Temporal evolution of main ambient PM2.5 sources in Santiago, Chile, from 1998 to 2012

10 The inhabitants of Santiago, Chile have been exposed to harmful levels of air pollutants for decades. The city’s poor air quality is a result of steady economic growth, and stable atmospheric conditions adverse to mixing and ventilation that favor the formation of oxidants and secondary aerosols. Identifying and quantifying the sources that contribute to the ambient levels of pollutants is key for designing adequate mitigation measures. Estimating the evolution of source contributions to ambient pollution levels is also paramount to evaluating the effectiveness of pollution reduction measures that have been implemented over the past decades. 15 Here, we quantify the main sources that have contributed to fine particulate matter (PM2.5) between April 1998 and August 2012 in downtown Santiago by using two different source-receptor models (PMF 5.0 and UNMIX 6.0), that were applied to elemental measurements of 1243 24-hour filter samples of ambient PM2.5. PMF resolved six sources that contributed to ambient PM2.5, with UNMIX producing similar results: motor vehicles (37.3±1.1%), industrial sources (18.5±1.3%), copper smelters (14.4±0.8%), wood burning (12.3±1.0%), coastal sources (9.5±0.7%), and urban dust (3.0±1.2%). Our results show that over the 15 years 20 analyzed here, four of the resolved sources significantly decreased [95% Confidence Interval]: motor vehicles 21.3% [2.6, 36.5], industrial sources 39.3% [28.6, 48.4], copper smelters 81.5% [75.5, 85.9], and coastal sources 58.9% [38.5, 72.5], while wood burning didn’t significantly change, and urban dust increased by 72% [48.9, 99.9]. These changes are consistent with emission reduction measures, such as improved vehicle emission standards, cleaner smelting technology, introduction of low sulfur diesel for vehicles and natural gas for industrial processes, public transport improvements etc. However, it is also apparent that the 25 mitigation expected from the above regulations has been partially offset by the increasing amount of private vehicle use in the city, with motor vehicles becoming the dominant source of ambient PM2.5 in recent years. Consequently, Santiago still experiences ambient PM2.5 levels above the annual and 24-hour Chilean and World Health Organization standards, and further regulations are required to reach ambient air quality standards. 30