METROPOLITAN NEW YORK CITY (NYC), LIKE MANY URBAN AND DOWNWIND AREAS, FACES CHALLENGES WITH OZONE EXCEEDANCES AND FINE PARTICULATE MATTER (PM2.5) LEVELS THAT SURPASS W.H.O. GUIDELINES. OUR RECENT RESEARCH HIGHLIGHTS TEMPERATURE-DRIVEN INCREASES IN BOTH POLLUTANTS. ADDITIONALLY, THE CHEMICAL MAKEUP OF PM2.5 IS SHIFTING DUE TO POLICY CHANGES AND WEATHER-RELATED FACTORS. PRELIMINARY FINDINGS FROM OUR AC4-FUNDED NEW YORK CITY METROPOLITAN MEASUREMENTS OF EMISSIONS AND TRANSFORMATIONS (NYC-METS) CAMPAIGN (SUMMER 2022/23) REVEAL THAT REDUCTIONS IN INORGANIC PM OVER THE 21ST CENTURY HAVE LED TO A SUMMERTIME PM COMPOSITION DOMINATED BY SECONDARY ORGANIC AEROSOL (SOA). SOA AND SULFATE CONCENTRATIONS, ALONG WITH SOA OXIDATION STATE, SHOW CLEAR TEMPERATURE-SENSITIVE INCREASES. THESE POLLUTANTS ARE ALSO INFLUENCED BY FACTORS SUCH AS RELATIVE HUMIDITY (RH) AND REGIONAL TRANSPORT PATTERNS. THIS POSES SIGNIFICANT CHALLENGES FOR AIR QUALITY, ESPECIALLY WHEN HEATWAVES CAN AMPLIFY THESE EFFECTS. MOREOVER, KNOWLEDGE GAPS IN GAS-PHASE ATMOSPHERIC CHEMISTRY REMAIN DUE TO COMPLEX BIOGENIC-ANTHROPOGENIC POLLUTANT INTERACTIONS, AS WELL AS THEIR NONLINEAR, MULTIPHASE CHEMICAL PROCESSES, ALL OF WHICH CONTINUE TO EVOLVE WITH POLLUTION CONTROL MEASURES. THE PROJECT WILL ANALYZE DATA FROM TWO DISTINCT NYC-METS SUMMER CAMPAIGNS IN MANHATTAN (AS PART OF AEROMMA) USING A CHEMICALLY-DETAILED DATASET FROM THE YALE-ARI TEAM. KEY OBJECTIVES ARE: (1) EXAMINE SHIFTS IN GAS-PHASE ORGANIC COMPOUND COMPOSITION AS A FUNCTION OF CHANGES IN TEMPERATURE AND DURING SUSTAINED HEATWAVES AND (2) INVESTIGATE THE INDIVIDUAL AND COMBINED EFFECTS OF TEMPERATURE, NOX LEVELS, AND RH, ON SECONDARY GAS-PHASE CHEMICAL PROCESSES, INCLUDING AQUEOUS-PHASE CHEMISTRY AND THE PRODUCTION OF HIGHLY-OXIDIZED AND MULTIFUNCTIONAL COMPOUNDS.