An appropriate representation of the NO_x/NO₂ ratio and NO_x lifetime is essential for satellite-based NO_x emissions estimation. We introduce a lightweight approach that applies variable NO_x/NO₂ ratios and piecewise fitting based on the directional derivative approach (DDA) to estimate regional and point-source NO_x emissions. Our method directly captures nonlinear NO_x chemistry and serves as an efficient alternative to both bottom-up inventories and computationally demanding top-down models.