Megacities are large sources of fine primary aerosols and trace gases that can lead to the formation of secondary aerosols. These primary and secondary aerosols are known to be important in determining the radiative balance of urban, regional, and global scale atmospheres, but the magnitude and even the sign of the effects depend on their chemical composition and size as well as their spatial distributions in the atmosphere. . Mexico City is one of the largest megacities in the world and is known for its high levels of air pollution (ozone and aerosols). In April of 2003, as part of the Mexico City Metropolitan Area (MCMA) 2003 air quality study we obtained measurements of ammonia at the CENICA facility in Iztapalaca. Ammonia is an important reactive gas that can act to increase secondary aerosols when it reacts with nitric acid vapor (formed from photochemical oxidation of nitrogen dioxide) to produce ammonium nitrate aerosol. Ammonia was measured using a near-IR tunable diode laser system using a telescope and retroreflector system using an open-path of 244 m. Fifteen minute averages of the data clearly showed significant levels of ammonia in the Mexico City air and evidence of reaction with nitric acid during high photochemical situations. The data will be presented and discussed in light of potential sources of ammonia in megacities.

This work was supported by the DOE Atmospheric Science Program.