A prototype instrument has been developed for online analysis of water-soluble Fe(II) (WS_Fe(II)) in atmospheric aerosols using a particle-into-liquid-sampler (PILS), which concentrates particles into a small flow of purified water, coupled with a liquid waveguide capillary cell (LWCC) and absorbance spectrophotometryto detect iron-ferrozine colored complexes. The analytical method is highly precise (<3% RSD), and the overall measurement uncertainty and limit of detection for the complete PILS-LWCC system are estimated at 12% and 4.6 ng m(-3), respectively. The online measurements compared well with those of 24 h integrated filter samples collected at two different sampling sites (n=27, R2 = 0.82, slope 0.90 +/- 0.08, and intercept 3.08 +/- 1.99 ng m(-3)). In urban Atlanta, fine particle WS_Fe(II) concentrations measured every 12 min exhibited large variability, ranging from below the detection limit (4.6) to 370 ng m(-3) during a 24 day period in June 2008. This instrument provides new capabilities for investigating the sources and atmospheric processing of fine particle WS_Fe(II) and may prove useful in studies ranging from effects of particle WS_Fe(II) on human health to effects of particle WS_Fe(II) on atmospheric chemistry and ocean biogeochemistry.