Hybrid Mass Balance/4D-Var Joint Inversion of NO _x and SO₂ Emissions in East Asia

Accurate estimates of NO _x and SO₂ emissions are important for air quality modeling and management. To incorporate chemical interactions of the two species in emission estimates, we develop a joint hybrid inversion framework to estimate their emissions in China and India (2005-2012). Pseudo observation tests and posterior evaluation with surface measurements demonstrate that joint assimilation of SO₂ and NO₂ can provide more accurate constraints on emissions than single-species inversions. This occurs through synergistic change of O₃ and OH concentrations, particularly in conditions where satellite retrievals of the species being optimized have large uncertainties. The percentage changes of joint posterior emissions from the single-species posterior emissions go up to 242% at grid scales, although the national average of monthly emissions, seasonality, and interannual variations are similar. In China and India, the annual budget of joint posterior SO₂ emissions is lower, but joint NO _x posterior emissions are higher, because NO _x emissions increase to increase SO₂ concentration and better match Ozone Monitoring Instrument SO₂ observations in high-NO _x regions. Joint SO₂ posterior emissions decrease by 16.5% from 2008 to 2012, while NO _x posterior emissions increase by 24.9% from 2005 to 2011 in China-trends which are consistent with the MEIC inventory. Joint NO _x and SO₂ posterior emissions in India increase by 15.9% and 19.2% from 2005 to 2012, smaller than the 59.9% and 76.2% growth rate using anthropogenic emissions from EDGARv4.3.2. This work shows the benefit and limitation of joint assimilation in emission estimates and provides an efficient framework to perform the inversion.