ANTHONY D.A. HANSEN, Magee Scientific Company, Berkeley, California 94703
This project is part of the Antarctic Environmental Research Program, helping to assess the possible environmental impacts of human activities in Antarctica on the pristine surroundings. One of these impacts is due to the emission of "black carbon" (BC) particles in the combustion exhaust from diesel-powered generators, oil-fired space heaters, and vehicles used to support antarctic operations. When deposited to the snow and ice, BC has a number of environmental effects:
Previous work showed that extremely small, but nonetheless detectable, concentrations of BC aerosol are brought to the South Pole in the "background" atmosphere, consistent with models of long-range atmospheric circulation. A 1986 study of the soot concentrations in surface snow and ice at the South Pole showed a clear plume of deposition downwind of the station. In this project, we installed equipment upwind and downwind of the Amundsen-Scott South Pole Station to re-establish the background aerosol measurements and to monitor the station's combustion emissions.
I deployed to Antarctica (I was at South Pole from 1 to 10 February 1997) and installed two Aethalometer™ instruments at the South Pole Station to measure the concentration of optically absorbing aerosol BC particles suspended in the air. The instruments are automatic and require no operator attention other than periodic checking. Approximately every 2 weeks, the data files from each instrument are transmitted by e-mail, assembled into spreadsheets, and merged with the station's meteorology data.
The upwind instrument at the Atmospheric Research Observatory (ARO) records data on a timebase of 1 hour. Very low concentrations of BC are measured, representing the transport of polluted air to the pole from lower latitudes. Figure 1 shows the smoothed time series of data from this upwind location. Mean aerosol BC concentrations decreased from 3 nanograms per cubic meter (ng/m3) at the end of austral summer to less than 0.1 ng/m3 in the middle of winter. The reduction is due to the formation of the polar vortex, which prevents incursions of polluted air from lower latitudes. Occasional events of elevated BC occur from time to time, due either to a shift in prevailing wind direction so that the station's plume blows toward the ARO site or perhaps to vehicle operation near the building.
The downwind instrument at the Balloon Inflation Facility (BIF) records data every 10 minutes. The BIF is the farthest away downwind building that is heated and powered during the winter season: the sampling inlet is approximately 270 meters from the main exhaust stacks of the central generating plant. High concentrations of BC are measured whenever the wind blows the exhaust plume over the sampling location. Figure 2 shows a typical 2-week record from the downwind instrument. When the wind is not directly from the exhaust stacks, the BC concentrations are low. Whenever the exhaust plume passes over the measurement point, we see giant excursions in the BC data.
We merged the wind speed and direction data from the meteorology files with our BC concentration results. The emission source strength—i.e., the number of grams of BC emitted per second from the diesel engines—is expected to be approximately constant, although the exhaust is dispersed into a wind of varying speed. Consequently, rather than examining BC concentration versus wind direction, we calculate the flux of BC passing the measurement point by multiplying the concentration and the wind speed.
Figure 3 shows the BC flux data gathered into 10-degree sectors. The vast majority of data points lie in the vector range of +90° to -45°, representing the prevailing wind direction. Individual flux measurements up to almost 350 micrograms per square meter per second (µg/m2/sec) are calculated when the wind direction lies in the sector from +10° to +30°, which corresponds to the +22° grid angle from the exhaust stack to the BIF building. The solid line shows the sector-averaged BC flux, peaking at about 20 µg/m2/sec for the 10-20° sector. The broken line shows the number of data points represented in each sector and clearly shows the dominance of the prevailing wind. Occasional high flux values are recorded for wind directions up to +90°. Very few data points are seen with wind directions outside this dominant range, and none of those show high BC values. These results confirm our expectation of the impact of the exhaust plume from the generator stack passing the measurement point.
We had earlier estimated that the emission rate of aerosol BC from the main diesel generators at the South Pole Station is on the order of 1,000 µg/sec, based on fuel consumption rates and typical emission indices. The highest 10-minute average flux computed from the downwind instrument's data is over 300 µg/m2/sec and is consistent with the emission estimate.
The data suggest that the emission rate of aerosol BC from the main generators of the South Pole Station is indeed on the order of 1 milligram per second (mg/sec). The upwind data show that the BC concentration in the background atmosphere is in the range of 0.1 to 0.3 ng/m3 for several months. Because the background air is so clean, the emissions will be detectable in a plume expanding to a width of some tens of kilometers, extending some hundreds of kilometers downwind. The plume of combustion exhaust particles from the South Pole Station would sweep around with the wind direction and be detectable over background—i.e., represent a measurable environmental impact—at great distances from the station itself.
This impact represents only that from year-round, winter season activities—i.e., the steady emission of soot from the main generators. It is my personal belief that summer season activities are likely to lead to far greater emissions of soot from aircraft operations, intensive vehicle operations, heating of outlying buildings, and so forth. As this project proceeds, our ongoing data collection will contribute to the assessment of the environmental impact from combustion emissions at the South Pole Station.
This research was supported by the Antarctic Environmental Research Program of the Office of Polar Programs, National Science Foundation, under grant OPP 95-30428.