Why do Models Overestimate Surface Ozone in the Southeastern United States?

Area of Science:

• Atmospheric Chemistry
• Air Quality Modeling
• Environmental Science

Background:

• Ozone pollution in the Southeast US is a complex issue driven by nitrogen oxides (NOx) and isoprene emissions.
• Current models often overestimate surface ozone concentrations, hindering effective emission control strategies.
• Accurate modeling is crucial for meeting air quality standards.

Purpose of the Study:

• To investigate the factors controlling surface ozone in the Southeast US using detailed chemical observations.
• To evaluate and improve the accuracy of the GEOS-Chem chemical transport model for this region.
• To assess the impact of NOx emissions on ozone formation.

Main Methods:

• Utilized data from the SEAC4RS aircraft campaign (August-September 2013).
• Employed the GEOS-Chem chemical transport model at high resolution (0.25°×0.3125°).
• Compared model outputs with NOx, ozone, nitrate wet deposition, and satellite NO2 column observations.

Main Results:

• The US Environmental Protection Agency's National Emission Inventory for NOx was found to be too high.
• A 30-60% reduction in NOx emissions from mobile and industrial sources is suggested.
• Lightning's contribution to upper tropospheric NO2 requires careful consideration for surface NOx estimation.

Conclusions:

• Reduced NOx emissions improve GEOS-Chem's simulation of ozone, aligning with aircraft observations.
• The model still shows a high bias in surface ozone, potentially due to excessive vertical mixing.
• Further refinement of model physics, particularly vertical transport, is needed for accurate surface ozone prediction.