The PMTACS-NY EPA Supersite project generated a wealth of data that continues to be utilized to 1) better understand aerosol distributions and composition; 2) identify aerosol production processes and emission sources; 3) evaluate and provide technology transfer for aerosol measurement methods and instrumentation; and 4) provide critical data and analysis for health effects researchers and policymakers. This study has focused narrowly on some of the winter/summer contrasts observed during two intensive field campaigns at Queens College, New York City. Important findings related to this topic include: Largest changes in chemical composition for winter versus summer were observed for nitrate (17%/5%), particle bound water (2%/7%), and chloride (1.8%/0.1%). Particle sizes measured by the SMPS and the AMS were shifted to significantly smaller sizes in the winter, with mean mode mass sizes ranging from 350-400 nm in summer and 150-200 nm in winter. Summer organic aerosol mass concentrations averaged nearly three quarters OOA and one quarter HOA, while winter organics were nearly half and half OOA and HOA. Empirical estimates of SOA production at the QC site based on measured VOC aerosol precursors and OH radical concentrations indicate summertime production rates roughly four times greater than during the winter.