Sunday, 22 January 2012
Semi-Empirical Functions Describing the Response of Short-Lived Radicals to Their Driving Forces in the WRF/Chem Model
Hall E (New Orleans Convention Center )
Short-lived radicals play a key role in the chemistry of the lower atmosphere. They 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, tropospheric pollution would accumulate to much higher levels and have a stronger negative effect on our climate and health. To understand short-lived radicals, i.e., hydroxyl radical (OH), we evaluated 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, whose slopes represent the “normalized sensitivity coefficients”. These coefficients describe how sensitive a radical is with respect to changes in the individual driver, e.g., atmospheric water vapor (H2O), volatile organic compounds (VOCs), nitrogen oxides (NOx), and the ultraviolet photolysis coefficient for NO2 (J(NO2)). Using the output from the WRF/Chem model and the sensitivity coefficients calculated from the NCAR Master Mechanism box model, we evaluated the chemical concentrations with respect to the SEF and found 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 origins were traced to WRF/Chem's use 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 with field campaign observations (like those of MIRAGE) for evaluating the representation of fast radical photochemistry in three-dimensional chemistry-transport models (such as WRF/Chem).