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MEASUREMENTS OF BLACK CARBON IN CHICAGO: IMPLICATIONS FOR CONTROLS ON DIESEL EMISSIONS
Impact/Purpose:
Large cities contain major sources of fine particulates that can influence human health and the radiative balance of the atmosphere on urban, regional, and global scales. In this context, black carbon (BC) is a key aerosol species. Use of heavy diesel engines is a significant source of diesel soot, an important contributor to BC levels in urban centers and to primary BC emissions into the atmosphere on regional and global scales.
This project was devoted to measuring BC levels in Chicago, where this summary highlights two periods before and during a major holiday. We measured the abundance of BC at a time resolution of 2-5 minutes by using a seven-channel aethalometer (Thermo Anderson). Data before and during the July 4 holiday weekend in 2004 were analyzed to assess changes in aerosol loadings as a function of vehicle traffic levels. The objective is to evaluate day-to-day changes in the abundance of BC that can be attributed to corresponding changes in the use of heavy diesel engines. By inference, we then estimate the reduction in BC levels that could result from future controls on these emissions.
Description:
Black carbon measurements were made in Chicago from the roof of the Hinds Laboratory at the University of Chicago, in the southern part of the city. The BC content of fine aerosols was measured by using a seven-channel aethalometer with a sample inlet designed to admit aerosols in the size range 0.1-2 μm. The aerosols in the air sample are collected within the instrument by continuous filtration through a paper tape strip. The optical transmission of the deposited aerosol particles is then measured sequentially at seven wavelengths (370, 450, 520, 590, 660, 880, and 950 nm). Because BC is a strongly absorbing aerosol species with a relatively constant absorption coefficient over a broad spectral region, the instrument can automatically calculate the BC content from the transmission measurements when we assume BC to be the main absorbing aerosol species in the samples with a mass-specific absorption coefficient of 19 m2 g-1, a value deduced in previous work.
The instrument is operated by an embedded computer with a display screen and keypad that controls all functions. Data are recorded to a built-in floppy diskette. Data were recorded for each of the seven channels at 2-minute time resolution. In addition, the analog output of the 520-nm channel was monitored continuously, and 1-minute averages from this channel were recorded separately. As the sample is deposited on the paper tape strip, light attenuation increases steadily. At high sample loadings, strong absorption causes a signal near the lower limit of detection. To prevent this, the instrument automatically advances the tape to a new sample spot when light attenuation becomes severe. After the tape advance, a background measurement is taken at each of the seven wavelengths to correct for variations in filter surfaces and source light intensities. The instrument also collects some sample on the filter before taking ongoing measurements. This minimizes artifacts due to light scattering from the clean filter surface. The instrument was operated in a dilution mode to minimize downtime due to excessively frequent tape advances caused by high BC loading.
Unlike other absorbing aerosol species (such as humic-like substances), the absorption of BC is relatively constant from the ultraviolet to the infrared. Thus, comparison of results from the different channels can give independent validation of the assumption that BC is the main absorbing species in the samples. For our sampling periods, all of the seven channels were in excellent agreement, with variation of only 1-2 percent, indicating that BC was indeed the major light-absorbing material present in the aerosol, and perhaps the only one.
For comparison, we made measurements of BC before and during the 4 July 2004 holiday period (Sunday, 4 July and Monday, 5 July). Figure 1 presents the BC data collected in Chicago during this time (Julian Days 186 and 187) based on the 880-nm channel. For comparison, data collected the previous Sunday and Monday (Julian Days 179 and 180) appear in Figure 2. The 24-hour averages and the maximum and minimum values for these dates, calculated for the 24-hour period from midnight to midnight, appear in Table 1.
The maximum BC measured in Chicago on 4 July 2004 was 1 μg m-3, while the maximum the previous Sunday was 2.3 μg m-3. The general traffic level in the city on the holiday was high because of crowds attending citywide celebrations. However, BC levels were low because of decreased heavy-duty diesel traffic. Maximum values for BC on Monday, 28 June were 3 μg m-3, as compared to the Monday, 5 July levels of 1.8 μg m-3. Diesel emissions clearly increased on 5 July over 4 July, but they were still below the value for the normal Monday, 28 June. Note also that the minimum values for the Mondays were about 0.3 μg m-3, while Sunday minimum levels were much lower at 0.06-0.02 μg m-3.
Table 1. Black Carbon Concentrations (μg m-3) Obtained in Chicago Using the 880-nm Aethalometer Channel on the Holidays Sunday, 4 July and Monday, 5 July 2004, as well as on Sunday, 27 June and Monday, 28 June 2004. Other wavelength channels yield similar results.
BC Concentration |
||||
Date (Day) |
Julian Day |
Max |
Min |
Avg |
27 June (Sunday) |
179 |
2.3 |
0.06 |
1.0 |
28 June (Monday) |
180 |
3.0 |
0.30 |
1.4 |
4 July (Sunday) |
186 |
1.0 |
0.02 |
0.4 |
5 July (Monday) |
187 |
1.8 |
0.30 |
0.8 |
Figure 1. Black Carbon Measurements in Chicago on Sunday July 4, 2004 (Top Panel, day 186) and Monday July 5, 2004 (Bottom Panel, day 187) Based on the 880-nm Aethalometer Channel