Photochemical ozone pollution in the Valparaiso Region, Chile

Abstract

The Valparaiso Region is characterized by the presence of air sheds that modulate the general atmospheric circulation with local winds. The ozone concentration in the region increases from the west coast toward the mountainous area, where the concentrations of ozone precursors are lower. In this study, surface ozone (O3), nitric oxide, nitrogen dioxide, nitrogen oxide (NOx), and volatile organic compounds (VOCs) were measured at four sites, including two upwind urban sites (Viña del Mar (VM) and Villa Alemana (VA)), an upwind suburban site (Quillota (QU)) and a downwind semirural site (Los Andes (LA)) during 01 March-07 April 2010. The purpose of this study is to improve our understanding of the photochemistry of ozone in urban and semirural areas of the Valparaiso Region and the transport of ozone pollution in downwind rural areas. The results show that, in the VM and VA sites, higher concentrations of ozone precursors, NOx, and VOCs, are observed in the early morning hours, which is related to emissions from vehicle traffic, while comparatively lower ozone precursor concentrations are observed in QU and LA. In contrast, higher ozone concentrations are found in LA and QU, and lower concentrations are found in VM and VA. VOC analysis was performed by gas chromatography, and the results show that the VOC composition in the studied sites is similar to and coincident with vehicle emissions. An ozone formation regime limited by VOCs is found in the area. The leading contributors to the ozone-forming potential were ethene, propene, xylene, and toluene, which are also anthropogenic species and account for more than 70 % of O3 formation. Thus, benzenes–toluene–xylenes played the most important role in O3 formation, followed by alkenes, alkanes, aromatic, biogenic, and alkynes during the study periods. The m-, p-xylene/ethylbenzene relation confirms the reception of aged air masses in LA. The results of the present study demonstrate that ozone pollution is a regional problem and that O3 regional transport is associated with such pollution. Finally, the results indicate the need to adopt mitigating measures for ozone pollution that consider the reactivity characteristics of VOCs.