Daewon Byun's professional career, while sadly shortened, is nevertheless quite remarkable and distinguished. His contributions are numerous and along with his great intellect, and international leadership and collaborations he has had a major impact on air quality modeling as we now know it. I had the opportunity help advise him on his PhD, to work with him on the RADM system and then as his supervisor when he joined us at NOAA and EPA. His role and contributions to the successful implementation of the prototypic CMAQ modeling system can now be viewed in retrospect, as the critical initial step towards the evolution from concept to maturation of CMAQ (and similar modeling systems) for its intended purpose, a multi-purpose, multi-issue, research based one-atmosphere state-of-science tool for air quality assessments and decision support applications. This legacy continues because he designed and brought into the core of CMAQ a comprehensive and fundamental science bases inclusive of all critical atmospheric dynamics, physical and chemical processes (as codes) into a computational modeling framework, evident in the widely referenced Models-3 Science documentation. His interactions with the meteorology community has contributed to the evolution of MM5 to WRF, soas to provide a fundamentally improved bases for the meteorological inputs for CMAQ and its eventual air quality forecasting use. After leaving EPA, we continued to collaborate in air quality modeling studies applying CMAQ at neighborhood scales for exposure assessment purposes. I will briefly review the impact of this collaboration on current fine grid scale modeling using CMAQ and WRF.