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 (CO_an) and carbon dioxide (CO_2an) (β^–1 ratios) in Cookeville, Tennessee (36.1628° N, 85.5016° 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 β^–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 R² value). The winter season is characterized by relatively high CO_an than biospheric CO₂ signals (CO_2Bio) 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 β^–1 values. For 2017, β^–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 β^–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.