Inverse modelling of NO_x emissions over eastern China: uncertainties due to chemical non-linearity

Abstract. Satellite observations of nitrogen dioxide (NO₂) have often been used to derive nitrogen oxides (NO_x =  NO + NO₂) emissions. A widely used inversion method was developed by Martin et al. (2003). Refinements of this method were subsequently developed. In the context of this inversion method, we show that the local derivative (of a first-order Taylor expansion) is more appropriate than the “bulk ratio” (ratio of emission to column) used in the original formulation for polluted regions. Using the bulk ratio can lead to biases in regions of high NO_x emissions such as eastern China due to chemical non-linearity. Inverse modelling using the local derivative method is applied to both GOME-2 and OMI satellite measurements to estimate anthropogenic NO_x emissions over eastern China. Compared with the traditional method using bulk ratio, the local derivative method produces more consistent NO_x emission estimates between the inversion results using GOME-2 and OMI measurements. The results also show significant changes in the spatial distribution of NO_x emissions, especially over high emission regions of eastern China. We further discuss a potential pitfall of using the difference of two satellite measurements to derive NO_x emissions. Our analysis suggests that chemical non-linearity needs to be accounted for and that a careful bias analysis is required in order to use the satellite differential method in inverse modelling of NO_x emissions.