Seasonal snapshots of the isotopic (¹⁴C, ¹³C) composition of tropospheric carbon monoxide at Niwot Ridge, Colorado

Clean continental air, collected near Niwot Ridge, Colorado (3.75 km altitude), over a two-year period (1991-1992), has been investigated for the composition of atmospheric carbon monoxide. Its chemical and isotopic signatures, distributed through time and space, along with a detailed look at gridded meteorological parameters, summarized in 5-day back-trajectories, have shed some light on the distribution of CO in tropospheric air and what the likely source regions are that contribute to background CO mixing ratios. The temporal distribution of CO and CO mixing ratios exhibit a seasonal trend with a maximum during the winter/spring and a minimum during the late summer. CO mixing ratios peak at about 19 molecules cm of air (at STP) with a minimum between years of about 9 molecules cm. A comparable seasonal pattern was observed for CO mixing ratios with a maximum of 240 nmol mol and a minimum of 100 nmol mol. An observed trend of increasing CO with increasing CO mixing ratios suggests an input of recycled living carbon. The δC data show a winter/spring peak in 1992; however, too few data points exist for 1991 to indicate a clear seasonal cycle. δC values range from -26.3‰ to -30.5‰ over the two years of data. The seasonally averaged CO and δC data show autumn values of 116 ± 11 (std. dev.) nmol mol and -27.85 ± 0.87‰, respectively; winter values 159 ± 19 nmol mol and -28.0 ± 1.7‰; spring values 181 ± 45 nmol mol and -27.1 ± 1.6‰; and summer values 142 ± 37 nmol mol and -29.4 ± 1.2‰. Five-day back trajectories and on-site meteorology for each sampling day show that, with some exceptions, winds were on average from the west or northwest. Potential CO sources on these dates were in the northwestern US. CO sampled during these periods may reflect continental CO sources such as oxidized vegetative emissions and biomass burning. On the other hand, there were days when the winds were either from the south and southwest or from northern Canada. Thus, urban air from the Boulder/Denver metropolitan area to the east and southeast was probably not sampled to any significant extent.