Changes in mixing ratio and isotopic composition of CO2 in urban air from the Los Angeles basin, California, between 1972 and 2003

Atmospheric CO2 mixing ratios and C and O isotopic compositions are reported for the Los Angeles basin in southern California, a region renowned for its air pollution. Air samples collected midday on the Caltech campus in Pasadena, California, contained ∼30 ppm more CO2 in 1998–2003 than in 1972–1973 (averaging 397 ppm in 1998–2003 and 366 ppm in 1972–1973) compared to a 47 ppm change in background air CO2, yet the ranges of the carbon and oxygen isotopic compositions remained essentially constant. Because the 1998–2003 data show a significant progression through time, analysis was done on data from 2002 to 2003 complete calendar years (CO2 mixing ratios increased 41 ppm between 1972 and 1973 and 2002–2003). Both 1972–1973 and 2002–2003 data sets display significant correlation between δ 13C and 1/[CO2] with local CO2 source end-member δ 13C values of −30.9 ± 0.5‰ for 1972–1973 and −29.9 ± 0.2‰ for 2002–2003 (1σ errors). Mass balance calculations explain that this apparently coincidental similarity reflects a change in the relative proportion of natural gas and petroleum products burned in the region combined with a change in the origin, and thus isotopic composition, of the petroleum burned. The δ 13C of the average CO2 inventory in Pasadena can be explained by local addition to background air of 38 ± 4 ppm CO2 in 1972–1973 and 29 ± 3 ppm in 2002–2003 from anthropogenic sources, in seeming contradiction to the known increase in CO2 emissions between these two time periods.