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<p>An air-to-air ultrafine particle concentrator (Aerosol Dynamics Inc. concentrator; ADIc) has been designed to enhance online chemical characterization of ambient aerosols using aerosol mass spectrometry. The ADIc employs a three-stage, moderated water-based condensation growth tube coupled to an aerodynamic focusing nozzle to concentrate fine particles into a portion of the flow. The system can be configured to sample between 1.0 and 1.7&thinsp;L&thinsp;min<span class="inline-formula">⁻¹</span>, with an output concentrated flow between 0.08 and 0.12&thinsp;L&thinsp;min<span class="inline-formula">⁻¹</span>, resulting in a theoretical concentration factor (sample flow&thinsp;/&thinsp;output flow) ranging from 8 to 21. Laboratory tests with monodisperse particles show that the ADIc is effective for particles as small as 10&thinsp;nm. Laboratory experiments conducted with the Aerosol Mass Spectrometer (AMS) showed no shift in the particle size with the ADIc, as measured by the AMS particle time-of-flight operation. The ADIc-AMS system was operated unattended over a 1-month period near Boston, Massachusetts. Comparison to a parallel AMS without the concentrator showed concentration factors of <span class="inline-formula">9.7±0.15</span> and <span class="inline-formula">9.1±0.1</span> for sulfate and nitrate, respectively, when operated with a theoretical concentration factor of <span class="inline-formula">10.5±0.3</span>. The concentration factor of organics was lower, possibly due to the presence of large particles from nearby road-paving operations and a difference in aerodynamic lens cutoff between the two AMS instruments. Another field deployment was carried out in Helsinki, Finland. Two <span class="inline-formula">∼10</span>&thinsp;d measurement periods showed good correlation for the concentrations of organics, sulfate, nitrate and ammonium measured with an Aerosol Chemical Speciation Monitor (ACSM) with the ADIc and a parallel AMS without the concentrator. Additional experiments with an AMS alternating between the ADIc and a bypass line demonstrated that the concentrator did not significantly change the size distribution or the chemistry of the ambient aerosol particles.</p>