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 δ¹³C and 1/[CO₂] With local CO₂ source end-member δ¹³C 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 δ¹³C 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.