Abstract
Abstract. 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, 60 km from Mexico City were analyzed with respect to the origin of the air masses. In the current evaluation, the temperature, water vapor mixing ratio (WMR), ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2) and acyl peroxy nitrate (APN) are simulated with the WRF-Chem chemical transport model and compared with the measurements at the mountain site. Comparisons between the model and measurements are also evaluated for particle size distributions (PSDs) of the mass concentrations of sulfate, nitrate, ammonium and organic mass (OM). The model predictions of the diurnal trends in temperature, WMR and trace gases were generally well correlated; 13 of the 18 correlations were significant at a confidence level of <0.01. Less satisfactory were the average hourly differences between model and measurements that showed predicted values within expected, natural variation for only 10 of the 18 comparisons. The model performed best when comparing with the measurements during periods when the air originated from the east. In that case all six of the parameters being compared had average differences between the model and measurements less than the expected standard deviation. For the cases when the air masses are from the southwest or west northwest, only two of the comparisons from each case showed differences less than the expected standard deviation. The differences appear to be a result of an overly rapid growth of the boundary layer predicted by the model and too much dilution. There also is more O3 being produced, most likely by photochemical production, downwind of the emission sources than is predicted by the model. The measured and modeled PSD compare very well with respect to their general shape and the diameter of the peak concentrations. The spectra are log normally distributed with most of the mass in the accumulation mode centered at 200 ± 20 nm and little observed or predicted changes with respect to the time when the RML is above the Altzomoni research station. Only the total mass changes 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 coagulation after six hours of aging downwind of the major sources of anthropogenic emissions in Mexico's Megapolis. This could greatly simplify parameterization in climate models although it is not known at this time if this invariance can be extended to other megacity regions.
Highlights
The role of aerosol particles in modulating climate and their link to various health related issues has been clearly documented with numerous observational and theoretical studies (Molina et al, 2010 and references therein)
The fidelity of the model with respect to its accuracy in predicting the temporal properties of the atmospheric constituents is evaluated in two ways: (1) by comparing the relative trends in the measured, diurnal averages made at the Altzomoni research site with averages from the simulation, both in hourly intervals and (2) by comparing the average difference between the measured and modeled parameters over lines) are shown here for time periods when air originates from (a) the west northwest (Mexico City), (b) southwest (Cuautla) and (c) east (Texmelucan)
The primary focus of the evaluation is on the time evolution of the size distributed mass concentrations measured with the aerosol mass spectrometer (AMS) compared to the log normal particle size distributions (PSDs) produced by the model
Summary
The role of aerosol particles in modulating climate and their link to various health related issues has been clearly documented with numerous observational and theoretical studies (Molina et al, 2010 and references therein). Megacities are a major source of these particles and in particular Mexico City. Ochoa et al.: Physical and chemical properties of the regional mixed layer of Mexico’s Megapolis Part II has been the focus of a number of major field programs since the early 1990’s that studied the sources and evolution of these particles, as well as other anthropogenic emissions. The third experiment was the Mexico City Metropolitan Area field experiment (MCMA-2003), an intensive 5-week campaign that took place in the spring of 2003 (31 March–4 May), with the goal of investigating the atmospheric chemistry of the MCMA, with particular focus on secondary PM formation (Molina et al, 2007). The Megacity Initiative: Local And Global Research Observations (MILAGRO) project was carried out in March 2006 (Molina et al, 2010) during which time continuous observations of gas and particle concentrations were taken at various locations across Mexico City and its surroundings
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