Project at a Glance

Title: How new chemistry findings affect our understanding of the weekend effect : a modeling study

Principal Investigator / Author(s): Dabdub, Donald

Contractor: UC Irvine

Contract Number: 04-333

Research Program Area: Atmospheric Processes

Topic Areas: Chemistry & Reactivity, Modeling, Monitoring


High ozone mixing ratios are a serious concern of public health. While ozone concentrations are high on weekdays due to anthropogenic emissions, they are often higher on weekends despite lower emissions. This phenomenon has been named the weekend effect. This study uses the UCI-CIT air quality model to assess the weekend effect in the South Coast Air Basin (SoCAB) of California. The weekend effect is reproduced by the model using an emissions inventory that includes representative weekday and weekend emissions. There are four main objectives in this study. First, to examine the influence of renoxification process on the weekend effect. Second, to quantify the impact of heterogeneous/multiphase chlorine reactions on the weekend effect. Third, to analyze the contribution of distributed generation (DG) to the weekend effect in the year 2010. Finally, to study the consequences of reducing NOx emissions on ozone concentration aloft and the subsequent impact on the weekend effect. The Caltech Atmospheric Chemistry Mechanism (CACM), used in the UCI-CIT model, is modified to accommodate these scenarios by introducing new heterogeneous reactions involving nitrogen oxides and chlorine. The emissions inventory from the 2003 Air Quality Management Plan is used to simulate a one-week episode in 2010. Results demonstrate that both renoxification and chlorine chemistry lead to a net decrease in the average weekend effect intensity. With the implementation of DG, the weekend effect intensity estimated for 2010 is significantly lower than in 1997, although it is still present even though the emissions for 2010 are significantly lower than in 1997. These results suggest that the SoCAB will still be under a VOC-limited regime in the year 2010. Emissions from DG contribute to a small percentage of the total basinwide emissions. In the study of the pollutants aloft, the weekend effect is shown to be more prominent at the layer immediately above the ground level. The increasing weekend effect from ground level to altitudes up to 670m is heavily attributed to the decrease in NOx emissions and the increase in the VOC/NOx ratios in altitudes between 38 154m from weekdays to weekends.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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