Preliminary Results of in-situ Measurements of Aerosol Optical and Water Uptake Properties from the ARM Mobile Facility in Niger
| Jefferson, Anne | NOAA CMDL |
| Ogren, John | NOAA/CMDL |
Category: Field Campaigns
The second deployment of the ARM Mobile Facility was in November of 2005 to Niamey, Niger. The year-long deployment is part of the AMF Radiative Divergence using AMF, GERB and AMMA Stations (RADAGAST) field campaign. The goal of this campaign is to provide direct estimates of the divergence of solar and thermal radiation across the atmosphere as well as study the role of clouds, aerosol and water on surface solar radiation. The site in Niger is located in the Sahel at the southwestern edge of the Sahara Desert. The aerosol of this region is a mix of desert dust and smoke from biofuel burning. Changes in the air mass between the wet and dry seasons will allow us to look at the influence of the aerosol on direct, clear-sky, radiative forcing as well as how cloud processing and wet deposition influence aerosol properties. We show preliminary results from the first three months of in-situ aerosol measurements of aerosol light scattering and absorption, water uptake properties of the particles and their ability to act as cloud condensation nuclei. The winter months are during the dry season, when the ambient daytime relative humidity is less than 10%. Aerosol loading is relatively high with average scattering coefficients for the sub 10 um size aerosol at 550 nm near 100 Mm-1. Most of the aerosol scattering resides in the super micron size mode. The Ǻngstrom exponent for the 550/700 nm pair for sub10 um size aerosol is between 0.0 and 0.5, which indicates a large particle size. The submicron Ǻngstrom exponent varies between 0.5 and 2.0. The aerosol single scattering albedo at 550 nm indicates a relatively dark aerosol with values ranging from 0.55-0.8 for subum size aerosol and 0.7 to 0.9 for sub 10um size aerosol. The variation of the aerosol absorption coefficient across three wavelengths of the visible spectrum will be shown. The water uptake properties of the particles are important in determining both direct and indirect aerosol radiative forcing. The hygroscopic growth factor, which is a measure of the increase in the aerosol scattering coefficient for a relative humidity increase from 40 to 85%, is remarkably low for both sub10 um and submicron size particles, averaging about 1.0 and 1.2, respectively. Cloud condensation nuclei (CCN) are measured at five supersaturations. We observe a distinct diurnal variation in the CCN number concentration which coincides with observer observations of night time biofuel burning.
www.arm.gov/sites/amf/niamey.stm
This poster will be displayed at the ARM Science Team Meeting.
