WE WILL USE MULTI-DECADAL RECORDS OF LONG-LIVED TRACE GASES TO ADDRESS QUESTIONS ABOUT OZONE TREND ATTRIBUTION STRATOSPHERIC CIRCULATION CHANGE AND THE SENSITIVITY OF O3 PROJECTIONS TO SIMULATED DYNAMICAL VARIABILITY. THESE RECORDS IMPLICITLY CONTAIN THE LOWER STRATOSPHERIC DYNAMICAL COMPONENT OF OZONE TRENDS THAT REGRESSION ANALYSES ATTEMPT TO ACCOUNT FOR. THEY ALSO PROVIDE A REFERENCE FRAME FOR IDENTIFYING FUTURE CIRCULATION CHANGE. OUR WORK IS NOVEL BECAUSE WE ANALYZE SPECIES OTHER THAN O3 TO INDEPENDENTLY INFORM O3 TREND ATTRIBUTION. OZONE TRENDS MAY CONTAIN SIGNATURES OF OZONE-DEPLETING SUBSTANCE (ODS) DECLINE CIRCULATION CHANGE AND DYNAMICAL VARIABILITY. TREND STUDIES AGREE ON O3 INCREASES ATTRIBUTABLE TO DECLINING ODSS IN REGIONS WHERE CHLORINE CHEMISTRY PLAYS A LARGE ROLE BUT FIND EITHER NEGATIVE OR NO TREND IN LOWER STRATOSPHERIC (LS) O3. LS O3 IS LONG-LIVED AND ITS VARIABILITY IS MAINLY INFLUENCED BY WINTER WAVE-DRIVEN TRANSPORT. MIDLATITUDE COMPOSITION IS AFFECTED BY THE QUASI-BIENNIAL OSCILLATION (QBO) MERIDIONAL CIRCULATION WHICH CREATES TRACE GAS ANOMALIES IN THE LOWER AND MIDDLE STRATOSPHERE WINTER HEMISPHERE THAT PERSIST THROUGH SUMMER. THE PROXIES USED BY MULTIPLE LINEAR REGRESSION (MLR) TREND ANALYSES ARE UNABLE TO ACCOUNT FOR THE IMPACT OF THIS DYNAMICAL VARIABILITY ON O3. WE USE A GMI CHEMISTRY TRANSPORT MODEL SIMULATION WITH MERRA2 METEOROLOGY AND BOUNDARY CONDITIONS REFLECTING ODS DECLINE (BASE) AND ONE WITH IDENTICAL METEOROLOGY AND ODSS FIXED AT 1995 LEVELS (FIXED). THE FIDELITY OF TRANSPORT AND CHEMISTRY IN THE BASE SIMULATION FROM 2004-2017 HAS BEEN DEMONSTRATED BY COMPARISONS WITH VARIOUS TRACE GAS RECORDS. WE WILL USE DIFFERENCES BETWEEN THE BASE AND FIXED SIMULATIONS TO QUANTIFY THE EFFECT OF DECLINING ODSS ON O3 DURING THE PAST 2 DECADES. WE WILL QUANTIFY INTERANNUAL VARIATIONS IN EARLY WINTER ANTARCTIC O3 (PRIOR TO THE ONSET OF DEPLETION) AND THEIR IMPACT ON ANTARCTIC SPRING O3. THIS PREVIOUSLY UNACCOUNTED FOR SOURCE OF VARIABILITY AFFECTS ESTIMATES OF OZONE HOLE RECOVERY THAT RELY ON THE TREND IN SEPTEMBER O3 MEASUREMENTS. THIS WILL RECONCILE THE REMAINING UNCERTAINTIES ABOUT DYNAMICAL CONTRIBUTIONS TO ANTARCTIC TRENDS. MLS N2O DATA SHOW LS CIRCULATION CHANGES SINCE 2004 CONSISTENT WITH INCREASING MEAN AGE IN THE NORTHERN LS AND DECREASING MEAN AGE IN THE TROPICAL UPPER STRATOSPHERE. IS THIS AN INDICATION OF CIRCULATION CHANGE OR MIGHT THESE TRENDS BE REVERSED IN A LONGER RECORD? WE WILL ADDRESS THIS USING 20+ YEAR (ONGOING) RECORDS OF COLUMN HNO3 FROM ~10 EXTRATROPICAL NDACC STATIONS. THE VARIATIONS IN THEIR ANNUAL MEAN COLUMNS QUANTIFY LS CIRCULATION VARIABILITY. NDACC HCL AND CLONO2 RECORDS FOR ~2002 FORWARD (I.E. AFTER LOSS OF METHYL CHLOROFORM) WILL BE ANALYZED FOR CONSISTENCY. THESE ANALYSES WILL SHOW WHETHER THERE HAS BEEN A NET CHANGE IN THE EXTRATROPICAL LS CIRCULATION IN EITHER HEMISPHERE SINCE THE MID-1990S. THE RESULTS WILL INDICATE HOW LONG A DATA RECORD LENGTH AND HOW LARGE A CIRCULATION TREND MUST BE TO BE DETECTED ABOVE NATURAL VARIABILITY. COMBINED WITH THE MODEL RESULTS WE WILL ESTIMATE WHEN ATTRIBUTION OF LS O3 INCREASES DUE TO DECLINING ODSS WILL BE POSSIBLE. CHEMISTRY CLIMATE MODELS (CCMS) PROJECT FUTURE STRATOSPHERIC O3 BUT SIMULATED FIELDS ARE LESS VARIABLE THAN OBSERVED IN PART BECAUSE MANY CCMS DO NOT SIMULATE THE QBO. THE GEOSCCM USES A SIMPLE PARAMETERIZATION FOR TROPICAL NON-OROGRAPHIC GRAVITY WAVE DRAG (NOGWD) AND GENERATES A QBO WITH REALISTIC PERIOD AND AMPLITUDE BUT HAS LESS DYNAMICAL VARIABILITY THAN OBSERVED. WE WILL COMPARE GEOSCCM SIMULATIONS WITH MORE REALISTIC NOGWD WITH CIRCULATION VARIABILITY OBTAINED FROM NDACC AND MLS ANALYSES AND ASSESS THE O3 SENSITIVITY TO THAT FORCING. THE DYNAMICALLY-DRIVEN COMPOSITION VARIABILITY IN THE DATA RECORDS WILL PROVIDE AN IMPORTANT BENCHMARK FOR EVALUATING CCM PERFORMANCE IN 21ST CENTURY PROJECTIONS.