On particle-bound polycyclic aromatic hydrocarbons (PPAH) and links to gaseous emissions in Mexico city

Abstract

Mexico City is a densely populated megacity with average PM10 and PM2.5 levels frequently above those indicated as health-threatening for humans by the World Health Organization. Polycyclic aromatic hydrocarbons (PAH) can be emitted by anthropogenic sources as the result of the incomplete combustion of organic matter and fossil fuels and they represent an important risk for human health under poor air quality conditions. High concentrations of PAHs have been linked with cancer and other respiratory diseases. Particle-bound PAHs (PPAHs) have been previously studied in Mexico City; however, their temporal scales are not fully understood given the short sampling periods (no more than five weeks) during which time they were evaluated.Recent automated and continuous measurements of PPAH concentration made in Mexico City during one full year (April 2016 to March 2017) show an average PPAH concentration of 31.7 ± 29.9 ng m−3 at the sampling location. The highest concentrations were observed in the winter corresponding to the cold dry season. On a daily basis, the maximum concentrations were observed between 06:00 and 11:00 am, as high as 495 ng m−3. The average PPAHs levels measured in the spring of 2016 were found to be ca. 150% higher than those that were measured 13 years previously in the spring of 2003, in approximately the same location and with the same equipment. A conceptual model is introduced to explain the primary processes behind the daily evolution of PPAHs that are produced from primary emissions but whose concentrations decrease due to evaporation and dilution with rising temperatures.Given the lack of instrumentation to measure PPAH at most air quality monitoring stations, a proxy is developed based upon correlations with the concentrations of carbon monoxide (CO) and nitrous oxides (NOx), which are typically measured at urban monitoring stations worldwide and that are co-emitted with PAHs. An example application of parametrizations with this proxy suggest that people living in the North and Northwest of Mexico City are at greater health risk given that in these locations the PPAH concentrations are expected to be higher than in the rest of the city.