Ambient concentrations and insights on organic and elemental carbon dynamics in São Paulo, Brazil

Abstract

The São Paulo Metropolitan Area (SPMA) is a megacity with about 20 million people and about 8 million vehicles, most of which are fueled with a significant fraction of ethanol - making it a unique case worldwide. This study presents organic and elemental carbon measurements using thermal-optical analysis from quartz filters collected in four sampling sites within the SPMA. Overall Organic Carbon (OC) concentration was comparable at all sites, where Street Canyon had the highest concentration (3.37 μg m−3) and Park site the lowest (2.65 μg m−3). Elemental Carbon (EC), emitted as result of incomplete combustion, has been significantly higher at the Street Canyon site (6.11 μg m−3) in contrast to all other three sites, ranging from 2.25 μg m−3 (Downtown) to 1.50 μg m-3 (Park). For all sampling sites, the average OC:EC ratio are found on the lower bound (<2) of what is usually observed for other megacities, highlighting the significant contribution of EC in Sao Paulo. At the Street Canyon site, average OC:EC ratio was 0.56, to our knowledge the lowest value ever observed for any urban site. An approach for apportionment between primary and secondary organic carbon based on primary OC:EC ratio was evaluated. The secondary OC was estimated to be 30–40% of total OC concentrations throughout the various sampling sites.The organic carbon dynamics has been further studied using each of the thermally-derived organic carbon fractions. Each of these has been studied regarding their correlation with EC and the correlation between different sites. The analyses have identified that the OC3 and OC4, i.e., the carbon fraction which evolves from the filter at temperatures above 450 °C, presents a regional behavior, with high correlation among all sites. Conversely, OC1, the first fraction to evolve, has depicted a more local characteristic. Furthermore, the fraction of OC which becomes char during the temperature increase under inert atmosphere (the Pyrolytic Carbon-PC) has been the only fraction not to present a significant correlation with EC. Since that EC is assumed to be a primary emission marker, it indicates that PC is not significant in traffic emissions. This study provided innovative insights of the organic aerosol content associated with air pollution dynamics in a megacity impacted by a unique vehicular fleet. It also shows the need of implementation of EURO VI technology and to improve mass transport systems such a metro and more bus corridors to allow better transport for 19 million people in the SPMA.