Water uptake by mineral dust particles
Abstract
Aerosols affect the physical and chemical characteristics of the troposphere in a number of different ways. Light scattering by suspended particles, the provision of cloud condensation nuclei, heating effects due to the absorption of visible or long-wavelength infrared radiation, and health effects are all important phenomena. In terms of atmospheric chemistry, aerosol particles provide sinks for important reactive intermediates, and may significantly affect gas-phase chemistry through other surface-gas interactions. Thus the size, composition, and phase of a given aerosol particle determines its chemical and radiative impact in the troposphere. For example, the amount of water incorporated into the aerosol will affect the chemical reactivity of the aerosol and consequently the chemical lifetimes of interacting gas-phase species. There is thus a need to measure hygroscopic growth factors for aerosols derived from different particles encountered in the troposphere. Mineral dust is the single most abundant aerosol in the troposphere. Estimations of the global source strengths vary from ∼200 to 5000Mtyr-1, representing ∼50% of the total production of tropospheric aerosols. In the present study, preliminary experiments have been carried out to assess the effect of relative humidity on the increase in the mean diameter of aerosol particles - hygroscopic growth - derived from CaCO_3 (a proxy for the alkaline component of mineral dust) and from Arizona mineral dust. Aerosols were produced by passing a stirred suspension, of the particle source of interest, through a nebuliser. The relative humidity of the aerosol stream was controlled using a combination of dehumidifying (silica gel adsorbant traps) and humidifying stages (mixture of dry N_2 and N_2 entraining water vapour from H_2O bubblers). In this way relative humidities from 2-80% were achieved. Aerosol size distributions were then measured using a differential mobility analyser (DMA) and the change in their mean diameter subsequently assessed. Initial experimental results show that the Arizona mineral dust aerosol has a slightly higher affinity for water than the model, CaCO_3 aerosol. It can thus be expected that their impact on tropospheric chemical processes will be correspondingly different.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....10805G