A preliminary analysis was conducted using readily available ambient
air quality data to compare observed isoprene concentrations with
concentrations predicted by the Ozone Transport Assessment Group
(OTAG) UAM-V photochemical model simulations for the July 10-18,
TABLE OF CONTENTS
A preliminary analysis was conducted using readily available ambient
air quality data to compare observed isoprene concentrations with
concentrations predicted by the Ozone Transport Assessment Group
(OTAG) UAM-V photochemical model simulations for the July 10-18,
1995 episode. These UAM-V predictions are based on biogenic emissions
inventory estimates generated by the BEIS2 model. It has been
suggested by some observers that OTAG BEIS2 isoprene emission
estimates, under which isoprene constitutes 70 - 80 percent of
total VOC emissions in the OTAG domain, may be over stated. OTAG
UAM-V model results show almost no sensitivity to VOC controls
except in some of the very largest urban areas (Northeast urban
corridor, Chicago). This result appears to be inconsistent with
trends reports such as those reviewed by Morris (1996) which indicate
that declines in ozone concentrations over the past ten years
or so in the eastern U.S. coincided with substantial reductions
in VOC emissions but little or no change in NOx emissions.
Furthermore, inventory evaluation studies suggest that anthropogenic
VOC emissions may be under-represented in the OTAG inventory,
yet ozone is significantly overpredicted under this inventory
in isoprene-rich areas such as the South. Previous evaluations
of BEIS2 based on direct comparisons of isoprene fluxes from vegetation
are based on a limited range of vegetation types and ambient conditions
and therefore may not be representative of the full range of conditions
represented by the OTAG model application. Reconciliation of the
BEIS2 isoprene inventory with ambient measurements is complicated
by the high reactivity of isoprene which must somehow be accounted
for. UAM-V isoprene concentration predictions account for atmospheric
dispersion, reactivity, and deposition processes. Therefore, comparison
of UAM-V predictions with isoprene measurements is the best methodology
currently available for performing real-world evaluation of the
BEIS2 emission estimates used in the OTAG modeling.
Ambient measurements of isoprene, ozone, total non-methane hydrocarbons
(TNMOC), and NOx were compared with UAM-V predictions
for the surface layer grid cell in which each ambient monitoring
site is located. The UAM-V OTAG surface-layer grid cells are approximately
12 km x 12 km wide by 50 m high. Comparisons were made for July
10-18, 1995 at 15 monitoring sites: 11 PAMS sites in the Northeast,
two PAMS sites in Baton Rouge, LA, one Southern Oxidants Study
site in Nashville, TN, and the Harvard Forest, MA research site.
Time series of observed and predicted concentrations were compared
for each species. Additional statistical comparisons were conducted
using data from the mid-morning to early evening (hours 11 - 19)
during which isoprene emissions peak and vertical mixing is usually
most vigorous, thus minimizing differences between surface-based
measurements and the volume-average predictions. Statistical distributions
of observed and predicted concentrations (unpaired in time) were
compared, and summary statistics (mean observed and predicted,
bias, bias as percent of mean observed, gross error, gross error
as percent of mean observed, and normalized bias and normalized
gross error above preselected threshold concentrations) were calculated
for values paired in time. All analyses were done on a site-by-site
For isoprene, biases as a percent of mean observed were less than
plus or minus 50 percent at eight out of the 15 monitoring sites.
Most (six) of the remaining sites exhibited large over predictions
with biases of between 150 to 250 percent at four of these sites.
One site (Ware, MA) exhibited a bias of less than -50 %. These
results are in qualitative but not quantitative agreement with
those from a parallel study being conducted by Dr. Eric Edgerton.
Reasons for the quantitative discrepancies between the two studies
have not been fully explored in this preliminary analysis. Although
preliminary and not yet conclusive, the results to date suggest
there may be a bias toward overestimating isoprene in the OTAG
For TNMOC, biases were within plus or minus 50 % at over half
of the sites examined; observed TNMOC values were significantly
less than predicted values at the two Baton Rouge sites; significant
overpredictions were noted at sites in Delaware and Maine. Paradoxically,
at many of the sites where isoprene was overpredicted, TNMOC was
underpredicted. Thus, the isoprene weight percents at these sites
were severely overpredicted. Bias in the Isoprene/TNMOC ratio
was within plus or minus 50 % at 7 of the 13 sites with available
TNMOC measurements; the ratio was underpredicted by 59 percent
at one site (Ware, MA where isoprene was underpredicted by just
over 50% ) and overpredicted at four sites.
A comparison of predicted and observed TNMOC/NOx ratios revealed
significant differences at some sites, whereas at others the TNMOC/NOx
ratios were comparable. When there were significant differences,
the OTAG Modeling System tended to overstate the observed TNMOC/NOx
Despite the relatively large discrepancies in observed and predicted
precursor concentrations and ratios noted above, UAM-V ozone predictions
were biased by more than plus or minus a relatively modest 15
percent at only 3 of the 13 sites with ozone measurements. However,
it is somewhat disturbing that at these 13 sites there is a tendency
toward systematic overestimation of the average afternoon observed
Further analysis of the OTAG Modeling System isoprene concentrations is ongoing. Data from additional sites are being included. In addition, comparisons aloft with the NARSTO-NE aircraft measurements are also planned.
Many States in the eastern U.S. performed preliminary photochemical
modeling for their 1994 State Implementation Plans (SIPs) which
suggested transported ozone and ozone precursors from upwind regions
were contributing significantly to ozone exceedances in their
nonattainment areas. The presence of significant amounts of transported
ozone from upwind made it difficult for many of the nonattainment
regions to demonstrate attainment with any reasonable level of
local emissions controls. In recognition of the transport problem,
the U.S. Environmental Protection Agency (USEPA) established a
twophase program for states to develop approvable ozone
SIPs. In a policy memorandum dated March 2, 1995 ("Ozone
Attainment Demonstrations"), USEPA outlined the major elements
of this program. Phase I requires states to complete preNovember
1994 SIP requirements, submit regulations sufficient to meet the
initial Rate of Progress requirements, and submit modeling analyses.
Phase II calls for a twoyear (19951996) consultative
process to assess national and regional strategies to deal with
ozone transport in the eastern U.S., and subsequent revisions
of local control plans, as necessary, based on any new national
or regional strategies. To accomplish the Phase II consultative
process, the Environmental Council of States (ECOS), in conjunction
with USEPA, established the Ozone Transport Assessment Group (OTAG).
OTAG is performing regional meteorological (RAMS and SAIMM), emissions
(EMS95), and photochemical (UAM-V) modeling of the eastern U.S.
to identify regional emissions control strategies to reduce ozone
transport into ozone nonattainment regions during periods of ozone
exceedances. Four episodes were selected for regional photochemical
The EPA, Midwest, Southeast, and Northeast Modeling Centers are
taking the lead for modeling, respectively, the 1988, 1991, 1993,
and 1995 ozone episodes.
Early on in the OTAG process, a decision was made to use BEIS2
biogenic emission estimates over BEIS1. BEIS2 uses more recent
biogenic emission measurements and is generally believed by the
scientific community to provide more accurate estimates of biogenic
emissions. However, preliminary regional model simulations performed
using BEIS2 exhibited serious overprediction of the observed ozone
concentrations (e.g., EPA ROM and MOCA UAM-V sensitivity simulations).
OTAG formed an Ad Hoc Biogenic Emissions Committee to investigate
the use of BEIS1 versus BEIS2. It was recommended that BEIS2 should
be used and that the CBM-IV isoprene chemistry be updated using
more recent smog chamber experiment data which would lower the
ozone formation potential of isoprene.
The latest (D2) UAM-V base case simulations for the four OTAG
episodes using BEIS2 and the new isoprene chemistry exhibited
"reasonable" performance in the Northeast and Midwest,
but still exhibited an overprediction tendency in the South, especially
over Atlanta. Biogenic emissions (mainly isoprene) constitute
70-80 percent of the total VOC emissions in the OTAG modeling
domain. The OTAG VOC/NOx control sensitivity tests exhibit almost
no ozone sensitivity to VOC controls, except for some of the very
largest urban areas such as Chicago and New York City. Clearly,
the BEIS2 isoprene emissions are driving the conclusions of the
OTAG emissions sensitivity simulations. There is circumstantial
evidence that BEIS2 may be overestimating biogenic isoprene emissions:
Several researchers (e.g., S.T. Rao of the NYSDEC, George Wolff of General Motors, B. Cox and S.-H. Chu of EPA, and K. Jones of Zephyr Consulting, see OTAG Review Report, Morris, 1996) have noted that over the last decade or so there is a definite downward trend in observed meteorology-adjusted ozone concentrations in the Northeast. Since NOx emissions in the region have been fairly stable but VOC emissions have been reduced substantially, this suggests that the reduction in ozone might be due to the reductions in VOC emissions. Yet the OTAG UAM-V ozone estimates exhibit very little sensitivity to reductions in VOC emissions.
Anthropogenic VOC emissions may be under-represented in the OTAG emissions inventory (e.g., absence of off-cycle VOC emissions from mobile sources), with Canadian emissions being particularly suspect, yet ozone is significantly overpredicted in the South. One possible hypotheses is that the biogenic emissions may be overstated compensating for low anthropogenic emissions in the Northeast and Midwest but, because biogenic emissions are much greater in the South, overcompensating for low anthropogenic VOC emissions in the South resulting in ozone overprediction tendency.
Given the importance of BEIS2 emissions estimates in driving the
OTAG VOC versus NOx emission control decision making, a real-world
evaluation of the BEIS2 biogenic emission estimates as derived
in the OTAG modeling is needed. There have been several studies
shich compred BEIS2 emission fluxes with measured fluxed off of
a forest canopy (e.g., Guenther et al., 1996). However, comparison
of the BEIS2 emissions estimates with special study measurements
of emissions from vegetation is complicated by the limited range
of vegetation types and ambient conditions for which the experiments
were performed. Further, these measurements were collected under
ideal conditions whereby microscale meteorological and landuse
characteristics were quantified which is not possible when generating
regional inventories as for the OTAG domain. There has been some
success in the past in performing emissions/ambient measurement
reconciliation, however implementation of such an approach for
BEIS2 would be problematic due to the high reactivity of isoprene;
any comparison of the BEIS2 emission estimates with ambient data
must account for the reactivity of isoprene. Since the UAM-V takes
isoprene reactivity into account and the OTAG UAM-V version contains
an update of the isoprene chemistry, the comparison of the UAM-V
isoprene estimates with ambient data provides the best methodology
currently available for performing a real-world evaluation of
the isoprene emissions estimates being used in the OTAG modeling.
Overview of Approach
The objective of this study is to evaluate the OTAG UAM-V/BEIS2
isoprene estimates against ambient measurements to determine whether
BEIS2 as applied in the OTAG modeling study accurately represents
observed isoprene (biogenic) emissions. Historically, speciated
VOC ambient measurements have only been collected during specialized
field studies (e.g., LMOS and SOS) or at specialized research
chemical measurement sites. However, for the 1995 OTAG modeling
period, there are several sources of isoprene measurements with
which to compare with the UAM-V/BEIS2 model estimates:
By 1995, many Photochemical Assessment Monitoring Sites (PAMS) were in operation providing a fairly large database of measured speciated VOC (and other species) concentrations, at short (hourly and 3-hourly) time resolution.
The NARSTO-Northeast 1995 field study included 9 air quality research stations in the eastern U.S. which collected speciated VOC data during Intensive Operating Periods (IOPs) (including July 12-16 of the 1995 OTAG episode). NARSTO-Northeast also operated aircraft which collected VOC samples aloft during the IOPs.
The Southern Oxidant Study (SOS) operated an extensive field study, with surface and aloft air quality sampling, in the Nashville area during the summer of 1995. Although data from the SOS study are slow in becoming available, data may be available upon request from some of the individual researchers (e.g., TVA).
There are other special study data associated with universities, laboratories, or States that may be available (e.g. Harvard Forest data).
This study was planned in Two Phases. In Phase I we would provide
a preliminary evaluation of the OTAG UAM-V/BEIS2 isoprene predictions
using currently available observed data that were available by
mid-December and did not involve extensive processing. If the
results from the preliminary evaluation are enlightening, then
the comparisons of the UAM-V/BEIS2 model predictions with a more
"complete" (as available) observational data set will
be performed and the result documented in a report. We will also
look at other comparisons of ozone and ozone precursors concurrent
with the isoprene measurements in order to obtain a complete and
comprehensive understanding of the photochemical system.
Speciated ambient VOC measurements in the eastern U.S. for the
July 1995 OTAG episode that were available by Mid-December 1995
were used in the preliminary (Phase I) analysis discussed in this
document. Unfortunately, the speciated surface and aircraft VOC
data from the NARSTO-95 field study were still undergoing quality
assurance at the time this study was performed and so could not
be included in the preliminary analysis. In addition, data from
many of the PAMS sites were not yet available or contained incomplete
or possibly suspect data and so could also not be included. Also,
except for one site, data from the SOS 1995 Nashville study have
not yet been released.
There have been several independent evaluations of BEIS2 that
bear mention before discussing this study's results. Some of these
studies have involved the collection of detailed VOC emissions
factors from different vegetation types which were used to develop
and evaluate the BEIS2 emission factors, whereas others collected
ambient speciated VOC and other species and microscale meteorological
measurements which were used along with detailed site specific
biomass to provide independent evaluation of the BEIS2 isoprene
and VOC emission fluxes from a canopy (e.g., Guenther et al.,
1993; 1994; 1996; Monson et al., 1995; Geron et al., 1995; Lawrimore
et al., 1995; Geron et al., 1994). There have also been some indirect
evaluations of the BEIS2 emissions estimates using photochemical
models whereby modeled ozone and precursor concentrations were
compared against observations using BEIS1 and BEIS2 biogenic emission
estimates (e.g., Sillman et al., 1995). However, none of the aforementioned
studies evaluated the biogenic emissions estimates being used
in the OTAG process. Currently, there are several groups performing
such analysis including each of the OTAG Modeling Centers, the
OTAG model performance evaluation contractor (STI), work by the
one of the NARSTO-NE contractors (STI), and work being performed
by Eric Edgerton formerly of Environmental Science Engineering
(ESE) for Southern Company Services (SCS). In this Chapter we
breifly discuss some of these studies to set the stage and aid
in the interpretation of our analysis presented in Chapter 3.
Comparison of Measured Versus BEIS2 Isoprenes Fluxes from a Forest Canopy
As noted above, there have been several studies comparing measured
versus model estimated isoprene fluxes from a forest canopy. Probably
the most detailed and extensive analysis was performed at a forested
site near Oak Ridge Tennessee during July and August 1992 using
8 different measurement techniques (Guenther et al., 1996). Individual
leaf emissions and midday forest isoprene fluxes were measured
along with meteorological eddy measurements on a tower in the
canopy. In addition, detailed information on vegetation biomass
was collected as a function of direction from the measurement
tower. When using the detailed information on meteorology within
the canopy (e.g., temperature and UV) and accounting for the biomass
distribution along the sector of forest that was upwind of the
tower, they found that the isoprene emission fluxes derived using
the BEIS2 emissions factors were within 25 percent of the measured
OTAG Model Performance Evaluation
As part of the model performance evaluation of the OTAG UAM-V
D2 base case simulations, comparisons were made between predicted
and observed ozone precursors (RHC, NOx, and NOy) at five NARTO-Northeast
monitoring sites: Holbrook PA; Arendtsville PA; Kunkeltown PA;
Brookhaven NY; and Truro MA (OTAG, 1996). In general, the model
overestimated the NOy concentrations and underestimated the RHC
concentrations. However, the comparisons at rural sites, where
the modeled RHC was biased low on average, were different than
at the urban PAMS sites, where an average overestimation bias
for RHC existed.
OTAG Isoprene Performance Evaluation by ESE/SCS
Dr. Eric Edgerton, formerly of ESE Environmental Inc, (ESE), is
performing a parallel study to this one for Southern Company Services
(SCS) comparing modeled and measured isoprene concentrations during
the July 1995 OTAG episode. Because ESE was the monitoring contractor
for the NARSTO-Northeast and SOS-Nashville studies, he had advanced
copies of the data before it underwent the final QA and release
to the public. At the December 1996 OTAG meeting, Dr. Edgerton
presented model/measured isoprene comparisons at 14 sites in the
OTAG domain as shown in Figure 2-1. Note that in this study we
initially looked at 23 potential sites with isoprene concentrations
of which only four sites overlapped with the Edgerton database.
Because the OTAG UAM-V isoprene predictions are 12 km x 12 km
grid cells averaged over a 50 m lowest layer, the focus of the
model/measured comparison was during the afternoon when the atmosphere
is "well-mixed" so that the predicted 50 m layer-average
isoprene concentrations are more representative of the surface
measured isoprene concentrations. Dr. Edgerton also performed
a height adjustment of the measured surface isoprene concentrations
to adjust to the average height of layer. However, in the afternoon
during vigorous vertical mixing, this height adjustment did not
significantly alter the surface concentration (just a few percent).
Figure 2-2 summarizes the average percent bias in the afternoon
OTAG UAM-V isoprene concentrations at the 14 sites analyzed. Because
of the uncertainties in the modeled grid cell average versus point
measurement and uncertainties in the emissions and meteorology,
Dr. Edgerton divided his sites into those that predicted the observed
afternoon isoprene on average within ± 50%, those with an
overprediction bias (> 50%), and those with an underprediction
bias (< -50%). He noted that 5 of the 14 sites (36%) had average
isoprene agreement within ± 50%. Of the remaining sites,
7 (50%) showed modeled isoprene concentrations that were substantially
higher than observed (53-311% overestimation on average), and
1 showed modeled concentrations substantially lower than observed.
As shown in Figure 2-3, Dr. Edgerton did not see a regional bias
in the OTAG/UAM-V isoprene over- or under-prediction bias. Dr.
Edgerton concluded that additional work is needed to understand
the magnitude and causes of the differences between model output
and field observations.
[Figures not currently available.]
Figure 2-1. Locations of the 14 monitors where model/observed
isoprene concentrations were made in the Edgerton SCS Study (Source:
Figure 2-2. Percent bias of afternoon predicted isoprene concentrations (100 BIAS/AVG OBS) from the Edgerton SCS Study (Source: Edgerton, 1996).
Figure 2-3. Spatial distribution of modeled isoprene bias
from the Edgerton SCS Study (Source: Edgerton, 1996).
There were three main sources of measured isoprene data for the
preliminary comparison with model estimates from the OTAG July
1995 UAM-V base case simulation:
As noted previously, it was hoped that data from the 1995 NARSTO-Northeast
monitoring study could be integrated into the analysis and more
of the data from the 1995 SOS-Nashville Study could be included.
However, by mid-December such data had not yet been released.
In addition, the number of PAMS sites where data were available
was less than anticipated. We processed data for 23 sites across
the OTAG region for which isoprene measurements were collected.
Because the speciated VOC sites where isoprene measurements were
collected tended to be enhanced chemistry sites (e.g., PAMS),
we also processed the measured data for total nonmethane hydrocarbons
(TNMOC), ozone, and oxides of nitrogen (NOx) for comparison with
the model estimates to aid in understanding the complete photochemical
oxidant system. For each of the 23 sites for which data were available,
we first plotted the time series of predicted and observed isoprene,
TNMOC, ozone, and NOx concentrations to determine the reasonableness
and data completeness for each site. Of the 23 sites processed,
8 were not used in the statistical comparisons either for having
insufficient data capture to calculate meaningful statistics,
or, in the case of measured data obtained on the UAM-V base case
tape from the OTAG Clearing House, data were of unknown origin
and appeared questionable.
Table 3-1 identifies each of the 23 sites originally processed
for inclusion in the OTAG UAM-V/BEIS2 isoprene evaluation study
and the reason for the elimination of the 8 sites from the statistical
comparisons. Most of the data that were available by mid-December
1996 were PAMS sites. There are four classifications of PAMS sites:
Type 1 = upwind sites; Type 2 = urban sites; Type 3 = downwind
ozone maximum sites; and Type 4 = downwind outer edge.
Figure 3-1 displays the locations of the resultant 15 sites where
statistical comparisons were made between the OTAG UAM-V/BEIS2
modeling system predicted and the measured values.
Table 3-1. Preliminary set of monitoring sites where speciated VOC measurements were collected that were processed and included in the OTAG UAM-V/BEIS2 isoprene model performance evaluation database.
Reason for Exclusion
E. Hartford, CT
Type 2 PAMS
No isoprene observations in database
Type 3 PAMS
Lums Pond, DL
Type 1,4 PAMS
Low capture, unknown source
Camp Logan, IL
Low capture, unknown source
Type 2 PAMS
Type 1 PAMS
Type 1 PAMS
Low isoprene data capture
Type 1,3 PAMS
No data on record
Type 2 PAMS
Harvard Forest, MA
Type 2 PAMS
Type 3 PAMS
Low isoprene data capture
Type 3 PAMS
Lake Clifton, MD
Type 2 PAMS
Cape Elizabeth, ME
Type 3 PAMS
Rider College, NJ
Type 3 PAMS
The Bronx, NY
Type 2 PAMS
No data currently (12/96) available
Type 2 PAMS
East Providence, RI
Type 2 PAMS
Youth Inc., TN
Insufficient data capture
Data Model Intercomparisons
Using the preliminary database described above, we performed model-data
comparisons between isoprene and related species. Figures showing
the complete comparison for all of the measures are provided in
the Appendices to this report. A few descriptive and summary figures
are presented in this section. The following comparisons were
made for this study using the preliminary (data available by mid-December
cut-off time) database:
Time Series Analysis
Appendix A contains time series of predicted and observed isoprene, TNMOC, ozone, and NOx concentrations at each of the 23 sites listed in Table 3-1.
East Hartford, Connecticut (CT HAR): At the time the data
from the CT HAR site were downloaded there was no total or speciated
VOC measurements available for the OTAG July 1995 modeling period.
The agreement between the predicted and observed ozone concentrations
is reasonable, except the model fails to capture the ozone spikes
on the high observed ozone days (e.g., July 13, 14, and 18). As
this is an urban PAMS site (Type 2), this difficulty of the model
to replicate the observed peaks may be due to the low model resolution
(12 km ) which may be insufficient to characterize urban plumes.
Stafford, Connecticut (CT STR): Stafford CT is one of four
sites, along with Chicopee MA (MA CHI), Harvard Forest MA (MA
HAV), and Ware MA (MA WAR), that are located fairly close to each
other in northern CT/middle MA (see Figure 3-1). There is a fairly
consistent trend in the model/observation comparison among the
four sites in this region. Although the model tends to track the
diurnal variations in the observed isoprene concentrations fairly
well at Stafford and Chicopee, it fails to capture the very highest
observed afternoon isoprene spikes (e.g., on July 14). The observed
isoprene peak at the MA CHI site that occurred around midnight
on July 18 is suspect as there should be minimal isoprene emissions
at this time. At the MA HAV and MA WAR sites, the model is underestimating
the observed daytime isoprene concentrations. In general, it appears
that isoprene concentrations are underestimated at these four
sites. The model does a poorer job in reproducing the observed
TNMOC concentrations at these four sites than for isoprene. The
modeled and observed diurnal TNMOC profiles appear to be out of
phase with each other. The comparisons of the predicted and observed
ozone concentrations range from fairly reasonable (e.g., MA WAR
and CT STR) to suspect (MA CHI). The predicted and observed comparisons
for NOx are more suspect due to uncertainties in the measurement
techniques (e.g., the measured values include more than just NO
and NO2) and there is also the potential for subgrid-scale
impacts of local plumes. Not surprisingly, the model fails to
capture localized spikes and, in general, the observed NOx is
higher than predicted, which may be partly due to additional observed
nitrogen species that are included in the observed "NOx".
Lums Pond, Delaware (DL LUM): At DL LUM the model estimated
isoprene concentrations have a curious double peak every day of
the episode, the expected one in the afternoon when isoprene emissions
are at the greatest and an additional one in the early morning.
On some days the observed diurnal profile also contains such a
double peak (e.g., July 18), but not on every day of the episode
as estimated by the model. It is speculated that this early morning
modeled isoprene spike may be due to inconsistencies between the
RAMS generated atmospheric mixing profiles and the BEIS2 derived
isoprene emissions. The RAMS meteorology was not used to develop
the BEIS2 emission estimates, thus, there may be inconsistencies
in the meteorology used (e.g., temperature and UV) to define biogenic
emissions and vertical mixing. For example, a time zone shift
or, different light algorithms, or presence/lack of clouds may
result in sufficient temperature and light for BEIS2 to estimate
isoprene emission fluxes which are not present in the RAMS meteorology
so there is very slow vertical mixing. The model fails to capture
the magnitude and diurnal variations in the observed TNMOC. The
model/observed ozone comparisons are reasonable on some days but
poor on others (e.g., July 15). The model estimates an earlier
onset of ozone production than observed, which may possibly be
related to the higher than observed predicted morning isoprene
Camp Logan and Braidwood, Illinois (IL CAM and IL BRA):
Isoprene observations for the two Illinois sites were included
on the data tape of the UAM-V base case model predictions from
the OTAG Clearing House. Only isoprene measurements were included
and these data were very sparse. Thus, data from these two sites
were not included in the statistical model/data comparisons.
Capitol and Pride, Louisiana (LA CAP and LA PRD): The model
exhibits very poor agreement with the magnitude and diurnal variations
in the observed isoprene, TNMOC, ozone, and NOx concentrations.
Care should be taken is using the model to evaluate alternative
control plans in this region. These sites are located in the outer
30 km resolution model grid.
Agawam and Borderland, Massachusetts (MA AGA and MA BOR):
Both of these sites suffer from poor data capture, so were not
used in the statistical model/observed comparison. For the one
day in which isoprene observations did exist at the MA AGA site
(July 14), the model agreed quite well with the observed values.
Lynn, Massachusetts (MA LYN): In general, the predicted
and observed hourly isoprene concentrations agree reasonably well,
with the exception of modeled very high afternoon isoprene concentrations
on some days (e.g., > 50 ppbC on July 14) that are not reflected
in the observed values. The model and observed TNMOC hourly concentrations
also agree reasonably well, however again it appears that the
model diurnal profile is out of phase with the observations. The
observed ozone diurnal profile on the two higher ozone days (July
13-14) are simulated quite well by the model.
Newbury, Massachusetts (MA NEW): For the few days in which
observed isoprene concentrations were available at this site,
the model tended toward overestimation. Data capture for isoprene,
TNMOC, and ozone were low at this site so it was not included
in the statistical performance evaluation.
Lake Clifton, Maryland (MD LKC): At the Maryland site near
Baltimore the model is significantly overestimating the observed
isoprene concentrations -- the two-peaked diurnal profile is present
in both the measurements and model estimated time series. TNMOC
appears to be also slightly overestimated, again the model and
observed diurnal profiles are quite similar. The modeled ozone
performance ranges from extremely good on some days (July 14)
to questionable on others.
Cape Elizabeth, Maine (ME CAP): During periods of very
low isoprene concentrations, both the model and observations agree
quite well (e.g., July 10-12, 16-19). However, during higher isoprene
concentration conditions it appears that the model overestimates
the observed afternoon peaks. Although the model estimated TNMOC
appears to be greater than observed over most hours, the observed
TNMOC spikes are not replicated by the model. Until the end of
the episode when the predicted and observed ozone concentrations
are both near background concentrations, the model does a poor
job in reproducing the observed hourly ozone concentrations.
Rider College, New Jersey (NJ RID): The diurnal variation
in the observed hourly isoprene concentrations are reproduced
by the model reasonably well at this site, with the exception
of underestimating the observed spikes on July 14 and July 15.
The Bronx, New York (NY BNX): As seen for some of the other
sites, the model estimated diurnal bimodal isoprene peaks do not
appear to be reflected in the observations. The modeled isoprene
concentrations vary from underestimation (e.g., July 12 and 17)
to overestimation (July 15). The diurnal variations in the observed
ozone concentrations have large amounts of hour-to-hour variability
reflecting the influences of local NOx sources at this urban site.
The modeled diurnal ozone profile is much smoother due to the
inability of the model to simulate the subgrid-scale impacts of
Arendtsville, Philadelphia (PA ARE): When data were acquired
and processed for this site, there was no data available during
the July 1995 OTAG episode.
Philadelphia, Pennsylvania (PA PHL): The model appears
to systematically overestimate the observed isoprene concentrations
in Philadelphia. Except for high (> 200 ppbC) TNMOC peaks on
July 10 and July 12, the model appears to reproduce the magnitude
of the observed TNMOC reasonably well. For most days the observed
hourly ozone concentrations are not simulated well by the model.
East Providence, Rhode Island (RI PRO): The model significantly
overestimates the observed isoprene concentrations at this site
on July 13-16. However, TNMOC is generally underestimated. The
general day-to-day variation in the observed ozone concentrations
are reproduced reasonably well, although the model fails to reproduce
the observed peak on July 14.
Youth Inc., Tennessee (TN YTH): Only isoprene concentrations
were acquired for the Tennessee site. Except for the underestimation
of the high observed isoprene concentrations on July 11 and 15,
the model reproduces the observations reasonably well.
Corbin Virginia (VA COB): Only one day of isoprene data
was available at this site (July 14) during which it appears that
the model exhibits significant overprediction. Due to the limited
data availability, this site was not included in the statistical
Statistical Comparisons of
Predictions and Observations
We performed two types of statistical comparisons of the predicted and observed concentrations and their ratios at the 15 selected sites:
Figure 3-2 displays the statistical model/data comparison for
isoprene at the four sites located in reasonably close proximity
in northern CT and mid-MA (see Appendix B for all sites). At these
four sites it appears that the model has a tendency toward underestimating
the observed isoprene concentrations. An examiniation of the Box
Plots reveals that at these sites the observed isoprene concentrations
have much greater variability than predicted. The variability
across the sites is also much greater in the observed values than
predicted; the average observed isoprene conentrations vary by
almost a factor of three at the four sites that are in reasonably
close proximty (11.5 ppbC at MA CHI to 30.8 ppbC at MA WAR). Whereas
the average observed values only varies by about 50% (8.1 ppbC
at MA CHI to 12.3 at MA WAR). Similar Box Plots and model performance
statistics for the remainder of the sites are provided in Appendix
B. More analysis is needed to fully interpret and understand the
results of these displays.
Isoprene Summary: Figure 3-3 summarizes the percent differences
between the predicted and observed afternoon (1100-1900) isoprene
concentrations at the 15 monitoring sites under study in the Phase
I preliminary analysis. Unlike the Edgerton analysis presented
in Figure 2-2, we see that over half of the sites (8 out of 15)
have average isoprene concentrations that agree within ±
50%. However, like the Edgerton analysis discussed in Chapter
2, there is more of a tendency toward significant (bias > 50%)
overprediction (6 out of 15 sites) than significant underprediction
(< - 50%) (1 out of 15 sites). Furthermore, at four of the
sites, the isoprene overprediction bias is quite large (150% to
250%). At the four sites that are in common between the Edgerton
and this analysis (DE LUM, MA LYN, ME CAP, and TN YTH), there
is qualitative agreement, but quantitative differences: (1) at
DE LUM both studies estimate an overprediction bias, but Edgerton's
analysis suggest that the overprediction bias (60%) is more significant
than this analysis (30%); (2) at MA LYN the reverse is true with
Edgerton estimating an approximate 30% bias whereas our analysis
is closer to 60%; (3) the two studies agree that the OTAG UAM-V/BEIS2
modeling systemn significantly overestimates isoprene concentrations
at the ME CAP sites by 200-230%; and (4) the Edgerton study calculates
a larger overprediction bias at the TN YTH site (55%) than this
study (10%). More details on the procedures used in the Edgerton
analysis are needed to resolve these differences.
TNMOC Summary: A summary of the percent bias for the comparison
of predictd and observed TNMOC concentrations are provided in
Figure 3-4, the complete comparison is provided in Appendix C.
As seen for isoprene, over half of the sites predict the average
observed TNMOC concentrations to within ± 50%. The model
appears to be understating the observed TNMOC concentrations at
the two Baton Rouge sites, even though isoprene was overestimated
at one of them (LA CAP). It is interesting to note that at many
of the sites where there was significant overestimation of isoprene
(e.g., LA CAP, MA LYN, MD LKC, PA PHL, RI PRO), the model underestimates
Isoprene-to-TNMOC Ratios: Appendix D compares the predicted
and observed isoprene-to-TNMOC ratios, with a summary of the percent
bias provided in Figure 3-5. The predicted isoprene fraction of
the TNMOC agrees to within ± 50% at over half of the sites
(7 out of 13). At the remaining six sites, there is one with a
slightly significant underestimation (-59% at MA WAR), one with
a slightly significant overestimation (68% at ME CAP), and four
with very signifiant overestimation (> 200%) of the observed
TNMOC-to-NOx Ratios: Because of the uncertaintis in the
NOx measurements and potential for subgrid-scale impacts at monitors,
care should be taken in the interpretation of the TNMOC-to-NOx
ratio comparisons. Further, just examining the summary figure
of percent bias in Figure 3-6 provides an incomplete picture of
the comparison; the magnitude of the ratios is much more important
as they suggest the chemical regime of the photochemical system
and whether VOC or NOx control will be more effective at reducing
ozone concentrations. For example, if both the predicted and observed
TNMOC-to-NOx ratios are above approximately 20, then the predicted
and observed photochemical systems are both in the NOx-sensitivity
regime even if there is signifiant bias. Similarly, a TNMOC-to-NOx
ratio of less than 10 suggests a VOC-sensitive regime. An examination
of the predicted and observed TNMOC-to-NOx ratios in Appendix
E suggests there are serious shortcomings of the model at the
DL LUM site where the observed ratio (3) is in the VOC-sensitive
regime whereas the predicted ratio (25) is in the NOx-sensitive
regime. At the two Baton Rouge sites, both the model and observations
have ratios greater than 20 which suggest NOx-sensivity regime.
The average observed TNMOC-to-NOx ratios at the three Massachusett
sites are all < 10 which suggest more VOC-sensitivity, but
except for one site (MA WAR) the predicted values (18, 10, and
30) are tilted more toward NOx-sensitivity conditions. The remaining
four sites (MD LKC, NJ RID, NY BNX, and RI PRO) exhibit reasonable
agreement between the predicted and observed TNMOC-to-NOx ratios.
Ozone Concentrations: The final model/data comparison is
for ozone, which is shown in Appendix F with the average bias
summarized in Figure 3-7. A more complete evaluation of the OTAG
UAM-V/BEIS2 modeling system ozone model performance evaluatuion
is provided in the OTAG model evaluation report (OTAG, 1996).
The comparison of the predicted and observed ozone concentrations
are much closer than seen for the ozone precursors. At only three
sites does the bias exceed 15%, the two Baton Rouge sites and
PA PHL site. Although the 13 sites analyzed represent just a small
fraction of available ozone measurements, it is somewhat disturbing
that at these 13 sites there is a tendency toward systematic overestimation
of the average afternoon observed ozone concentrations.
24-Hour Comparisons: Figures comparing the predicted and
observed isoprene, TNMOC, isoprene-to-TNMOC ratio, TNMOC-to-NOx
ratio, and ozone for the full 24-hour diurnal period are provided
in Appendix G. Given the difficulty in interpreting modeled 50
m concentrations with surface values under nocturnal stable comnditions
and the limited resources available for Phase I of this study,
at this time we have not interpeted these results.
Figure 3-1. Locations of the 15 monitoring sites where
statistical comparisons of predicted and observed isoprene, TNMOC,
isoprene-to-TNMOC ratios, TNMOC-to-NOx ratios, and ozone where
made in the Phase I preliminary analysis.
Figure 3-2b. Statistial comparison of predicted and observed
isoprene concentrations at the Chicopee, Massaschusetts (MA CHI)
monitoring site. Box Plots display the 5th , 25th, median, mean
(symbol), 75th, and 95the pecentile of the distribution.
Figure 3-4. Summary comparison of the percent average bias
between the OTAG UAM-V/BEIS2 predicted and observed afternoon
total nonmethane organic compund (TNMOC) concentrations at 13
monitoring sites (more details are found in Appendix C).
Figure 3-5. Summary comparison of the mean OTAG UAM-V/BEIS2
predicted and observed isoprene-to-TNMOC ratios at 13 monitoring
sites (more details are found in Appendix D).
Figure 3-6. Summary comparison of the mean OTAG UAM-V/BEIS2
predicted and observed TNMOC-to-NOx ratios at the 10 monitoring
sites (more details are found in Appendix E).
Figure 3-7. Summary comparison of the percent average bias
between the OTAG UAM-V/BEIS2 predicted and observed ozone concentrations
at the 13 monitoring sites (more details are found in Appendix
The preliminary model evaluation of the OTAG UAM-V/BEIS2 biogenic
(isoprene) emission estimates for the July 1995 D2 base case simulation
has raised several issues that need further investigation and
analysis. In addition, within the limited resources of Phase I
of this study, the results generated could not be analyzed in
as detailed fashion as desired. Additional data sources (e.g.,
NARSTO-Northeast 1995 data, more PAMS sites, SOS data) have also
become available since the Phase I work was initiated and need
to be integrated into the analysis. The following are areas that
we see require further investigation under Phase II of this work
Detailed Investigation into Procedures Used to Generate the OTAG Biogenic Emissions: We need to clearly understand the procedures used and assumptions made in running BEIS2 to generate the OTAG biogenic emission estimates. In our analysis we noticed that frequently there was an unusual modeled diurnal isoprene pattern that was not supported by the observation which may have been due to inconsistencies between the meteorology used for biogenic emissions (observations) versus the meteorology used to define mixing (RAMS/SAIMM). In order to interpret any discrepancies between the theoretical basis of BEIS2 and its evaluation under ideal circumstances versus the way it was applied in the OTAG study to generate a regional biogenic emission inventory, the procedures used to generate the OTAG biogenic emissions need to be clearly documented so that their implications on the OTAG modeling can be determined.
Comparison with Detailed Flux Measurements: OTAG biogenic emission estimates and estimated isoprene concentrations for the grid cells containing the locations where detailed vegetation emission fluxes were measured in the special studies (e.g., the site near Oak Ridge, TN as reported by Guenther and co-workers, 1996) should be extracted and compared with the historical detailed measured fluxes under similar meteorological conditions. This may help establish a link between the BEIS2 experimental derived emission factors and evaluation under ideal well-characterized conditions and the real-world application of the model for generating regional biogenic emission estimates as done for OTAG.
Isoprene Bias: The analysis suggest that there may be a net overprediction bias for isoprene concentrations in the OTAG UAM-V/BEIS2 modeling system. The more rural NARSTO sites used by Dr. Edgerton appear to confirm this hypothesis. More data sites need to be included in the analysis in order to have a more comprehensive and statistically significant comparison.
Reconciliation with Edgerton Analysis: Although this study and the Edgerton analysis produced qualitatively similar results for the four sites in common, there were some quantitative differences. Discussions are needed with Dr. Edgerton to obtain details on the procedures he used to make the isoprene comparisons so they can be resolved with this study's results.
Isoprene versus Other TNMOC: One of the most provocative findings of our Phase I work effort is that at many of the sites where isoprene was overestimated by the OTAG modeling system, TNMOC was underestimated. Although of lower mass concentrations than TNMOC, isoprene is much more reactive (approximately five times more reactive than automobile exhaust). The results to date suggest that isoprene (biogenic VOC) may be overstated in the OTAG modeling system, but may be compensated for by an understatement of anthropogenic VOC emissions. More investigations into this issue is needed (e.g., urban-rural stratification) to understand the validity of this hypothesis and its implications.
Spatial Representativeness/Subregional Analysis: Related to the urban-rural analysis is the need to perform spatial analysis of the model/data comparisons to identify any spatial trends. Additional subregional analysis may also be beneficial, for example the results at the two Baton Rouge sites raise serious questions concerning the adequacy of the model in that subregion.
TNMOC/NOx Ratios: Some of the model/data comparisons of the TNMOC-to-NOx ratios were quite disturbing. In several cases, the model estimated ratios that would characterize the photochemical system as NOx-sensitive, whereas the observations suggested VOC-sensitivity. A closer look at the TNMOC and, especially the NOx measurements is needed before determining the implications of model/data differences in the TNMOC-to-NOx ratios.
Site Characterization: Comparison of the individual site characteristics (e.g., local biomass, exposure, vegetation type) with the grid cell average may provide some insight into the model/data differences.
Comparisons Aloft: Comparison of the isoprene and other measured concentrations from the NARSTO aircraft observations with the measured values aloft will provide additional insight into any potential bias in the OTAG BEIS2 emissions estimates.
Each of the above items should be examined in more detail under
Phase II of this study. In addition, Phase II would generate a
report and presentation quality graphics.
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