Physical and chemical properties of the regional mixed layer of Mexico's Megapolis Part II: Evaluation of measured and modeled trace gases and particle size distributions
This study extends the work of Baumgardner et al. (2009) in which measurements of trace gases and particles, at a remote, high altitude mountain site, 60km from Mexico City were analyzed with respect to the origin of the air masses. In the current evaluation, the temperature, water vapor, ozone (O3), carbon monoxide (CO), acyl peroxy nitrate (APN) and particle size distributions (PSDs) of the mass concentrations of sul- fate, nitrate, ammonium and organic mass (OM) were simulated with the WRF-Chem chemical transport model and compared with the measurements at the mountain site. The model predictions of the diurnal trends of the gases were well correlated with the measurements before the regional mixed layer (RML) reached the measurement site but underestimated the concentration after that time. The differences are caused by an over rapid growth of the boundary layer by the model and too much dilution. There also is more O3 being actually produced by photochemical production downwind of the emission sources than predicted by the model. The measured and modeled PSDs compare very well with respect to their gen- eral shape and diameter of the peak concentrations. The spectra are lognormal with most of the mass in the accumulation mode and the geometric diameter centered at 200±20 nm, showing little observed or predicted change with respect to the time when the RML is above the Altzomoni research station. Only the total mass changed with time and air mass origin. The invariability of average diameter of the accumulation mode suggests that there is very little growth of the particles by condensation or coag- ulation past about six hours of aging downwind of the major sources of anthropogenic emissions in Mexico’s Megapolis. This could greatly simplify parameterization in cli- mate models although it is not known at this time if this invariance can be extended to other megacity regions.