Airborne observations of AOD-CCN relationship

Aerosol optical depth (AOD) and its variation weighted by the wavelength dependence (AI) are commonly used as a proxy for aerosol loading in model evaluation of aerosol indirect effects on climate. This can be justified only if these optical properties are related to the number concentration of cloud condensation nuclei (CCN) on which cloud microphysics rests. The objective of our study is to describe the AOD-CCN relationships observed during airborne experiments in various environments. We fit data (e.g., apply linear regression to logCCN vs. logAOD) and explain the deviation of individual data points from the result by examining the aerosol size distribution, extinction per dry particle, critical dry diameter (derived from CCN concentration and aerosol size distribution), humidity effect on extinction, vertical profile, horizontal variability and measurement noise. The Canada phase of the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission, which mainly sampled smoke from local forest fires, saw a factor of 2.5 deviation of the near-surface CCN concentration (averaged over 10s and adjusted to 0.4% supersaturation, per cm3) from the fit CCN = 2.2*10^3*AOD499nm^0.68. With the impact of vertical profile excluded, the deviation (i.e., the geometric standard deviation of the near-surface extinction to CCN ratio) is 2.0. We will compare the fit and deviation with separate observations in polluted marine air with generally higher particle hygroscopicity, high ambient humidity and diverse sources of particles.