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 (CO_2ff), but it is expensive and time-consuming to measure. Here, we used common hourly Air Quality Index (AQI) pollutants (AQI, PM_2.5, PM₁₀, and CO) to indirectly trace diurnal CO_2ff variations during certain days at the urban sites in Beijing and Xiamen, China, based on linear relationships between AQI pollutants and CO_2ff traced by ¹⁴C (CO2ff−14C) for semimonthly samples obtained in 2014. We validated these indirectly traced CO_2ff (CO_2ff-in) concentrations against CO2ff−14C concentrations traced by simultaneous diurnal ¹⁴CO₂ observations. Significant (p < 0.05) strong correlations were observed between each of the separate AQI pollutants and CO2ff−14C for the semimonthly samples. Diurnal variations in CO_2ff traced by each of the AQI pollutants generally showed similar trends to those of CO2ff−14C, 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 CO2ff−14C 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 CO_2ff averages at the sampling sites in Beijing and Xiamen were traced using CO concentration as a tracer. The monthly CO_2ff averages at the Beijing site showed a shallow U-type variation. These results indicate that CO can be used to trace CO_2ff variations in Chinese cities with CO_2ff concentrations above 5 ppm.