ARM Program Surface Measurements for Aerosol Profiles Shown to Represent Integrated Column Measurements
Andrews, E., University of Colorado
Aerosol Properties
Aerosol
Andrews, E., P. J. Sheridan, J. A. Ogren, and R. Ferrare (2004), In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 1. Aerosol optical properties, J. Geophys. Res., 109, D06208, doi:10.1029/2003JD004025.
Delle Monache, L., K. D. Perry, R. T. Cederwall, and J. A. Ogren (2004), In situ aerosol profiles over the Southern Great Plains cloud and radiation test bed site: 2. Effects of mixing height on aerosol properties, J. Geophys. Res., 109, D06209, doi:10.1029/2003JD004024.
A comparison of forcing calculated from aircraft and SGP surface measurements suggest that integrated surface measurements are representative of the column above. Dashed lines indicated 10% difference from 1 to 1 line; solid black lines are linear regression fit.
Understanding the effects of aerosol on the earth radiation balance requires knowledge of aerosol properties throughout the entire atmospheric column. The simplest way to measure the properties of atmospheric aerosols is to use instruments at the earth surface. Unfortunately, the correlation between aerosol properties aloft and surface aerosol properties is not well understood. Researchers funded by the Department of Energy's Atmospheric Radiation Measurement (ARM) Program have shed new light on this problem using multi-year ARM measurements. They report in the Journal of Geophysical Research (JGR, March 2004) that long-term surface aerosol measurements adequately capture column aerosol properties, but do not well represent day-to-day variations in the column. Based on in situ flight data and ground-based surface measurements from ARM's Southern Great Plains (SGP) site in Oklahoma, statistical plots of aerosol properties show that surface measurements can be a reasonable surrogate for integrated column measurements. A parallel effort to evaluate the effect of mixing height on aerosol properties (JGR, March 2004) resulted in similar conclusions with respect to the feasibility of using SGP surface based aerosol measurements to infer aerosol properties within the atmospheric boundary layer.
In this study, the researchers compared 2 years worth of surface measurements with corresponding aerosol data collected by a comparably instrumented aircraft. Between March 2000 and March 2002, the aircraft flew 253 vertical flight profiles over the SGP site, at altitudes ranging from 500 to 3,500 m. Flights occurred several days per week at 8am, noon, and 4pm CST, to capture variability in hourly and daily timescales and obtain a statistically representative data set of in situ aerosol vertical profiles. The researchers also compared aerosol optical depth (AOD) calculated from the in situ flight data (adjusted to ambient temperature, pressure, and relative humidity) with AOD from ground-based radiation instruments at the SGP.
The researchers divided aerosol characteristics into two categories: "intensive" properties, which are tightly coupled to aerosol chemistry and size, and "extensive" properties, which are related to the concentration or number of aerosols per unit volume of air. The working hypothesis is that the intensive properties should be relatively constant in an air column, while the extensive properties may change with height due to dilution of the aerosol concentration by atmospheric processes. The data analysis confirmed this hypothesis by showing that while median values of extensive parameters (light absorption, scattering, and extinction) tended to decrease with altitude, the median values of intensive parameters (single scattering albedo, Angstrom exponent and backscatter fraction) were relatively constant with altitude.
The SGP site is the only location in the world where ground-based and in situ airborne measurement of aerosol extensive and intensive properties are made on a routine basis in conjunction with measurements of the meteorological parameters necessary to estimate mixing height. Since 1992, ground-based instruments at the SGP site have continuously collected atmospheric data for the study of cloud and radiative feedback processes. These long-term data sets are critical for developing comprehensive statistical frameworks from which meaningful conclusions can be drawn. As demonstrated in this two-part study, these findings are important in evaluating the seasonal effects of aerosol optical properties, and the role they play in climate modeling.
