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 (FeO_x) in liquid water (C _NFeOx and C _MFeOx, respectively). The SP2 could selectively detect individual FeO_x particles in mixed wüstite–fullerene soot laboratory samples and melted Arctic snow samples. The nebulizer efficiency for FeO_x 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 FeO_x and black carbon in the mixed wüstite–fullerene soot suspensions and snow samples was approximately 3.0% and 10%, respectively. Overall uncertainty in total C _NFeOx and C _MFeOx (220–1400 nm) was approximately 19% and 18%, respectively. After storage at 4 °C for 16 months, the FeO_x particle size distributions in melted Arctic snow had remained stable, and C _NFeOx and C _MFeOx had changed by less than 19% and 1.0%, on average, respectively. Most of the FeO_x 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 C _NFeOx and C _MFeOx measurements will help to improve our quantitative understanding of the wet deposition of FeO_x and provide more accurate estimates of the effects of FeO_x on snow surface albedo.