Relating Ground-Based Aerosol Size Distributions and Vertical Mixing: Mexico City and Other Case Studies

In March 2006, during the MILAGRO field campaign in Mexico City, observations at the T0 location could often be characterized by morning conditions with high particle mass concentrations, low mixing heights, and good correlation between particle number and carbon dioxide, indicative that particle number is controlled by primary emissions. Using this correlation between particle number and carbon dioxide, an average number based-emission factor of 3.4 × 1013 #/vehicle-km has been determined. In the afternoon, the CO2 level drops during ventilation of the daily polluted layer, and the coupling between CO2 and particle number breaks down, with particle number sometimes increasing as CO2 decreases. New particle formation events were observed both simultaneous to and separately from the afternoon ventilation. While the distinction between morning conditions dominated by primary emissions and afternoon conditions dominated by mixing and photochemical particle production is particularly strong in Mexico City because of local meteorology and intense sources, we show that this pattern is common to several locations. Furthermore, the observed particle size distribution after ventilation of the nighttime inversion is a weighted average of nocturnal capping inversion and the morning boundary layer. Finally, using a vertically resolved new particle formation model, we test the hypothesis that growth rates (high in Mexico City at up to 15nm per hour) can be influenced by boundary layer growth rates in addition to condensable species concentrations, under certain assumptions about the vertical profile of new particle formation.