Semi-empirical Functions (SEF) Describing the Response of Short-lived Radicals to Their Driving Forces in the WRF/Chem Model

Short-lived radicals play a key role in the chemistry of the lower atmosphere. The short-lived radicals oxidize pollutants e.g., carbon monoxide (CO) and methane (CH₄), making them more soluble and therefore easier to remove by wet or dry deposition. Without these radicals, the tropospheric pollution would accumulate to much higher levels and have a negative effect on our climate and health. To understand short-lived radicals, i.e., hydroxyl radical (OH), we evaluate them against the environmental and chemical driving factors that, theoretically, should control them. We use a semi-empirical function (SEF) that is linear in log space, with the slopes representing the ‘normalized sensitivity coefficients’. These coefficients describe how sensitive a radical is with respect to changes in the individual driver. Using the output from the WRF/Chem model and the coefficients calculated from the NCAR Master Mechanism box model, we evaluate the chemical concentrations with respect to the SEF and find a reasonably linear relation with a reduction in scatter compared to the correlations with each individual driver. However, multiple correlation lines were identified and their origin was traced, preliminarily, to the use in WRF/Chem of different chemical mechanisms over land, ocean, and near the domain boundaries.  Therefore these correlations may provide a powerful technique for comparing models with one another and with measurements, and specifically for use in field campaign observations (e.g. MIRAGE) for evaluating the representation of fast radical photochemistry in three-dimensional chemistry-transport models (e.g. WRF/Chem).