Optical properties of atmospheric particles over an urban site in Mexico City and a peri-urban site in Queretaro

Optical properties of atmospheric particles at Mexico City (UNAM) and Queretaro (JQRO) were measured with a Photoacoustic Extinctiometer (PAX) at 870?nm. The Mexico City Metropolitan Area has around 21 million inhabitants and Queretaro Metropolitan Area has little more than a million. Observations of meteorological parameters (relative humidity, solar radiation, and wind speed) were used to identify the rainy and dry seasons and explain the daily and seasonal behaviors of particles optical properties. The measurements were made from November 1, 2014 to July 31, 2016. At UNAM, the mean values of the scattering coefficient (B_scat) in cold dry, warm dry, and rainy seasons were 35.8, 27.1, and 31.3?Mm^?1, respectively; while at JQRO were 10.9, 11.9, and 15.0?Mm^?1. The average values of the absorption coefficient (B_abs) at UNAM during the cold dry, warm dry, and rainy seasons were 14.5, 12.7, and 12.7?Mm^?1, respectively; whereas at JQRO were 4.9, 4.7, and 3.9?Mm^?1. Both absorption and scattering coefficients showed similar diurnal behaviors, but at UNAM they are three times higher than JQRO. Concentrations of criteria gases (O₃, NO, NO₂ and NO_x) were also measured. At UNAM no difference was observed between the seasonal values for the single scattering albedo (SSA); while in JQRO, the rainy season had the highest seasonal value, being 13% higher than in the dry seasons. The Mass Scattering Cross-Section (MSC) values at UNAM were close to 2 m²/g; on the other hand, at JQRO the MSC values were lower than 1 m²/g. The results suggest a seasonal variability in the aerosol optical properties in both sites, which should be verified with more long-term studies.