Meteorological factors associated with inhomogeneous ozone concentrations within the Mexico City basin

Meteorological processes associated with inhomogeneous ozone concentrations over Mexico City are examined by using observations from a recent field campaign and a mesoscale dynamics and dispersion modeling system. During this 4-week field campaign, meteorological measurements of the spatial flow structure within the Mexico City basin were obtained for the first time. A mesoscale model that employs four-dimensional data assimilation is used to create analyses that describe the boundary layer characteristics and local and regional circulations in the vicinity of Mexico City. The mesoscale analyses are then used to drive a Lagrangian particle dispersion model to simulate pollutant transport and diffusion. The resemblance between the calculated particle concentration fields and the observed spatial ozone patterns indicates that the mesoscale analyses, based on the model and the observed profiles of wind, temperature, and humidity, captured the main flow features responsible for the inhomogeneous ozone concentrations within the basin. The highest particle concentrations usually occurred in the vicinity of the peak ozone concentrations during the afternoon. The observations and mesoscale analyses provided evidence that the circulations are highly complex, and relatively weak upper level synoptic systems had an impact on the local and regional thermally driven flows in the area. Ln addition to horizontal advection and vertical diffusion, vertical wind shears, recirculation patterns associated with venting and entrainment processes, and mean vertical motions due to convergence within the basin also played an important role in producing the spatial variations in the near-surface particle distributions. The contribution of emissions from the previous day was found to be relatively minor for the periods examined in this study, even though the nocturnal wind speeds were light, suggesting that the high ozone concentrations are not due to multiday accumulation of pollutants.