Pollution in the Tropics: Multi-scale Variability in 30 Years of Ozone and Carbon Monoxide Observations across Mexico City

Surface aerosol concentrations in Mexico City have exceeded international health standards for over 30 years. As such, many targeted studies have examined the chemical composition, volatile reactions and secondary production, and source emissions of these airborne contaminants, finding that much of the observed contamination originates locally within the Mexican central basin. Other studies have explored the response of concentration levels to a variety of meteorological factors, including planetary boundary layer depth, precipitation, tropospheric ventilation, radiation, turbulent and mechanical mixing, and cloud cover. Despite these studies, however, the variability of two important pollutants, ozone and carbon monoxide, remains largely unknown. What is known is that the leading modes of atmospheric variability operating on the subseasonal to interannual time scales act to control many of the meteorological factors found to influence ozone and carbon monoxide. Thus, the primary hypothesis tested in this study was that ozone and carbon monoxide in Mexico City would vary on those time scales due to local influences of the larger-scale atmospheric modes.

Mexico City more closely resembles a tropical climate for about half of the year and a subtropical one for the other half of the year, making it an ideal site to study both local and teleconnected tropical variability. Hourly observations of ozone and carbon monoxide began in 1986 with the installation of automated monitoring stations in the System for Atmospheric Monitoring (SIMAT) network, and 30 years of data from five SIMAT sites were examined in this study. High spatial resolution reanalysis data, along with in-situ meteorological observations, also were examined, via compositing methods, to isolate the mean atmosphere. Particular emphasis was given to examining the different phases of the leading modes of subseasonal and interannual variability, the Madden-Julian Oscillation (MJO) and the El Niño-Southern Oscillation. A robust pattern of subseasonal ozone variability was found for the eight active phases of the MJO, with phases 1 and 8 associated with below-normal surface ozone concentrations and phases 4 and 5 associated with above-normal concentrations. Seasonality of these results, along with physical reasons behind the observed variability on the subseasonal and interannual time scales, are explored in more depth in the study.