Abstract
Abstract. The evolution of ozone (O3) and 13 volatile organic compounds (VOCs) in the boundary layer of Mexico City was investigated during 2000–2004 to improve our understanding of the complex interactions between those trace gases and meteorological variables, and their influence on the air quality of a polluted megacity. A tethered balloon, fitted with electrochemical and meteorological sondes, was used to obtain detailed vertical profiles of O3 and meteorological parameters up to 1000 m above ground during part of the diurnal cycle (02:00–18:00 h). VOCs samples were collected up to 200 m by pumping air to canisters with a Teflon tube attached to the tether line. Overall, features of these profiles were found to be consistent with the formation of an upper residual layer during nighttime carrying over a fraction of the O3 from the previous day that contributes to the background concentration in surrounding regions. At the same time the release of heat stored in the urban surface forms a shallow unstable layer close to the ground, where the nocturnal emissions are trapped. After sunrise an O3 balance is determined by photochemical production, entrainment from the upper residual layer and destruction by titration with nitric oxide, delaying the ground-level O3 rise by 2 h. The subsequent evolution of the conductive boundary layer and vertical distribution of pollutants are discussed in terms of the energy balance, the presence of turbulence and the atmospheric stability.
Highlights
Air overlying a region has layers of diverse history and diverse composition and chemistry, as well as complex vertical distributions of pollutants
This section examines the vertical distribution of O3 and volatile organic compounds (VOCs) in the context of the boundary layer evolution to understand their temporal and vertical variation throughout the course of the day
All profiles were examined by individual days; only a few measurement days covered the complete diurnal cycle, not all the profiles reached 1000 m as explained above
Summary
Air overlying a region has layers of diverse history and diverse composition and chemistry, as well as complex vertical distributions of pollutants. These vertical distributions are result of the horizontal and vertical transport due to the effects of the winds and turbulence produced by thermal fluxes and surface friction. Vertical profiles of pollutants are estimated by mathematical models. The drawback of these models is that they require ideal conditions; otherwise their predictions are more uncertain. In an urban area there are many buildings, which cause large inhomogeneities in the energy and wind profiles, and the vertical distributions of pollutants are difficult to model. The characterization of nighttime accumulation of pollutants in the lowest layers and their subsequent interactions in the following morning as the nocturnal boundary layer begins to break up are necessary to evaluate the effectiveness of pollution control measures, such as limiting stationary emissions and extending vehicular traffic restrictions at night, as well as instituting a later workday to shift the morning rush hour to later on after sunrise
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