SECONDARY ORGANIC AEROSOL FORMATION POTENTIAL IN
SOUTH GEORGIA
Comprehensive Exam Paper
Submitted by
Venus Dookwah.
March 17, 2003.
1
ABSTRACT
Organic aerosols comprise a significant fraction of the total atmospheric particle loading
and have strong correlations to climatic and health effects. Ambient aerosol is comprised
of both primary and secondary components. The fraction of secondary organic aerosol
was estimated for three cities in south Georgia by using ambient data collected and
estimates of background organic carbon/elemental carbon ratio. Calculated secondary
organic carbon is estimated to have contributed between 10 – 20 % of total PM2.5 mass
and 50 – 70 % of total organic carbon mass during the period sampled in these three
cities.
Estimates of the amount of secondary organic aerosol potentially contributed by species
found in mobile emissions in these cities were then determined using fractional aerosol
coefficients. The main contributor of secondary organic aerosol (SOA) production in
each city is toluene. It accounts for 48 % of the potential mobile emissions SOA loading.
These results are relevant to ozone and PM2.5 abatement strategies.
2
INTRODUCTION
Approximately 10-70 percent of the total dry fine atmospheric particulate matter, is
organic material [Turpin et al 2000]. PM2.5 is a US EPA regulated pollutant and the
current National Ambient Air Quality Standard for PM2.5 is :
Annual arithmetic mean of 15 g m-3
24 hour average of 65 g m-3
One of the main reasons that PM2.5 is regulated is because of its correlation to adverse
human health effects such as cardiopulmonary disease, morbidity and mortality [Pope et
al 1995]. PM2.5 bypasses our respiratory defenses, such as the ciliated mucous linings,
and is speculated to being easily absorbed into the lining of the respiratory pathway. The
organic constituent of PM2.5 is of particular significance in the mechanism of effecting
hazardous health effects since it is capable of reacting synergistically with trace metals
present on the same particle [Ron Wyzga EPA Supersite meeting]. The resulting
potentially harmful redox reactions are one of the main reasons that the organic
component of aerosols, which usually averages around 30-40 percent, requires study.
Another undesirous effect of PM2.5 relates to the possible effects of this pollutant on
agricultural production. Fine particles affect the flux of solar radiation passing through
the atmosphere by scattering and absorbing radiation. This can result in reduced
downward photosynthetically active radiation (PAR) resulting in reduced crop yield
[Chamedies et al, 1998]. For regions whose economies are strongly tied to agricultural
yields, such as China, this phenomenon can have serious implications.
3
Organics in aerosols can modify the thermodynamic and chemical properties of
atmospheric particles thereby, altering the role played by these particles in the
atmosphere. According to Saxena et al, 1995, particle phase organics can alter the
hygroscopic properties of the atmospheric particles. They reported that for non-urban
locations organics enhance water absorption whereas for urban locations, the presence of
organics inhibits water absorption of atmospheric particles.
Aerosols which serve as nuclei upon which water vapor condenses in the atmosphere are
called cloud condensation nuclei (CCN). As explained by solute effects, for small
particles, the higher the water solubility or wettability of an aerosol, the lower the
supersaturation at which it can serve as CCN [Wallace and Hobbs]. Hence, the
hygroscopic properties of organic aerosols are indeed important in this respect.
The optical and chemical properties of atmospheric particles and their ability to act as
cloud condensation nuclei (CCN) depend strongly upon their affinity for water [Saxena et
al 1995]. The albedo and radiative properties of clouds are determined largely by the
number density of cloud condensation nuclei. Novakov and Penner, 1993, reported that
organic aerosols accounted for a major part of both the total aerosol number
concentration and the CCN fraction and the role played by organic aerosols was at least
as important as sulphate aerosols in determining the climate effect of clouds.
Dickerson et al. 1997, reported that UV scattering by aerosols can have a substantial
positive impact on the production of ground level ozone. Aerosol scattering of UV
radiation was found to increase calculated boundary layer ozone mixing ratios by 20
ppbv or more and UV absorbing aerosol reduced calculated ozone mixing ratios by up to
24 ppbv.
4
In summary, organic aerosols are significant because:
-
they can contain toxins which can cause deleterious health effects, if inhaled
as the majority of fine aerosols are too small to be efficiently trapped in bronchial
passages and can reach the lungs and be absorbed into the mucous lining
-
visibility and climate forcing issues are strongly influenced by organic species
-
they play a role in cloud condensation nuclei, thereby affecting precipitation
patterns which affects the hydrological cycle
-
they contribute to photochemical reactions affecting tropospheric ozone formation
and removal of atmospheric oxidizing species such as OH, O3, and NO3.
Even though a significant fraction of atmospheric aerosols consists of organic substances,
little is known about source-reaction pathways and chemical composition of this organic
fraction. One main reason for this lack of knowledge is due to the fact that organic
particulate matter is really a complex aggregate of a wide variety of compounds which
have varying chemical and thermodynamic properties [Saxena and Hildermann, 1996].
Further complications are due to the presence of multiple phases of the organics, that is,
volatile, semi-volatile, and particle phases, which can interchange depending on the
prevailing ambient meteorological conditions and species concentrations. Also, no single
analytical technique can analyze the entire range of organics present in aerosols [Turpin
et al, 2000].
5
SOURCES OF ORGANIC AEROSOL
Primary organic aerosol particles are emitted directly into the atmosphere by a variety of
sources such as forest fires, biomass burning, oil refineries, chemical plants, pulp and
paper industries, vehicular emissions, producers and users of paints and solvents, meat
cooking and various agricultural activities, to name a few. Some primary aerosols are
emitted from many sources, for example, n-nonadecane (C19) can be emitted from
automobiles, road dust, vegetation, natural gas appliances, asphalt, boilers and wood
burning [Seinfeld & Pandis, 1998]. Some primary organics are emitted by one specific
type of activity and are, therefore, called tracer compounds or marker species for this
particular activity type, for example,
SOURCE
Meat cooking
Cigarette smoke
Biogenic sources
TRACER COMPOUND
Cholesterol
Anteisoalkanes
C27, C29, C31, C33, n-alkanes
REFERENCE
Rogge et al 1991
Rogge et al 1994
Mazurek & Simoneit 1984
Simoneit 1984
Rogge et al 1993
Secondary organic aerosols (SOA), like ozone, are formed as byproducts of gas-phase
photochemical oxidation of volatile organic compounds (VOCs), but whereas the
oxidation of most VOCs results in ozone formation, SOA is generally formed from the
oxidation of low vapor pressure VOCs, that is, those comprised of six or more carbon
atoms [Griffin et al. 1999 ; Grosjean and Seinfeld 1989]. Thus, for calculations of
secondary formation potential estimates, isoprene, benzene and all aliphatic compounds
with six or less carbon atoms, are not considered in this study.
6
Secondary organic aerosol is formed in the atmosphere by the oxidation of volatile
organic gases by oxidants such as OH radical, ozone and the nitrate radical. Oxidation
products which have low volatilities can condense onto existing particles in order to
establish equilibrium between the gas and aerosol phases, thereby forming secondary
organic aerosol via heterogeneous nucleation. Homogeneous nucleation is also a possible
SOA formation mechanism. For example, a stable reaction product of cyclohexene-ozone
oxidation is adipic acid. Assuming that for every 1 ppb of cyclohexene oxidation with
ozone, 0.01 ppb of adipic acid is formed. The saturation mixing ratio of adipic acid is
0.08 ppb, which, based on the previous assumption requires 8 ppb of cyclohexene to be
oxidized by ozone. When the adipic acid mixing ratio reaches saturation (0.08 ppb), then
further cyclohexene-ozone reaction will lead to supersaturation of the gas phase adipic
acid and the excess will condense onto any available aerosol particles or homogeneously
nucleate resulting in SOA production. SOA production, therefore, involves two stages:
1. gas phase oxidation of parent VOC, which is a chemical reaction and
2. partitioning of the oxidation product between gas and particulate phases, which is
a physicochemical process.
-
The chemical reaction pathways involved in stage 1 are complex and not fully
understood and the physicochemical processes leading to gas-to-particle
partitioning are also unclear but are speculated to involve absorption, adsorption
or some combination of these two processes.
Under peak photochemical smog conditions, when non-attainment of ozone and PM2.5
usually occurs, as much as eighty (80) percent of the observed organic particulate carbon
can be secondary in origin [Turpin & Huntzicker, 1995].
7
Organic particulate matter can be speciated using a number of analytical techniques such
as :







Gas Chromatography-Mass Spectroscopy [Rogge et al 1993]
Gas Chromatography-Flame Ionization Detector [Mazurek et al 1997]
Carbon isotope analysis [Johnson and Dawson, 1993; Kaplan and Gordon, 1994;
Hildemann et al., 1994]
Fourier Transform Infrared Spectroscopy (FTIR) [Mylonas et al., 1991; Pickle et
al., 1990]
High Pressure Liquid Chromatography-Ultraviolet/Visible detector [Gorzelska et
al., 1992]
MALDI – Matrix Assisted Laser Desorption/Ionization [Mansoori et al., 1996]
Thermal Desorption Particle Beam-Mass Spectroscopy [Ziemann and others
2003]
However, no analytical method by itself is able to distinguish between primary and
secondary organic material. This is due to the fact that some secondary products can also
be emitted by primary sources, for example, adipic acid is a by product of the
cyclohexene-ozone oxidation but is also emitted from meat cooking and wood burning
sources [Seinfeld and Pandis, 1998]. Hence, species can be identified but whether their
source is primary or secondary really cannot be determined by analytical methods only.
Additional assumptions must be used to make an estimate of the relative contribution of
primary and secondary organics to total PM2.5 mass.
Knowledge of the estimated secondary organic aerosol formation potential and the main
precursor species which contribute most to this fraction can lead to the institution of
better controls, especially during summertime periods when photochemical conditions are
ideal and exceedences are observed, and can mean the difference between attainment and
non-attainment.
8
Because of the complexity of SOA reaction pathways, the vast number of products
formed by photochemical oxidation of primary aerosol, and the costly analytical methods
required for speciation, indirect methods for quantitative assessment of SOA have
become very useful.
Literature review reveals three main empirical methods of estimating the secondary
organic aerosol (SOA) component of PM:

OC/EC ratio / EC tracer method [Turpin and Huntzicker, 1991]

Fractional Aerosol Coefficient method (FAC) [Grosjean, 1992]

Gas/Particle Partitioning method [Pankow, 1994; Odum et al., 1996]
The first method will be used in this study to estimate the contribution of SOA to total
PM2.5 mass in metropolitan cities in south Georgia and the second method will be used
to estimate the relative species contribution of compounds found in mobile emissions of
these cities to SOA formation.
OC/EC Ratio Method
Elemental carbon, (EC), is predominantly formed through combustion processes and is
emitted into the atmosphere in particulate form. It is, therefore, a good tracer for primary
carbonaceous aerosol of combustion origin. Organic aerosol can be emitted directly in
particulate form (primary organic aerosol) or formed in the atmosphere from products of
photochemical oxidation of precursor reactive gases called Volatile Organic Carbon
(VOCs) or Reactive Organic Gases (ROGs) by various authors. The latter aerosol type is
called secondary organic aerosol (SOA).
9
This method is based on the observation that background OC/EC ratios are much smaller
than OC/EC ratios found during peak photochemical periods. This is expected since EC
is unaffected by photochemical oxidation reactions whereas primary OC is the precursor
of secondary OC. By participating in oxidation reactions, the OC fraction is increased
resulting in an increased OC/EC ratio.
In order for this method to be used for secondary OC estimation, an estimate of the
primary OC/EC ratio is first needed. OC/EC emissions vary from source to source and
hence the primary OC/EC ratio will be influenced by local sources, meteorology, as well
as diurnal and seasonal fluctuations in emissions. Therefore, it is only possible to
determine the range in which the primary ratio is likely to fall rather than using a specific
OC/EC ratio. For this study, this primary OC/EC ratio will be determined by examination
of gas phase species such as NO, CO, and ozone alongside measured OC/EC ratios. The
primary OC/EC ratio will be chosen as the lowest ratio which coincides with the highest
NO, and CO and lowest ozone. The rationale for this choice will be discussed later in this
paper.
Experimental Procedure
The data used in this study were obtained during the “Fall Line Air Quality Study”
(FAQS) in summer 2000. The FAQS project was initiated in response to observed poor
air quality in Augusta, Macon and Columbus, which are metropolitan areas located south
of Georgia’s Fall Line. Table 1 provides details on the days during which poor air quality
was observed in these cities.
10
Table 1
Number of days with peak 8-hour averaged ozone concentrations exceeding 0.08 ppmv,
1997-1999.
Site
Augusta
Macon
Columbus – Airport
Columbus – Crime Lab
1997
5
12
1
2
1998
14
18
8
8
1999
8
18
9
13
Table 2: Summary of OC/EC sampling during period studied.
Site
Period of sampling
No. of OC/EC samples taken
Macon – Sandy Beach Park
Augusta – Ft.Gordon
Columbus – North Water Works
June 11-21
June 28-July 9
July 17-25
(9) 24 hr & (4) 12 hr
(9) 24 hr & (6) 12 hr
(7) 24 hr & (4) 12 hr
Facility
Location of sites:
Sandy Beach Park, Macon – 10 miles West of downtown Macon.
Ft. Gordon, Augusta – 12 miles SW of downtown Augusta
North Water Works, Columbus – 4 miles N of downtown Columbus
Oxbow Learning Center, Columbus – 5 miles S of downtown Columbus.
EXPERIMENTAL
Ambient VOC samples were collected four (4) times daily, at each of the sampling sites
during the sampling period, using evacuated canisters. The times selected for taking the
VOC samples were ~ 0:00, 08:00, 12:00 and 17:00. The VOC samples were analyzed by
The University of California, Irvine using gas chromatography / mass spectroscopy
(GC/MS).
11
The days during which sampling was conducted, and the number of samples taken at
each site is detailed in Table 2. OC/EC sampling was achieved using an insulated,
temperature controlled particle composition monitoring sampling box and pump. A
typical sampling setup can be seen in Figure 1.
In addition to the VOC samples and OC/EC samples, gas phase concentrations of NO,
NOy, CO and ozone were measured continuously over the entire sampling period at each
site. Meteorological parameters such as wind speed, wind direction, ambient temperature,
ambient pressure, solar irradiance and relative humidity were also measured continuously
at each location. The sampling setup used for OC/EC determination is illustrated in
Figure 1 below.
Figure 1
12
A cyclone separator was used at the sampler inlet to remove particles with aerodynamic
diameter of >2.5 micrometers. An XAD – coated glass denuder was plumbed
downstream of the cyclone head to remove volatile organic species from the sampled
aerosol. It is important to remove these volatile species from the aerosol sample since
they can be adsorbed onto the filter media resulting in an overestimation of organic
particulate mass (positive artifacts). Denuder techniques have been deployed for over a
decade by investigators such as Krieger and Hites, 1992; Eatough, 1999; Cui et al., 1998;
Eatough et al., 1995. The removal of gas species from the air stream, however, disturbs
the delicate equilibrium which exists between the gas and particle phases, and can result
in volatilization of particle phase organics (negative artifacts). Eatough et al., 1995; Cui
et al., 1998, have estimated the magnitude of both positive and negative artifacts using
diffusion denuder sampling systems at remote and urban locations and conclude that
evaporation is the dominant artifact (negative).
In order to correct for this negative artifact, a backup filter is deployed. Any mass
recorded on this backup filter is due to volatilization off of the front filter (negative
artifact) and is accounted for by adding this mass to the front filter’s mass [Eatough et al.,
1995].
An XAD – coated quartz filter, which has an enhanced affinity for volatile organic gases,
[Gundel and Lane, 1996], was used as the backup absorber in this study. After being
scrubbed for volatile organics, the particulate material is deposited onto a quartz filter,
and any gas phase organics which volatilize off of this first quartz filter is captured by an
XAD – coated quartz filter.
13
The mass of organic material measured on this backup XAD-coated quartz filter is added
to the OC mass found on the front (first) quartz filter in order to correct for any negative
artifacts that were created during sampling.
The system operated at an average flow rate of 16.7 litres per minute with a total sampled
volume of ~ 24 m-3 or 24,000 litres of air over a twenty-four hour period.
The Pallflex 2500 QAT-UP (47mm diameter) quartz filters are prepared for sampling by
pre-firing at 600 oC for 2 hours. The baked filters are then stored in Petri dishes at ~ - 10
degrees until they are fitted into filter packs to be used for sampling. Some of the baked
filters are coated with the XAD resin to be used as backup adsorbers. Following
collection, the filters were placed in air tight Petri dishes and stored at approximately – 10
degrees until analysis was conducted.
A thermal optical technique (TOT) was used to determine the organic carbon and
elemental carbon content of the samples. This technique has become very popular for
OC/EC analysis and is detailed in Birch and Cary, 1996.
Data Quality
Field blanks for each sample run were used. The blanks were handled and prepared
exactly like the actual sample. Any mass found on these blanks is, therefore,
representative of contamination due to handling, such as, storing, transporting, loading
and unloading of filters. From these blanks, the detection limit of OC/EC was determined
by using a two-tailed student’s t-distribution and an assumed 95% level of confidence.
The detection limit was calculated as follows:
DLn = cn,avg (B) + tN-1 . sn(B)
14
Where cn,avg (B) is the average blank concentration for species n, sn(B) is the standard
deviation of the blank distribution for species n, and tN-1 is the t-value for N-1 blanks
(N = total number of blanks) at 95% confidence level of a two-tailed student’s tdistribution.
Figure 2: Graph of sample TOT measurements of OC and EC versus NIST analysis of
the same samples as an accuracy determination.
a)
OC
b)
EC
OC
EC
20
250
y = 0.9035x
R2 = 0.9661
15
y = 1.2829x
R2 = 0.9911
-3
GIT (g m )
TOT Measurement ( g g-1)
200
150
100
10
5
50
y = 1.09x + 0.34
R2 = 0.87
y = 0.3664x
R2 = 0.808
0
0
0
50
100
150
200
0
250
-1
5
10
15
-3
NIST Standard (g g )
NIST (g m )
Accuracy estimates for EC (+9%) and OC(-10%) were obtained by comparison of
measurements obtained from TOT analysis of samples with that obtained from analysis
by National Institute for Standards and Technology (NIST) [Baumann et al., 2003] as
seen in Figure 2.
15
20
25
RESULTS
In order to determine the secondary contribution to total measured organic mass, the
primary OC/EC ratio must first be established. Turpin and Huntzicker, 1995, identified
secondary organic aerosol episodes by looking at 2 hour OC/EC data along with
meteorological data to determine periods that were influenced mostly by local sources
rather than from distant sources transported into the region of study (Los Angeles). They
selected as a primary OC/EC ratio, one observed during an early morning period, 0600 –
0800, when EC peaked without an OC peak and the sea breeze which transports
pollutants into the region did not develop until later on in the afternoon of that day
chosen.
Some other investigators, Cabada et al [AAAR presentation 2002] also used time
resolved OC/EC data to identify a primary OC/EC ratio. Using ~ 2 hour OC/EC
measurements along with corresponding time resolved CO, NOx, and NO measurements,
they identified a morning period, 0600 – 1000, as being heavily influenced by primary
emissions. These investigators also used 24 hour daily averaged samples of OC/EC along
with daily averaged NO, NOx, CO and ozone to determine periods that were dominated
by primary emissions. This group reported daily averaged concentrations of 17 ppb NOx,
4 ppb of NO, 0.2 ppb of CO and 30 ppb of ozone as primary dominated concentrations
and 20 ppb of NOx, 4 ppb of NO, 0.2 ppm of CO and 45 ppb of ozone as secondary
dominated concentrations in the Pittsburg region studied.
For this study of metropolitan areas in south Georgia, an examination of average daily
concentrations of NO, CO and ozone along with measured OC/EC ratios was used to
identify the lowest OC/EC ratio (that is, highest EC concentration relative to OC
16
concentration) that coincides with elevated NO, moderate CO concentrations and reduced
ozone concentrations. This ratio was selected as the primary OC/EC ratio (OC/EC pri) for
that specific site and sampling period.
Table 3a : GAS-PHASE SPECIES CONCENTRATION AND OC/EC RATIOS MEASURED DURING
DAYS SAMPLED IN AUGUSTA.
AUGUSTA
NO
(ppb)
DATE
OZONE
(ppb)
CO (ppb)
OC/EC
RATIO 24
hr
6/28/2000
0.42
33.22
136.8
6/29/2000
0.73
29.95
183.5
6/30/2000
0.62
53.63
284.43
14.20
7/1/2000
0.18
60.26
269.4
19.12
7/2/2000
0.23
55.49
266.1
7/3/2000
0.88
57.99
227.2
14.72
7/4/2000
0.12
50.5
207.5
21.56
7/5/2000
0.21
54.31
235.9
7/6/2000
0.38
49.64
219.6
17.80
7/7/2000
0.69
49.68
300.6
13.75
7/8/2000
0.43
48.47
251.8
12.12
7/9/2000
0.12
48.54
199.7
41.04
AVGE
0.42
49.31
231.88
STD DEV
0.26
9.11
46.38
OC/EC
RATIO 12
HR
DAYTIME
OC/EC
RATIO 12
HR
NIGHTTIME
9.05
7.42
56.12
59.33
25.97
8.02
3.94
Table 3b : GAS-PHASE SPECIES CONCENTRATION AND OC/EC RATIOS MEASURED DURING
DAYS SAMPLED IN MACON
MACON
NO
(ppb)
DATE
OZONE
(ppb)
CO (ppb)
OC/EC
RATIO 24
hr
6/11/2000
0.09
42.76
223.9
14.48
6/12/2000
0.23
29.64
214.3
11.64
6/13/2000
0.16
38.13
145.9
13.51
6/14/2000
0.48
28.11
153
6/15/2000
0.39
31.13
150.6
6/16/2000
0.49
25.46
148.6
6/17/2000
1.12
28.41
195.7
19.96
6/18/2000
0.83
26.71
212.9
74.57
6/19/2000
0.27
21.59
148.7
21.58
6/20/2000
0.21
26.02
165.9
14.77
6/21/2000
0.49
32.65
183.1
19.53
AVGE
0.43
30.06
176.60
STD DEV
0.31
6.01
30.39
OC/EC
RATIO 12
HR
DAYTIME
OC/EC
RATIO 12
HR
NIGHTTIME
3.17
25.62
13.31
5.63
15.46
17
Table 3c : GAS-PHASE SPECIES CONCENTRATION AND OC/EC RATIOS MEASURED DURING
DAYS SAMPLED IN COLUMBUS
COLUMBUS
NO
(ppb)
DATE
OZONE
(ppb)
CO (ppb)
OC/EC
RATIO 24
hr
7/17/2000
2.48
49.21
294.6
7/18/2000
3.43
56.87
319.1
7/19/2000
2.12
50.25
340.6
17.94
7/20/2000
0.85
59.6
280.9
26.81
7/21/2000
1.37
45.67
282.9
40.70
7/22/2000
0.89
40.89
299.5
18.26
7/23/2000
0.36
47.21
246.9
16.61
7/24/2000
0.97
39.98
268.3
7/25/2000
0.37
50.2
271.7
AVGE
1.43
48.88
289.39
STD DEV
1.04
6.51
28.09
OC/EC
RATIO 12
HR
DAYTIME
OC/EC
RATIO 12
HR
NIGHTTIME
19.56
8.51
7.74
9.72
28.42
30.06
Tables 3a - c show that the OC/EC ratio which is most likely dominated by primary
emissions occurs in the daytime period as reported by other investigators mentioned
above, however, a more accurate determination of the time at which this occurs requires
more time resolved OC/EC data.
The secondary component of the organic aerosol can be calculated as follows:
Where
OCsec = OCtot - OCpri
eq 1
OCpri = EC(OC/EC)pri
eq 2
OCtot is measured OC, OCpri is primary OC, EC is measured EC, and (OC/EC)pri is the
primary ratio estimate.
Using primary OC/EC ratios for Augusta, Columbus and Macon as 7.42, 9.72 and 3.17
respectively and equations 1 & 2 above, the secondary organic carbon component was
calculated for each day sampled. The results are depicted in Figures 3a – c.
18
Figure 3a: Graph showing calculated secondary OC (SOC) as a percentage of total OC
(TOC) measured as well as total PM2.5 mass for the Columbus metropolitan area.
Graph of calculated Secondary Organic Carbon as
a percentage of Total Organic Carbon and of Total
PM2.5 mass in Columbus
90
80
SOC %
70
60
50
SOC % of TOC
40
SOC % OF Total PM
mass
30
20
10
7/
16
/
2
7/ 000
17
/2
7/ 00
18 0
/2
7/ 000
19
/2
7/ 000
20
/2
7/ 00
21 0
/2
7/ 000
22
/2
7/ 000
23
/2
7/ 00
24 0
/2
7/ 000
25
/2
00
0
0
Date
Figure 3b: Graph showing calculated secondary OC (SOC) as a percentage of total OC
(TOC) measured as well as total PM2.5 mass for the Macon metropolitan area.
Graph of Secondary Organic Carbon as a percentage
of Total Organic Carbon and Total PM2.5 mass in
Macon
120
SOC % of
TOC
80
60
SOC % of
Total PM2.5
mass
40
20
0
6/
11
/2
00
6/
12 0
/2
00
6/
13 0
/2
00
6/
14 0
/2
00
6/
15 0
/2
00
6/
16 0
/2
00
6/
17 0
/2
00
6/
18 0
/2
00
6/
19 0
/2
00
6/
20 0
/2
00
6/
21 0
/2
00
0
SOC %
100
Date
19
Figure 3c: Graph showing calculated secondary OC (SOC) as a percentage of total OC
(TOC) measured as well as total PM2.5 mass for the Augusta metropolitan area.
100
90
80
70
60
50
40
30
20
10
0
SOC % of
TOC
SOC % of
Total PM2.5
mass
6/
28
/2
6/ 000
29
/2
6/ 00
30 0
/2
0
7/ 00
1/
20
7/ 00
2/
20
7/ 00
3/
20
7/ 00
4/
20
7/ 00
5/
20
7/ 00
6/
20
7/ 00
7/
20
7/ 00
8/
20
7/ 00
9/
20
00
SOC %
Graph of Calculated Secondary Organic Carbon as a
percentage of Total Organic Carbon and Total PM2.5 mass
in Augusta
Date
The estimated secondary organic carbon contribution to total organic carbon was found to
average around 53 % on the days sampled in Columbus, 72 % on days sampled at Macon
and 51 % on days sampled at Augusta, using respective primary OC/EC ratios of 9.72,
3.17 and 7.42 (see Tables 3a-c). Using these estimations of secondary organic carbon, the
resulting contribution to total PM2.5 mass was calculated to be 16%, 20% and 11% for
Columbus, Macon and Augusta respectively.
Cabada et al. reported an average SOC contribution to total organic carbon of 22 %
during the July to August 2001 period studied in Pittsburg. The averages of SOC
contribution to TOC concentration obtained for this study are between 50 – 72 %.
20
This higer SOC contribution can possible be due to the lower NO, and CO levels but
higher ozone levels found in these cities as compared to Pittsburg.
Further analysis is conducted below to determine the major SOA precursor species.
FRACTIONAL AEROSOL COEFFICIENT (FAC)
The Fractional Aerosol Coefficient approach for determining secondary organic aerosol
yield is based on measurements of the total aerosol formed in smog chamber reactions of
a specific precursor species and a specific oxidant. Since the reaction mechanism is not
known, the kinetics and reaction rate constants are also not known. The smog chamber
data are, therefore, used to empirically derive the reaction stoichiometry, that is, to
determine the amount of condensed matter formed per gram of reactant. This quantity is
called the fractional aerosol coefficient or fractional aerosol yield. The aerosol yield can
be expressed on a molar, mass or carbon concentration basis [Grosjean and Seinfeld,
1989]. This dimensionless ratio of mass concentration is defined by Grosjean, 1992 as:
FAC = aerosol from VOC (gm-3) / initial VOC (gm-3)
With this definition, and knowing the VOC emission rate and the fraction of VOC that
has reacted in the atmosphere, the amount of aerosol formed from each VOC can be
calculated as:
Amount of aerosol produced = (amt. of VOC emitted) x (fraction of VOC reacted)
x (FAC)
21
Analysis of VOC samples collected at the three sites identified the following species:
Figure 4: Chemical groups of compounds analyzed from VOC canister samples collected
in Macon: 11-21st June, Augusta: 28th June – 9th July and Columbus: 17th – 25th July 2000
are listed below.
Halo-organics
F-12
CH3Cl
F-114
H-1211
MeBr
F-11
F-113
CH2Cl2
CHCl3
MeCCl3
CCl4
C2Cl4
MeI
I-PrONO2
C2HCl3
CH2Br2
CHBr3
Alkanes
MeONO2
EtONO2
n-PrONO2
ethane
propane
i-butane
n-butane
i-pentane
n-pentane
hexane
heptane
octane
2-methylpentane
3-methylpentane
2,2-dimethylbutane
2,3-dimethylbutane
2,4-dimethylpentane
2,3-dimethylpentane
2-methylhexane
3-methylhexane
n-heptane
2,5-dimethylhexane
2,3,4-trimethylpentane
2-methylheptane
3-methylheptane
nonane
2,2,4-trimethylpentane
Alkenes & alkynes
Ethene
Ethyne
Propene
1,3-butadiene
t-2-butene
cis-2-butene
1-butene
3-methyl-1-butene
1-pentene
2-methyl-1-butene
t-2-pentene
c-2-pentene
2-methyl-2-butene
t-2-pentene
c-2-pentene
2-methyl-2-butene
4-methylpentene
2-methyl-1-pentene
Aromatics
Benzene
Toluene
Ethylbenzene
m-xylene
p-xylene
o-xylene
1,2,3-trimethylbenzene
isopropylbenzene
propylbenzene
3-ethyltoluene
4-ethyltoluene
2-ethyltoluene
1,3,5-trimethylbenzene
1,2,4-trimethylbenzene
1,3-diethylbenzene
1,4-diethylbenzene
1,2-diethylbenzene
Biogenics
Isoprene
Limonene
Alpha-pinene
Secondary organic aerosol can be formed by parent VOC species which have a carbon
chain greater than six but generally less than ten (10>C>6) since species with high
molecular weights (C>10) tend to be present only at low concentrations while those with
low molecular weights (C<6) have high saturation vapor pressures [Isidorov, 1990].
Therefore, from the aromatic species identified in the VOC samples analyzed, those with
carbon chains greater than six were selected analysis using the FAC approach.
The FAC is a very crude first order approximation to SOA formation.
22
It summarizes the complicated oxidation-condensation processes that govern SOA
formation into one constant for each precursor VOC species. It is, however, very useful
since secondary organic aerosol can be treated as primary emissions by applying the FAC
method. It is noteworthy to mention that aerosol formation varies with many factors such
as oxidant concentration, temperature, relative humidity, and existing aerosol
concentration in the ambient air. Thus, the results obtained from this study are estimates
of secondary organic aerosol formation potentials rather than quantification of SOA
formation.
In this study, the FAC method was applied to aromatic species which were common to
both the VOC samples analyzed and the mobile emissions profile species. Such analysis
allows for the identification of those species present in mobile emissions which have the
greatest impact on atmospheric loading of SOA. Such information is useful as model
input data for the simulation of the effect of reducing the atmospheric concentration of
this species on overall PM2.5 mass.
Data input required for this analysis were daily county mobile VOC emissions which was
obtained from Georgia EPD, Air Protection Branch. Mobile source emissions are
currently determined by US EPA by using a processing software called SMOKE. This
program’s input data set includes county specifics such as road types, vehicle miles
traveled (VMT), gridded emissions of pollutants (VOC, NOx, CO), and emission factors
such as fuel volatility, fuel type, speeds, temperature, mode of operation of vehicle, to
name a few [www.epa.gov/ttn/chief/software/speciate].
23
From the VOC mobile emissions data, a speciation profile [Sagebiel et al., 1996; Sigsby
et al., 1987] for gasoline and diesel fuel was applied to determine the emission of the
aromatic aerosol forming species from these two fuel types.
The road classifications and vehicle type classifications are as follows:
ROAD CLASSES
Rural Interstate
Rural Principal Arterial
Rural Minor Arterial
Rural Major Collector
Rural Minor Collector
Rural Local
Urban Interstate
Urban Freeway
Urban Principal Arterial
Urban Minor Arterial
Urban Collector
Urban Local
VEHICLE TYPES
Light duty gasoline vehicles
Light duty gasoline trucks 1
Light duty gasoline trucks 2
Heavy duty gasoline vehicles
Light duty diesel vehicles
Light duty diesel trucks
Heavy duty diesel vehicles
Motorcycles
A speciation profile for each vehicle type would yield more accurate species emission
calculations. However, such data is not yet available, thus, a generic speciation profile for
gasoline fueled vehicles and diesel fueled vehicles was used [Sagebiel et al., 1996;
Sigsby et al., 1987].
24
TABLE 4: VOC EMISSION RATES AND AMOUNT OF DAILY SOA PRODUCED IN EACH
COUNTY FOR 6 HOUR EPISODE BY PARENT VOC SPECIES.
SPECIES
1012 x
k(298K)
(cm3
molec-1
s-1)
Fraction
of
species
reacted
Daily
Emission
in kg
Fractional
aerosol
coefficient
Species
nonane
10.2
n-heptane
7.15
8.02E01
8.57E01
2-methylheptane
9.8
8.09E01
3-methylheptane
9.9
octane
8.68
toluene
5.96
8.07E01
8.29E01
8.79E01
Amt. of
aerosol
produced
(kg [6 hr
episode])
Muscogee
Amt. of aerosol
produced (kg [6 hr
episode])Richmond
Amt. of aerosol
produced (kg [6 hr
episode])Columbia
Amt. of
aerosol
produced
(kg [6 hr
episode])
Bibb
Muscogee
Richmond
Columbia
Bibb
7.70E-03
0.015
9.27E-05
1.21E-04
4.98E-05
1.11E-04
5.94E-02
0.0006
3.05E-05
3.00E-05
1.64E-05
3.68E-05
4.93E-02
0.005
2.00E-04
2.65E-04
1.29E-04
2.40E-04
4.39E-02
0.005
1.77E-04
2.35E-04
9.54E-05
2.13E-04
2.71E-02
0.0006
1.35E-05
1.79E-05
7.26E-06
1.62E-05
8.62E-01
0.054
4.09E-02
5.45E-02
2.21E-02
4.93E-02
1.70E-01
0.054
7.90E-03
1.05E-02
4.26E-03
9.52E-03
3.20E-01
0.047
9.02E-03
1.20E-02
4.86E-03
1.09E-02
3.20E-01
0.016
3.75E-03
5.00E-03
2.02E-03
4.52E-03
m-xylene
23.6
p-xylene
14.3
o-xylene
13.7
8.58E01
6.01E01
7.34E01
7.44E01
2.45E-01
0.05
9.11E-03
1.21E-02
4.91E-03
1.10E-02
19.2
6.61E01
7.71E-02
0.063
3.21E-03
4.27E-03
1.73E-03
3.86E-03
12.1
7.70E01
8.51E-02
0.025
1.64E-03
2.17E-03
8.82E-04
1.97E-03
135-TMBenzene
57.5
2.89E01
1.02E-01
0.029
8.55E-04
1.14E-03
4.61E-04
1.03E-03
2-ethyltoluene
12.3
7.67E01
1.93E-01
0.026
3.85E-03
5.12E-03
2.07E-03
4.63E-03
32.5
4.96E01
3.23E-01
0.017
2.72E-03
3.62E-03
1.47E-03
3.28E-03
32.7
4.93E01
8.59E-02
0.014
5.93E-04
7.88E-04
3.20E-04
7.14E-04
isopropylbenzene
6.6
8.67E01
2.98E-02
0.007
1.81E-04
2.41E-04
9.76E-05
2.18E-04
n-propylbenzene
5.8
8.82E01
5.04E-02
0.007
3.11E-04
4.14E-04
1.68E-04
3.75E-04
ethylbenzene
3-ethyltoluene
4-ethyltoluene
124-TMBenzene
123-TMBenzene
7.1
Rate constants were obtained from Grosjean and Seinfeld, 1989.
Fraction of species reacted was calculated using rate constants and an assumed [OH] = 1 x 106
molecules cm-3.
25
Daily emissions obtained from daily mobile emissions data generated for these counties by
Georgia EPD (using a SMOKE model). Emissions were given for eight vehicle types and twelve
road types giving a total of ninety-six categories. These were then divided into gasoline operated
and diesel operated vehicles and the respective species profile was then applied to determine the
species emissions.
Fractional Aerosol Coefficients for these species were obtained from Grosjean and Seinfeld,
1989.
FIGURE 4 : ESTIMATED DAILY AMOUNT OF AEROSOL PRODUCED DURING
A 6 HOUR EPISODE IN EACH COUNTY STUDIED
6.00E-02
5.00E-02
4.00E-02
M uscogee
3.00E-02
Richmond
2.00E-02
Columbia
1.00E-02
Bibb
oc
ta
ne
to
lu
en
et
e
hy
lb
en
ze
ne
m
-x
yl
en
e
pxy
le
ne
oxy
le
3ne
et
hy
lto
lu
4en
et
e
hy
lto
13
lu
en
5TM
e
Be
nz
en
2et
e
hy
lto
12
lu
en
4TM
e
Be
12
nz
3en
TM
e
Be
is
n
op
z
e
ro
ne
py
lb
en
nze
pr
ne
op
yl
be
nz
en
e
0.00E+00
no
na
ne
nhe
2pt
an
m
et
e
hy
lh
ep
3ta
m
ne
et
hy
lh
ep
ta
ne
Aerosol produced [kg per 6 hr episode]
Daily Am ount of Aerosol Produced (kg [6 hr episode]
Species
The Augusta metropolitan area lies midway between Columbia and Richmond counties
hence these two counties’ mobile emissions would impact on Augusta. These were,
therefore both considered in the calculations performed. As can be seen from the above
graph, toluene is estimated to be the major SOA precursor species in each county studied,
accounting for approximately 48% of the total SOA potential atmospheric loading.
26
This finding is important since toluene is a carcinogen and is a toxic/hazardous air
pollutant. Reducing the toluene content of gasoline would, therefore, have beneficial
health effects and possibly lower PM2.5 mass concentrations.
GAS-PARTICLE PARTITIONING METHOD
Secondary organic aerosol is formed by the partitioning of gas phase products of
photochemical oxidation into the particle phase. Extensive work has been done by
Pankow, 1994, to develop a gas/particle partitioning absorption model. This work has
been incorporated into the empirical fractional aerosol yield method by Odum et al 1996,
as a more accurate means of calculating SOA yields. This group developed the following
expression for fractional aerosol yield (Y):
Yi = Mo [i Kom,i / (1 + Kom,i Mo)]
Yi is the yield of an individual product, i is the proportionality constant relating the
concentration of VOC that reacts to the total concentration of product i that is formed.
Kom,i (m3 g-1) is the partitioning coefficient for species i, and Mo is the absorbing organic
mass concentration (g m-3).
This expression is an improvement on the empirical FAC method since it incorporates
partitioning coefficients, temperature dependence, activity coefficients, particulate
concentration and vapor pressure factors, all of which affect the partitioning process.
The application of this method is limited to mathematical modeling since that particulate
mass concentration is constantly changing as more gas phase material partitions into the
particulate phase. Calculations using this method must, therefore, be iterative.
27
The usefulness of this method is also dependent on the availability of data such as
partitioning vapor pressure, activity coefficients and so on.
CONCLUSIONS
From this study, it is estimated that secondary organic aerosol (SOA) contributes
significantly to the total PM2.5 mass, averaging around 10 – 20 %, on most of the days
sampled. These metropolitan areas studied have significant vegetation coverage and the
SOA contribution to total organic carbon was estimated to be 50 - 70% during the period
studied. One possible explanation for this observation can be due to the significant
biogenic emissions found in these counties as compared to highly urbanized metropolitan
areas. The lack of mass transit facilities in Macon, Augusta and Columbus, implies that
commuting is achieved primarily via personal vehicles. The combined effect of high
aerosol forming biogenic VOCs with mobile NOx sources could create the right
conditions for ozone and SOA formation. However, this statement is purely speculative
since transport of pollutants from the Atlanta area and other neighbouring counties is also
a possible reason for the observed SOA.
Of the mobile emissions studied, toluene is estimated to be the largest potential SOA
contributor. The details of SOA formation and its chemical composition are only partially
known but empirical data can be used to estimate the formation potential of precursor
gases, if their source species profile is available as well as their emissions data.
28
For quantitative estimates of ambient SOA concentrations, the empirical FAC approach
neglects important variables such as timescales involved in SOA formation, transport
factors, relative humidity influences, competition between VOC species, synergistic
reactions of VOC species and other possibilities that exist in ambient gas mixtures that do
not exist in controlled chamber studies.
Despite these limitations, FACs can be used to compare the relative importance of VOC
sources for SOA formation. This study was limited to mobile sources of VOC since
species profiles were available. It would be extremely useful to conduct similar type
calculations for biogenic emissions, however, more research is needed to determine the
species profiles for the main vegetation types that exist and impact on these cities.
OPTIONS FOR REDUCING SOA
Despite the difficulties in quantification, some qualitative conclusions can be drawn from
this study:
1. The reduction of toluene content in gasoline is a possible means of reducing SOA.
Toluene is a toxic/hazardous air pollutant as well as a major SOA contributor and
it may well be more than worthwhile to investigate the possibilities of reducing
the toluene content of gasoline. However, detailed cost-effect analyses must first
be conducted to determine the feasibility of this option, similar to the studies that
lead to implementation of reformulated gasoline in Georgia in 1995.
29
2. The photochemistry of VOC is highly dependent on OH availability which is
coupled to NOx availability. The same is true for ozone photochemistry. OH and
NOx concentrations are dependent on VOCs with less than 6 carbons. Thus, it
seems that strategies for reducing ozone would result in a corresponding decrease
in SOA. Therefore, targeting selected VOC species might not be as important as it
fist seems since this system can be NOx limited. It is not yet possible to decide
which of these two given options, reducing selected VOC species versus reducing
ozone (NOx), is more effective or whether some combination of the two
approaches is a better option. More research, analysis and detailed modeling
efforts are needed to draw further conclusions.
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