Constraints on urban VOC emissions from day of week measurements of column NO2

The ratio of NO2 column observed on weekdays to that observed on weekends (i.e., the NO2 weekend effect) has been used extensively to constrain NOx emissions from sources that do not operate on weekends. For instance, ~40 % less NO2 is observed on the weekend by OMI over the Los Angeles Basin (~ 150 x 100 km2) indicating the reduction of heavy-duty diesel emissions on the weekends. The decreases observed are not uniform however. OMI observes decreases of column NO2 as low as ~5 % directly over downtown Los Angeles and as high as 60 % further downwind (125 km inland). We show that chemistry is sufficient to explain this observed variability using WRF-CHEM. Furthermore, we show that agreement between simulation and observation is improved significantly when emissions of hydrocarbons (VOC) are increased.