Sensitivity of ozone concentrations to diurnal variations of surface emissions in Mexico City: A WRF/Chem modeling study

Sensitivity of ozone (O3) concentrations in the Mexico City area to diurnal variations of surface air pollutant emissions is investigated using the WRF/Chem model. Our analysis shows that diurnal variations of nitrogen oxides (NOx = NO + NO2) and volatile organic compound (VOC) emissions play an important role in controlling the O3 concentrations in the Mexico City area. The contributions of NOx and VOC emissions to daytime O3 concentrations are very sensitive to the morning emissions of NOx and VOCs. Increase in morning NOx emissions leads to decrease in daytime O3 concentrations as well as the afternoon O3 maximum, while increase in morning VOC emissions tends to increase in O3 concentrations in late morning and early afternoon, indicating that O3 production in Mexico City is under VOC-limited regime. It is also found that the nighttime O3 is independent of VOCs, but is sensitive to NOx. The emissions of VOCs during other periods (early morning, evening, and night) have only small impacts on O3 concentrations, while the emissions of NOx have important impacts on O3 concentrations in the evening and the early morning.This study suggests that shifting emission pattern, while keeping the total emissions unchanged, has important impacts on air quality. For example, delaying the morning emission peak from 8 am to 10 am significantly reduced the morning peaks of NOx and VOCs, as well as the afternoon O3 maxima. It suggests that without reduction of total emission, the daytime O3 concentrations can be significantly reduced by changing the diurnal variations of the emissions of O3 precursors.