Measurement of Number and Mass Size Distributions of Light-absorbing Iron Oxide Aerosols in Liquid Water with a Modified Single-Particle Soot Photometer

Here, we used a modified single-particle soot photometer (SP2) coupled with a concentric pneumatic nebulizer to measure the size-resolved number and mass concentrations of light-absorbing iron oxide aerosols (FeOx) in liquid water (CNFeOx and CMFeOx, respectively). The SP2 could selectively detect individual FeOx particles in mixed wustite-fullerene soot laboratory samples and melted Arctic snow samples. The nebulizer efficiency for FeOx particles was about 50% within the 70-650 nm diameter range, as derived from the ratio of the volume of ammonium sulfate before and after extraction by the nebulizer and the size-resolved transmission efficiency in the nebulizer-SP2 sampling line. Uncertainty from the boundary lines empirically drawn to discriminate the scatterplots of FeOx and black carbon in the mixed wustite-fullerene soot suspensions and snow samples was approximately 3.0% and 10%, respectively. Overall uncertainty in total CNFeOx and CMFeOx (220-1400 nm) was approximately 19% and 18%, respectively. After storage at 4 degree C for 16 months, the FeOx particle size distributions in melted Arctic snow had remained stable, and CNFeOx and CMFeOx had changed by less than 19% and 1.0%, on average, respectively. Most of the FeOx on dust particles measured by this system was estimated to be in the diameter range smaller than 1000 nm, considering the nebulizer efficiency for dust particles. The high accuracy of the CNFeOx and CMFeOx measurements will help to improve our quantitative understanding of the wet deposition of FeOx and provide more accurate estimates of the effects of FeOx on snow surface albedo.