Detailed Mesometeorological Studies of Air Pollution Dispersion in the Chicago Lake Breeze

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  • 1 Col1ege of Engineering and Applied Science and Center for Great Lakes Studies, The University of Wisconsin—Milwaukee
  • | 2 Department of Atmospheric Science, Oregon State University, Corvallis, Oreg.
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Abstract

The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensional divergence values defined the approximate u, w streamline patterns with time. Convergence values in a narrow (1–2 km) zone at the lake-breeze front exceeded 200×10−3 s−1, and calculated upward motions reached 125 cm/s. Optically tracked tetroons yielded Lagrangian trajectory data that confirmed the basic pattern. Most importantly, the tetroons recirculated within the lake-breeze cell, describing a helical trajectory roughly centered on the shoreline. This strongly suggests that air pollutants will likewise be at least partially recirculated over the shoreline, accumulating to levels higher than would otherwise be expected.

An NCAR Queen Air instrumented aircraft took continuous cross sections of particulate concentrations and temperature through the lake-breeze life cycle. The smaller suspended particulates (0.5–3.0 µm), which essentially float with the air, clearly suggest that a significant, fraction of the pollutants released from nearshore sources move inland within the inflow, rise aloft, at the front, advect lakeward in the return flow layer, and then sink back down into the inflow layer offshore. By contrast, larger particles (7–9 µm), having significant terminal velocities, fall out of the cell while over the lake and do not appear to take part in the recirculation phenomena. The role of continuous fumigation of plumes from elevated point sources is also discussed. A schematic model of the lake breeze and its effects on pollutant transport is presented.

Now at Sveriges Meteoroldgiska Och Hydrologiska Institut, Stockholm, Sweden

Abstract

The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensional divergence values defined the approximate u, w streamline patterns with time. Convergence values in a narrow (1–2 km) zone at the lake-breeze front exceeded 200×10−3 s−1, and calculated upward motions reached 125 cm/s. Optically tracked tetroons yielded Lagrangian trajectory data that confirmed the basic pattern. Most importantly, the tetroons recirculated within the lake-breeze cell, describing a helical trajectory roughly centered on the shoreline. This strongly suggests that air pollutants will likewise be at least partially recirculated over the shoreline, accumulating to levels higher than would otherwise be expected.

An NCAR Queen Air instrumented aircraft took continuous cross sections of particulate concentrations and temperature through the lake-breeze life cycle. The smaller suspended particulates (0.5–3.0 µm), which essentially float with the air, clearly suggest that a significant, fraction of the pollutants released from nearshore sources move inland within the inflow, rise aloft, at the front, advect lakeward in the return flow layer, and then sink back down into the inflow layer offshore. By contrast, larger particles (7–9 µm), having significant terminal velocities, fall out of the cell while over the lake and do not appear to take part in the recirculation phenomena. The role of continuous fumigation of plumes from elevated point sources is also discussed. A schematic model of the lake breeze and its effects on pollutant transport is presented.

Now at Sveriges Meteoroldgiska Och Hydrologiska Institut, Stockholm, Sweden

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