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

Abstract

Sensitivity of ozone (O 3) 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 (NO x = NO + NO 2) and volatile organic compound (VOC) emissions play an important role in controlling the O 3 concentrations in the Mexico City area. The contributions of NO x and VOC emissions to daytime O 3 concentrations are very sensitive to the morning emissions of NO x and VOCs. Increase in morning NO x emissions leads to decrease in daytime O 3 concentrations as well as the afternoon O 3 maximum, while increase in morning VOC emissions tends to increase in O 3 concentrations in late morning and early afternoon, indicating that O 3 production in Mexico City is under VOC-limited regime. It is also found that the nighttime O 3 is independent of VOCs, but is sensitive to NO x . The emissions of VOCs during other periods (early morning, evening, and night) have only small impacts on O 3 concentrations, while the emissions of NO x have important impacts on O 3 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 NO x and VOCs, as well as the afternoon O 3 maxima. It suggests that without reduction of total emission, the daytime O 3 concentrations can be significantly reduced by changing the diurnal variations of the emissions of O 3 precursors.