A single particle soot photometer (SP2), an instrument that measures the optical size and refractory black carbon (BC) mass content of individual aerosol particles, was modified to include a compact humidification system, described here. This permits quantification of water uptake by BC-containing particles, an important process that can affect their optical properties and lifetime. A Mie and kappa-Kohler theory framework was developed to relate measured humidity-dependent changes in BC aerosol optical size to the hygroscopicity parameter (kappa) of the non-BC content in the particles (which is responsible for water uptake by these particles). Laboratory testing of this experimental and theoretical system with both homogeneous non-light-absorbing particles and BC-containing particles was carried out Agreement between the theoretical predictions and laboratory measurements for the homogenous aerosols validates the experimental methodology. For BC with a 70-nm thick coating of ammonium sulfate, reasonable agreement (equivalent to similar to 20% in kappa) between measurements and theoretical predictions were observed over a span of RH from 70% to 90%. Two SP2s were configured to sample in parallel, one dry and one humidified, permitting continuous monitoring of water uptake by BC-containing aerosol. Operational refinements in SP2 setup to optimize the optical size measurement of BC-containing aerosol, and the consistency between the two SP2s are presented. This system was flown on the NASA DC8 research aircraft during the 2012 DO and 2013 SEAC4RS campaigns, providing engineering data included here that demonstrate the system's performance under challenging sampling conditions. Finally, SP2-scattering lookup tables used in the theoretical portions of this work are provided for reference. (C) 2014 Elsevier Ltd. All rights reserved.