Changes in Concentration and Isotopic Ratios of CO2 in Air in the Los Angeles Basin, California, Between 1972 and 2003

Air in the Los Angeles Basin (California) is known for its high levels of pollution, due in large part to burning of fossil fuels for industry and transportation. CO₂ is a major product of the combustion process. We compare concentrations and isotopic compositions of CO₂ in air samples collected in 1998-2003 with samples collected in 1972-1973 in almost the same location on the Caltech campus in Pasadena. We also compare our results to analyses of air from unpolluted areas reported by the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) from oceanic sites at the same latitude. CO₂ concentrations have increased by ∼50 ppm (averaging 355 ppm in 1972-1973 and 402 ppm in 1998-2003), but the ranges of the carbon and oxygen isotopic ratios have not changed significantly (δ ¹³C_VPDB was -7.9 to -13.5‰ in 1972-1973 and -8.0 to -14.1‰ in 1998-2003; δ ¹⁸O_VPDB-CO2 was -4.35 to -0.60‰ in 1972-1973 and -4.29 to 0.15‰ in 1998-2003). Each data set displays a significant correlation between δ ¹³C and 1/ CO₂ concentration. The intercepts at infinite CO₂ are very similar for both 1972-1973 (-31.7‰ ) and 1998-2003 (-30.6‰ ). Mass balance calculations show that this is consistent with the changing mix of natural gas and petroleum products burned in the region provided the countries of origin of the petroleum products are taken into account. The seasonally averaged 1998-2003 CO₂ inventory in Pasadena can be explained by a local contribution of 20-39 ppm (i.e., 5-11% relative) to clean air for this latitude (based on the NOAA data). This additional CO₂ is characterized by an average δ ¹³C of -30.3‰ and δ ¹⁸O -9.20‰ . Because of the mild climate in southern California, larger amounts of isotopically light fossil fuel are consumed for energy generation for air conditioning during the summer than during the winter for heating. This offsets the effects of vegetation due to photosynthesis/respiration and masks the periodic seasonal variation observed in clean air sites at the same latitude.