Atmospheric Chemistry and Physics Optical particle counter measurement of marine aerosol hygroscopic growth

Abstract. A technique is developed for the determination of the hygroscopic growth factor of dry particles with di-ameter between 0.3 and 0.6µm and is applied to measure-ments made during the second Dynamics and Chemistry of Marine Stratocumulus experiment (DYCOMS-II). Two opti-cal particle counters are utilized, one measures the aerosol size spectrum at ambient relative humidity and the other si-multaneously dries the aerosol prior to light scattering de-tection. Growth factors are based on measurements made in the region of the Mie scattering curve where scattered light intensity increases monotonically with dry and wet particle diameter, i.e. D<0.9µm. Factors influencing the accuracy of the measurement are evaluated, including particle drying, refractive index and shape. Growth factors at 90±3 % ambi-ent relative humidity in marine airmasses 400 km west of San Diego, California range between 1.5 and 1.8. This suggests that a significant fraction of the particle mass, between 40 and 70%, is either non-hygroscopic or weakly hygroscopic.