Vertical ozone variation over urban and rural sites in California Central Valley

Ozone in the planetary boundary layer is a photochemical oxidant that has an adverse effect on human and the environment. As with most pollutants in the lower troposphere, ozone concentration varies in spatial and temporal scale depending on sources of pollution, environmental conditions and the boundary layer dynamics. Among the several factors that influence ozone variation, the seasonal changes in meteorological parameters and availability of ozone precursors are crucial because they control ozone formation and decay. In California Central Valley, a region where regulatory state and national standards of ozone are frequently exceeded, there is discrepancy between the rural and urban areas in emission sources of ozone precursors. In the rural areas ozone precursors are largely volatile organic compounds(VOCs) from the vast agricultural operations that dominate the regional landscape, while in the urban areas they are mostly nitrous oxide (NOx) and VOCs from vehicle emissions. Understanding how the difference in emission sources affect vertical ozone transport is considered crucial to the improvement of regional ozone forecast for the area. The purpose of this paper is to present the results of a study done to characterize vertical ozone transport in urban and rural boundary layers in California Central Valley. The study, which was conducted over a five year period, from 2007 to 2012, involved vertical profile measurements of ozone using electrochemical ozonesondes and meteorological sondes tethered to a 9 cubic meters helium balloon. On each day of the data collection, multiple balloon launches were made over a period representing different stages of the boundary layer development. The results show ozone profiles measured at the urban site are significantly different from those at the rural site, and while at the urban site the highest ozone concentration occurred near the surface during the growing phase of the boundary layer, at the rural site the profiles display some unique features which suggest unusual destruction of ozone near the surface regardless of the time of day.