Deployment of a GC/MS System to Measure C₂ to C₅ Carbonyls, Methanol and Ethanol Aboard Aircraft

Eric C. Apel - NCAR/ACD
Daniel D. Riemer - UM/RSMAS

   We will deploy a gas chromatograph/mass spectrometer (GC/MS) instrument to measure £ C₅ carbonyl compounds, methanol and ethanol on the NASA DC-8 aircraft. The time response for the measurement of these compounds will be 5 minutes yielding 12 measurements per hour. The system is based on a Hewlett-Packard 5973 Mass Spectrometer. Air samples will be drawn into the introduction system where preconcentration occurs. Helium carrier gas will rapidly transfer the preconcentrated compounds to a gas chromatograph fitted with a HP-624 column. The carbonyl and alcohol compounds of interest will elute rapidly from the column and into the mass spectrometer operating in the single ion mode. This method provides unambiguous identification because the compounds are chromatographically separated and mass selected. The sensitivity is estimated to be between 2 and 10 pptv depending on the compound.

   It is well known that these trace gases are extensively involved in atmospheric photooxidation processes, but they also represent classes of compounds that are not routinely measured. As a result of photodecomposition processes and reactions with OH, carbonyl compounds generate substantial amounts of HO₂, RO₂, and RC(O)O₂ radicals in the atmosphere. These radicals react with NO to produce NO₂ with the subsequent formation of O₃. The RC(O)O₂ radicals can react with NO₂ to form the reservoir PAN and PAN type compounds which can be transported over large distances in the troposphere. Thus, the carbonyl compounds are important participants in the tropospheric ozone formation and the HO_x cycle.

   Recent experiments have shown a discrepancy between measured formaldehyde and modeled formaldehyde in continentally influenced air masses. Under these conditions, measurements tend to be higher (by as much as a factor of 2 (Alan Fried, personal communication)) than modeled results indicating a source of formaldehyde which is not included in the models. Unmeasured compounds, such as methanol and carbonyl compounds are potential candidates for the missing source.

   There is still much uncertainty in our understanding of tropospheric ozone formation and the HOx cycle. A large contributor to this uncertainty is likely the unmeasured carbonyl and alcohol species. Additional uncertainty may be attributed to measurement techniques that have not been well characterized.

In summary,

1. We will measure £ C₅ carbonyl compounds, methanol and ethanol aboard the DC-8 aircraft with a time response of 5 minutes.  

2. We will investigate the direct emissions of these compounds from the Asian continent.

    3. We will investigate the extent to which photochemical evolution is important to the formation of carbonyl compounds in air transported from the Asian continent.

    4. We will compare our technique for measuring £ C₅ carbonyls and methanol and ethanol to other available techniques involved in TRACE-P.