Comparison of surface and column measurements of aerosol scattering properties over the western North Atlantic Ocean at Bermuda

摘要

Light scattering by size-resolved aerosols in near-surface air at TudorHill, Bermuda, was measured between January and June 2009. Verticaldistributions of aerosol backscattering and column-averaged aerosol opticalproperties were characterized in parallel with a micro-pulse lidar (MPL) andan automated sun–sky radiometer. Comparisons were made between extensiveaerosol parameters in the column, such as the lidar-retrieved extinction at400 m and the aerosol optical depth (AOD), and scattering was measured with asurface nephelometer. Comparisons were also made for intensive parameterssuch as the Ångström exponent and calculations usingAERONET(Aerosol Robotic Network)-derivedaerosol physical parameters (size distribution, index of refraction) and Mietheory, and the ratio of submicron scattering to total scattering for size-segregated nephelometer measurements. In these comparisons the wasgenerally around 0.50. Data were also evaluated based on back trajectories.The correlation between surface scattering and lidar extinction was highestfor flows when the surface scattering was dominated by smaller particles andthe flow had a longer footprint over land then over the ocean. Thecorrelation of AOD with surface scatter was similar for all flow regimes.There was also no clear dependence of the atmospheric lapse rate, asdetermined from a nearby radiosonde station, on flow regime. TheÅngström exponent for most flow regimes was 0.9–1.0, but for thecase of air originating from North America, but with significant time overthe ocean, the Ångström exponent was 0.57 ± 0.18. The submicronfraction of aerosol near the surface () was significantlygreater for the flows from land (0.66 ± 0.11) than for the flows whichspent more time over the ocean (0.40 ± 0.05). When comparing and the column-integrated submicron scattering fraction,, the correlation was similar, = 0.50, but was generally less than , indicating more largeparticles contributing to light scattering at the surface, contrary toconditions over continents and for polluted continental transport over theocean. In general, though, the marginal correlations indicate that the columnoptical properties are weakly correlated with the surface opticalmeasurements. Thus, if it is desired to associate aerosol chemical/physicalproperties with their optical properties, it is best to use optical andchemical/physical measurements with both collected at the surface or bothcollected in the column.