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.本研究采用改装后的单颗粒炭黑光度计（single-particle soot photometer, SP2）结合同心气动雾化器，对液态水中吸光性氧化铁气溶胶（light-absorbing iron oxide aerosols, FeOx）的尺寸分辨数浓度（CNFeOx）与质量浓度（CMFeOx）进行测定。该SP2可在方铁矿-富勒烯炭黑混合实验室样品与融化北极雪样中，选择性检出单个FeOx颗粒。针对70~650 nm粒径范围内的FeOx颗粒，雾化器的萃取效率约为50%，该效率通过雾化器萃取前后硫酸铵的体积比，以及雾化器-SP2采样管路的尺寸分辨传输效率推导得出。为区分方铁矿-富勒烯炭黑悬浮液与雪样中FeOx与黑碳的散点图所经验绘制的边界线带来的不确定度，在两类样品中分别约为3.0%与10%。220~1400 nm粒径段总CNFeOx与CMFeOx的总不确定度分别约为19%与18%。融化北极雪样中的FeOx粒径分布在4℃储存16个月后保持稳定，CNFeOx与CMFeOx的平均变化率分别低于19%与1.0%。考虑到雾化器对粉尘颗粒的萃取效率，本系统测得的粉尘颗粒表面FeOx大多处于粒径小于1000 nm的范围。CNFeOx与CMFeOx测量的高精度，将有助于增进我们对FeOx湿沉降过程的定量认知，并可更精准地估算FeOx对雪面反照率的影响。