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FDP - CENTER FOR ENVIRONMENTAL SCIENCE
Impact/Purpose:
This project is to establish a Center for Environmental Science as a joint effort of the University of Chicago and Argonne National Laboratory. The broad mission of the Center is to enhance understanding of ways in which human activity, particularly urbanization, alters the chemical and physical environments to which large populations are exposed, with emphasis on local and regional atmospheric and surface conditions. Specific scientific objectives are (1) to develop empirical descriptions of the radiative, chemical and thermal environments of the Chicago area, and (2) to identify and evaluate the physical mechanisms responsible for altered air chemistry and regional climatic conditions.
Description:
The suite of instruments that supported the research described here, collectively called the “Urban Atmosphere Observatory” (UAO), was located on the roof of the Geophysical Science Building on the campus of the University of Chicago. The following sensors operated at UAO during all or part of the 3-year period encompassed by the grant: (1) a Multifilter Rotating Shadowband Radiometer (MFRSR) that recorded solar irradiance, including the direct and diffuse components separately in five visible bands and one near-infrared wavelength band; (2) a Brewer Spectrophotometer that recorded ultraviolet solar spectral irradiance over the 2 wavelength range 300-350 nm; (3) a seven-channel, and later a two-channel, aethalometer that measured the abundance of atmospheric black carbon; (4) a nephelometer for monitoring atmospheric particulate matter abundances; (5) a Kipp and Zonen net radiometer that measured total solar irradiance and total thermal infrared irradiance, including the upward and downward components of each; and (6) a weather station that provided ongoing data on air temperature, relative humidity, wind speed and wind direction in support of the other datasets. A second net radiometer and weather station, identical to those listed above, operated for several months on the roof of a low-rise building surrounded by an urban canyon in downtown Chicago.
A Quality Assurance Plan was approved by the U.S. Environmental Protection Agency (EPA) shortly after the start of funding in 2003. Instruments located at UAO underwent periodic checks to ensure proper operation. As part of their standard operating sequences, trace gas and particle monitors perform internal calibrations using known quantities of the constituent of interest. Consistency of measurements from the two net radiometers was evaluated by direct comparison of the signals recorded when the instruments were co-located.
The following sections briefly identify the objectives and relevance of each research effort, while additional details appear in the separate reports for each project.
Attenuation of Ultraviolet Solar Radiation by Cloudy Skies: Links to Urban Air Quality (Grant No. CR830890C003)
This project examined the dataset obtained by the Brewer Spectrophotometer that operated at the University of Chicago as part of the national network formerly supported by EPA. The initial goal was to evaluate the influence of local cloudiness on spectral irradiances received at the ground, emphasizing the wavelength band 300-350 nm. However, a close examination of the measurements revealed a wavelength-dependent attenuation in the presence of thick cloud cover that could not be explained by scattering alone or by absorption associated with elevated ozone amounts in the cloud-free portion of the troposphere. Comparison with a set of radiative transfer calculations demonstrated that the magnitude and wavelength dependence of the excess absorption were consistent with absorption by ozone located in the interstitial air of a cloud. The multiple scattering that occurs in the interior of a cloud produces a long effective geometrical path through the absorbing medium, which in the case of ozone implies greater attenuation as wavelength decreases from 350 nm toward 300 nm. While the magnitude and wavelength dependence of the excess absorption were compatible with typical tropospheric ozone amounts in an urban area, a contribution from absorption by carbon-containing aerosols cannot be ruled out.
Measurements of Black Carbon in Chicago: Implications for Controls on Diesel Emissions (Grant No. CR830890C004)
This project involved performing measurements of the black carbon (BC) abundances in urban air, initially using a seven-channel aethalometer. The instrument pumps outdoor air through a chamber where particulate BC is deposited on a translucent tape. The measured transmissions of visible beams of light (at seven wavelengths) incident on the tape, taken with the laboratory calibration, yield estimates of the BC abundance in urban air. Data samples, obtained at 2- to 5-minute intervals for over 1 year, showed typical BC abundances in Chicago to lie in the range of 1 to 3 micrograms per cubic meter of air.
Diesel engines are likely a major source, but not the only source, of atmospheric BC. The number of diesel-powered vehicles on Chicago’s highways varies between weekdays and holidays, and this difference provides a means to estimate the contributions of local traffic to observed BC concentrations. Examination of data selected to refer to these different periods revealed BC abundances on holidays that were one-third to one-half those on work days. This observation provides an estimate of the permanent reductions in urban BC that could be expected in response to future controls on diesel emissions.
Attenuation of Visible Sunlight by Limited Visibility and Cloudiness (Grant No. CR830890C005)
This project examined the dataset obtained by the MFRSR with the goal of characterizing the attenuations of visible sunlight associated with limited visibility and cloudiness, including any spectral dependence. The role of visibility is of particular interest since this form of atmospheric opacity varies in response to the particulate matter (PM) abundances in an urban area. Elevated PM concentrations, in combination with high values of relative humidity, lead to formation of liquid droplets and the urban haze that restricts visibility.
An enhanced atmospheric optical depth for scattering is accompanied by reduced ground-level solar irradiances in the visible portion of the spectrum. Statistical analyses revealed a significant correlation (at better than the 1% level of confidence) between visible irradiances measured at UAO and visibility reported in standard meteorological observations at Midway Airport, located approximately 12 kilometers west of the University of Chicago. The results show that a reduction in visibility from 10 miles to 5.7-5.8 miles is, typically, accompanied by a decrease in solar irradiance of about 10 percent at all wavelengths from 416 to 868 nm. Although the influence if changing visibility is readily detected in the MFRSR’s database for clear skies, the attenuation associated with clouds is far more significant when averaged over time periods of several days to weeks.
The Energetics of Urban Microclimates (Grant No. CR830890C006)
This project combined measurements of solar radiation, terrestrial radiation, and meteorological variables to estimate the energy fluxes that heat and cool the underlying surface, with a focus on the impact of urban development in localized areas, referred to as “microclimates.” The objective was to determine the effects of urban development on the energy balance of selected surfaces and the consequent influence on temperatures. This, in turn, influences the demand for energy. Instrumentation consisted of the Kipp and Zonen Net Radiometer, which measures both total solar and total terrestrial irradiance in both the upward and downward directions, and the Davis Instruments weather station, which measures air temperature, relative humidity, wind speed, and wind direction. Two identical sets of sensors were deployed. One set was located at UAO and the other was mobile, operating at both rural and heavily urbanized sites. The resulting datasets were combined with a statistics-based model to describe the energy balances of various types of surfaces. The fluxes associated with heating and cooling were either measured directly (upward and downward, solar and terrestrial irradiances) or estimated via regression techniques by adopting algebraic forms based on measured surface temperature, air temperature and winds.
A comparison of the energy balance of a low-rise rooftop surrounded by buildings 30 or more stories tall (the “urban canyon site”) in downtown Chicago with that of a concrete roof with no nearby obstructions (the “control site” at UAO) showed the following: (1) blocking of sunlight by tall structures in the urban canyon depressed air temperatures during the day; while (2) thermal emission from the vertical faces of buildings suppressed cooling overnight. The result is a diurnal cycle of air temperature in the urban canyon which had smaller amplitude than that at the control site.
A separate comparison of the green garden rooftop portion of Chicago’s City Hall with the concrete roof of UAO’s control site demonstrated the importance of evaporative cooling in reducing the surface temperature of the former location. Solar and terrestrial radiation, acting together, led to a net heating of surfaces at both sites in a 24-hour average sense. In response, the concrete roof achieves its energy balance by warming to a high temperature relative to adjacent air. The surface-to-air temperature contrast leads to a large flux of sensible heat from the solid surface to the atmosphere. The green roof, however, cools by evaporation of water stored in the soil. This allows the green surface to maintain an energy balance while having a temperature that differs only slightly from that of the surrounding air.
These results have implications for the design of livable, energy-efficient urban spaces. The techniques developed here could be used to evaluate the energy budget of any localized area. When combined with the energy balance model, it is possible to estimate the energy savings available from actions such as altering the albedos of surfaces exposed to the sun, replacing solid roofs by green roofs, and the addition of manmade pools of water. The government of DuPage County, Illinois, which is part of the Chicago metropolitan area, has expressed interest in incorporating the methodology and results of this research into its long-range planning for urban growth.