The empirical relationship between satellite-derived tropospheric NO<sub>2</sub> and fire radiative power and possible implications for fire emission rates of NO<sub>x</sub>

Abstract

Abstract. Nitrogen oxides (NO x ) play key roles in atmospheric chemistry, air pollution, and climate. While the largest fraction of these reactive gases is released by anthropogenic emission sources, a significant amount can be attributed to vegetation fires. In this study, NO 2 from GOME-2 on board EUMETSAT's MetOp-A and OMI on board NASA's Aura as well as fire radiative power (FRP) from the measurements of MODIS on board NASA's Terra and Aqua satellites are used to derive fire emission rates (FERs) of NO x for different types of vegetation using a simple statistical approach. Monthly means of tropospheric NO 2 vertical columns (TVC NO 2 ) have been analyzed for their temporal correlation with the monthly means of FRP for five consecutive years from 2007 to 2011 on a horizontal 1 • × 1 • grid. The strongest correlation is found to be largely confined to tropical and subtropical regions, which account for more than 80 % of yearly burned area, on average, globally. In these regions, the seasonal variation of fire intensity, expressed by the FRP data, is similar to the pattern of TVC NO 2 . As chemical models typically require values for the amount of NO x being released as a function of time, we have converted the retrieved TVC NO 2 into production rates of NO x from fire (P f ) by assuming a constant lifetime of NO x . The comparison between P f and NO x emissions from the Global Fire Emissions Database (GFEDv3.1) over 5 characteristic biomass burning regions in the tropics and subtropics shows good agreement. By separating the monthly means of P f and FRP according to land cover type, FERs of NO x could be derived for different biomes. The estimated FERs for the dominating types of vegetation burned are lowest for open shrublands and savannas (0.28-1.03 g NO x s −1 MW −1 ) and highest for croplands and woody savannas (0.82-1.56 g NO x s −1 MW −1 ). This analysis demonstrates that the strong empirical relationship between TVC NO 2 and FRP and the following simplified assumptions are a useful tool for the characterization of NO x emission rates from vegetation fires in the tropics and subtropics. Possible factors affecting the magnitude of the obtained values are discussed.

Inverse Modeling of Nitrogen Oxides Emissions from the 2010 Russian Wildfires by Using Satellite Measurements of Nitrogen Dioxide

“…The NOx lifetime values corresponding to the optimal estimates of the conversion factor are 4.2 and 3.1 h for the "VOCbyNOx" and "VOCbyCO" cases. These values are within a rather broad range of lifetimes (2-6 h) assumed in earlier studies of BB NOx emissions [11][12][13][14].…”

“…The DOMINO version 2 data product was evaluated against ground-based (MAX-DOAS) measurements [25], and only a minor negative bias (~10%) was found in the satellite data. NO2 data retrieved from the OMI measurements (although with slightly different algorithms) were found to be useful in earlier studies of BB NOx emissions [7,11,12,14].…”

“…We further note that the term on the lefthand side of Equation (7) NO concentrations being the weights), while the term on the right-hand part of Equation (7) is analogous to the term on the right-hand part of Equation (5). The main difference between Equation (7) and Equation (5) NO that have counterparts in the satellite data, because the primary purpose of evaluation of τnox in this study was to investigate the effects of chemical nonlinearities on the BB NOx emission estimates that can be derived from satellite measurement using either our method or simpler techniques involving a priori assumptions regarding the NOx lifetime [12][13][14]. Note also that the relationship between α (1) and α (0) depends not only on the NOx lifetime but also on the NO2/NO ratio: this ratio, in turn, depends on ozone concentration and can be affected by changes in the NOx emissions.…”

“…Nonetheless, we believe that the estimates of τnox provided below can be helpful in characterizing the processes determining the relationship between the NO2 columns and NOx emissions. Furthermore, such estimates can eventually contribute to reducing uncertainties associated with FER or EC estimates [7,[11][12][13][14] based on the analysis of the empirical relationships between FRP and tropospheric NO2 columns. However, we do not recommend using our estimates of τnox outside of this specific context.…”

“…Nonetheless, several studies [7,[11][12][13][14] have recently examined the relationship between the satellite-retrieved fire radiative power (FRP) and the tropospheric NO2 columns over fire spots and reported reasonable estimates of the fire emission rate (FER) [7,11] or the emission coefficient (EC) [12][13][14], both of which allow one to calculate NOx emissions, given the observed values of FRP. The measurement-based estimates of FER or EC for different regions and different types of biophysical model applications (BioMA) are useful, as they provide insight into the capability of available parameterizations to capture spatial and temporal variability of NOx emissions.…”