UAM-V model output for the 1995 OTAG episode was compared with field observations from 16 rural and suburban sites operated by N-NE,
SOS/Nashville and PAMS. Afternoon values were compared in order to eliminate, or at least minimize, vertical gradients and transitory
excursions in isoprene concentration.
Photochemical ozone (O3) formation is a by-product
of the atmospheric oxidation of hydrocarbons and reactive nitrogen
species (e.g., NO and NO2). In a highly simplified
view, hydrocarbon oxidation provides a source of oxygen in the
form of oxy- and peroxy- radicals. Reactive nitrogen acts as a
catalyst to release oxygen atoms from peroxy- radicals, which
can then react with atmospheric oxygen (O2) to produce
O3. The process is extremely complicated and is dependent
on the mix and relative abundance of hydrocarbons and reactive
nitrogen (Atkinson, 1990).
Isoprene is a highly reactive, naturally produced hydrocarbon.
Important sources of isoprene include oak forests and a variety
of woodland landscapes (Guenther et al., 1994). Recent estimates
suggest that isoprene is the most abundant hydrocarbon (anthropogenic
or biogenic) across much of the eastern U.S. (Geron et al. 1994).
Analyses of isoprene reactivity, in the context of ozone formation,
suggests that isoprene is the dominant hydrocarbon in the rural
atmosphere and an important hydrocarbon in the urban atmosphere
(Chameides et al., 1992; Cardelino and Chameides, 1990).
This paper presents a comparison of isoprene measurements with
UAM-V model calculations for the July 1995 OTAG episode. The scope
of this analysis was restricted to isoprene because: 1) isoprene
is generally abundant and, hence, relatively easy to measure;
2) isoprene has a distinct emission inventory system within the
UAM-V model; 3) isoprene is treated explicitly in the UAM-V chemical
reaction scheme; and 4) the reactivity of isoprene is sufficiently
high that a bias or uncertainty in concentration estimates could
have a significant impact on model sensitivity to control strategies
or other factors.
The July 1995 OTAG episode was selected for analysis because it
provided the most robust set of field measurements available.
The episode coincided with two major field experiments (NARSTO-Northeast
and the SOS-Nashville Intensive) which provide unprecedented spatial
coverage, especially in rural areas. In addition, the PAMS network
had reached a stage of maturity where it could contribute high
quality information for a variety of suburban locations. None
of the other OTAG episodes enjoyed such a wealth of concurrent
As with any comparison of measurements and model calculations,
perfect agreement between the two quantities is not possible and
is not expected. The primary reason for this involves a mismatch
between point measurements and volumetric model calculations.
Measurements at field sites are influenced by local features that
cannot be resolved with a regional scale model. By definition,
surface measurements are ordinarily taken within 5-15 meters of
ground level. Model calculations, in contrast, attempt to represent
the atmosphere as a series of vertical layers, including a surface
layer which is 50 meters deep. Horizontal and vertical gradients,
to the extent they exist, have significant potential to influence
comparisons. Although these fundamental differences between observations
and model calculations cannot be eliminated, we have attempted
to minimize them in the data selection process.
Isoprene data were obtained from NARSTO-NE (N-NE), SOS and PAMS
data archives according to network specific data exchange protocols.
Data were examined for completeness and the characteristics of
monitoring sites were evaluated for representativeness. As a rule,
only rural and suburban sites with at least five days of data
during the 1995 OTAG episode were chosen for analysis. A total
of 16 sites satisfied the above selection criteria.
The sites selected cover a broad geographic range and form a diagonal
across the eastern U.S. from the Gulf of Mexico to the Gulf of
Maine (see Figure 1). The density of sites is particularly high
in the northeast and Tennessee-Kentucky area, due to field experiments
conducted by N-NE and SOS. In contrast, sites are sparse, or non-existent,
along the Atlantic Coast from North Carolina to Florida, in the
deep south and throughout the midwest. The comparability of field
measurements and model calculations thus cannot be determined
for certain parts of the OTAG domain.
Site characteristics and sampling information are presented in
Table 1 (below). N-NE and SOS each operated 5 sites, while PAMS operated
6 sites. The N-NE and SOS sites were almost entirely rural and
the PAMS sites were, with one exception, suburban. Land-use was
predominantly hardwood forest for the rural sites and predominantly
transportation or mixed for the suburban sites.
|SOS||Youth, Inc., TN||F/T||continuous||1H||H|
|SOS||Cove Mtn., TN||HF||canister||1H||D|
|PAMS||Baton Rouge, LA||T/I||canister||3H||3H|
|PAMS||So. DeKalb, GA||T||continuous||1H||H|
|PAMS||Lums Pond, DE||A/T||continuous||1H||H|
|PAMS||Cape Elizabeth, ME||MF/C||continuous||1H||H|
|Setting Codes: S=suburban; R=rural.|
|Land Use Codes: T=transportation; HF=hardwood forest; P=pasture; A=agricultural; MF=mixed forest; C=coastal.|
|Duration Codes: 1H=1-hour; 3H=3-hour; V=3-hour(day) or 6-hour(night).|
|Frequency Codes: H=hourly (i.e., continuous); 3H=every 3 hours; D=1/day; E=episodic.|
Isoprene sampling and analysis protocols differed in several important
respects from network to network. N-NE sites collected 3-hour
(daytime) or 6-hour (nighttime) canister samples on selected intensive
operating periods during the 1995 field season. These periods
included the July12-July 16 portion of the 1995 OTAG episode.
Canister samples were analyzed at a central laboratory facility
following the 1995 field season.
SOS sites generally collected a single canister sample each day
of the SOS/Nashville Intensive. Samples were collected over a
1-hour period typically from 1200 to 1300 local standard time.
Sample dates covered the entire 1995 OTAG episode. Canister samples
were analyzed within a few days of collection at a central laboratory
facility in Nashville that was set up for the purposes of the
study. The suburban site at Youth, Inc. collected and analyzed
samples on roughly a 1-hour cycle using a manually operated, on-site
gas chromatograph-mass spectrometer system. Manual samples were
collected on most intensive days from approximately 0500 to 2300
local standard time.
PAMS sites generally collected and analyzed hourly samples using
automated, on-site gas chromatographic systems equipped with flame
ionization detectors. One site, Baton Rouge, LA, collected 3-hour
canister samples that were subsequently analyzed at a central
laboratory. PAMS samples were collected around the clock at all
sites throughout the OTAG episode.
For comparison purposes, model output from the 1995 D2 basecase
was obtained and data corresponding to sampling intervals at each
site were extracted. Isoprene concentrations from the model were
then converted from units of parts per billion-volume (ppb-V)
to part per billion-carbon (ppb-C), in conformance with the reporting
conventions of the analytical laboratories. Finally, data from
the N-NE and PAMS sites were averaged over the hours 1100-1700
to provide a single daily value for comparison. These hours were
selected to isolate periods of highest isoprene emission and relatively
well-mixed atmospheric conditions (i.e., minimal vertical gradients).
Additional reasons for selection of the 1100-1700 time period
are illustrated in Figure 2, which shows a 3-day time series of
isoprene concentrations for the N-NE site at Kunkletown, PA. Modeled
concentrations at this site show a number of interesting features.
The most prominent features are the significant spikes that occur
around sunrise and sunset each day. These appear to be the result
of interaction between emissions and boundary layer structure,
as portrayed in the model. The morning spike apparently results
from the onset of emissions before break-up of the nocturnal boundary
layer, while the afternoon spike results from the continuing emissions
of isoprene as the nocturnal boundary layer becomes established.
These periods were excluded from the comparison because they represent
transient situations that the model may not be designed to handle.
The other prominent feature of the time series is the routine
decrease of isoprene overnight to near zero concentration. This
occurs at virtually all sites, both in the modeled values and
in the observations. The nightly return to baseline reflects the
degradation of isoprene by ozone (and possibly nitrate radical)
in the absence of surface emissions. Although it is reassuring
to see similar behavior in nighttime concentrations, nighttime
hours were excluded from the comparison because isoprene emissions
are negligible in the absence of sunlight.
3.0 RESULTS AND DISCUSSION
Means and 95-percent confidence intervals for observed isoprene,
modeled isoprene and various comparison statistics are shown in
Table 2 (below). In general, results show a broad range of concentrations
across sites, as well as significant differences between observed
and modeled isoprene. Mean observed isoprene ranged from 2.1 ppb-C
at Cape Elizabeth, ME to 28.6 ppb-C at Land Between the Lakes,
KY. Modeled isoprene concentrations ranged from 4.3 ppb-C at Holbrook,
PA to 48.2 ppb at Truro, MA.
|95 % CI|
|95 % CI|
|Shen. NP, VA||Mean|
|95 % CI|
|95 % CI|
|95 % CI|
|95 % CI|
|Mamm. Cave, KY||Mean|
|95 % CI|
|95 % CI|
|95 % CI|
|95 % CI|
|Baton Rouge, LA||Mean|
|95 % CI|
|So. DeKalb, GA||Mean|
|95 % CI|
|Lums Pond, DE||Mean|
|95 % CI|
|95 % CI|
|95 % CI|
|Cape Eliz., ME||Mean|
|95 % CI|
The overall average observed isoprene concentration was 10.2 ppb-C,
while the overall average modeled isoprene was 15.7 ppb-C. Modeled
and observed isoprene were significantly different (95-percent
confidence interval) for 10 of the 16 sites, and mean differences
were within 2 ppb-C for only 2 sites. Mean percent differences
(PDs) were generally large and ranged from -71.4 percent at Brookhaven,
NY to 244.8 percent at Cove Mountain, TN.
Absolute differences (ADs) and absolute percent differences (APDs)
provide useful measures of typical comparability between two or
more quantities, because positive and negative deviations are
not allowed to cancel. Mean ADs between observed and modeled isoprene
ranged from 1.1 ppb-C at Holbrook, PA to 34.9 ppb-C at Truro,
MA and averaged 10.7 ppb-C over the 16 sites. Mean APDs ranged
from 32.9 at Holbrook, PA to 245 percent at Cove Mountain, TN
and averaged 119 for the study. In other words, modeled and observed
isoprene were typically only within about a factor of two of each
Figure 3 shows mean modeled and mean observed isoprene for all
sites. Also shown is the line of ideal agreement (1:1) and envelopes
of agreement within +/- 25 percent and +/- 50 percent. Results
show that overall agreement between the model and observations
is relatively poor. Of the 16 sites, only 2 (Holbrook, PA and
South DeKalb, GA) agreed within +/- 25 percent and only 6 agreed
within +/- 50 percent. Of the 10 remaining sites, 9 fell above
and 1 (Brookhaven, NY) fell below the +/- 50 percent envelope.
Mean PDs between modeled and observed isoprene are shown in Figure
4. Brookhaven, NY and Chicopee, MA exhibit significantly and moderately
negative PDs, respectively, while all other sites exhibit positive
PDs. Five of the 16 sites have mean PDs between -50 percent and
+ 50 percent and another 5 sites have PDs between 50 and 100 percent.
PDs in excess of 100 occur at 5 sites, representing each of the
networks and covering the entire geographic range of the study
(i.e., from Louisiana to Maine). Model overpredictions thus are
not linked to any particular sampling regime or restricted to
a particular part of the model domain. By the same token, sites
that exhibit better agreement between observations and model predictions
(i.e., +/- 50%) represent all networks (sampling protocols) and
a broad geographic range.
PDs between mean observed and mean modeled concentrations are
depicted in Figure 5. The overall range of PDs is similar to Figure
4; however, the overall distribution is somewhat different. Six
sites exhibit PDs of +/- 50 and these are evenly divided between
positive and negative values. One site (Brookhaven, NY) is below
-50 and 5 sites are above 100.
The scattergram of mean difference versus mean modeled isoprene
concentration is shown in Figure 6. Also shown is the linear least
squares regression line through the data and corresponding regression
statistics. Results show a surprisingly linear relationship (R2=0.78)
and one that suggests an increasing tendency to overpredict with
increasing concentration. For modeled concentrations below 10
ppb-C, there is significant scatter around a mean difference of
zero. Eight sites have mean modeled concentrations of less than
10-ppb-C and 3 of these exhibit an underprediction, on average.
In contrast, eight sites have mean modeled concentrations of 10
ppb or greater and all but one of these show significant overpredictions.
For modeled concentrations of 20 ppb-C or higher, 4 of 5 sites
show overpredictions of at least 10 ppb-C.
The physical significance of the regression statistics shown in
Figure 6 is not entirely clear. The slope of the regression line
suggests that, in the limit, model overpredictions could amount
to roughly 80 percent. A negative intercept, on the other hand,
implies that some underprediction may occur at the low end of
the concentration range. Given the limited number of data points
in the regression, a literal interpretation of the regression
statistics appears to be unwarranted. Nevertheless, the tendency
for the model to overpredict seems clear. It is the precise nature
of the overprediction that is uncertain. Additional sites, especially
those with high predicted isoprene concentrations, are needed
to confirm, or refine, the present observations.
The complexity of the UAM-V modeling system is such that a wide
variety of factors could be responsible for isoprene overpredictions.
These factors include, among others, vegetation specific emission
rates, chemical destruction rates and meteorological advection
and dilution rates. Site representativeness is another factor
that must be considered in any model evaluation or comparison
exercise. We have selected rural and suburban sites for this analysis
based on the assumption that these sites are better characterized
with respect to isoprene emissions than urban sites. We have also
selected midday data to avoid large concentration excursions that
may occur when the nocturnal boundary layer is breaking up or
forming. Nevertheless, it must be recognized that we are using
point measurements near the surface to represent a three-dimensional
cell approximately 12 kilometers on a side and 50 meters deep.
Two potential sources of bias that we have been able to evaluate
to a limited extent include vertical gradients and surface temperature
error. The first of these addresses the ability to represent a
finite layer with a measurement near the bottom of that layer.
As noted earlier, isoprene measurements were taken at a height
of about 5-10 meters above ground level (agl), whereas the model
calculates an average concentration through a layer 50 meters
deep. Andronache and Chameides (1993) have examined near-surface
concentration gradients of isoprene. Based on available data and
theoretical considerations, the authors concluded that isoprene
typically exhibits an exponential decay with height that is related
to rates of vertical mixing and chemical destruction. The resulting
vertical gradient produces a 50 percent reduction in isoprene
concentration between the surface and roughly 300 meters. However,
the reduction between the surface and 50 meters (i.e., the top
of the lowest UAM-V layer) is much less. A measurement at 10 meters
is expected to be only 2-6 percent higher than the average through
a 50 meter layer. Not only is the difference small relative to
model overpredictions, it is of the wrong sign to moderate the
overpredictions. In other words, a gradient of the form predicted
by Andronache and Chameides (1993) would only increase differences
between the model and observations.
The other area of investigation involved surface temperature measurements
used to drive the isoprene emissions module. Temperature data
for the isoprene module come from measurements made at roughly
1.5 meters agl. This height may be appropriate for short vegetation
(e.g., grass and crops) but may not be appropriate for taller
vegetation such as forest canopies. Examination of delta temperature
information from the CASTNet network (a rural ozone and meteorology
network of 40-50 sites) shows that there is typically a daytime
gradient of about -0.5 to -1.0 degrees C (surface warmer) between
2 meters and 9 meters agl. Based on the temperature correction
factors of Guenther et al. (1993), a temperature gradient of this
magnitude implies a possible error (overestimate) in emissions
on the order of 7-15 percent. This source of bias is not negligible,
but it is generally much smaller than differences between observations
and model output reported here.
4.0 SUMMARY AND CONCLUSIONS
UAM-V model output for the 1995 OTAG episode was compared with
field observations from 16 rural and suburban sites operated by
N-NE, SOS/Nashville and PAMS. Afternoon values were compared in
order to eliminate, or at least minimize, vertical gradients and
transitory excursions in isoprene concentration. The following
conclusions obtain from the present study:
5.0 POLICY IMPLICATIONS
Isoprene concentrations are seriously overpredicted by UAM-V for
the 1995 modeling episode. In the absence of other information,
similar overpredictions are expected for the 1988, 1991 and 1993
episodes as well. Isoprene overpredictions can predispose the
model to a NOx limited condition and can predispose
the model to overstate the frequency and spatial extent of NOx
limitation. This, in turn, makes the model overly sensitive to
NOx control strategies and not sensitive enough to
VOC control strategies. Control strategy results must be interpreted
in light of this model predisposition.
Atkinson, R. 1990.Gas-phase tropospheric chemistry of organic
compounds: A review. Atmos. Environ. 24A:1-41.
Guenther, A., P. Zimmerman and P. Harley. 1994. Natural volatile
organic compound emission rate estimates for U.S. woodland landscapes.
Atmos. Environ. 28A:1197-1210.
Geron, C.A., A. Guenther and T. Pierce. 1994. An improved model
for estimating emissions of volatile organic compounds from forests
in the eastern U.S. J. Geophys. Res. 99:12773-12791.
Chameides, W. L., F. Fehsenfeld, M. O. Rodgers and C. Cardelino.
1992. Ozone precursor relationships in the ambient atmosphere.
J. Geophys. Res. 97:6037-6055.
Cardelino, C. A. and W. L. Chameides. 1990. Natural hydrocarbons,
urbanization and urban ozone. J. Geophys. Res. 95:13971-13979.
Andronache, C. and W. L. Chameides. 1994. Vertical distribution
of isoprene in the lower boundary layer of the rural and urban
southern United States. J. Geophys. Res. 99:16989-16999.
Guenther, A., P. Zimmerman and P. Harley. 1993. Isoprene and monoterpene
emission rate variability: Model evaluations and sensitivity analyses.
J. Geophys. Res. 98:12609-12617.
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