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. Earth's radiation budget is affected by new particleformation (NPF) and the growth of these nanometre-scale particles to largersizes where they can directly scatter light or act as cloud condensationnuclei (CCN). Large uncertainties remain in the magnitude and spatiotemporaldistribution of nucleation (less than 10 nm diameter) and Aitken (10–60 nmdiameter) mode particles. Acquiring size-distribution measurements of theseparticles over large regions of the free troposphere is most easilyaccomplished with research aircraft. We report on the design and performance of an airborne instrument, thenucleation mode aerosol size spectrometer (NMASS), which providessize-selected aerosol concentration measurements that can be differenced toidentify aerosol properties and processes or inverted to obtain a full sizedistribution between 3 and 60 nm. By maintaining constant downstreampressure the instrument operates reliably over a large range of ambientpressures and during rapid changes in altitude, making it ideal for aircraftmeasurements from the boundary layer to the stratosphere. We describe the modifications, operating principles, extensive calibrations,and laboratory and in-flight performance of two NMASS instruments operatedin parallel as a 10-channel battery of condensation particle counters (CPCs)in the NASA Atmospheric Tomography Mission (ATom) to investigate NPF andgrowth to cloud-active sizes in the remote free troposphere. An inversiontechnique to obtain size distributions from the discrete concentrations ofeach NMASS channel is described and evaluated. Concentrations measured by the two NMASS instruments flying in parallel areself-consistent and also consistent with measurements made with an opticalparticle counter. Extensive laboratory calibrations with a range of particlesizes and compositions show repeatability of the response function of theinstrument to within 5–8 % and no sensitivity in sizing performance toparticle composition. Particle number, surface area, and volumeconcentrations from the data inversion are determined to better than 20 %for typical particle size distributions. The excellent performance of theNMASS systems provides a strong analytical foundation to explore NPF aroundthe globe in the ATom dataset.

Fast time response measurements of particle size distributions in the 3–60 nm size range with the nucleation mode aerosol size spectrometer