Determination of urban volatile organic compound emission ratios and comparison with an emissions database

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Determination of urban volatile organic compound emission ratios
and comparison with an emissions database
C. Warneke,1,2S. A. McKeen,1J. A. de Gouw,1,2P. D. Goldan,1W. C. Kuster,1
J. S. Holloway,1,2E. J. Williams,1,2B. M. Lerner,1,2D. D. Parrish,1M. Trainer,1
F. C. Fehsenfeld,1S. Kato,3E. L. Atlas,4A. Baker,5and D. R. Blake5
Received 16 August 2006; revised 7 March 2007; accepted 27 March 2007; published 15 May 2007.
[1] During the NEAQS-ITCT2k4 campaign in New England, anthropogenic VOCs and
CO were measured downwind from New York City and Boston. The emission
ratios of VOCs relative to CO and acetylene were calculated using a
method in which the ratio of a VOC with acetylene is plotted versus the photochemical
age. The intercept at the photochemical age of zero gives the emission ratio.
The so determined emission ratios were compared to other measurement
sets, including data from the same location in 2002, canister samples collected inside
New York City and Boston, aircraft measurements from Los Angeles in 2002, and the
average urban composition of 39 U.S. cities. All the measurements generally
agree within a factor of two. The measured emission ratios also agree for most compounds
within a factor of two with vehicle exhaust data indicating that a major source of
VOCs in urban areas is automobiles. A comparison with an anthropogenic emission
database shows less agreement. Especially large discrepancies were found for the
C2-C4alkanes and most oxygenated species. As an example, the database overestimated
toluene by almost a factor of three, which caused an air quality forecast model
(WRF-CHEM) using this database to overpredict the toluene mixing ratio by a factor of 2.5
as well. On the other hand, the overall reactivity of the measured species and the reactivity
of the same compounds in the emission database were found to agree within 30%.
Citation: Warneke, C., et al. (2007), Determination of urban volatile organic compound emission ratios and comparison with an
emissions database, J. Geophys. Res., 112, D10S47, doi:10.1029/2006JD007930.
1.Introduction
[2] Volatile organic compounds (VOCs) are emitted into
the atmosphere in large quantities from a variety of different
natural and anthropogenic sources [Brasseur et al., 1999;
Hewitt, 1999]. VOCs are key ingredients in the formation of
ozone and aerosols in polluted air, and play a significant
role in determining regional air quality, in the chemistry of
the global troposphere, and possibly in the global carbon
cycle. On a global scale the biogenic VOC emissions,
mainly isoprene, a- and b-pinene and methanol [Guenther
et al., 1995, 2006], dominate over the anthropogenic
sources. On a regional scale, in and around urban areas,
the anthropogenic emissions, which are in large part caused
by-production, storage and use of fossil fuels, usually are
more important.
[3] In July and August of 2004, a large-scale atmospheric
chemistry and transport study was conducted over North
America and Europe within the framework of the ICARTT
collaboration (International Consortium for Atmospheric
Research on Transport and Transformation). As part of
the NOAA contribution to ICARTT, the NEAQS-ITCT
2k4 (New England Air Quality Study–Intercontinental
Transport and Chemical Transformation) study was con-
ducted, which involved airborne measurements using the
NOAA WP-3 research aircraft based out of Portsmouth,
New Hampshire and ship-based measurements using the
NOAA research vessel Ronald H. Brown in the Gulf of
Maine. Research goals of the NEAQS-ITCT 2k4 study
included a detailed characterization of (1) the primary
emissions of gas phase and aerosol species on the North
American continent, including emissions from eastern U.S.
cities (Boston and New York City), forest vegetation, and
point sources such as power plants; (2) the chemical
transformation leading to the formation of secondary pollu-
tants (ozone and aerosol); (3) the transport processes
involved, including local and long-range transport to
Europe; and (4) the evaluation of air quality forecast
models.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D10S47, doi:10.1029/2006JD007930, 2007
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1Earth System Research Laboratory, NOAA, Boulder, Colorado, USA.
2Also at Cooperative Institute for Research in Environmental Sciences,
University of Colorado, Boulder, Colorado, USA.
3Department of Chemistry, University of Colorado, Boulder, Colorado,
USA.
4Rosenstiel School of Marine and Atmospheric Science, University of
Miami, Miami, Florida, USA.
5Department of Chemistry, University of California, Irvine, California,
USA.
Copyright 2007 by the American Geophysical Union.
0148-0227/07/2006JD007930$09.00
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[4] The quality of the forecast of ozone and secondary
organic aerosol (SOA) formation strongly depends on the
accurate knowledge of the primary VOCs emissions that act
as precursors for ozone and SOA. Extensive data sets that
provide gridded VOC emissions of a large number of
species are used in these models. Recent examples for the
importance of the accurate knowledge of anthropogenic
emission data sets for air quality forecasts are modeling
papers for the Houston area [Jiang and Fast, 2004; Lei et
al., 2004; Zhang et al., 2004].
[5] In this paper, we will focus on the determination of
primary anthropogenic VOC emission ratios from mainly
Boston and New York City. Emission ratios of a large
number of VOCs versus acetylene and CO will be deter-
mined using the ship-based measurements in the Gulf of
Maine downwind of the urban areas. The resulting emission
ratio data will be compared to data from a previous
campaign in 2002 in the same area, to canister samples
collected in Boston and New York City and to data from
Los Angeles in 2002. The measured emission ratios will
also be used to test an anthropogenic emissions database,
which is based on the EPA NEI-99 (Environmental Protec-
tion Agency National Emissions Inventory-99) database.
This database includes four categories of emissions: on-
road, off-road, area and point sources. We will compare our
results to the sum of those four categories, because the VOC
measurements were made downwind of the urban areas and
should include all the source categories. The database is
used in various regional air quality forecast models such as
the WRF-CHEM (Weather Research and Forecasting-
Chemical Model). The results from WRF-CHEM are com-
pared to the measurements from the NEAQS-ITCT 2k4
study to demonstrate that the use of too high emission ratios
will cause an overprediction in the model result.
2.
2.1. Measurements
[6] The NOAA research vessel Ronald H. Brown
(RHB) operated in the Gulf of Maine between Boston,
Massachusetts, and Nova Scotia from 5 July until 12 August
2004. The NOAA WP-3 aircraft conducted 18 research
flights between 5 July and 15 August 2004, from Ports-
mouth, New Hampshire. The RHB and the WP-3 both
carried an extensive set of instruments to characterize the
gas phase and aerosol properties of the atmosphere. A full
description of the payload of the WP-3 and the RHB will be
given elsewhere. Here only the relevant instruments are
described briefly.
[7] The measurements from Los Angeles were performed
using the WP-3 aircraft as part of the ITCT2k2 (Intercon-
tinental Transport and Chemical Transformation 2002)
study [Nowak et al., 2004]. The same relevant instrumen-
tation as described below was used on the WP-3 during this
campaign.
2.1.1. Ship-Based Instrumentation
[8] VOC measurements on the RHB were performed
using an online GC-MS. A detailed description of this
instrument and its analysis procedure is given elsewhere
[Goldan et al., 2004]. The GC-MS instrument analyzed
350 mL air samples with a 5-min acquisition time every
30 min. More than 100 VOCs including many oxygenated
Methods
compounds, hydrocarbons, halocarbons and alkyl nitrates
were identified and quantified with this instrument. The
detection limit of the GC-MS instrument is <0.5 pptv and
the measurement precision is about 2%. The overall accu-
racy is mainly dependent on the quality of the calibration
standards used and is within 10% [Goldan et al., 2004].
[9] CO was measured via a modified AeroLaser GmbH
[Garmisch-Partenkirchen, Germany] AL5002 Ultra-Fast CO
analyzer, a commercially available vacuum-UV resonance
fluorescence instrument [Gerbig et al., 1999]. For the
campaign, data were collected at 1 Hz and averaged to a
1-min resolution; the total uncertainty is estimated at 3%,
with a limit of detection of 1.5 ppbv.
2.1.2. Airborne Instrumentation
[10] High time resolution measurements of oxygenated
VOCs, aromatics, acetonitrile, isoprene and monoterpenes
were made on the WP-3 aircraft with a PTR-MS instrument
from Ionicon Analytik [de Gouw et al., 2003a]. VOCs were
measured for 1 s every 17 s. During a ship-based intercom-
parison PTR-MS measurements have been compared with
the online GC-MS instrument [de Gouw et al., 2003b], and
possible interferences have been studied by combining
PTR-MS with a gas chromatographic preseparation method
[de Gouw et al., 2003a, 2003c; Warneke et al., 2003]. For a
detailed description of the PTR-MS instrument, the reader is
referred to these references. The PTR-MS was calibrated for
many VOCs between the flights using a standard mixture
containing 500 ppbv of each compound that was diluted to
sub-ppbv levels. The calibration accuracy is estimated to be
better than 15% for each compound. The detection limit of
the PTR-MS is dependent on the compound and ranges
from 30 pptv to 340 pptv [de Gouw et al., 2006].
[11] During every flight, up to 80 whole air samples
(WAS) were collected in electropolished stainless steel gas
canisters. The canisters were filled within 5s to 15s depen-
dent on the altitude at variable intervals. The canisters were
transported to the NCAR laboratory in Boulder, where they
were analyzed within a few days for hydrocarbons, halo-
carbons and C1- to C5- alkyl nitrates using several gas
chromatography techniques. The sampling and the subse-
quent analysis of the canisters is described elsewhere
[Schauffler et al., 1999, 2003]. The overall accuracy for
the VOC measurements is about 10% and the detection limit
is about 5 pptv. In the analysis presented here mainly the
acetylene measurements were used.
[12] CO was determined on the WP-3 aircraft every
second using a vacuum ultraviolet fluorescence measurement
[Holloway et al., 2000]. The precision of the measurements
is estimated to be 2.5%. Variability in the determination of
zero levels results in an absolute uncertainty of about 1 ppbv
in the values reported. The field standard was compared to
NIST Standard Reference Material (SRM) 2612a (10 ppmv
nominal CO in air). The concentration of the calibration
standard is known to within 2%. The overall accuracy of the
1s measurements is thus estimated to be 5%.
2.2. Anthropogenic Emissions Database
[13] The anthropogenic VOC emissions database (4 km
horizontal resolution) is based upon the U.S. EPA’s 1999
National Emissions Inventory (NEI-99, version 3) released
November 2003 (updated to March 2004 revisions), and the
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4 km horizontal resolution spatial surrogates released by
EPA in September of 2003. The emissions are representa-
tive of a typical summer day (average of weekday and
weekend days). VOC and PM2.5compounds, along with the
7 primary species are divided into 24 average hourly
emissions. This database is designed for regional-scale
photochemical models of North America that require emis-
sions data for NOx, VOC, CO, SO2, NH3, PM2.5and PM10.
Spatial partitioning within the U.S. and Canadian provinces
is based on spatial surrogates and source classification code
(SCC) assignments recommended by the U.S. EPA.
Anthropogenic VOCs are partitioned into 41 individual
and lumped VOC compounds, with several of the lumped
VOC compounds defined by the SAPRC-99 (Statewide Air
Pollution Research Center) condensed photochemical mech-
anism [Carter, 2000]. A more detailed description of the
anthropogenic emissions database is given by Frost et al.
[2006] and the Web site within that reference (http://
ruc.fsl.noaa.gov/wrf/WG11/anthropogenic.htm). The emis-
sion of a particular VOC from a single source involves a
three-step process: each SCC specific VOC emission
is converted to a total organic gas (TOG) emission, each
SCC emission is linked to one of about 700 TOG profiles,
and then each TOG profile is linked to about 760 individual
VOC. The data sets used in each of these steps is available
either directly through the U.S. EPA speciation Web site
(http://www.epa.gov/ttn/chief/emch/speciation/) or through
the SPECIATE version 3.2 software also available at that
Web site. Emissions of several individual VOC (e.g.,
ethane, propane, ethylene, propylene, acetylene, styrene,
benzene, toluene, acetone, and acetaldehyde) are extracted
for the database, and are used for comparisons in this study.
Emissions of the lumped VOC species are determined by
aggregating the individual VOC following the condensed
SAPRC-99 mechanism species assignments [Carter, 2000],
excluding the individual VOC separated out before hand
from their lumped categories.
2.3. NOAA WRF-CHEM Model
[14] The multiscale air pollution prediction system used
here is based on the Weather Research and Forecasting
(WRF) model, which is coupled with the RADM2 chemical
mechanism [Stockwell et al., 1990] with updated reaction
rates and NO3chemistry consistent with the RACM mech-
anism [Stockwell et al., 1997]. Biogenic and anthropogenic
emissions, dry deposition, convective and turbulent chem-
ical transport, photolysis, and advective chemical transport
are all treated simultaneously with the meteorology
‘‘online.’’ WRF-CHEM results presented here are from
the ‘‘reference case’’ simulations documented by Frost et
al. [2006]. The model domain covers the eastern U.S. and
southeastern Canada, and the 15 July to 15 August 2004
time period during the ICARTT 2k4 field experiment with
27 km horizontal resolution. Details of the model setup,
meteorological initial and boundary conditions, and physi-
cal options used within WRF-CHEM are given by Frost et
al. [2006]. Anthropogenic VOC emissions are based on the
database described in the previous section with emissions
aggregation into RADM2 species assignments [Stockwell et
al., 1990]. Boundary conditions for chemical species, and
details of the numerical treatment of various transport,
physical, photochemical, radiative and aerosol processes
are documented by Grell et al. [2005]. Statistical evalua-
tions of WRF-CHEM model performance for O3relative to
several other models during the ICARTT 2k4 study period
are given by McKeen et al. [2005].
3.
3.1. Determination of Emission Ratios
[15] VOC emission ratios are determined from data
collected on board the RHB in the Golf of Maine downwind
of Boston and New York City. For this analysis all data in
the Golf of Maine were used and not filtered to select
periods, when the ship was directly downwind of Boston
and/or New York City and our analysis includes the whole
northeastern seaboard. The two cities were the overwhelm-
ing sources for the periods with high VOC enhancements
and therefore we call the emission sources in the following
analysis Boston and New York City. The emission ratios are
determined using a method that was introduced by de Gouw
et al. [2005]. In this method, which will be summarized in
the following, the ratio of a VOC with acetylene is plotted
versus the photochemical age as is shown in Figure 1a for
ethyl benzene. The photochemical age was estimated using
the measured ratio between toluene to benzene in the
sampled air as described by Roberts et al. [1984]:
Results and Discussion
Dt ¼
1
OH
½
? ln
? ktoluene? kbenzene
toluene
½
benzene
½
ðÞ? ln
toluene
benzene
½?
½ ?jt¼0
???
?
?
? ??
;
ð1Þ
where ktolueneand kbenzeneare the rate coefficients for the
reaction with OH (ktoluene= 5.63 ? 10?12cm3molecule?1s?1
and kbenzene= 1.22 ? 10?12cm3molecule?1s?1) [Atkinson
et al., 2005]. [OH] is the average concentration of the
hydroxy radical. The emission ratio of benzene and toluene
(toluene
benzene
½
fresh plumes from Boston and New York City during
several flights with the WP-3 aircraft. Plumes were assumed
to be fresh, when the NOx/NOyratio was larger than 80%.
During eight different flights 18 fresh plumes were
encountered, 11 of which were sampled during the night.
The value of 4.25 for (toluene
benzene
½
recent years benzene emissions have decreased [Fortin et
al., 2005; Harley et al., 2006]. The measured ratio will also
depend on the time of year and on the measurement region.
[16] The [ethyl benzene]/[acetylene] ratio decreases by an
order of magnitude over the course of two days, because the
OH rate coefficient of ethyl benzene (k = 7 ? 10?12cm3
molecule?1s?1) is larger than that of acetylene (k = 0.83 ?
10?12cm3molecule?1s?1) [Atkinson et al., 2005]. The data
in Figure 1 can be described with:
½?
?jt=0) was set to 4.25 and determined by looking at
½?
?jt=0) seems rather large, but in
VOC
C2H2
½
½
?
?¼ ERVOC? exp ? kVOC? kC2H2
ðÞ OH
½?Dt
½?;
ð2Þ
with [VOC] and [C2H2] being the volume mixing ratios of a
primary anthropogenic VOC (ethyl benzene in Figure 1)
and acetylene, ERVOCthe emission ratio of the VOC with
acetylene, kVOC and kC2H2the rate coefficients of those
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compounds with OH [Atkinson et al., 2005] and Dt the
photochemical age. In Figure 1a [OH] was set to 2.1 ?
106molecules cm?3, which was calculated for this data set
in the same way as described by Warneke et al. [2004].
[17] From the intercept on the y axis of the linear fit (red
line in Figure 1a) the emission ratio ERVOCand from the
slope kVOCcan be determined as described by equation (2).
For this method of determining the emission ratios four
assumptions must be valid:
[18] 1. The speciation of VOCs is the same for all sources
upwind of the measurement area. For the presented data set
the VOC sources are mainly Boston and New York City.
The magnitude of the emission is proportional to the
emission of acetylene, which is used here as an inert tracer.
Acetylene is emitted mainly by automobile exhaust [Fortin
et al., 2005; Harley et al., 1997]. Small other sources of
acetylene, such as welding, certain chemical industries, and
fires, might exist, but are assumed to be of little influence
[Makar et al., 2003; Whitby and Altwicker, 1978]. Regional
differences in the VOC speciation are generally small as
will be shown in this paper. The RHB is an ideal platform to
study the emission ratios of Boston and New York City,
because the urban plumes are usually transported mainly
over the ocean before being encountered at the ship and
therefore no fresh anthropogenic VOCs are added to the
plumes during the transport. The air masses measured at the
ship’s location are frequently influenced by both Boston and
New York City simultaneously and therefore the emissions
from the two cities cannot be separated.
[19] 2. The removal of VOCs is controlled only by
reactions with OH radicals. Reactions of VOCs with NO3
radicals at night and with ozone were a minor sink of
anthropogenic VOCs in the Gulf of Maine [Warneke et al.,
2004]. Some of the alkenes react with NO3, but their loss is
small compared to the OH loss [Warneke et al., 2004].
Losses due to wet and dry deposition are assumed to be
negligible.
[20] 3. Anthropogenic/urban emissions are the only sour-
ces of the investigated primary hydrocarbons in this region.
Other sources of VOCs and CO such as biomass burning
and local sources (e.g., power plants and ship plumes) were
observed but have been eliminated from this analysis.
Biomass burning sources were eliminated by only consid-
ering data for which acetonitrile was lower than 150 pptv
[Warneke et al., 2006]. Power plant and ship plumes were
eliminated by looking at CO and NOx data combined with
FLEXPART back trajectory calculations [Warneke et al.,
2006]. The total elimination excluded less than 10% of the
data.
[21] 4. The photochemical age is described by formula
(1). Limitations of using hydrocarbon ratios to determine
the photochemical age by the mixing of air masses with a
different age were pointed out in several studies [McKeen
and Liu, 1993]. An error in the photochemical age results in
a small error for the emission ratios as shown in Figure 1b.
The black curve in Figure 1b is the same as in Figure 1a
with the best estimate of the photochemical age. The red
curve is with [OH] reduced by a factor of 2 and the green
curve has the
benzene
½
the [OH] concentration has no effect on the resulting
emission ratios. The initial estimate of the emission ratio
of toluene versus benzene on the other hand, clearly has an
influence. Toluene/benzene ratios depend on the fuel com-
position and must be carefully measured for this method and
may be dependent on the location (different fuel types) and
the time of year (different fuel composition in winter versus
summer). We therefore estimate the error in the emission
ratios to be about 30% (15% from the measurement uncer-
tainty of benzene and toluene and additional 15% from the
determination of the ratio).
[22] The emission ratios for all measured anthropogenic
hydrocarbons versus acetylene determined in a manner
similar to ethyl benzene as shown in Figure 1a are presented
in Table 1 together with the emission ratios versus CO,
which is used in many studies as the anthropogenic emis-
sions marker instead of acetylene. Determining the emission
toluene
½?
?jt=0ratio set to 50% (2.125). Varying
Figure 1. (a) Ethyl benzene/acetylene ratio plotted versus
the photochemical age of the air mass. The measurements
include all data on the RHB from 5 July until 12 August
2004 filtered for biomass burning and power plants (N =
1608). The red line is a linear fit where the emission ratio is
determined at photochemical age zero. (b) Emission ratios
determined using photochemical ages determined with OH/2
(red points with the red line as linear fit) and
(green points with the green line as linear fit). The black
points and the black line are the data from Figure 1a.
½toluene?
½benzene?jt=0/2
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Table 1. Emission Ratios of Anthropogenic VOCs From Boston/New York City, Los Angeles, and an Anthropogenic Emissions Inventory Based on EPA Dataa
Compound
Boston/New York
City 2004, pptv
[pptv C2H2]?1
Boston/New York
City 2002, pptv
[pptv C2H2]?1
Boston/New York
City CO 2004, pptv
[ppbv CO]?1
Los Angeles
2002, pptv
[ppbv CO]?1
Boston/New York
City (Baker et al.,
submitted manuscript,
2006), pptv [ppbv CO]?1
39 US Cities
[Seila et al., 1989],
pptv [pptv C2H2]?1
Vehicle Exhaust
[Harley et al., 1992],
pptv [pptv C2H2]?1
Database, pptv
[ppbv CO]?1
Alkanes
Ethane
3.097
2.738
11.616
7.84
10.37
1.806
0.967
1.571
Propane
2.187
2.505
7.733
12.39
5.50
1.214
0.023
0.357
n-butane
0.482
0.660
1.688
5.4
2.39
1.562
0.758
iso-butane
0.287
0.373
1.012
2.58
1.80
0.574
0.086
n-pentane
0.463
0.555
1.548
3.01
1.69
0.682
0.250
iso-pentane
1.192
1.509
3.991
6.38
3.65
1.404
0.583
Cyclohexane
0.092
0.068
0.285
0.057
0
Methyl cyclopentane
0.180
0.135
0.566
0.165
0.060
n-hexane
0.335
0.175
1.072
1.13
0.284
0.116
2-methyl pentane
0.341
0.272
1.106
0.385
0.244
3-methyl pentane
0.394
0.186
1.276
0.276
0.151
2, 2-dimethyl butane
0.033
0.050
0.120
0.070
2, 3-dimethyl butane
0.082
0.074
0.265
0.098
0.116
Methyl cyclohexane
0.065
0.118
0.202
0.075
0.041
n-heptane
0.126
0.157
0.398
0.59
0.104
0.030
2-methyl hexane
0.124
0.133
0.385
0.162
3-methyl hexane
0.148
0.156
0.460
0.131
2, 3-dimethyl pentane
0.080
0.115
0.252
2, 4-dimethyl pentane
0.053
0.071
0.171
0.049
2, 2, 3-trimethyl butane
0.009
0.012
0.031
n-octane
0.062
0.085
0.197
0.08
0.050
0.018
3-methyl heptane
0.042
0.082
0.131
0.043
2-methyl heptane
0.054
0.102
0.171
0.048
2, 2, 4-trimethyl pentane
0.148
0.576
0.476
0.132
2, 3, 4-trimethyl pentane
0.055
0.170
0.171
0.048
2, 3, 3-trimethyl pentane
0.061
0.209
0.194
n-decane
4e-05
1e-4
Alkenes
Ethylene
1.343
1.372
4.564
4.92
5.33
1.659
2.107
7.534
Propylene
0.408
0.393
1.363
0.76
1.37
0.398
0.595
0.949
trans-2-pentene
0.032
0.057
0.097
0.090
cis-2-pentene
0.016
0.029
0.050
0.112
2-methyl-1-butene
0.076
0.102
0.250
3-methyl-1-butene
0.018
0.037
0.058
1-butene
0.041
0.058
0.139
0.21
0.229
0.071
1-pentene
0.035
0.049
0.112
cis-2-butene
0.017
0.030
0.059
0.10
0.107
trans-2-butene
0.015
0.027
0.053
0.10
0.097
0.054
Acetylene
1.000
1.000
3.6
4.99
3.94
1.000
1.271
Aromatics
Styrene
0.007
0.012
0.026
0.023
Ethyl benzene
0.099
0.108
0.314
0.22
0.114
0.123
(m + p)-xylene
0.387
0.346
1.159
0.64
0.351
0.330
o-xylene
0.149
0.134
0.459
0.45
0.25
0.140
0.123
1, 2, 4-trimethyl benzene
0.116
0.129
0.350
0.183
0.158
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ratios by plotting VOC/CO versus the photochemical age
would depend on the atmospheric background mixing ratio
of CO and subtracting a constant CO background might
lead to an error in the emission ratios. Plotting the ratio of
acetylene with CO (with an atmospheric background of
75 ppbv subtracted [Warneke et al., 2006]) versus the
photochemical age as described earlier yields an emission
ratio of 3.9 pptv/ppbv. This value strongly depends on
the estimated CO atmospheric background and on the
different atmospheric life times (CO rate coefficient with
OH is 2.4 ? 10?13cm3molecules?1s?1(1 atm and 298 K)
[Atkinson et al., 2005]), which introduces a large uncertainty.
We have therefore used the ratio between acetylene and CO
of 3.6 pptv/ppbv as shown in Figure 2 to calculate the
emission ratios versus CO. The data in Figure 2 are all
the data measured on the RHB and WP-3 aircraft during the
entire campaign; only biomass burning and power plants
were eliminated. In Figure 2 the atmospheric background of
CO of 75 ppbv that complicates the photochemical age
method can also be seen. As will be shown in Figure 3,
using the slope from the correlation plot of acetylene versus
CO might cause a small error, which we estimate to be
about 10% yielding a total error of the VOC/CO emission
ratio of 40%.
[23] The standard method to determine emission ratios is
to calculate the slope of the linear fit of the scatterplot of
two compounds. Figure 3a shows the case of ethyl benzene
versus acetylene. The color code indicates the photochem-
ical age and it can be seen that the [ethyl benzene]/
[acetylene] ratio is higher at younger ages. The linear fit
through the data is given with the solid black line, which is
usually used as the emission ratio. The emission ratio from
the photochemical age method is shown with the red line in
Figure 3a and lies close to all the points that have not
been photochemically aged. The slope from the linear fit
was determined for all the primary hydrocarbons listed in
Table 1. The results are plotted in Figure 3b versus the
photochemical age method from above. It can be seen that
the values determined from the scatterplots are, for all
compounds except two, lower than the method used here.
Mixing with older air will cause the slopes from the
correlation plots to change dependent on the lifetime and
the mixing ratio of the investigated compounds in the
aged air. The relative difference ([ER at photochem. Age
t = 0]?[ER from DX])/[ER from DX] of the two methods is
plotted versus the rate coefficient with OH in Figure 3c and
it is obvious that there is a clear dependence on the lifetime
of the investigated compound. The relative error gets larger
the shorter-lived the compounds are. It is very difficult to get
close enough to emission sources in ambient air measure-
ments that neither mixing nor chemistry had changed
the ratio of two compounds. We therefore will use the data
determined with the photochemical age method for the
further analysis in this paper. The [toluene]/[benzene] and
[acetylene]/[CO] ERs, which cannot be determined with
the photochemical age method, might therefore be an
underestimation.
3.2. Emission Ratios of Oxygenated VOCs
[24] The calculation of the emission ratios of oxygenated
VOCs (oxyVOCs) is further complicated by their photo-
chemical production and possible biogenic sources
Table 1. (continued)
Compound
Boston/New York
City 2004, pptv
[pptv C2H2]?1
Boston/New York
City 2002, pptv
[pptv C2H2]?1
Boston/New York
City CO 2004, pptv
[ppbv CO]?1
Los Angeles
2002, pptv
[ppbv CO]?1
Boston/New York
City (Baker et al.,
submitted manuscript,
2006), pptv [ppbv CO]?1
39 US Cities
[Seila et al., 1989],
pptv [pptv C2H2]?1
Vehicle Exhaust
[Harley et al., 1992],
pptv [pptv C2H2]?1
Database, pptv
[ppbv CO]?1
1, 2, 3-trimethyl benzene
0.022
0.031
0.069
0.059
0.042
1, 3, 5-trimethyl benzene
0.030
0.038
0.091
0.052
0.108
1-ethyl-2-methyl benzene
0.033
0.036
0.100
0.050
1-ethyl-(3 + 4)-methyl-benzene
0.116
0.124
0.349
0.140
iso-propyl benzene
0.008
0.010
0.025
n-propyl benzene
0.026
0.029
0.081
Benzene
0.171
0.210
0.617
0.95
1.09
0.326
0.474
0.599
Toluene
0.846
0.792
2.622
3.51
3.79
0.749
0.967
6.439
aThe estimated error on the measured emission ratios is 30%. Data in the first three columns were determined with the photochemical age method (see text) and therefore are the most accurate.
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[de Gouw et al., 2005; Li et al., 1997]. Therefore no linear
fit can be used in a semilogarithmic plot of the [oxyVOC]/
[acetylene] ratio versus photochemical age. The evolution
of oxyVOCs can be described using the expression
[de Gouw et al., 2005]:
oxyVOC
½ ? ¼ ERoxyVOC? C2H2
? exp ? koxyVOC? kC2H2
þ ERprecursor? C2H2
?exp ?kprecursorsOH
þ biogenics
½?
??OH
koxyVOC? kprecursors
?Dt
exp kC2H2OH
½
? þ background
½?Dt
kprecursors
??
½ ? ?
½
??? exp ?koxyVOCOH
?Dt
?
½?Dt
??
ðÞ
½½ðÞð3Þ
where the first term represents the removal of oxyVOCs by
OH as described in equation (1). The second term represents
the removal and production of secondary anthropogenic
oxyVOCs. The third term represents biogenic emissions,
including photochemical production from biogenics, plus
the local background mixing ratios. ERoxyVOC and
ERprecursorare the emission ratios of the oxyVOC and the
precursor and kprecursorthe rate coefficient with OH from the
precursor to form oxyVOCs. koxyVOCis the rate coefficient
of the oxyVOC, which was taken from [Atkinson et al.,
2005]. Photolysis reactions of the oxygenated species are not
taken into account, because the photolysis loss rate is small
compared to the OH loss rate [Warneke et al., 2004].
ERoxyVOC, ERprecursor, kprecursor, ([biogenics] + [background])
are determined from a linear least squares fit that minimizes
the difference between the measured oxyVOC mixing ratio
and those calculated from equation (3) [de Gouw et al.,
2005]. The results of this analysis for the emission ratios of
the measured oxyVOCs on the RHB are shown in Table 2.
All other parameters that can be determined from this fit are
irrelevant to the analysis presented here and very similar to
our previous study [de Gouw et al., 2005] and are therefore
not given. The non negligible influence of biomass burning
from forest fires in Alaska and Canada [Warneke et al.,
2006] and the substantial ocean uptake of oxyVOCs further
Figure 2. Scatterplot of acetylene versus CO for all the
RHB ship (N = 1608) and all the WP-3 aircraft (N = 1065)
measurements. The data were filtered for biomass burning
and power plant plumes.
Figure 3. (a) Scatterplot of ethyl benzene versus acetylene
(same data as Figure 1a). The black line is the linear fit, and
the red line the emission ratio determined with the photo-
chemical age method. (b) Emission ratios determined from
scatterplots versus the ones determined with the photoche-
mical age method for the 51 VOC species in Table 1, where
eachdatapointrepresentsonecompound.Theemissionratios
from the scatterplots are generally lower, because of mixing
with aged air masses. (c) Relative difference of the two
methods ([ER at photochem. Age t = 0] ? [ER from DX])/
[ER from DX] versus the rate coefficient with OH. For
shorter-lived compounds the difference is larger.
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complicates the description of oxyVOCs versus the photo-
chemical age and might result in an error in the determined
emission ratios. We have therefore also calculated the
emission ratios of the oxyVOCs measured with PTR-MS
using the slopes of the linear fits of the scatterplots with CO
from the fresh New York City and Boston plumes measured
on board the WP-3 aircraft as explained previously. This is
not possible for the hydrocarbons presented in Table 1
because of the limited number of canister samples taken in
the plumes. The averaged emission ratios from all plume
crossings are also given in Table 2. All of the emission ratios
determined from the scatterplots are higher than from the
photochemical age method, possibly indicating that sub-
stantial photochemical production had already increased the
mixing ratios of the oxyVOCs even in the plumes thought to
be ‘‘fresh.’’ The large difference shows that the emission
ratios for oxyVOCs cannot be determined accurately from
scatterplots.
[25] Here it should be mentioned that the emission ratios
determined in this work are only valid for the total emissions
of an urban area, which have a large contribution from on-
road automobile emissions, but also include other urban
sources such as emissions from off-road vehicles, point
sources such as industrial facilities and area sources such
as natural gas emissions and gasoline evaporation during
transport and storage. The data presented here were collected
downwind of the urban areas and therefore the different
source categories cannot be distinguished, only the total
‘‘urban’’ emission ratios are determined.
[26] In the following section the results from Tables 1
and 2 are compared with other data sets and with the
emission database used in air quality forecast models.
3.3. Comparison With Other Recent Data Sets
3.3.1. Comparison With Boston and New York City
in 2002
[27] In 2002 the emission ratios relative to acetylene were
determined on a cruise of the RHB in roughly the same area
downwind of Boston and New York City [de Gouw et al.,
2005]. The comparison with the 2004 data is shown in
Figure 4 and the values are given in Tables 1 and 2. The
emission ratios for all four classes of compounds in both
years generally agree within a factor of two as indicated by
the shaded areas. Only the alkanes show some variability,
which is not surprising, since a large part of the alkane
emissions are area sources and are not completely dominated
by automobile emissions, as will be discussed later. Also the
oxyVOCs show some variability, which might be caused by
the ocean uptake or the larger uncertainties for those ERs.
The agreement within a factor of two leads to three con-
clusions: (1) The emission ratios did not change signifi-
cantly from 2002 to 2004, (2) the emissions were rather
homogeneous in the whole New England region because in
Table 2. Emission Ratios of Oxygenated VOCs
Compound
ERoxyVOC
(2004), pptv
[pptv C2H2]?1
ERoxyVOC
(2002), pptv
[pptv C2H2]?1
ERoxyVOC(Slope
Fresh Urban
Plumes), pptv
[ppbv CO]?1
ERoxyVOC
(2004), pptv
[ppbv CO]?1
ERoxyVOC
(Los Angeles),
pptv [ppbv CO]?1
ERoxyVOC
(Vehicle Exhaust),
pptv [pptv C2H2]?1
ERoxyVOC
(Database), pptv
[ppbv CO]?1
Acetaldehyde
Propanal
Acetone
MEK
Methanol
Ethanol
Acetic acid
0.2
0.2
0.8
0.2
1.1
1.6
0.0
0.8
0.2
1.2
0.3
2.3
1.0
0.0
5.0
n/a
5.8
2.0
9.0
n/a
3.2
0.7
0.7
2.9
0.8
4.0
5.8
0.0
9.7
n/a
14.2
1.5
8.4
n/a
n/a
0.1
0.01
0.1
0.01
0.5
0.5
0.1
Figure 4. Comparison of the emission ratios from the 2002
and2004cruisewiththeRHB,whereeachdatapointrepresents
one compound. The shaded area gives the agreement within a
factor of 2. The blue lines are linear fits.
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2004 the RHB spent significantly more time further north in
the Gulf of Maine than in 2002, and (3) the method used
here to determine the emission ratios yields results within a
factor of two for different data sets.
3.3.2. Comparison With Los Angeles in 2002
[28] In the spring of 2002 the WP-3 conducted a research
flight over the Los Angeles Basin [de Gouw et al., 2003c;
Neuman et al., 2003]. During this flight the emission ratios
for all available VOCs were determined using the slopes
of the correlation plots with CO. The comparison with
the results from Boston and New York City is shown in
Figure 5 and the values are given in Tables 1 and 2. For a
better comparison, the oxyVOCs plotted in Figure 5 from
Boston and New York City were also determined from the
slopes (column 3 in Table 2). The agreement is within a
factor of two, except for the butanes, which might be
strongly influenced by area sources and therefore dependent
on the location, and for acetone and acetaldehyde. It is also
interesting to note that during this flight the alkyl nitrates,
which are photochemically produced, were rather low. This
shows that photochemistry had only a small influence on
the measured emissions ratios presented here. Even with
slow photochemistry, some of the emission ratios from Los
Angeles were higher than from Boston and New York City
(both determined from the slopes). On the other hand,
differences between cities are to be expected.
3.3.3. Comparison With Canister Samples From
Boston and New York City
[29] A comparison with data taken from A. Baker et al.
(Measurements of NMHCs in U.S. cities, submitted to
Journal of Geophysical Research, 2006, hereinafter referred
to as Baker et al., submitted manuscript, 2006) is shown in
Figure 6 and the values are given in Table 1. Baker et al.
(submitted manuscript, 2006) collected whole air canister
samples at different ground sites in 28 US cities including
Boston and New York City. Samples were collected in
Boston (22 canisters) and New York City (21 canisters)
during August 2003 and analyzed using various GC tech-
niques. Each canister was filled at a unique sampling site,
selected to be removed from point sources, and these sites
were distributed throughout each city. The emission ratios
were determined by the slope of the VOCs versus CO from
all the canisters collected in both cities. The results of the
two measurements correlate well (R = 0.97) with a slope of
S = 0.88. The small difference in the slope might be caused
by the difference in the methods used to determine the
emissions ratios as was described previously in Figure 3b.
3.3.4. Comparison With Average Urban Air
Composition and Vehicle Exhaust
[30] A comparison with the average urban air composi-
tion from 39 US cities published by Seila et al is shown in
Figure 7 [Seila et al., 1989] and the data are given in Table 1.
Even though this study is nearly two decades old, it is one
of the most comprehensive measurements of urban air
composition published, in contrast to tunnel studies that
give detailed information about the vehicle exhaust. More
recent studies of ambient urban air, such as by [Millet et al.,
2005] did not include acetylene or CO mixing ratios and
cannot be used to compare to the data set presented here. It
can be seen that our measured emission ratios also compare
well with the Seila et al data set. Mainly ethane and propane
are higher in the New England data and n-butane is lower
than the average composition. Also shown in Figure 7 and
Tables 1 and 2 is a comparison with measurements from
vehicle exhaust [Harley et al., 1992]. The C2-C4alkane
emission ratios measured here are higher than in the vehicle
exhaust, because evaporation of gasoline and other area
sources are an important source of these VOCs in an urban
area [Harley et al., 1992; Rubin et al., 2006]. Most of the
alkenes and aromatics are also within a factor of two with
Figure 5. Comparison of the 2004 emission ratios with
CO with data from a flight in the Los Angeles Basin in
2002, where each data point represents one compound. The
solid red line is a linear fit through the data. The grey
shaded area shows an agreement within a factor of two.
Figure 6. Comparison of the emission ratios with CO
from 2004 with data from Baker et al. (submitted manu-
script, 2006), which are canister samples collected in
Boston and New York City. Each data point represents
one compound, and the solid red line is a linear fit through
the data. The grey shaded area shows an agreement within a
factor of two.
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the vehicle exhaust measurements showing that mobile
sources are a main source for those compounds in urban
air. The oxyVOCs in the vehicle exhaust are smaller than in
the urban measurements confirming that only small quan-
tities are emitted by automobiles [de Gouw et al., 2005].
[31] The emission ratios determined in this work gener-
ally agree well with recent measurements from urban areas
even though these were done in different years or cities as
shown in Figures 4–7. For alkenes and aromatics they also
agree generally well with vehicle exhaust measurements.
For individual compounds there are clear differences
between the cities, years and seasons and in the Figures 4–7
the agreement of within a factor of 2 that we consider as a
good agreement is shown with the grey shaded area. These
differences can be caused for example by different fuel
compositions, different industrial facilities, different evapo-
rative sources, which are also dependent on ambient tem-
perature, and many other possible differences between cities
or years. Furthermore uncertainties in the measurements and
the analysis can cause differences between the various data
sets. A discussion about differences of individual species is
beyond the scope of this paper and can be found elsewhere
(Baker et al., submitted manuscript, 2006).
[32] From the different data sets given in Table 1 the most
accurate and up-to-date urban VOC emission ratios are
determined by the photochemical age method, which are
given in the first three columns. Those will be used in the
further analysis.
3.4. Comparison With the Anthropogenic Emissions
Database
[33] The anthropogenic VOC emissions database, based
on the NEI-99 data, separates emissions into four catego-
ries: on-road, nonroad, point, and area sources. The on-road
emissions are mobile sources, which are mainly highway
gasoline and diesel cars and trucks, the nonroad are mobile
sources, which are off-highway vehicles such as 2-stroke
lawn and garden equipment or construction, farming and
mining equipment, the area sources are evaporation during
storage and transport of petroleum products like gasoline
service stations or nonindustrial solvent evaporation, and
the point sources are industrial operations or petroleum and
solvent evaporation from, for example, surface coating
operations. Small point sources are included in the area
sources.
[34] To compare the measured emission ratios with the
VOC emissions database we have integrated the gridded
VOC emissions for all available species and CO in squares
that cover Boston (latitude 42.1 to 42.5 and longitude ?71.4
to ?71.0, 1440 km2), New York City (latitude 40.5 to 41.3
and longitude ?74.6 to ?73.5, 8096 km2) and Los Angeles
(latitude 33.5 to 34.3 and longitude ?118.5 to ?117.7,
5824 km2), respectively. The relative speciation of the four
categories for the three cities, the total emissions and the
emissions per square kilometer are given in Table 3. In
Boston and New York City the area sources and in Los
Angeles the on-road sources dominate. The point sources in
Figure 7. Comparison of the 2004 emission ratios with
acetylene with average data of 39 U.S. cities [Seila et al.,
1989] and vehicle exhaust measurements [Harley et al.,
1992]. Each data point represents one compound, and the
solid lines are linear fits through the data. The grey shaded
area shows an agreement within a factor of two.
Table 3. Relative Contributions of the Different Source Categories to the VOC Emissions, the Total VOC Emissions, and the VOC
Emissions per Square Kilometer of Boston, New York City, and Los Angeles
City
On-Road VOC
Emission, %
Nonroad VOC
Emission, %
Area VOC
Emission, %
Point VOC
Emission, %
Total VOC
Emission, tons/day
VOC Emissions,
kg/day/km2
Boston
New York City
Los Angeles
28
32
40
24
17
25
45
47
32
3
4
3
211
1210
908
139
149
148
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all three cities are very small. In all three cities the emissions
per square kilometer are about the same, but New York as
the largest city has the largest total emissions.
[35] After summing up the four categories, the total VOC/
CO ratios for individual and lumped species were calculated.
The ratios for all the available individual species are given in
Table 1 and the comparison for Boston/New York and Los
Angeles with the measurements is shown in Figure 8. The
measurements and the database do not agree well, especially
the alkanes and oxygenated VOCs are very different.
[36] The main source for the alkanes are the area sources,
which are the most difficult to determine accurately. In the
emissions database the area sources are generally larger than
the on-road emissions, but the good agreement of the
measurements with the vehicle exhaust might indicate that
the contribution of the area sources is overestimated in the
database. Only recently it was shown that oxyVOCs are also
emitted in large quantities from urban areas [e.g., de Gouw
et al., 2005; Jacob et al., 2002, 2005; Li et al., 1997; Singh
et al., 2001]. The sources of oxyVOCs in urban areas is not
well understood yet, but they are likely not primary auto-
mobile emissions [de Gouw et al., 2005]. The emissions
database does not yet include those recent findings of large
oxyVOC emissions, which cause the large discrepancies
with the presented oxyVOC emission ratios.
[37] The measured emission ratios for different years and
cities, as presented in Figures 4–7, certainly have a vari-
ability for individual species of up to a factor of two or more
for certain species and different locations, but the differ-
ences are still small as compared to the difference with the
emissions database from Figure 8.
[38] The emission input of a certain VOC into a chemical
forecast model is crucial for the quality of the prediction of
this species and as a result also for ozone and secondary
organic aerosols. The prediction of toluene and CO from the
WRF-CHEM model is compared in Figure 9 with the
measurements of those two species. In Figure 9 an altitude
profile of all the measurements from the WP-3 during the
NEAQS-ITCT2004 campaign are compared with the WRF-
CHEM results along the flight track. CO agrees fairly well,
whereas toluene is overpredicted by almost a factor of 3,
which is close to the difference in the emission ratios from
the database and the measurements.
[39] The WRF-CHEM toluene results shown in Figure 9
are for the lumped RADM2 aromatic compound assigned to
toluene. However, within the RADM2 reactivity weighted
assignment to individual VOC emissions for this lumped
species, more than 80% is due purely to toluene for the
individual urban areas considered in this study, and for the
entire U.S.
3.5. OH Reactivities
[40] The OH reactivity of a compound is calculated by
multiplying its concentration with the OH rate coefficient
(taken from Atkinson et al. [2005]). For this calculation the
emission ratio and a CO enhancement of 100 ppbv was
used, which is conservatively low for typical observed
urban CO enhancements during NEAQS-ITCT2004. Shown
in Figure 10 are the reactivities of CO (100 ppbv) and the
different classes of compounds calculated from the mea-
sured emission ratios of the 2004 data. The total reactivity
of all VOCs is about 50% larger than of CO. The alkanes,
alkenes and aromatics contribute about equally to the VOC
Figure 8. Comparison of the measured emission ratios
with CO with an anthropogenic emissions database based
on EPA NEI-99 data that is used in various air quality
forecast models. Each data point represents one compound,
and the solid lines are linear fits through the data. The grey
shaded area shows an agreement within a factor of two.
Figure 9. Measuredandmodeledaltitudeprofilesoftoluene
and CO.
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reactivity and the oxyVOCs only play a minor role at the
time of the emission. The alkenes have the shortest lifetimes
followed by the aromatics and the alkanes and therefore the
reactivity in the early stages of the plume evolution will be
dominated by the alkenes. Later as the plume ages and the
short-lived alkenes and aromatics are reacted away the total
reactivity will be smaller and dominated by the alkanes and
oxyVOCs [Goldan et al., 2004].
[41] Also shown in Figure 10 is the total reactivity of all
the VOCs in the emissions database. The total reactivity of
the database is about 30% larger than of the sum of the
measured VOCs. The database includes more compounds in
the lumped species than the measurements, but the ones that
contribute most to the reactivity are measured. Using this
database it might be difficult to predict individual com-
pounds accurately as seen in Figure 9, but the rather small
difference in total reactivity will help in the prediction of
ozone.
4.Conclusion
[42] Emission ratios with CO of a large number of VOCs
were determined for the urban areas Boston/New York City
and Los Angeles and compared to other data sets. The
measurements presented here compared well to other mea-
surement data from urban areas and also to measurements of
vehicle exhaust indicating that a large source of VOCs in
urban areas is automobile exhaust. On the other hand the
measured emission ratios did not compare well (R = 0.29,
slope of 0.57) with a frequently used anthropogenic emis-
sions database, especially the alkanes and the oxygenated
VOCs showed discrepancies of up to an order of magnitude.
The urban emissions in the database are dominated by area
sources and not by the on-road emissions. The toluene
mixing ratio calculated with the WRF-CHEM model in
the New England using this database is overpredicted by
about a factor of three. This is about the same difference as
between database and measurements. Using measured emis-
sion ratios for urban areas instead of inventories could
therefore in some cases help improve the quality of regional
air quality forecast models. It is also important in future
studies to find out the reasons for the discrepancies and
apply the appropriate corrections to the speciation profiles
of VOCs used in the inventories.
[43] Acknowledgments.
Ronald H. Brown and our NEAQS-ITCT 2004 collaborators.
We thank the crews of the NOAAWP-3 and
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? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
E. L. Atlas, Rosenstiel School of Marine and Atmospheric Science,
University of Miami, Miami, FL 33149, USA.
A. Baker and D. R. Blake, Department of Chemistry, University of
California, Irvine, CA 92697, USA.
J. A. de Gouw, F. C. Fehsenfeld, P. D. Goldan, J. S. Holloway, W. C.
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80305, USA. (carsten.warneke@noaa.gov)
S. Kato, Department of Chemistry, University of Colorado, Boulder,
CO 80309, USA.
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