Indoor and outdoor urban atmospheric CO2: Stable carbon isotope constraints on mixing and mass balance

From July to November 2009, concentrations of CO2 in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ13C values varied from −8.9‰ to −19.4‰. The CO2 from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ13C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO2 exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ13C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO2 gas. Collectively, the δ13C values of the indoor CO2 samples were linearly correlated with the reciprocal of CO2 concentration, exhibiting an intercept of −21.8‰, with r2 = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO2 data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ13C value for human-exhaled CO2 demonstrates simple mixing between two inputs: (1) outdoor CO2 introduced to the interior spaces by ventilation systems, and (2) CO2 exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO2 input from the local ventilation system and CO2 in the ambient air of the room) could be a reasonable estimate of the average δ13C value of the CO2 exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO2 that can be used to determine the average proportions of C3 and C4-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C4-derived C appears to have constituted ∼40% of the average diet. erage concentration of outdoor Dallas atmospheric CO2 was ∼17 ppm higher than the average of CO2 concentrations measured on the same campus 10 a ago. In addition, Dallas outdoor CO2 concentrations at both times were higher than the contemporaneous global atmospheric CO2 concentrations. This observation, plus the fact that the increase of ∼17 ppm in the average concentration of Dallas outdoor CO2 was comparable to the global increase of ∼18 ppm over the same 10-a interval, is consistent with a significant role for urban CO2 “factories” in the global atmospheric CO2 budget.