Ground-Based Atmospheric Measurements of CO:CO2 Ratios in Eastern Highland Rim Using a CO Tracer Technique

We present ground-based atmospheric measurements of ratios between anthropogenic carbon monoxide (COan) and carbon dioxide (CO2an) (beta(-1) ratios) in Cookeville, Tennessee (36.1628 degrees N, 85.5016 degrees W), a medium-sized city located within the Eastern Highland Rim region of the United States, obtained using a continuous wavelength-scanned cavity ring-down spectrometer (CRDS). In contrast to the summer season, the winter and spring beta(-1) values are reasonably high, where a tight correlation between the above-background mole fractions of CO and CO2 is exhibited (as given by the high R-2 value). The winter season is characterized by relatively high COan than biospheric CO2 signals (CO2Bio) due to fossil fuel heating sources and reduced biospheric uptake of CO2. The lowest estimated seasonal background CO2 and CO mole fractions during the study period are 407.1 +/- 11.3 ppm and 144.9 +/- 2.1 ppb, respectively. During the summer, the biogenic CO source from isoprene oxidation collocates with direct anthropogenic CO sources, leading to uncertainties in the calculated summertime beta(-1) values. For 2017, beta(-1) values (ppb:ppm) of 9.7 +/- 0.4, 5.3 +/- 0.4, and 2.0 +/- 0.2 were obtained for the winter, spring and summer seasons, respectively. In 2018, a similar seasonal variability in the beta(-1) ratios was obtained with values of 8.7 +/- 0.5, 7.4 +/- 0.7, and 2.6 +/- 0.5 for winter, spring, and summer seasons, respectively. While the estimated percentage contribution of the oxidation reaction between the OH radical and isoprene (CH2=C(CH3)-CH=CH2 + OH) to the total summertime CO budget is less than 50%, the relative amounts may be significant enough to imply that any future study based on the CO as a tracer of combustion emission should account for its photochemical production through biogenic volatile organic compounds in the summertime.