Tuesday, 16 January 2007: 8:30 AM
Vertical variation of nocturnal nitrogen chemistry in Mexico City during MILAGRO
212A (Henry B. Gonzalez Convention Center)
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
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 NOx and HONO in Mexico City. The impact of nocturnal chemistry on urban air quality will also be discussed.