Influence of sea-salt activated chlorine and surface-mediated renoxification on the weekend effect in the South Coast Air Basin of California

Abstract

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. This phenomenon has been named the weekend effect. This study uses the University of California, Irvine-California Institute of Technology (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. Additionally, this study modifies the Caltech Atmospheric Chemistry Mechanism (CACM), used in the UCI-CIT model, by introducing new heterogeneous reactions involving nitrogen oxides and chlorine. Eight modeling scenarios that include the nitrogen oxide renoxification and heterogeneous/multiphase chlorine reactions are presented to quantify how these reactions impact the weekend effect. The renoxification reaction and chlorine chemistry are found to increase ozone levels during weekdays and weekend days. However, increases in weekdays are generally larger than these increases that occur in the weekend. As a result, renoxification and chlorine chemistry lead to a net decrease in the average weekend effect intensity. The influence of renoxification on the weekend effect depends on the reaction probability ( P), and the impact on the weekend effect is significant for P larger than 0.1. The influence of chlorine chemistry on the weekend effect depends strongly on the sea-salt source function that activates the chlorine chemistry. An amplification factor of 10 for the sea-salt source function, which produces the best agreement with observed chlorine levels in the SoCAB, leads to a basin-wide overall decrease of 29% in the weekend effect intensity with respect to the base case.