Cloud Impacts on Photolysis and Ozone Production Rates in Urban Southeast Texas

With reductions in the 8-hour NAAQS ozone standard to 75 ppbv or lower, periods other than the traditional summer/fall ozone season will become increasingly important as policy makers work to develop regional control strategies and State Implementation Plans to comply with the new standards. In April & May of 2009 an intensive measurement campaign, the Study of Houston Atmospheric and Radical Precursors (SHARP) was conducted in the greater Houston, TX area. A primary goal of the campaign was to examine the processes involved in the spring time ozone exceedances in southeast Texas. The work presented here examines the impact of clouds on ozone production and loss rates calculated using the NASA Langley Research Center (LaRC) 0-D photochemical box model. To assess the impacts of changes in actinic flux on ozone production and loss rates, the LaRC model was run with photolysis rates from both measured and modeled actinic fluxes. Measured actinic fluxes were made using a Scanning Actinic Flux Spectroradiometer and modeled actinic fluxes were calculated using the Tropospheric Ultraviolet and Visible (TUV) radiative transfer model. During this study net ozone production rates peak mid-morning and consequently cloudy conditions during this time period have greater impacts on peak ozone levels. Several case studies will be examined when meteorological models incorrectly forecasted cloud formation and the subsequent poor simulation of peak ozone mixing ratios. Similarly, our results show that changes in UV levels due to clouds also impact steady-state mixing ratios of HONO and HCHO, the primary OH radical sources during the morning rapid ozone formation period. These results are also compared to results from other urban air quality studies conducted in recent years.