Satellite radiative power-based emission factors for nitrogen oxides from wildland fires in California

Wildfire emissions are a significant source of nitrogen oxides, the primary precursor for ozone formation. Fire emission factors (EFs) are generally reported as the ratio of pollutant mass emitted to total biomass burned (g/kg) and are based on limited observations. However, EFs (in g/MJ) based on time-integrated fire radiative power (FRP), a property measured by satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Aqua and Terra satellites, have great potential for use in atmospheric modeling due to the global, near-continuous measurement of this property on fine spatio-temporal scales. Here we derive nitrogen dioxide EFs for three land types (forest, shrub, or grass) over California and Nevada using MODIS FRP and a mass emission rate calculated using tropospheric nitrogen dioxide column measurements from the Ozone Monitoring Instrument (OMI) and wind vectors from the North American Regional Reanalysis dataset. Linear regressions of daily radiative power vs. mass emission rate were performed to derive EFs. Correlation coefficients (R-squared) were on the order of 0.5-0.7, and further averaging over multi-day fire events yielded even better correlations, with R-squared values on the order of 0.7-0.9. EFs were comparable in order of magnitude to literature values of radiative-energy based nitrogen dioxide EFs from laboratory fires. The largest source of error in these calculations is likely due to the OMI air mass factor calculations, which rely on assumptions about the vertical profile of nitrogen dioxide and aerosol loading that are violated under fire conditions.