Changes in CO2 Composition in the Air of the Los Angeles Megacity Coinciding with the Prolonged Drought

California has been in a severe drought for five years affecting weather, vegetation, and policy. Unfortunately, the last year's strong El Nino conditions did not bring the expected extra rainfall relief to southern California, and we are just finishing a year of water rationing, concentrating on limiting irrigation of outdoor landscape. What changes in the atmospheric CO2 composition in the Los Angeles, CA (LA) megacity have occurred during this period? We investigate changes reflected in the radiocarbon (Δ14C) and stable carbon isotopic composition (δ13C) of the CO2 in LA. We use the long-term records at our Pasadena (18 years, 11 with Δ14C) and Palos Verdes (7 years) sites to put the last five years in context. Pasadena is inland and a good receptor site for emissions in the LA basin for much of the year when the prevailing winds are from the ocean passing over LA. Palos Verdes is on the coast and usually receives winds from the ocean except during Santa Ana conditions, when dry, warm winds come from the north to northeast from the desert over the mountains and across the basin. Radiocarbon measurements give information on the fossil fuel and biosphere components of the local emissions. The amplitude of the observed seasonal cycle of CO2 derived from fossil fuel emissions (CO2ff) has decreased in Pasadena, while the amplitude of the biosphere's seasonal cycle has increased since the beginning of the drought, at both the Pasadena and Palos Verdes sites. There is evidence that the δ13C signature of the biosphere in Pasadena may have changed in the last year or so, which is consistent with an increase in the proportion of C4 vegetation surviving the drought and being planted to replace less drought-resistant C3 plants.