Philadelphia Fine Particle Concentrations
and Visibility Study
By:
Rudolf B. Husar, Stefan R. Falke, and William E. Wilson
June 13, 1997
Abstract
Fine particle data are typically monitored in 6-day or 3-day increments and at limited number of
urban sites. On the other hand, the estimation of exposure for epidemiological studies requires
daily fine particle concentration values. The purpose of this analysis is to estimate the daily fine
particle concentration in Philadelphia for the period 1979-1983. The temporal interpolation
method requires the estimation of fine mass for two days sandwiched in between the 3-day
sampling intervals. The interpolation utilizes airport visibility observations and the regression
analysis that relates the weather corrected extinction coefficient to fine mass. Further
understanding of the fine mass spatial pattern in Philadelphia was gained from the analysis of the
data from the saturation monitoring study and from data collected at a Philadelphia station from
1992-95.
Table of Contents
Introduction ..................................................................................................................................... 1
Methodology of PM2.5 Estimation................................................................................................. 1
Visibility Derived Extinction Coefficient for Philadelphia ........................................................ 1
Philadelphia Saturation Study, September-October 1994 .......................................................... 2
Fine Mass-BEXT Relationship During the Saturation Study ............................................... 10
The 1979-1982 Monitoring Data Set ........................................................................................ 15
Relationship between PM2.5 and Extinction during 1979-82 .............................................. 18
The 1992-95 Monitor Site Data Set .......................................................................................... 22
Relationship between FBext and Rel. Humidity and Temperature during 1992-95............. 23
References ..................................................................................................................................... 25
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Introduction
Epidemiological studies that relate fine particles to mortality and morbidity are frequently
hampered by incomplete aerosol concentration data for exposure estimates. The data limitations
are both spatial and temporal. Spatial coverage is limited to a few sites, some of which may be
influenced by local sources. Sometimes, such source-oriented sampling is desirable to
characterize the exposure levels near a specific source.
The temporal data limitations arise from the fact that due to cost considerations aerosol sampling
and weighing is conducted in 3-day or 6-day increments, rather than daily. The purpose of this
work is to provide a continuous, daily PM2.5 concentration estimate for Philadelphia 1979-1983.
The Philadelphia metropolitan area has been a subject of several intensive fine particle sampling,
epidemiological analysis since the late 70s (Burton et al., 1996; Tropp et al., 1996). For this
analysis we are using monitoring data collected during the 1979-1983 period for multiple sites in
the Philadelphia area, monitoring data collected at a single site from 1992-1995, the Aerosol
Saturation Monitoring Study, September-October, 1994, and the visibility derived extinction
coefficient for the Philadelphia International Airport.
Methodology of PM2.5 Estimation
Estimating the PM2.5 concentration during the days without sampling is based on the visual
range data as a surrogate for fine mass. The steps in the analysis are listed below:
Visibility Derived Extinction Coefficient for Philadelphia
The first step was to examine the pattern of the light extinction coefficient in Philadelphia. The
long-term 1960-1995 trends allow placing the 1979-1983 period into a historical perspective.
Since the extinction coefficient depends on natural obstructions to vision as well as on fine
particles, the extinction data require filtering due to fog, precipitation, and high humidity events.
FBEXT represents the noon extinction coefficient without the data when fog, precipitation, or
RH>80% occurred. RHBEXT is where f(RH) is a relative humidity dependent correction term.
1
Figure 1.1 Long-term trend of extinction coefficient for Philadelphia 1960-1995.
Long-term trend extinction coefficient data for Philadelphia Figure 1.1 FBEXT*f(RH) shows a
decline from about 0.17 km-1 in 1960 to about 0.12 km-1 in 1980. Since 1980, the extinction has
remained roughly constant. In the early 1960s the extinction coefficient was high during both
winter and summer, and lower during spring and fall Figure 1.2a. By 1980 the winter peak in
extinction has diminished, and the summer peak has increased Figure 1.2b. By 1995, the
extinction coefficient was also summer peaked Figure 1.2c. From the extinction coefficient data
it is evident that by 1980 the shift from a winter peaked to a summer peak of the fine particle
concentration has taken place. In addition, the data suggest that since 1980, the seasonality of the
extinction coefficient has remained the same.
The role of relative humidity correction for bext is examined in Figure 1.3 a and b. The
correlation charts indicate that the relative humidity correction constitutes only about 10% of the
extinction values. For this reason, it is recommended that in the following analysis the FBEXT
rather than RHBEXT values are used.
Philadelphia Saturation Study, September-October 1994
The spatial pattern of fine mass throughout the Philadelphia metropolitan area (Figure 2.1) is
best examined using the 1994 Saturation Monitoring Study (Tropp et al., 1996). Data at 15
monitoring sites (Figure 2.2) were collected during the time period September 11-October 9,
1994.
2
Figure 1.2a Seasonal extinction coefficient for Philadelphia, 1962
Figure 1.2b Seasonal extinction coefficient for Philadelphia, 1980
3
Figure 1.2c Seasonal extinction coefficient for Philadelphia, 1995
Figure 1.3a The relationship between RH corrected BEXT (RHBEXT) and FBEXT throughout the year.
4
Figure 1.3b The relationship between RH corrected BEXT (RHBEXT)
and FBEXT during June, July, August.
The time series of the Saturation Monitoring Study data are shown in Figure 2.2a-c. Figure 2.2a
shows all the monitoring data for the 15 sites. It is evident that throughout the month of
monitoring, there was a PM2.5 episode (September 17, 1994) of 65  g/m3 that was present at all
monitoring sites. During the remaining days, many of the sites were clustered around the same
value. Notable exceptions were sites 9, 10, 11, and 12 that exhibited anomalous fluctuations
factor of two above the concentration of the clustered sites. The four anomalous sites were
located in an industrial area to the northwest of the city.
Removing the 4 industrial sites, 9, 10, 11, and 12 yielded a time series where all the sites were
within about 10  g/m3 from each other (Figure 2.2b). The single exception was site 7 in
downtown Philadelphia. In past studies this site was identified as being strongly influenced by
diesel exhaust (Tropp et al., 1996). Removing the single downtown monitoring site (7) yields a
time series Figure 2.2c for the remaining 10 non-urban-industrial sites, whose concentrations are
within  5  g/m3.
Figure 2.3a-c shows the relative deviations of fine mass concentrations from the regional mean
aerosol level. The regional mean concentration was obtained by averaging the values for the 10
non-industrial-downtown sites. Figure 2.3a shows that at the industrial sites the fine mass
concentration frequently exceeds the regional values by 20-50  g/m3. The excess concentration
at the downtown site is about 5-15  g/m3. The remaining ten sites 2.3c the concentrations are
within about  5  g/m3 from the regional values.
5
Figure 2.1 Map of the Philadelphia monitoring locations in the Saturation Study
Figure 2.2a Philadelphia Saturation Study, all sites
6
Figure 2.2b Philadelphia Saturation Study, industrial sites removed.
Figure 2.2c Philadelphia Saturation Study, industrial and downtown sites removed.
7
It is clear that the fine mass concentration in Philadelphia during the Saturation Monitoring
Study was dominated by regional aerosol that was uniform throughout the metropolitan area.
Within the urban center with the highest population density, there is evidence of 5-15  g/m3 of
excess PM2.5 over the regional PM2.5 levels. From previous studies, it can be inferred that the
downtown excess is largely due to diesel exhaust. The spatial and temporal characteristic of the
industrial aerosol (stations 9, 10, 11, 12) clearly indicates that the impact of these industrial
emissions in the northeastern Philadelphia does not influence the populated areas of the city to
the south and southwest. The possible explanation lies in the prevailing wind direction, which is
from the city towards the industrial area.
The results Saturation Monitoring Study suggest that most of the population in Philadelphia is
exposed to the regional PM2.5 concentrations, except in the downtown street canyons and near
the industrial area. Hence, the detailed characterization of the industrial PM2.5 source areas may
not be necessary for population exposure studies.
Figure 2.3a Philadelphia Saturation Study, deviation from regional concentration, all sites
8
Figure 2.3b Philadelphia Saturation Study, deviation from regional concentration, industrial sites removed.
Figure 2.3c Philadelphia Saturation Study, deviation from regional concentration,
industrial and urban sites removed.
9
Fine Mass-BEXT Relationship During the Saturation Study
The relationship between fine mass concentration at 15 city sites and the extinction coefficient at
the airport is first examined by overlaying of time series, Figure 2.4. The average PM2.5
concentration for the 10 non-industrial sites is compared to FBEXT and RHBEXT. The temporal
correspondence between PM2.5 and extinction coefficients are evident. Both the peaks and the
valleys coincide.
The statistical relationship between the measured PM2.5 concentration at 10 non-industrialdowntown sites and the extinction coefficient is shown in Figure 2.5. The correlation coefficient
is R2 = 0.71. Hence, about 70% of the regional PM2.5 variance can be explained using the
extinction coefficient data. It should be noted that this good correspondence is applicable to the
conditions in 1990s, but may not be applicable for the 1970s and before when urban-industrial
primary fine particle sources may have been more abundant.
The Saturation Monitoring Study also provides a unique opportunity to examine the spatial
representativeness of airport visibility observations. In the following 15 Figures (Figure 2.6) each
figure shows the correlation plots between fine mass and FBEXT. The resulting correlations
indicate that for the ten non-industrial/downtown sites the correlation ranges between 0.73 and
0.78. For the four industrial sites the correlation is much less, ranging between 0.1 and 0.5. For
the downtown site, 7, the correlation is 0.66. These results indicate that the visibility
observations indeed yield representative regional concentrations that are applicable throughout
the Philadelphia metropolitan area, except for the immediate vicinity of smelters and the
downtown site, Market Street.
10
Figure 2.4 Average PM2.5 and extinction coefficient for the Saturation Study.
Figure 2.5 Extinction coefficient and fine mass correlation for nonindustrial sites.
11
12
13
Figure 2.6 Fine mass and extinction coefficient correlation plots for Saturation Monitoring Study sites.
14
The 1979-1982 Monitoring Data Set
The primary purpose of the present analysis is to provide a time-interpolation for the missing days
during 1979-82.
The 1979-1982 dataset consists of 9 monitoring sites (Figure 3.1) with somewhat sporadic time
coverage. Two sites, S.E. Water Treatment Plant and Gratz College had only a few days of data. For
this reason, these were eliminated from further consideration.
Figure 3.1 Map of the monitoring locations for the Philadelphia 1979-1983 study.
The examination of the spatial representativeness began with time series for all data from June 14
through October 3,1979. Figure 3.2 shows all available data during the June 14-October 3, 1979
period. Throughout the period four sites were operating: Presbyterian Home, Belmont, Temple
University, and N.E. Airport. For a brief intensive period, August 25 –September 9, 1979,
additional monitoring sites were operating. For the following analysis, only the four longer-term
monitoring sites were utilized. The time series for these four sites is shown in Figure 3.3. All the
data points in the Figure are connected with lines regardless whether on not they were missing days
between the sampling periods. These four locations and their time series were taken as
representative for the PM2.5 concentration in the greater Philadelphia area.
15
Figure 3.2 Fine mass observations from June 14-October 3, 1979.
Figure 3.3 Fine mass concentrations for Presbyterian Home, Belmont, Temple University, and N.E. Airport.
16
The average concentration for the four sites is shown in Figure 3.4. The Figure also contains the
aggregate fine mass concentration provided by W. Wilson. It is clear that the aggregate
concentrations from these four sites and the Wilson aggregate are virtually identical. This is further
illustrated in Figure 3.5 which correlates the regional average and Wilson aggregate concentrations
during June 14-Ocober 3, 1979.
Figure 3.4 Calculated regional fine mass and Wilson aggregate fine mass for June 14-October 3, 1979.
17
Figure 3.5 Calculated regional fine mass and Wilson aggregate fine mass correlation.
Relationship between PM2.5 and Extinction during 1979-82
The particular goal of this analysis is to establish the relationship between fine particulate mass and
light extinction coefficient in Philadelphia during 1979-82.
The time series for the regional (Presbyterian, Belmont, Temple, NEAirport) average fine mass
concentration and the light extinction coefficient is shown in Figure 3.6. It s evident that there is a
good correspondence in the fine mass and extinction coefficient time series. However, the two
extinction coefficients for July 14 and August 28, 1979 are substantially higher than expected. This
is further illustrated in the scatter plot of fine mass and FBext in Figure 3.7 which shows good
correlation except for these two days. The correlation coefficient, R2 equal 0.46 and the slop is
0.0059. As a test, these two data points were removed from the correlation and the results are shown
in Figure 3.8. The correlation coefficient has improved to R2 =0.6 and slope of 0.0051.
18
Figure 3.6 Regional fine mass and extinction coefficient for June 14-October 3, 1979.
Figure 3.7 FBext and PM2.5 correlation for Presbyterian Home, Belmont, Temple U. and N.E. Airport
19
Figure 3.8 FBEXT and PM2.5 correlation with high FBEXT (0.4 km-1) on 7/14 and 8/28/79 removed.
Examination of the two days with the anomolously high extinction coefficient has revealed that the
relative humidity was only 65% and 75% respectively, hence the RH could not be responsible for
the high extinction coefficients. The weather data for both days has indicated haze as the
obstruction to vision which is normal for visibilities of 5 km or less. Consequently, there is no
apparent reason for the anomoulously high extinction coefficients.
It is worth noting that the slope of the extinction coefficient versus fine mass in the 1979 data is
0.0059. This is substantially higher than the slope of 0.0032 for the 1994 saturation monitoring
study. On the other hand the offset in the 1979 data is 0.01 km-1 while in 1994 the offset is 0.08 km1
. Again, the reasons for these deviations are not apparent.
The 1979-82 data set includes some coverage throughout the 1979-82 period. The fine mass - light
extinction relation at the four "representative" sites (Presbyterian, Belmont, Temple, NEAirport) is
shown in Figure 3.9. The scatter chart clearly indicates that the strength of the relationship declines
when the entire period is considered. The correlation is R2 =0.4 and the slope is 0.0042.
20
Figure 3.9 FBEXT and PM2.5 correlation for Presbyterian, Belmont, Temple, N.E. Airport during 1979-82.
Figure 3.10 displays the seasonal of fine mass-light extinction relationship for the four sites used in
this part of the analysis. The winter (Dec., Jan., Feb.) shows the lowest correlation coefficient, R2,
of 0.23 and the lowest slope of 0.0015. The spring (Mar., Apr, May) has the highest R2 of 0.40 and
slope of 0.0046.The summer (Jun., Jul., Aug.) has similar in correlation coefficient (R2=0.36) and
slope (0.0045) as the spring. The fall months (Sep., Oct., Nov) have an R2 = 0.31 and slope of
0.0027 which is comparable to the slope calculated from the saturation study data (slope=0.0032)
(Figure 2.5)
From the seasonal analysis, it is evident that the light scattering-PM2.5 ratio depends on the season.
Therefore, a temporal interpolation would require the incorporation of seasonally adjusted
regression coefficients. However, the available data from the 1979-1982 study are inadequate to
establish a set of robust regression coefficients. Additional data sets would be desirable.
21
Figure 3.10 Seasonal FBEXT and PM2.5 correlations for Presbyterian, Belmont, Temple, and N.E. Airport.
The 1992-95 Monitor Site Data Set
An additional PM2.5 data set was available for a single site in Philadelphia from 1992 to 1995. The
fine mass data is composed of observations at the Presbyterian Home site. It provides every other
day measurements in May, 1992 and daily measurements from June 1, 1992 through April 12, 1995.
22
Relationship between FBext and Rel. Humidity and Temperature during 1992-95
Figure 3.11 displays the seasonal of fine mass-light extinction relationship for the Presbyterian site.
All seasons show similar slopes for the fitted linear equation (0.0023-0.0030). The winter (Dec.,
Jan., Feb.) has the highest correlation coefficient, R2, of 0.31. The summer (Jun., Jul., Aug.) has a
correlation coefficient of 0.29. The spring (Mar., Apr, May) and fall have R2 values on the lower
end, 0.18 and 0.17 respectively. The summer exhibits the most scatter but each season has a
number of outliers.
Figure 3.11 Seasonal FBEXT and PM2.5 correlations for 1992-95.
Causes for the outliers seen in the correlation plots for FBEXT and PM2.5 (Figure 3.11) could be
the influence of relative humidity and temperature on the measurement of FBEXT. The ratio of
extinction to fine mass was correlated against relative humidity and temperature. Figure 3.12 shows
that relative humidity tends to increase the amount of extinction per fine mass concentration
especially during the spring and summer. Figure 3.13 indicates that the Fbext/PM2.5 ratio
decreases as temperature increases, most noticeably during the spring and summer.
23
The influence of temperature and relative humidity on the extinction coefficient measurement needs
to be investigated further. The goal is to derive a correction factor dependent upon relative
humidity and temperature. This factor would then be used to adjust the measured Fbext values.
Figure 3.12 FBEXT/PM2.5 and Relative Humidity scatter plots from May 12, 1992 through April 12, 1995.
24
Figure 3.13 FBEXT/PM2.5 and Temperature scatter plots from May 12, 1992 through April 12, 1995.
References
R.M. Burton, H.H. Suh and P. Koutrakis, Spatial Variation in Particulate Concentrations within
Metropolitan Philadelphia. Environ. Sci. & Technol. 30, 400-407, 1996.
R.J. Tropp, S.F.Sleva, W. Ramadan, C.J. Harris, and N.J Berg, Jr., Results of the 1994 Philadelphia
PM2.5 and PM10 Saturation Study, Presented at the 89 Annual Air & Waste Management Meeting
& Exhibition, 96-MP3.03, Nashville, TN, June 23-28, 1996.
25