Monday, 10 January 2005
Surface and Aloft Measurements of Aerosol Concentrations in a Wintertime Boundary Layer at a Mid-Atlantic Site
A detailed examination of the structure and evolution of the wintertime boundary layer was conducted from 3 January - 14 February 2004 near Lancaster, PA in support of the research objectives of the Mid-Atlantic/Northeast - Visibility Union (MANE-VU). Two tethered balloons were used to deploy meteorological sensors, condensation particle counters, laser-diode scatterometers, and filter samplers to altitudes of 750 m AGL, while a suite of ground-based instruments measured trace gas and particle concentrations and meteorological parameters, and where used for calibration and intercomparison with balloon-borne instruments. Sensors onboard tethered balloons used in conjunction with surface measurements have the unique ability to sample the boundary layer with high spatio-temporal resolution and characterize the coupling between the surface and free atmosphere. Measurements were primarily limited to times when progressive anticyclones moved over the site, bringing clear skies, strong nocturnal radiational cooling, and wind speeds not in excess of 12 ms-1 for durations ranging from several hours to 2-3 days. It is during these episodes of large-scale subsidence and relatively light winds that particulates can accumulate in the boundary layer, and may deleteriously affect human health. This study focuses on the characterization of particulate distributions in a wintertime mid-Atlantic boundary layer. The tethered atmospheric sounding system consisted of a 21 m3 blimp with a free lift capacity of 7.5 kg, which was used for vertical profiling, and a 4-meter diameter balloon, also capable of lifting 7.5 kg in calm winds (considerably more in higher wind) for long-duration time series. Over 120 vertical profiles were performed over the six-week project duration. Each platform carried a meteorological sensor package (T, p, q, vector wind), optical scatterometer (DustTrak Model 8520) and condensation particle counter (TSI Model 3007), and had the capability of long duration (8-12 hours) integrated aloft sampling using SKC PEMs with PM2.5 size cuts. Surface instruments included a portable meteorological package, a suite of trace gas analyzers (O3, NO/NO2/NOX, SO2, and CO) and a TSI 3-wavelength nephelometer (ƒ"-coeff). In addition, an Aethalometer provided BC and UV-C concentrations, while a MetOne Model 9012 Ambient Aerosol particle sizer yielded particle concentrations in six size bins. Eta and WRF gridded data, satellite and radar imagery, surface and upper data, EPA PM2.5 data, and HYSPLIT trajectories were archived in order to later place the site measurements into a regional context.
January 2004 was characterized by a very active synoptic pattern that frequently brought Arctic air into the mid-Atlantic region and resulted in this being the 10th coldest January on record. Daytime conditions were marked by the rapid development of the nearly adiabatic mixed layer of uniform winds extending to a depth of 500 - 700 m AGL and capped by a subsidence inversion. As is typical of mixed layers, particulate concentrations remained relatively uniform with height, however considerable differences from day to day were observed as a function of wind direction. The nighttime periods were considerably more complex with stratification embedded within and above the inversion, and significant variability in wind speed and direction, water vapor mixing ratio, particle concentration, and scattering coefficient observed across layers that were often only tens of meters thick. Depending on the atmospheric condition around sunset and the rate of development of the nocturnal inversion, high concentrations of particles were found trapped near the surface and/or in shallow stable layers within the inversion. The tethered balloons were deployed to capture this detail by first performing a vertical profile to examine the boundary layer structure. Once potential layers of interest were identified in the profile, a second balloon was tethered and parked at that altitude for durations up to 12 hours in order to gather a time series of meteorological variables and particle concentrations. Preliminary results reveal high temporal variability at the surface with particle counts ranging from 12,000 - 50,000 cm-3, and episodes when black carbon peaked at 3.00 micrograms per cubic meter and greater. Although the site was considered rural, increased traffic along a road adjacent to the site produced signatures during the higher frequency periods in the morning and early evening. Wood smoke also contributed to diurnal variability, and the profiles were able to capture the stratification of wood smoke and other airborne particulates during boundary layer stabilization. These data can be used to validate boundary layer parameterization schemes used in numerical models and help in gleaning a better understanding of wintertime transport and mixing in the lower atmosphere.