An improved estimate for the δ 13 C and δ 18 O signatures of carbon monoxide produced from atmospheric oxidation of volatile organic compounds
2019
Abstract. Atmospheric carbon monoxide (CO) is a key player in global atmospheric
chemistry and a regulated pollutant in urban areas. Oxidation of volatile
organic compounds (VOCs) is an important component of the global CO budget
and has also been hypothesized to contribute substantially to the summertime
urban CO budget. In principle, stable isotopic analysis of CO could
constrain the magnitude of this source. However, the isotopic signature of
VOC-produced CO has not been well quantified, especially for the oxygen
isotopes. We performed measurements of CO stable isotopes on air samples
from two sites around Indianapolis, US, over three summers to investigate
the isotopic signature of VOC-produced CO. One of the sites is located
upwind of the city, allowing us to quantitatively remove the background air
signal and isolate the urban CO enhancements. as well as the isotopic
signature of these enhancements. In addition, we use measurements of Δ 14 CO 2 in combination with the CO:CO 2 emission ratio from fossil fuels to constrain the fossil-fuel-derived CO and thereby isolate the
VOC-derived component of the CO enhancement. Combining these measurements
and analyses, we are able to determine the carbon and oxygen isotopic
signatures of CO derived from VOC oxidation as - 32.8 ‰ ± 0.5 ‰ and 3.6 ‰±1.2 ‰ , respectively. Additionally, we analyzed CO stable
isotopes for 1 year at Beech Island, South Carolina, US, a site thought
to have large VOC-derived contributions to the summertime CO budget. The
Beech Island results are consistent with isotopic signatures of VOC-derived
CO determined from the Indianapolis data. This study represents the first
direct determination of the isotopic signatures of VOC-derived CO and will
allow for improved use of isotopes in constraining the global and regional
CO budgets.
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