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We present a new method of determining the size and composition ofCCN-active aerosol particles. Method utility is illustrated through a seriesof ambient measurements. A continuous-flow thermal-gradient diffusionchamber (TGDC), pumped counterflow virtual impactor (PCVI), and Aerodynetime-of-flight mass spectrometer (AMS) are operated in series. Ambientparticles are sampled into the TGDC, where a constant supersaturation ismaintained, and CCN-active particles grow to ~2.5 ± 0.5 μm.The output flow from the TGDC is directed into the PCVI, where a counterflowof dry N<sub>2</sub> gas opposes the particle-laden flow, creating a region ofzero axial velocity. This stagnation plane can only be traversed by particles withsufficient momentum, which depends on their size. Particles that haveactivated in the TGDC cross the stagnation plane and are entrained in thePCVI output flow, while the unactivated particles are diverted to a pump.Because the input gas is replaced by the counterflow gas with better than99 % efficiency at the stagnation plane, the output flow consists almostentirely of dry N<sub>2</sub> and water evaporates from the activated particles.In this way, the system yields an ensemble of CCN-active particles whosechemical composition and size are analyzed using the AMS. Measurements ofurban aerosol in downtown Toronto identified an external mixture ofCCN-active particles consisting almost entirely of ammonium nitrate andammonium sulfate, with CCN-inactive particles of the same size consisting ofa mixture of ammonium nitrate, ammonium sulfate, and organics. We alsodiscuss results from the first field deployment of the TGDC-PCVI-AMS system,conducted from mid-May to mid-June 2007 in Egbert, Ontario, asemirural site ~80 km north of Toronto influenced both by clean airmasses from the north and emissions from the city. Organic-dominatedparticles sampled during a major biogenic event exhibited higher CCNactivity and/or faster growth kinetics than urban outflow from Toronto,despite the latter having a higher inorganic content and higher O:C ratio.During both events, particles were largely internally mixed

Analysis of cloud condensation nuclei composition and growth kinetics using a pumped counterflow virtual impactor and aerosol mass spectrometer