A particle‐into‐liquid sampler (PILS) was coupled to a total organic carbon (TOC) analyzer for 3 s integrated measurements of water‐soluble organic carbon (WSOC) in PM1 ambient particles. The components of the instrument are described in detail. The PILS‐TOC was deployed on the NOAA WP‐3D aircraft during the NEAQS/ITCT 2004 program to investigate WSOC sources over the northeastern United States and Canada. Two main sources were identified: biomass burning emissions from fires in Alaska and northwestern Canada and emissions emanating from urban centers. Biomass burning WSOC was correlated with carbon monoxide (CO) and acetonitrile (r2 > 0.88). These plumes were intercepted in layers at altitudes between 3 and 4 km and contained the highest fine particle volume and WSOC concentrations of the mission. Apart from the biomass burning influence, the lowest WSOC concentrations were recorded in rural air masses that included regions of significant biogenic emissions. Highest concentrations were at low altitudes in distinct plumes from urban centers. WSOC and CO were highly correlated (r2 > 0.78) in these urban plumes. The ratio of the enhancement in WSOC relative to CO enhancement was found to be low (∼3 μg C/m3/ppmv) in plumes that had been in transit for a short time, and increased with plume age, but appeared to level off at ∼32 ± 4 μg C/m3/ppmv after ∼1 day of transport from the sources. The results suggest that the production of WSOC in fine particles depends on compounds coemitted with CO and that this process is rapid with a time constant of ∼1 day.