Interannual variability of nitrogen oxides emissions from boreal fires in Siberia and Alaska during 1996–2011 as observed from space

Abstract

Past studies suggest that forest fires contribute significantly to the formation of ozone in the troposphere. However, the emissions of ozone precursors from wildfires, and the mechanisms involved in ozone production from boreal fires, are very complicated. Moreover, an evaluation of the role of forest fires is prevented by the lack of direct observations of the ozone precursor, nitrogen oxides (NOx), and large uncertainties exist in the emissions inventories currently used for modelling. A comprehensive understanding of the important processes and factors involving wildfires has thus been unobtainable. We made 16 year consistent analyses of NOx emissions from boreal wildfires by using satellite observations of tropospheric nitrogen dioxides (NO 2 ) from 1996 to 2011. We report substantial interannual variability of tropospheric NO 2 originating from large boreal fires 2009. Monthly comparisons of NO 2 enhancements with fire radiative power (FRP) show reasonably strong correlation, suggesting that FRP is a better proxy than burned area for boreal fire NOx emissions. We provide space-based constraints on NOx emission factors (EFs) for Siberian and Alaskan fires. Although the associated uncertainty is relatively large, the derived EFs fall into a in reasonably agreeable range with those previously determined by in situ ground-based and airborne observations over these regions.

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].…”

“…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.…”

“…However, such estimates are presently rather uncertain, particularly because they involve explicit quantitative assumptions regarding the NOx lifetime and NO2/NO ratio. For example, it has been argued [13] that as the NOx lifetime is likely to vary across individual fire plumes between 2 and 6 h, the estimates of EC can be uncertain at least by a factor of 3.…”