Atmospheric fossil fuel CO2 traced by 14CO2 and air quality index pollutant observations in Beijing and Xiamen, China

Radiocarbon (¹⁴C) is the most accurate tracer available for quantifying atmospheric CO₂ derived from fossil fuel (CO₂ff), but it is expensive and time-consuming to measure. Here, we used common hourly Air Quality Index (AQI) pollutants (AQI, PM₂.₅, PM₁₀, and CO) to indirectly trace diurnal CO₂ff variations during certain days at the urban sites in Beijing and Xiamen, China, based on linear relationships between AQI pollutants and CO₂ff traced by ¹⁴C ([Formula: see text]) for semimonthly samples obtained in 2014. We validated these indirectly traced CO₂ff (CO₂ff₋ᵢₙ) concentrations against [Formula: see text] concentrations traced by simultaneous diurnal ¹⁴CO₂ observations. Significant (p < 0.05) strong correlations were observed between each of the separate AQI pollutants and [Formula: see text] for the semimonthly samples. Diurnal variations in CO₂ff traced by each of the AQI pollutants generally showed similar trends to those of [Formula: see text], with high agreement at the sampling site in Beijing and relatively poor agreement at the sampling site in Xiamen. AQI pollutant tracers showed high normalized root-mean-square (NRMS) errors for the summer diurnal samples due to low [Formula: see text] concentrations. After the removal of these summer samples, the NRMS errors for AQI pollutant tracers were in the range of 31.6–64.2%. CO generally showed a high agreement and low NRMS errors among these indirect tracers. Based on these linear relationships, monthly CO₂ff averages at the sampling sites in Beijing and Xiamen were traced using CO concentration as a tracer. The monthly CO₂ff averages at the Beijing site showed a shallow U-type variation. These results indicate that CO can be used to trace CO₂ff variations in Chinese cities with CO₂ff concentrations above 5 ppm.