VOC Monitoring to Understand Changes in Secondary Pollution in Mexico City

Abstract:

Previous studies have documented the distribution, diurnal pattern, magnitude, and reactivity of the volatile organic compounds (VOCs) within and downwind of Mexico City. These studies have provided valuable data, but their duration has been restricted to a few weeks since the majority have been part of intensive field campaigns. With the aim of addressing the VOC pollution problem during longer monitoring periods and evaluating control measures to reduce the production of ozone and secondary aerosols, the environmental authorities of Mexico City through its Air Quality Monitoring Network have developed a program to monitor over 50 VOC species every hour in selected existing air quality monitoring stations inside and outside the urban sprawl. The program started with a testing period of six months in 2012 covering the ozone-season (Mar-May). Results of this first campaign are presented in this paper.

Using as reference VOC data collected in 2003, reductions in the mixing ratios of light alkanes associated with the consumption of liquefied petroleum gas and aromatic compounds related with the evaporation of fossil fuels and solvents were observed. In contrast, a clear increase in the mixing ratio of olefins was observed. This increase is of relevance to understand the moderate success in the reduction of ozone and fine aerosols in recent years in comparison to other criteria pollutants, which have substantially decreased. Particular features of the diurnal profiles, reactivity with the hydroxyl radical and correlations between individual VOCs and carbon monoxide are used to investigate the influence of specific emission sources. The results discussed here expect to highlight the importance of monitoring VOCs to better understand the drivers and impacts of secondary pollution in large cities like Mexico City.