An improved estimate for the <i>δ</i><sup>13</sup>C and <i>δ</i><sup>18</sup>O signatures of carbon monoxide produced from atmospheric oxidation of volatile organic compounds

Abstract

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 Δ14CO2 in combination with the CO:CO2 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.

Wildfire smoke in the lower stratosphere identified by in situ CO observations

“…h (Vimont et al, 2019). Table based on the most recent compilation of source signatures by Vimont et al (2019).…”

“…The total amount of NMHCs in the stratosphere is on the order of several ppt (Scheeren et al, 2003). Furthermore, estimates of the NMHCs source signature suggests that the oxygen signature is in the range 0-3.6 ‰ (Brenninkmeijer and Röckmann, 1997;Vimont et al, 2019). The NMHCs produced in the fire have a mean enhancement ratio to CO of the order of ppt ppm −1 (Mauzerall et al, 1998), and thus result in a very small in situ source of CO that has a very small effect on the isotopic composition.…”

Wildfire smoke in the lower stratosphere identified by in situ CO observations

“…These trace gases and aerosols affect the radiative transfer properties of the atmosphere and lead to the formation of tropospheric ozone. Not only the troposphere is affected, but the smoke also occasionally reaches the lower stratosphere (Waibel et al, 1999;Fromm et al, 2000;Fromm and Servranckx, 2003;Jost et al, 2004;Fromm et al, 2010), enhancing aerosol levels and ozone (Fromm et al, 2005), with potential global effects (Peterson et al, 2018).…”

“…Nevertheless, wildfire smoke has been observed from in situ aircraft measurements as well. First, Waibel et al (1999) reported a CO plume in the Northern Hemisphere (NH) extratropical lowermost stratosphere at 10 km altitude. The plume was associated with the extensive 1994 burning season.…”

“…In situ observations of wildfire smoke are typically identified by an increase in mole fractions of CO (Waibel et al, 1999;Jost et al, 2004;Cammas et al, 2009). In addition to CO, Cammas et al (2009) measured O 3 , NO x , and PAN.…”