The stability of the nocturnal boundary layer (NBL) leads to strong vertical variations of the atmospheric composition in urban areas. The emission of NO by traffic at the ground leads to a destruction of ozone and a formation of NO₂ in the lower NBL. Similarly the NO + NO₃ reaction leads to an increase of NO₃ and N₂O₅ levels with altitude. Heterogeneous reactions at the ground or buildings lead to a deposition loss of ozone and NO₂, while forming other nitrogen compounds such as HONO. The ground formation in combination with the slow vertical transport can lead to strong vertical concentration gradients of HONO in the lower NBL.

Here we present observations of the vertical distribution of O₃, NO₂, NO₃, and HONO in Mexico City during the MILAGRO experiment. A long-path Differential Optical Absorption Spectroscopy system was set up at the roof of a six story building, aiming at retroreflector arrays placed at a distance of 3.9 km at three altitudes (92 m, 156 m, and 202 m). The measurements show clear vertical profiles of all compounds at night. We will discuss these observations in the context of the chemistry and the budgets of NO_x and HONO in Mexico City. The impact of nocturnal chemistry on urban air quality will also be discussed.