Aerosol climatology over the Mexico City basin: Characterization of optical properties

Abstract Climatology of Aerosol Optical Depth (AOD), Single Scattering Albedo (SSA), and aerosol particle-size distribution were analyzed using a 15-year (1999–2014) dataset from AErosol RObotic NETwork (AERONET) observations over the Mexico City (MC) basin. The atmosphere over this site is dominated by two main aerosol types, represented by urban/industrial pollution and biomass-burning particles. Due to the specific meteorological conditions within the basin, seasons are usually classified into three as follows: Dry Winter (DW) (November–February); Dry Spring (DS) (March–April), and the RAiny season (RA) (May–October), which are mentioned throughout this article. Using a CIMEL sun photometer, we conducted continuous observations over the MC urban area from January 1999 to December 2014. Aerosol Optical Depth (AOD), Angstrom exponent (α 440–870 ), Single Scattering Albedo (SSA), and aerosol particle-size distribution were derived from the observational data. The overall mean AOD 500 during the 1999–2014 period was 0.34 ± 0.07. The monthly mean AOD reached a maximal value of 0.49 in May and a minimal value of 0.27 in February and March. The average α 440–870 value for the period studied was 1.50 ± 0.16. The monthly average of α 440–870 reached a minimal value of 1.32 in August and a maximal value of 1.61 in May. Average SSA at 440 nm was 0.89 throughout the observation period, indicating that aerosols over Mexico City are composed mainly of absorptive particles. Concentrations of fine- and coarse-mode aerosols over MC were highest in DS season compared with other seasons, especially for particles with radii measuring between 0.1 and 0.2 μm. Results from the Spectral De-convolution Algorithm (SDA) show that fine-mode aerosols dominated AOD variability in MC. In the final part of this article, we present a classification of aerosols in MC by using the graphical method proposed by Gobbi et al. (2007), which is based on the combined analysis of α and its spectral curvature δα.