Evaluation of a Method for Measurement of the Concentration and Size Distribution of Black Carbon Particles Suspended in Rainwater

Abstract

We make detailed evaluations of a method of measuring the mass concentration and size distribution of black carbon (BC) particles suspended in water for the purpose of application to rainwater samples. Water samples were aerosolized with an ultrasonic nebulizer. Generated water droplets were dried in order to extract airborne BC particles. The mass of each BC particle was then measured by a Single Particle Soot Photometer (SP2), based on the laser-induced incandescence technique. Under the optimized operating conditions of our measurement system, the overall efficiency of the extraction of BC particles from rainwater was determined to be 11.4% ± 0.83% by mass, resulting in a total uncertainty of about ±20% for the measurement of the BC concentration in rainwater. The change in efficiency due to water-soluble species was found to be negligibly small for rainwater samples analyzed in this study. The determination of the efficiency by using standard BC solutions was necessary at an appropriate frequency because changes in the overall conditions of the system lead to changes in the efficiency in the long term. The size of BC particles in rainwater can be overestimated because of the coagulation of BC particles in the process of extraction by the nebulizer. This effect increased with the increase in BC concentration in rainwater. The effect can be minimized by dilution of rainwater samples by pure water. The volume of a rainwater sample required for BC measurement was less than 5 mL, enabling measurement for weak rain or highly time-resolved measurements during each rain event.