Abstract
Abstract. A small airplane made 597 aerosol optical property (light absorption and light scattering) vertical profile measurements over a rural Oklahoma site between March 2000 and December 2007. The aerosol profiles obtained during these 8 yr of measurements suggest significant seasonal differences in aerosol loading (scattering and absorption). The highest amounts of scattering and absorbing aerosol are observed during the summer and the lowest loading occurs during the winter. The relative contribution of aerosol absorption is highest in the winter (i.e., single scattering albedo is lowest in winter), particularly aloft. Aerosol absorption generally decreased with altitude below ~1.5 km and then was relatively constant or decreased more gradually above that. Aerosol scattering decreased sharply with altitude below ~1.5 km but, unlike absorption, also decreased at higher altitudes, albeit less sharply. Scattering Ångström exponents suggest that the aerosol was dominated by sub-micron aerosol during the summer at all altitudes, but that larger particles were present, especially in the spring and winter above 1 km. The seasonal variability observed for aerosol loading is consistent with AERONET aerosol optical depth (AOD) although the AOD values calculated from in situ adjusted to ambient conditions and matching wavelengths are up to a factor of two lower than AERONET AOD values depending on season. The column averaged single scattering albedo derived from in situ airplane measurements are similar in value to the AERONET single scattering albedo inversion product but the seasonal patterns are different – possibly a consequence of the strict constraints on obtaining single scattering albedo from AERONET data. A comparison of extinction Ångström exponent and asymmetry parameter from the airplane and AERONET platforms suggests similar seasonal variability with smaller particles observed in the summer and fall and larger particles observed in spring and winter. The observed seasonal cycle of aerosol loading corresponds with changes in air mass back trajectories: the aerosol scattering was higher when transport was from polluted areas (e.g., the Gulf Coast) and lower when the air came from cleaner regions and/or the upper atmosphere.
Highlights
Understanding the temporal and spatial variability of aerosol optical properties is important for relating aerosols to their sources, quantifying the effects of transport and transformation on the aerosol and understanding the contribution of aerosol to such wide-ranging concerns as health, visibility and climate
Park et al (2003) used a combination of elemental carbon measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and model simulations to determine the spatial distribution of carbonaceous aerosol concentrations across the US
There were no IMPROVE sites in Oklahoma at the time of their study, so the predicted values of EBC in the region of the Southern Great Plains (SGP) site were the result of filling the gap in measurements between the eastern and western US
Summary
Understanding the temporal and spatial variability of aerosol optical properties is important for relating aerosols to their sources, quantifying the effects of transport and transformation on the aerosol and understanding the contribution of aerosol to such wide-ranging concerns as health, visibility and climate. Of particular interest for climate change is the question of the vertical distribution of absorbing aerosol in the atmosphere (e.g., Haywood and Ramaswamy, 1998; Ramanathan et al, 2001). NOAA and Department of Energy/Atmospheric Radiation Measurement (DOE/ARM) developed a light aircraft-based aerosol profiling platform for year-round, long term measurements of climatically important aerosol properties. This platform was implemented at the DOE site in Oklahoma (e.g., Andrews et al, 2004) and a similar platform operated over a NOAA surface site in central Illinois between 2006–2009 (Sheridan et al, 2011).
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