PNNL

Abstract

An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite during the Mexico City Metropolitan Area field study from March 29-May 4, 2003. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR PM1) with high time and size resolution. Measurements of Black Carbon (BC) using an aethalometer, and estimated soil concentrations from Proton-Induced X-Ray Emission (PIXE) analysis of impactor substrates are also presented and combined with the AMS in order to include refractory material and estimate the total PM2·5 mass concentration at CENICA during this campaign. In Mexico City, the organic fraction of the estimated PM2·5 at CENICA represents 55% of the mass, with the rest consisting of inorganic (mainly ammonium nitrate and sulfate/ammonium salts) compounds, BC, and soil. Inorganic compounds represents 27.5% of PM2·5; BC mass concentration is equivalent to about 11%; while soil represents about 7%. The NR species and BC have diurnal cycles that can be qualitatively interpreted as the interplay of direct emissions, photochemical production in the atmosphere followed by condensation and gas-to-particle partitioning, boundary layer dynamics, and/or advection. Bi and trimodal size distributions are observed, with a small primary organic combustion (likely traffic) particle mode and an accumulation mode that contains mainly organic and secondary inorganic compounds. The AMS and BC mass concentrations, size distributions, and diurnal cycles are found to be qualitatively similar to those from other field measurements in Mexico City.