At the core of all atmospheric models is closed set of governing equations that serve as the basis for integrating mass, energy, and momentum over 3-dimensional spatial domains. In the early days of numerical weather prediction (NWP) there was little concern for accurate conservation of these quantities because of simplifications to the dynamical equations and limited time integrations. With the advent of fully compressible non-hydrostatic atmospheric models for NWP in the 1990s with finer resolution and longer forecasts, more conservative computational systems were needed. The coupling of air quality models to non-hydrostatic NWP models placed further demands on mass conservative formulations. Daewon Byun was a strong advocate for designing atmospheric models based on conservative variables. He was also one of the first model developers to formulate chemical transport models for use with fully compressible non-hydrostatic meteorology models. Daewon also developed a generalized coordinate system for the Community Multiscale Air Quality (CMAQ) model. Air quality modelers still wrestle with these issues of reconciling trace chemical mass with the air density and momentum fields. In addition to recounting some of Daewon's contributions in this area, new techniques for mass consistency in air quality modeling will be discussed.