Non-Methane Hydrocarbons, Halocarbons, and DMS

D. Blake

University of California, Irvine

NMHC and halocarbons act as tracers for air-mass origin as well as providing insight into average oxidation rates (OH concentration) along their transit path. The inclusion of DMS as part of the UC measurement effort reflects the fact that the combination of DMS, CHBr3, CH3I, and CH3ONO2 is one of the most convincing means available for assessing whether an air mass being sampled has recently had an exposure to a marine environment. However, in addition to this tracer role, DMS is one of the dominant sources of sulfur in Antarctica. DMS will be measured at the SP station (2003), on the Twin Otter (2003), and on the C-130 (2005).
During 2003, approximately 80 cans will be collected at SP and another 168 cans on the Twin Otter (8 per flight for 21 flights). In excess of 600 whole air samples will be collected onboard the NSF C-130 during the 2005 study. Don Blake will be responsible for integration of equipment on the aircraft and he and/or other research group members will collect samples on the C-130 at a rate of at least one per degree of latitude during the transits to and from Antarctica. On average, about 36 samples are planned for each local flight. All filled samples will be transported back to our UCI laboratory where their contents will be analyzed. Each sample will be analyzed for more than 50 trace gases comprising hydrocarbons, halocarbons, dimethyl sulfide (DMS), and alkyl nitrates. In brief, each sample of 1520 ± 1 cm3 (STP) of air is preconcentrated in liquid nitrogen, warmed by ~80 ºC water, and then flushed into the carrier flow where it is quantitatively split into five streams, each stream being directed to a different gas chromatograph with a specific column and detector combination. The sample contacts only stainless steel from the sample canister to the 5-port splitter and is connected to the columns via Silcosteel tubing (0.53 O.D.; RESTEK Corporation). The columns are all cryogenically cooled during injection and then follow the temperature ramp programs described previously. The sample split is highly reproducible as long as the specific humidity of the injected air is above a certain level, estimated to be 2 g H2O/kg air. This is ensured by adding ~2.4 kPa of water into each evacuated canister just before they are sent out to the field.
The NMHCs are separated by a J&W Scientific silica plot column (30m, 0.53 mm) connected to a flame ionization detector (FID). The detection limit of each NMHC is 1 pptv. The alkyl nitrates are separated by a Restek 1701 capillary column (60 m, 0.25 mm, 1µm) connected to an electron capture detector. The detection limit for the alkyl nitrates is 0.1 pptv. All NMHCs are calibrated against whole air working standards, which have been calibrated against NIST and Scotty standards. The precision of our C2-C4 NMHC analysis is ± 3% when compared to NIST standards during the Non-Methane Hydrocarbon Intercomparison Experiment (NOMHICE). Alkyl nitrates are calibrated against whole air working standards, which have been previously calibrated to an accuracy of better than 2% against a synthetic standard provided by Elliot Atlas and Frank Flocke of the National Center for Atmospheric Research. The alkyl nitrate precision is better than ±5% for all compounds. Carbon monoxide (CO) is also measured in the canister samples, using a packed column GC separation of CO followed by reduction to methane on a nickel catalyst and detection by FID.
Methyl nitrate, CH3I, and CHBr3 are separated analytically employing three of the five column-detector combinations. The first combination (abbreviated as ‘DB5ms/MSD’) is a DB-5ms column (J&W; 60 m, 0.25 mm I.D., 0.5 µm film thickness) output to a quadrupole mass spectrometer detector (MSD) (HP-5973). The second combination (Restek1701/ECD) is a RESTEK 1701 column (60 m, I.D. 0.25 mm, film 0.5 mm), which is output to an electron capture detector (ECD). A third combination (DB5-Restek1701/ECD) is a DB-5 (J&W 30 m, I.D. 0.25 mm, film 1 mm) column connected in series to a RESTEK 1701 column (5 m, I.D. 0.25 mm, film 0.5 mm), and output to an ECD. The DB5ms/MSD, Restek1701/ECD, DB5-Restek1701/ECD, and Plot/FID combinations receive 10.1%, 7.2%, 6.8%, and 47% of the sample flow, respectively. Dimethyl sulfide is also analyzed on the DB5ms/MSD. For DMS, the precision is approximately 3% at mixing ratios >25 pptv, and 1 pptv or 15%, whichever is greater, at mixing ratios <10 pptv. The typical alkyl nitrate detection limit is 0.02 pptv (0.01 pptv for methyl nitrate) and the precision is ±5% at mixing ratios above 5 pptv and ±10% below 5 pptv. The detection limits for CH3I, CHBr3, and DMS are 0.01, 0.01, 1.0 pptv, respectively. The measurement precision for CH3I and CHBr3 is 1.1 and 1.6% respectively. It is anticipated that several California Alliance for Minority Participation (CAMP) students will be involved in the analysis and possibly the measurement portion of this project.