Hybrid Mass Balance/4D‐Var Joint Inversion of NO(x) and SO(2) Emissions in East Asia

Accurate estimates of NO(x) and SO(2) 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 SO(2) and NO(2) can provide more accurate constraints on emissions than single‐species inversions. This occurs through synergistic change of O(3) 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 SO(2) emissions is lower, but joint NO(x) posterior emissions are higher, because NO(x) emissions increase to increase SO(2) concentration and better match Ozone Monitoring Instrument SO(2) observations in high‐NO(x) regions. Joint SO(2) 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 SO(2) 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.