**4. Conclusions**

Potential N and C sources of PM2.5 in an industrial sector and a commercial/residential zone of Mexico City were evaluated during three dry season periods (March and November) using stable nitrogen and carbon isotopes. Daily PM2.5 concentrations ranged between 10 and 70 μg/m3 and were inversely correlated to wind speed. The percentage of N in PM2.5 also increased at low wind speeds and suggests gas-to-particle conversion of nitrogen compounds. In contrast, the % C increased with high wind speeds and low PM2.5 concentrations reflecting direct carbon emissions. Based on δ 13C values, the principal carbon sources of PM2.5 are fossil fuel emissions, although geological material is an important component when easterly winds resuspend dust from the nearby Texcoco dry lakebed. δ 15N-PM2.5 values ranged between **−**9.9 and 21.6 ‰, similar to values from other megacities. δ 15N values were consistently lower at the industrial site and suggest isotopic fractionation of NOx emissions during particle accretion. The δ 15N-PM2.5 values presented here provide a nitrogen isotope baseline for Mexico City airborne fine particles as no previous data have been collected for this megacity.
