Modeling weekday to weekend changes in emissions and ozone in the Los Angeles basin for 1997 and 2010

In this study, we used numerical models to investigate weekday to weekend ozone differences in the Los Angeles (LA) basin in 1997 and 2010. We developed relatively complete descriptions of weekday to weekend emission changes for nitrogen oxides (NOx) with 80% of the weekday NOx emissions receiving a weekend adjustment in 1997 and 67% in 2010. Weekend NOx decreases of 34% on Saturday and 45% on Sunday relative to weekdays for 1997 are predicted to become slightly greater by 2010 and are dominated by NOx decreases for on-road vehicles (especially heavy-duty diesel vehicles) and off-road construction equipment. Weekend anthropogenic reactive organic gas (ROG) decreases for 1997 (12% on Saturday and 16% on Sunday) are smaller than for NOx and become even smaller in 2010. However, there is less certainty in the weekend ROG emission changes because only 47% of the weekday ROG emissions received a weekend adjustment in 1997 and only 21% in 2010. The ozone modeling used multiple representations of the meteorology and chemistry in order to identify results that are consistent across these inputs. Weekend ozone increased in central portions of the basin in response to weekend NOx and ROG decreases for both 1997 and 2010, with both the SAPRC99 and CB4 chemical mechanisms, and with all meteorological data sets. Weekend ozone decreased in other portions of the basin in all models. Comparing 2010 to 1997, weekend ozone increases are smaller and confined to smaller portions of the LA basin near the Pacific coast, whereas weekend ozone decreases are larger and more widespread in 2010. For 2010, all models show lower 8-h ozone on Sunday than weekdays in the Riverside/San Bernardino area. First-order sensitivity analysis (via the decoupled direct method) shows that the weekday/weekend ozone changes are mainly due to the NOx emission changes. Also, the weekday/weekend ozone changes for 2010 are sensitive to the choice of initial and boundary concentrations.