Department of Chemistry, University of California, Irvine, CA 92697*
To whom correspondence should be addressed. E-mail: rowland@uci.edu.
Contributed by F. Sherwood Rowland, August 15, 2003 Other Sections▼
Abstract
Methods
Results
Discussion
Conclusion
References
AbstractLight alkane hydrocarbons are present in major quantities in
the near-surface atmosphere of Texas, Oklahoma, and Kansas during both
autumn and spring seasons. In spring 2002, maximum mixing ratios of
ethane [34 parts per 109 by volume (ppbv)], propane (20 ppbv), and n-
butane (13 ppbv) were observed in north-central Texas. The elevated
alkane mixing ratios are attributed to emissions from the oil and
natural gas industry. Measured alkyl nitrate mixing ratios were
comparable to urban smog values, indicating active photochemistry in
the presence of nitrogen oxides, and therefore with abundant formation
of tropospheric ozone. We estimate that 4–6 teragrams of methane are
released annually within the region and represents a significant
fraction of the estimated total U.S. emissions. This result suggests
that total U.S. natural gas emissions may have been underestimated.
Annual ethane emissions from the study region are estimated to be 0.3–
0.5 teragrams. Other Sections▼
Abstract
Methods
Results
Discussion
Conclusion
References
 We have performed two regional studies in different seasons of
hydrocarbon and halocarbon mixing ratios in surface-level air sampled
within the southwestern United States. Elevated atmospheric mixing
ratios of C1-C4 alkanes and C2-C4 alkyl nitrates (RONO2) were measured
over much of the region during both studies. The alkyl nitrate
enhancements show that significant photochemistry analogous to urban
smog formation is occurring within the source region. The release of
hydrocarbons into the atmosphere contributes to photochemical ozone
(O3) production, with related adverse health effects, reduction in
plant growth, and climate change (1–3). The production, storage, and
transport of oil and natural gas are a major global source of
hydrocarbons into the atmosphere (4), and the southwestern states have
some of the largest oil and natural gas reserves in the United States.
Although the U.S. natural gas industry has been estimated to account
for ≈20% of the total U.S. anthropogenic methane (CH4) emissions (5),
the global budgets of light (C2-C4) alkanes, including their emissions
from the oil and natural gas industry, are more poorly assessed.The C2-
C4 alkanes have globally averaged lifetimes ranging from ≈2 months for
ethane to several days for the butanes (6). Because of their short
lifetimes, the atmospheric concentrations of light alkanes are
variable and depend on the number and strength of nearby emission
sources. By contrast, CH4 is by far the most abundant hydrocarbon in
the atmosphere, in part because of its 8-year atmospheric lifetime
(7), which allows it to be widely distributed throughout both the
northern and southern hemispheres. The greater reactivity of C2-C4
alkanes relative to CH4 ensures that a much larger fraction of the
former will react in the area where the emissions occur, making the
combined C2-C4 alkane contributions more important for local and
regional O3 formation than the influence of the incremental local
increases in CH4.In the troposphere, photochemical O3 production
begins with the attack of parent hydrocarbons (RH) by tropospheric
hydroxyl radicals (HO), and proceeds through the following key
reactions (2):  [1]
 [2]  [3a] [3b]  [4] [5]
where R¡ is an alkyl radical, and  and RO¡ are its alkylperoxy and
alkoxy counterparts, respectively. Hydroxyl radicals have a strong
seasonal concentration dependence, peaking in the summer in temperate
and polar regions (8). Therefore, if emissions are independent of
season, the shorter-lived alkanes should show maximum local mixing
ratios during late winter in each hemisphere and a minimum during
summer when there are higher HO concentrations and longer days.
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