air parcel travel times from various source regions [Smyth et al, 1996b], ratios near 1 pptv/ppbv correspond to moderately processed air (e.g., equivalent to at least several days of photochemical processing after being released), and ratios less than approximately 0.5 pptv/ppbv correspond to very well processed air (i.e., air parcels that have not been significantly impacted by fresh combustion emissions within the equivalent of at least a week or more). Plate 16 shows the observed Some of the largest NO mixing ratios were observed in the subtropical western Pacific in connection with the deep convection outflows, as discussed earlier, in section 4.2.…”