Relations between cloud condensation nuclei and aerosol optical properties relevant to remote sensing

Abstract

Clouds affect the Earth's energy balance by reflecting incoming sunlight and trapping outgoing heat. It has been hypothesized that various cloud properties (e.g., reflectivity, coverage, lifetime and precipitation efficiency) partly depend on the number concentration of aerosols that serve as cloud condensation nuclei (CCN) during cloud growth. The CCN number varies widely throughout the world over time, and is poorly known. Therefore, a means to convert satellite derived aerosol optical properties to CCN concentration would help evaluate this important indirect effect of aerosols on radiation balance. In order to assess whether remote sensing of CCN is feasible, this dissertation explores the relationships among aerosol size distribution, chemical composition and optical properties measured during aircraft experiments over USA and Mexico.The aircraft data illuminate four reasons why remote sensing of CCN concentration can potentially achieve a small relative error (<20%) over Mexico. First, many pollution particles are large enough to significantly affect light extinction. They even dominate it in absence of dust. Consequently, the correlation between extinction and CCN number is high, once the extinction due to dust is excluded using the wavelength dependence of extinction. Second, the ambient humidity is usually low, which reduces the error in the estimated response of particle extinction to humidity changes. Third, because many CCN contain black carbon, light absorption also provides a measure of the CCN concentration. The fourth reason is pertinent to the chemical properties of CCN. The organic fraction of volatile mass of submicron particles (OMF) was found to be anti-correlated with the wavelength dependence of extinction. This anti-correlation provides a potential tool for remote-sensing not only OMF but also the CCN activity, if the relationship between these two properties is tight and known. The concurrent aircraft measurement of aerosol size distribution suggests that the observed anti-correlation reflects preferential uptake of anion (sulfate and nitrate) by coarse dust. This chemical action may significantly reduce solubility of accumulation-mode particles and, consequently, CCN concentration. All of these four features are weaker over USA and the surrounding oceans than in Mexico.