Trace gas mixing ratios in the United States: Urban and regional studies

Mixing ratios of a suite of trace gases were determined for more than 1500 whole air canister samples collected in twenty U.S. cities during the period from August 1999 to May 2003. Mixing ratios of methane, carbon monoxide, and twenty-six non-methane hydrocarbons (NMHCs) and alkyl nitrates are reported here. Levels of halogen compounds are also discussed. Source samples collected directly from or nearby potential emission sources were also collected in order to determine which gases were given off by various emission sources. This represents one of the largest urban trace gas sampling campaigns in the world.; Several regional trends were seen in different areas of the U.S. These include enhanced levels of methane and halogen compounds in northeastern cities, high light alkane levels in the southwestern U.S., high biogenic emissions in the southeast, and a dominance of combustion byproducts in many western cities. These differences illustrate the importance of various emissions sources throughout the country. Seasonal trends, daily trends, hourly trends, sampling location within the city, and meteorological conditions were all examined and found to play important roles in observed hydrocarbon mixing ratios. Our urban hydrocarbon results show generally lower mixing ratios than those collected during previous U.S. city studies carried out from the 1970s to 1990s, which could be the consequences of either stricter emission controls or the differences in sampling protocol.; Results from two separate regional studies in the southwestern U.S. are also presented and discussed. During these studies, over 350 mainly rural samples were collected to determine the extent and source of enhanced regional light alkanes. These gases were found to be present in high concentrations throughout a wide swath of the southwestern U.S. The elevated light alkane mixing ratios are attributed to emissions from the oil and natural gas industries. Annual methane and ethane emissions from the region are estimated to be 4--6 and 0.3--0.5 teragrams respectively. Overall methane emissions in the southwest and the relative importance of oil and natural gas emissions found in our studies are significantly higher than current EPA estimates.