WE PROPOSE TO PROVIDE PHOTOCHEMICAL MODELING SUPPORT TO ASSIST THE DYNAMICS AND CHEMISTRY OF THE SUMMER STRATOSPHERE (DCOTSS) MISSION IN ADDRESSING THREE OF THE FOUR CHEMISTRY QUESTIONS GIVEN IN SECTION 1.2.2 OF THE INVESTIGATION IMPLEMENTATION PLAN. THE DCOTSS MISSION WILL OBTAIN MEASUREMENTS OF THE CHEMICAL COMPOSITION OF THE UPPER TROPOSPHERE AND LOWER STRATOSPHERE USING INSTRUMENTS ON BOARD THE NASA ER-2 AIRCRAFT VIA A SERIES OF FLIGHTS THAT WILL OCCUR IN SUMMER 2020 AND SUMMER 2021 FROM SALINA KANSAS. DCOTSS WAS SELECTED UNDER THE NASA EARTH SYSTEM SCIENCE PATHFINDER (EESP) VENTURE-CLASS SUBORBITAL-3 (EVS-3) COMPETITION. THE THREE CHEMISTRY QUESTIONS UPON WHICH WE PROPOSE TO FOCUS OUR CONTRIBUTION TO DCOTSS ARE: SOW Q1: HOW MUCH CHLORINE FROM VERY SHORT LIVED (VSL) SOURCES IS PRESENT IN THE LOWER STRATOSPHERE OVER NORTH AMERICA IN SUMMER? SOW Q2: WHAT CHEMICAL CHANGES TAKE PLACE IN THE STRATOSPHERE DUE TO CONVECTION IN THE NORTH AMERICAN MONSOON ANTICYCLONE? SOW Q3: WHAT WILL BE THE STRATOSPHERIC CHEMICAL RESPONSE TO AN EXPLOSIVE VOLCANIC ERUPTION? WE PROPOSE TO ADDRESS THESE THREE QUESTIONS USING EXISTING PHOTOCHEMICAL STEADY STATE (PSS) BOX MODELS (E.G. SECTION 2.2 OF WALES ET AL. 2018) THAT WE HAVE DEVELOPED AND APPLIED TO DATA COLLECTED DURING NUMEROUS PRIOR NASA AIRBORNE FIELD CAMPAIGNS AS WELL AS A SUITE OF CHEMISTRYCLIMATE MODELS (CCMS) (E.G. SECTION 4.1 OF WALES ET AL. 2018) DEVELOPED BY GROUPS WITH WHICH WE MAINTAIN ACTIVE COLLABORATIONS. OF THESE CCMS WE HAVE PARTICULARLY CLOSE TIES TO THE GEOS COMPOSITION FORECAST (GEOS-CF) MODEL DEVELOPED AT THE GLOBAL MODELING AND ASSIMILATION OFFICE (GMAO) OF NASA GODDARD SPACE FLIGHT CENTER.