Apportionment of observed aerosol optical thickness to aerosol dry matter and water

Arjan van Beelen

In this project I work under supervision of Dr. Geert-Jan Roelofs (IMAU), Prof. Thomas Röckmann (IMAU) and Dr. Otto Hasekamp (SRON). Our goal is to find a method to separate the contributions of aerosol dry mass and water to aerosol optical thickness (AOT), to improve (remote sensing) estimates of the (dry) aerosol content in the atmosphere. The effects of aerosols are one of the largest uncertainties in our understanding of the climate system. Measurements of AOT from the ground or by satellites are often used to validate the aerosol-climate models and assess the effects (e.g. radiative forcing) of natural and anthropogenic aerosol on climate. The relation between AOT, aerosol dry mass and chemical composition is however not ambiguous. Aerosols, often consisting of hygroscopic matter will tend to accumulate water, altering its optical properties with significant effects on AOT. This might be an important reason for the current discrepancies between model and satellite derived estimates of the aerosol radiative forcing. For the separation we will use AOT and refractive indices from the Advanced Polarimetric Sounder (APS), and atmospheric humidity and backscatter vertical profiles taken from global aerosol-climate simulations. A model will be used to construct different atmospheric profiles of aerosol mass, hygroscopicity and aerosol water, and with a minimization technique the optimal profiles are determined. Separating the contributions of water and aerosol dry mass on retrieved AOT will lead to more efficient use of remote sensing measurements and might improve estimates of aerosol climate forcing and validation of models. It will also increase our understanding of aerosol-humidity interactions, assess the increase of aerosol water due to anthropogenic aerosols and reduce discrepancies between satellite derived and simulated estimates of the aerosol radiative forcing.