**Abstract**

Stable nitrogen and carbon isotopes were used to trace the interaction between atmospheric particles < 2.5 mm in diameter (PM2.5) with atmospheric physical variables and atmospheric chemical species in an urban environment. PM2.5 were collected daily at two sites in Mexico City during three dry seasons during two-week periods. PM2.5 varied between 10 and 70 μg/m3 , with the highest concentrations occurring during low-speed southerly winds and the lowest during high-speed easterly winds. Stable carbon isotope composition (δ 13C) showed that the main carbon source of PM2.5 included emissions from fossil fuel combustion, along with low-molecular-weight carbon emissions and suspended dust. Stable nitrogen isotope values (δ15N) in PM varied between −9.9 and 21.6‰. The most 15N-enriched particles generally occurred during low wind speeds and correlated significantly with hourly averaged ambient NOx and NO2 concentrations. Simultaneous samples from MER (commercial site) and XAL (industrial site) showed that PM2.5 mass concentration was generally lighter and the carbon and nitrogen isotopic compositions were heavier at the commercial site relative to the industrial site. The δ15N of PM2.5 increased with the %N in PM2.5 concordant with an isotopic fractionation during gas-to-particle condensation. Results suggest that wind speed, along with the nitrogen emission source, determines the nitrogen isotopic composition of PM2.5.

**Keywords:** PM2.5, δ 13C, δ 15N, Mexico City, ambient air
