INTRA-URBAN AND INTRA-PIXEL VARIABILITY IN THE CONCENTRATIONS OF SHORT-LIVED ATMOSPHERIC TRACE GASES COMPLICATES THE INTERPRETATION AND APPLICATION OF SATELLITE-BASED OBSERVATIONS. URBAN-SCALE VARIABILITY IN SURFACE CONCENTRATIONS OF NITROGEN DIOXIDE (NO2) IS FREQUENTLY ACKNOWLEDGED IN AIR POLLUTION RESEARCH BUT HOW THIS GROUND-LEVEL VARIABILITY MANIFESTS IN THE VERTICAL COLUMN AND AFFECTS SATELLITE REMOTE SENSING IS NOT WELL UNDERSTOOD. EVEN LESS IS KNOWN ABOUT INTRA-URBAN VARIABILITY OF TROPOSPHERIC FORMALDEHYDE (HCHO) ANOTHER IMPORTANT ATMOSPHERIC TRACE GAS WITH A SHORT ATMOSPHERIC LIFETIME. THE DISCOVER-AQ CAMPAIGNS CLEARLY SHOWED THAT LOCAL VARIATIONS IN NO2 MEASURED BY GROUND-BASED INSTRUMENTS ARE NOT CAPTURED BY CURRENT SATELLITE-BASED INSTRUMENTS. THESE CAMPAIGNS ALSO DEMONSTRATED THAT THERE ARE STILL UNCERTAINTIES IN THE RETRIEVALS THAT CAN RESULT IN DISAGREEMENT. IN THE END THE NEED FOR SPATIALLY DENSE AND LONG-TERM GROUND-BASED MEASUREMENTS OF TROPOSPHERIC COMPOSITION EMERGED AS A KEY THEME FROM DISCOVER-AQ AND HAS COME UP REPEATEDLY IN DISCUSSIONS ABOUT FUTURE GEOSTATIONARY OBSERVATIONS. THIS PROPOSAL WILL ADDRESS THE SCIENTIFIC NEED TO QUANTIFY AND EXPLAIN INTRA-URBAN VARIABILITY IN TROPOSPHERIC COLUMN DENSITIES OF TWO KEY REACTIVE TRACE GASES NO2 AND HCHO AND TO RECONCILE SATELLITE REMOTE SENSING WITH GROUND-BASED MEASUREMENTS OF SURFACE AIR QUALITY. TO ACHIEVE THIS I PROPOSE TO ESTABLISH A NETWORK OF FIVE GROUND-BASED PANDORA SPECTROMETERS WITHIN A SINGLE URBAN REGION (IN BOSTON MA) CAPABLE OF RETRIEVING CONTINUOUS NO2 AND HCHO TROPOSPHERIC COLUMNS DURING THE DAY. THE NETWORK WILL HAVE COLLOCATED GROUND-LEVEL IN SITU MEASUREMENTS OF NO2 AT EACH LOCATION AND LIDAR-DERIVED AEROSOL PROFILES AT A CENTRAL SITE TO DERIVE URBAN BOUNDARY LAYER HEIGHT. IN ADDITION TO INSTRUMENTATION ALREADY AVAILABLE IN MY LAB I WILL LEVERAGE EXISTING INSTRUMENT CAPABILITIES FROM COLLABORATORS AT SEVERAL INSTITUTIONS IN THE REGION AND FROM GOVERNMENT-RUN AIR QUALITY MONITORING STATIONS. THE NEW GROUND-BASED OBSERVATIONS WILL BE COMBINED WITH SATELLITE-BASED REMOTE SENSING OF TROPOSPHERIC NO2 AND HCHO FROM OMI GOME-2 AND EVENTUALLY TROPOMI TO EVALUATE THE SATELLITE RETRIEVALS AND TO EXPLORE THE IMPORTANCE OF HETEROGENEITY WITHIN A SATELLITE FOOTPRINT. I AM INTERESTED IN DETERMINING WHETHER ROBUST RELATIONSHIPS BETWEEN TROPOSPHERIC COLUMN DENSITY AND SURFACE CONCENTRATIONS CAN BE DEMONSTRATED AT THE INTRA-URBAN SCALE. I WILL EXPLORE INTRA-URBAN SPATIAL SCALING RELATIONSHIPS AND DETERMINE HOW PERSISTENT THESE RELATIONSHIPS ARE ACROSS TIME SCALES OF HOURS TO SEASONS. A STATE-OF-THE-ART HIGH-RESOLUTION POLLUTANT EMISSION INVENTORY BASED ON LOCAL VEHICLE DATA WILL PROVIDE HOURLY ESTIMATES OF SURFACE NOX SOURCES AND OUTPUT FROM A REGIONAL ATMOSPHERIC CHEMISTRY MODEL (WRF-CHEM) WILL HELP INTERPRET THE IMPORTANCE OF ATMOSPHERIC CHEMISTRY TRANSPORT AND MIXING. TOGETHER THESE DATASETS WILL BE UNIQUELY POSITIONED TO ANSWER IMPORTANT UNRESOLVED SCIENCE QUESTIONS ABOUT INTRA-URBAN VARIABILITY IN AIR POLLUTANT CONCENTRATIONS WITHIN THE TROPOSPHERIC COLUMN AND AT THE SURFACE AND ADVANCE THE APPLICATIONS OF SATELLITE REMOTE SENSING OF SURFACE AIR QUALITY.OVERALL THE MAIN OBJECTIVES OF THE PROPOSED RESEARCH WILL BE TO: 1) QUANTIFY AND EXPLAIN INTRA-URBAN VARIABILITY IN AIR QUALITY AT THE SURFACE AND IN THE TROPOSPHERIC COLUMN AND RECONCILE THESE OBSERVATIONS BY CONSIDERING SURFACE EMISSIONS METEOROLOGY AND PLANETARY BOUNDARY MIXING AND CHEMISTRY 2) DETERMINE THE EXTENT TO WHICH LIMITED AGREEMENT BETWEEN SATELLITE- AND GROUND-BASED TROPOSPHERIC COLUMN OBSERVATIONS RESULTS FROM A LACK OF SPATIAL REPRESENTATION VERSUS UNCERTAINTY IN THE INDIVIDUAL OBSERVATIONS 3) EVALUATE PREDICTIONS OF INTRA-URBAN POLLUTANT VARIABILITY FROM A STATE-OF-THE ART EMISSIONS INVENTORY AND CHEMICAL TRANSPORT MODELING AND EXPLORE HOW HOURLY-RESOLVED SATELLITE OBSERVATIONS COULD INFORM AIR QUALITY RESEARCH AND MONITORING