Atmos. Chem. Phys. Discuss., 7, 14569–14601, 2007

www.atmos-chem-phys-discuss.net/7/14569/2007/

© Author(s) 2007. This work is licensed

under a Creative Commons License.

Atmospheric

Chemistry

and Physics

Discussions

Global modeling of secondary organic

aerosol formation from aromatic

hydrocarbons: high- vs low-yield

pathways

D. K. Henze1, J. H. Seinfeld1, N. L. Ng1, J. H. Kroll2, T. -M. Fu3, D. J. Jacob3, and

C. L. Heald4

1Department of Chemical Engineering, California Institute of Technology, Pasadena,

California, USA

2Aerodyne Research, Inc., Billerica, Massachusetts, USA

3School of Engineering and Applied Sciences and Department of Earth and Planetary

Sciences, Harvard University, Cambridge, Massachusetts, USA

4Center for Atmospheric Sciences, University of California, Berkeley, California, USA

Received: 1 August 2007 – Accepted: 9 October 2007 – Published: 15 October 2007

Correspondence to: D. K. Henze (daven@caltech.edu)

14569

Abstract

Formation of SOA from the aromatic species toluene, xylene, and, for the first time,

benzene, is added to a global chemical transport model. A simple mechanism is presented that accounts for competition between low and high-yield pathways of SOA

5 formation, wherein secondary gas-phase products react further with either nitrogen

oxide (NO) or hydroperoxy radical (HO2) to yield semi- or non-volatile products, respectively. Aromatic species yield more SOA when they react with OH in regions where the

[NO]/[HO2] ratios are lower. The SOA yield thus depends upon the distribution of aromatic emissions, with biomass burning emissions being in areas with lower [NO]/[HO2

]

10 ratios, and the reactivity of the aromatic with respect to OH, as a lower initial reactivity allows transport away from industrial source regions, where [NO]/[HO2

] ratios are

higher, to more remote regions, where this ratio is lower and, hence, the ultimate yield

of SOA is higher. As a result, benzene is estimated to be the most important aromatic

species with regards to formation of SOA, with a total production nearly equal that of

15 toluene and xylene combined. In total, while only 39% percent of the aromatic species

react via the low-NOx pathway, 72% of the aromatic SOA is formed via this mechanism.

Predicted SOA concentrations from aromatics in the Eastern United States and Eastern Europe are actually largest during the summer, when the [NO]/[HO2

] ratio is lower.

Global production of SOA from aromatic sources is estimated at 3.5 Tg/yr, resulting

20 in a global burden of 0.08 Tg, twice as large as previous estimates. The contribution

of these largely anthropogenic sources to global SOA is still small relative to biogenic

sources, which are estimated to comprise 90% of the global SOA burden, about half

of which comes from isoprene. Compared to recent observations, it would appear

there are additional pathways beyond those accounted for here for production of an25 thropogenic SOA. However, owing to differences in spatial distributions of sources and

seasons of peak production, there are still regions in which aromatic SOA produced via

the mechanisms identified here are predicted to contribute substantially to, and even

dominate, the local SOA concentrations, such as outflow regions from North America

14570
and South East Asia during the wintertime, though total SOA concentrations there are

small (∼0.1 μg/m3).

1 Introduction

Organic aerosols play an important role in global climate (Kanakidou et al., 2005). A

5 significant fraction of organic aerosol material results from the gas-phase oxidation of

volatile hydrocarbons to yield semivolatile products that condense into the particulate

phase; this is referred to as secondary organic aerosol (SOA). At present, levels of

organic aerosol measured in field campaigns tend to exceed those predicted by global

chemical transport models (Heald et al., 2005, 2006). While it is predicted that, on the

10 global scale, secondary organic aerosol from biogenic sources substantially exceeds

that from anthropogenic sources (Tsigaridis et al., 2006; Tsigaridis and Kanakidou,

2007), data from recent field studies suggest that SOA from anthropogenic hydrocarbons might be more significant than previously thought (de Gouw et al., 2005; Volkamer

et al., 2006).

15 Among anthropogenic hydrocarbons, aromatic compounds are generally considered

to be the most important SOA precursors. It has recently been established that the

SOA yields (aerosol yield is defined as the ratio of the mass of organic aerosol produced to the mass of parent hydrocarbon reacted) from aromatics, as well as those

from a variety of other hydrocarbons, depend critically on the prevailing level of nitrogen oxides (NOx

20 ) (Hurley et al., 2001; Martin-Reviejo and Wirtz, 2005; Presto et al.,

2005; Song et al., 2005; Johnson et al., 2004, 2005; Ng et al., 2007). This discovery led to a systematic re-evaluation of aromatic SOA yields (Ng et al., 2007) from

the historical yields of Odum et al. (1996, 1997) that were measured under high-NOx

conditions. In particular, aromatic yields under the low-NOx conditions typical of most

25 of the global atmosphere significantly exceed those under high-NOx conditions typical

of urban cores (and of past laboratory chamber experiments). However, given that

sources of aromatics are likely co-located with sources of NOx, the significance of this

14571

finding on the global SOA burden is not readily apparent.

Initial global modeling studies of SOA formation from toluene and xylene (Tsigaridis

and Kanakidou, 2003) used the empirical yield parameters from Odum et al. (1996,

1997). More recently, this model was updated to include the NOx dependence of yields

5 for toluene and xylene based upon the xylene yields in the work of Song et al. (2005).

Lack et al. (2004) also included aromatic species in their lumped calculation of SOA

formation from industrial sources, using yield coefficients from (Odum et al., 1997) and

(Cocker et al., 2001). Calculation of SOA formation from benzene has been identified in

the box model studies of Pun and Seigneur (2007) as an important additional pathway.

10 The recent data (Ng et al., 2007) on SOA yields from all of these aromatic hydrocarbons

prompt a re-evaluation of the contribution of aromatic SOA to the global SOA production

rate and burden. In the current study, the GEOS-Chem global chemical transport model

is updated to include abbreviated aromatic oxidation chemistry and SOA formation from

aromatics.

15 2 Summary of SOA yields from aromatic hydrocarbons

The SOA-forming potentials of m-xylene, toluene, and benzene have been measured in

a series of laboratory chamber experiments (Ng et al., 2007). Atmospheric reaction of

these aromatics with the hydroxyl radical (OH) initiates a complex series of gas-phase

reactions (Calvert et al., 2002; Koch et al., 2007). As noted above, a crucial factor

20 governing the nature of the gas-phase chemistry and subsequent aerosol formation is

the NOxlevel. Experiments were conducted under both low- and high-NOx conditions;

these correspond to NOxlevels of less than 1 parts-per-billion by volume (ppb) and

several hundred ppb, respectively.

SOA yields of the three aromatics studied are highly dependent on the prevailing NOx

25 level. Under high-NOx conditions, measured yields for toluene and m-xylene are in the

range of 5 to 10% with a strong dependence on the amount of organic aerosol present,

in general agreement with those reported by Odum et al. (1996, 1997). Under low14572
NOx conditions, all three compounds exhibit high (>30%) constant yields, exceeding

yields from high-NOx conditions. The first studies of SOA yields from benzene reported

yields in the range of 10% for both high and low-NOx conditions (Martin-Reviejo and

Wirtz, 2005); Ng et al. (2007) found a considerably higher yield for benzene, 28% under

5 high-NOx conditions. The discrepancies between the two reported low-NOx yields in

(Martin-Reviejo and Wirtz, 2005) and the one yield in Ng et al. (2007) may be a result

of higher SOA yields in the presence of an inorganic seed in the experiments of Ng

et al. (2007). Additional factors, such as the uncertainty in the benzene threshold

concentrations in Martin-Reviejo and Wirtz (2005) make comparisons between these

10 results difficult. While additional studies of the low-NOx yields of benzene are clearly

called for, use of inorganic seed particles in Ng et al. (2007) affords decoupling of the

NOx effect from the effects of an “induction period” caused in their absence (Kroll et al.,

2007), which will be shown to facilitate use of such data for modeling SOA formation.

The mechanisms for atmospheric oxidation of both biogenic and anthropogenic hy15 drocarbons are complex and not fully understood; it does appear, however, that the

observed low- and high-NOx behavior of SOA formation (Song et al., 2005; Ng et al.,

2007) hinges on the competitive reactions of the bicyclic peroxy radicals (ROHO2) that

result from initial attack of the aromatic species (R) by OH radicals followed quickly

by O2 addition and cyclization (Johnson et al., 2005; Koch et al., 2007), written here

20 simply as

R + OH·

kOH −→ ROH  ̇

O2 −→ ROHO2

· . (R1)

As explained in detail in Ng et al. (2007), the alternative reaction of the aromatic-OH

adduct with NO2is not expected to play a significant role in the chamber conditions

(Koch et al., 2007), nor are levels of NO up to ppm expected to prevent formation of the

25 bicyclic peroxy radical from its isomer that initially results from O2 addition (Zhao et al.,

2005; Fan and Zhang, 2006). The peroxy radicals react predominantly with either hydroperoxy radicals (HO2

) or NO, depending on the relative concentrations of HO2 and

NO. Reaction of the peroxy radicals with themselves is found to be unimportant under

14573

chamber conditions (the rate constant for this is relatively small and HO2 concentrations are high) in the kinetic simulations of Ng et al. (2007). The role of such competing

reactions in the atmosphere is assessed later in the present work.

While it is common to refer to the oxidative conditions in environmental chambers

according to absolute NOxlevels or VOC:NOx5 ratios (as those are typically what is

measured), it is often the ratio of [NO]/[HO2] that determines the fate of secondary

gas-phase products and which is more important than absolute NOx concentrations

for extrapolating results from environmental chamber conditions to atmospheric conditions. Under low-NOx conditions, the levels of HO2

radical in the chamber are such that

reaction with HO210 radical is favored, and the resulting products, including hydroperoxides, are generally less volatile than those that result from the NO reaction path. This

competition can be represented as follows:

ROHO2· +HO2·

kH −→ αH SOGH + · · · (R2)

ROHO2· +NO kN −→ α1SOG1 + α2SOG2 + · · · (R3)

15 where SOG designates secondary organic gas-phase semivolatile products, and the

α’s are mass-based stoichiometric coefficients (i.e. for every gram of peroxy radical

that reacts, α grams of SOG are formed).

The fact that, for all three aromatics, the SOA yield under low-NOx conditions is constant with respect to changes in available substrate implies that the semivolatile prod20 ucts are essentially nonvolatile, at least at the level of aerosol mass concentrations

in the chamber; thus, the ROHO2+HO2 pathway can be represented as leading to a

single nonvolatile product, SOGH=SOAH . Since the high-NOx pathway exhibits yields

that depend on the total amount of absorbing organic aerosol, we use the customary two-product model for SOA formation, originally formulated by Odum et al. (1996,

1997); SOG1 and SOG225 represent these products, which have associated gas-particle

14574
partitioning equilibrium constants, K1 and K2

,

[SOGj]=

[SOAj]

KjM

,

where M is the concentration of total available substrate in μg m−3. Parameters describing yields under both sets of NOx

levels are given in Table 1. Equilibrium constants

5 and stoichiometric coefficients are based on laboratory studies of Ng et al. (2007),

where here the latter are multiplied by the ratio of the molecular weight of the parent

aromatic to that of the peroxy radical to reflect formation of SOG species directly from

the peroxy radical. The assumption that the low-NOx product is nonvolatile is specific

to aromatic species; in general, sets of α’s and K’s can be derived for semivolatile

10 products from both pathways.

3 Aromatic SOA formation in GEOS-Chem

In the current study, the GEOS-Chem global chemical transport model (version 7-

04-11 with a horizontal resolution of 2◦×2.5

◦

and 30 layers up to 0.01 hPa, GEOS-4

meteorological fields) is used to simulate one year of present day conditions (2004).

15 This model includes detailed simulation of gas-phase tropospheric chemistry (e.g., Bey

et al., 2001; Hudman et al., 2007) in addition to external mixtures of several aerosol

components (Park et al., 2004, 2006). Previous versions have been implemented with

a gas-particle partitioning model of SOA formation from terpenes, alcohols, sesquiterpenes (Chung and Seinfeld, 2002; Heald et al., 2005) and isoprene (Henze and Se20 infeld, 2006). As such, estimates of organic carbon aerosol have been notably low

compared to measurements during springtime 2001 measurements in the free troposphere of the Asian continental outflow region (ACE-Asia) (Heald et al., 2005) and have

failed to capture the variance in observations in the free troposphere during the summer 2004 campaign (ICARTT) in the northeastern United States (Heald et al., 2006);

14575

in comparison, such overwhelming inconsistencies have not been found in yearly average estimates of surface organic aerosol concentrations compared to filter samples

(Liao et al., 2007). The addition of abbreviated aromatic oxidation chemistry and SOA

formation from aromatics to the chemical reactions and SOA module in GEOS-Chem

5 is described in the following sections.

3.1 Aromatic global emissions

Global emissions of benzene, toluene and xylene from industrial and fossil fuel sources

are taken from the Emission Database for Global Atmospheric Research (EDGAR

V2.0) (Olivier et al., 1996, 1999) for 1990 and scaled to the year 2000 using liquid fossil

10 fuel usage from the Global Emission Inventory Activity (GEIA) project (Benkovitz et al.,

1996) following Bey et al. (2001). Emissions of aromatics from biofuel and biomass

burning are calculated by applying emission ratios to emissions of CO, where sources

of CO from biofuel burning are from the GEIA database, while biomass burning sources

of CO are from the Global Fire Emissions Database version 2 (GFEDv2) (Giglio et al.,

15 2006; van der Werf et al., 2006). The emissions ratios, listed in Table 3, are taken

from Andreae and Merlet (2001) with updates from M. O. Andreae (personal communication, 2006). Total emissions are 5.6, 6.9 and 4.7 Tg C/yr for benzene, toluene and

xylene, respectively. The breakdown by source type is given in Table 4, and the yearly

average emission fluxes of each aromatic species, and the combined total, are shown

20 in Fig. 1. The largest global sources are road transport and solvent use (for toluene

and xylene), and biofuel and biomass burning (for benzene); road transport is the main

source of benzene in urban areas.

3.2 Implementation of aromatic SOA formation

Gas-phase oxidation of each parent aromatic hydrocarbon (R1) and subsequent reaction of the peroxy radical product with HO2

25 (R2) and NO (R3) is calculated online as

an additional part of the tropospheric chemical reaction mechanism. By explicitly cal14576
culating the branching ratio between these two pathways, this implementation allows

natural transitions between low and high-yield environments as governed by temperature, and HO2

radical and NO concentrations. This approach, initially suggested in

the work of Presto and Donahue (2006), has the advantage of avoiding prior delineation of oxidative regimes based upon VOC/NOx

5 ratios (Song et al., 2005; Tsigaridis

et al., 2006; Presto and Donahue, 2006). Parameters for calculating the reaction rate

constants for these steps are listed in Table 2. Kinetic parameters for reaction of the

aromatic species with OH (the rate limiting step in peroxy radical formation) are from

Calvert et al. (2002). Rate constants for peroxy radical reactions, (R2) and (R3), are

10 from Atkinson et al. (1997), assuming similar temperature dependence as peroxy radical reactions with isoprene, as most reactions of hydrocarbons with NO and HO2 have

similar kinetics (Lightfoot et al., 1992; Eberhard and Howard, 1997). All forms of xylene

are assumed to behave as m-xylene for both gas and aerosol processes.

Calculation of reversible SOA formation follows the approach outlined in Chung and

15 Seinfeld (2002). The empirical yield parameters (Table 1) are used to estimate the

mass of SOA formed per mass of peroxy radical that reacts. An important aspect of

this treatment is specification of the available substrate, M, which affects gas-particle

equilibrium. For this work, M is taken to consist of the total mass of primary and secondary organic material.1 As GEOS-Chem tracks only the carbonaceous component

20 of primary organic aerosol, the ratio of organic to total carbon aerosol mass must be

specified in order to calculate M – here the ratio is assumed to be 2.1. Though this

differs from previous studies that used a ratio of 1.4 (Henze and Seinfeld, 2006; Liao

et al., 2007; Heald et al., 2006; Zhang et al., in press; van Donkelaar et al., 2007),

2.1 is recommended for non-urban aerosols (Turpin and Lim, 2001). This adjustment

25 alone causes a nearly 20% increase in total SOA production; hence, specification of

the carbon content of SOA is clearly an important source of uncertainty in these calculations. The Clausius-Clapeyron equation is used to extrapolate equilibrium constants

1Condensation directly on sulfate aerosol, considered in some versions of GEOS-Chem and

other studies (Tsigaridis and Kanakidou, 2003), is not included in these simulations.

14577

to tropospheric temperatures. The enthalpy of vaporization for SOA, a major source of

uncertainty in such calculations (Tsigaridis and Kanakidou, 2003; Henze and Seinfeld,

2006), is assumed to be 42 kJ mol−1for all species, in the range of limited available

experimental data (Offenberg et al., 2006). Dry deposition of all types of SOA is cal5 culated using a resistance-in-series model (Wesely, 1989; Wang et al., 1998); wet

scavenging follows Liu et al. (2001) with an assumed scavenging efficiency of 80%

(Chung and Seinfeld, 2002).

Additional sources of uncertainty include the extent to which yields determined in

chamber studies in the presence of high organic aerosol loadings (∼30 μg m−3) can

10 be extrapolated to the atmosphere (Presto and Donahue, 2006; Pathak et al., 2007),

particularly given the assumption of irreversible SOA formation at low NOx. Further, the

low-NOx yield parameters for benzene were fit to a time series of data points from a

single photooxidation experiment in Ng et al. (2007) which, while well suited for cleanly

assessing the SOA yields via Reaction (R2), await additional experimental verification.

15 Finally, as noted previously, application of this abbreviated mechanism for aromatic

oxidation chemistry does not include reaction of the peroxy radicals with themselves, a

pathway for which SOA yields are not known, nor reaction of the aromatic-OH adduct

with NO2instead of O2. With average concentrations of NO2in the 2◦×2.5◦grid cells of

the global model used here being <100 ppb, reaction of the aromatic-OH adduct with

NO220 is not likely an important pathway (Volkamer et al., 2002; Koch et al., 2007). Self

reaction of peroxy radicals may be an important reaction in areas where the peroxy

radical concentrations exceed those of NO and HO2 by much more than an order of

magnitude as the rate constant at 298 K is slower (likely <1×10−14 cm3 molecule−1s

−1

given their level of substitution) than kN (8.5 × 10−12 cm3 molecule−1s

−1

) or kH (1.5 ×

10−11 cm3 molecule−1s

−1

25 ). However, this may be rare since estimated concentrations

of even the parent aromatic species are typically on the order of only a few ppb near

source regions, where NOx concentrations are of order 0.1 ppb and/or HO2 are 10 ppt.

14578
4 Simulation of global aromatic SOA

The predicted contributions of benzene, toluene, and xylene to global aromatic SOA

via high- and low-yield pathways (the opposite of the NOxlevels) and the percentage

of the aromatics that react via each pathway are given in Fig. 2. Thus, 51% of globally

5 emitted benzene, for example, reacts via Reaction (R2), the high-yield path, whereas

only 26% of xylene follows this route. The location of the sources plays an important

role in determining the total SOA yield from each species. Aromatics, primarily benzene, emitted from biofuel and biomass burning sources are more likely to react via the

high-yield pathway as these emissions are less associated with sources of NOxthan

10 the industrial and fossil fuel emissions. Additionally, for emissions from industrial and

fossil fuel sources, it is the less reactive aromatics, as measured by their OH reaction

rate constants, that actually produce more SOA. The explanation is that lower reactivity affords the parent hydrocarbon more time to be transported to regions of lower

[NO]/[HO2] ratios, where ultimately, once reacted, the SOA yield is larger. Proportionately more of a reactive molecule like xylene is consumed in regions in which the NOx

15

levels are more characteristic of the source emission areas. On a global average, 39%

of the total aromatics proceed through the high yield pathway. As a result, 72% of

global aromatic SOA is produced via the high-yield (low-NOx) pathway.

Figure 3 shows the predicted seasonally averaged surface-level concentrations of

20 aromatic SOA. The concentrations generally reflect the distribution of areas with substantial anthropogenic emissions in northern mid-latitudes, which are themselves aseasonal, and biomass burning in the Southern Hemisphere, which have strong seasonal

cycles. The resulting SOA concentrations show considerable seasonal variability, even

in the northern mid-latitudes. Concentrations in the eastern parts of the United States

25 and Europe are lowest during the Northern Hemisphere winter months, and highest

during the Northern Hemisphere summer months. Figure 4 shows the contrast between the estimated concentrations of NO and HO2

radical during DJF and JJA, the

cycles of which both contribute to more of the aromatics in these areas reacting via the

14579

high-yield pathway during the summer. The combined effects on the fate of toluene are

shown in panels (e) and (f), which are the log of the branching ratio,

log10 

Number of toluene peroxy radicals that proceed via reaction with NO

Number of toluene peroxy radicals that proceed via reaction with HO2



.

For example, yellow colors indicate areas where the NO pathway is estimated to dom5 inate by two or more orders of magnitude while blue colors indicate areas where an

order of magnitude more toluene molecules are calculated to proceeded via the HO2

pathway. Values where the total toluene peroxy radical concentrations are less that 1%

of the maximum global surface concentration are not shown. The HO2 pathway entirely

dominates only rarely; however, it is clear that SOA formation via this low-NOx pathway

10 is much more important in the eastern United States and Europe during JJA than DJF.

Also note that during JJA, SOA production in these areas is not inhibited by increased

summertime temperatures as formation of SOA via the low-NOx pathway is treated as

irreversible. Biogenic sources of organic aerosol are also elevated during the summer. This additional substrate increases SOA production from aromatics via the (R3)

15 pathway, but this enhancements is found to be less than 5%. This demonstrates the

potential of NOx variations, whether perturbed by changes in anthropogenic emissions

or modulated by natural seasonal cycles, to influence global SOA yields.

Total aromatic emissions and resulting SOA production rates and burdens are given

in Table 5. The burden of SOA from aromatic compounds is estimated here to be

20 0.08 Tg. Similarly, recent work by Tsigaridis and Kanakidou (2007) estimated SOA

burdens from aromatics at 0.04 Tg, considering only toluene and xylene. Using the

same yields and mechanism reported here, Heald et al. (2007)2explored the response

of SOA estimates to future climate, land-use and emissions changes in a general circulation model and estimated present day production of SOA from anthropogenic sources

2Heald, C. L., Henze, D. K., Horowitz, L. W., et al.: Predicted change in global secondary

organic aerosol concentrations in response to future climate, emissions, and land-use change,

submitted, 2007.

14580
to be 3.4 Tg C/yr. Each of these studies highlights the importance of accounting for the

NOx dependent yields of these species. Overall, the range of estimated global burdens

of aromatic SOA from recent simulations (0.04–0.10 Tg) is more than twice as high as

earlier estimates of 0.01–0.03 Tg (Tsigaridis and Kanakidou, 2003).

5 5 Anthropogenic vs biogenic SOA

Table 5 also presents the emissions and resulting production rates and burdens of SOA

from biogenic species, where for simplicity we consider all aromatic sources to be anthropogenic, though undoubtedly some of the biomass burning sources are naturally

occurring. Even though aromatic SOA is appreciable, the global SOA burden is domi10 nated by biogenic sources, about 50% of which is attributable to isoprene, as predicted

based on current SOA yields from laboratory chamber studies. As such, addition of

aromatic SOA does not significantly affect global estimates of biogenic SOA through

nonlinear effects, though it may have a stronger influence in future emissions scenarios

(Tsigaridis and Kanakidou, 2007, and Heald et al., 20072).

15 While the bulk of the total modeled SOA is biogenic in origin, there are regions of

the global distribution where concentrations of SOA from aromatics are predicted to

be equal or larger. Figure 5 shows the log of the ratio of aromatic to biogenic SOA

concentrations,

log10 

[SOAarom]

[SOAbiogenic]

!

excluding locations where the total SOA concentration is less than 0.05 μg/m320 . During

DJF and MAM, much of the outflow regions in the Northern Hemisphere are dominated by anthropogenic SOA. Nevertheless, SOA concentrations in these areas are

still small, <0.1 μg/m3, as can be seen by comparison to Fig. 3. Further, concentrations in these regions are typically much smaller than the total (primary plus secondary)

14581

organic aerosol. For example, Fig. 6 shows the profile of SOA concentrations at 70◦

W, an area identified in Fig. 5 where the SOA is predominantly anthropogenic in origin.

While SOA concentrations from aromatics, see panel (a), are greater than from other

sources, panel (b), the total SOA concentrations are small, particularly in comparison

5 to total carbon aerosol concentrations, panels (c)–(d). As such, the revised estimates

of total carbonaceous aerosol from this work alone do not likely explain the “missing

source” of SOA noted in previous model estimates of OC aerosol in ACE-Asia (Heald

et al., 2005), nor the magnitude and variability of soluble OC from observations during

the ICARTT campaign of summer, 2004 (Heald et al., 2006). Over the Northeastern

10 United States during JJA, the estimated SOA from aromatics via the pathways considered here is still smaller than the anthropogenic source of SOA suggested by de Gouw

et al. (2005). While it is tempting to directly compare the results of the present calculation to the observations of SOA formation in Mexico City reported by Volkamer et al.

(2006), the results of this global modeling study are too coarse to resolve specific urban scale pollution events, and the NOx

15 levels in such cases can greatly exceed those

within the regime covered by the global model developed here. However, it would appear that additional mechanisms for anthropogenic SOA formation still exist beyond

those presently considered.

6 Conclusions

20 The global chemical transport model GEOS-Chem is updated to include simulation of

SOA formation from the aromatic species benzene, toluene and xylene. Following the

suggestion of Presto and Donahue (2006), a simple mechanism is presented that accounts for the competition between low and high-NOx pathways on SOA formation in a

continuous fashion. Depending upon the immediate chemical environment, secondary

peroxy radicals from photooxidation of aromatics by OH react with either NO or HO225

radical. Formation of SOA from reaction with NO leads to reversible formation of SOA

following the two-product model of Odum et al. (1996, 1997) using empirically deter14582
mined yield and partitioning coefficients from the high-NOx studies of Ng et al. (2007).

Aromatic peroxy radicals that react with HO2radical are treated as forming SOA irreversibly, following the low-NOx

results of Ng et al. (2007). This approach to treating

NOx dependence is general and could (with enough empirical data) be extended to ad5 dress the NOx dependence of other species that are associated enough with sources

of NOxto warrant discrimination between the two pathways. For the aromatic species,

explicitly considering both pathways was found to be important as substantial amounts

(25% or higher) of each aromatic species proceed via each pathway. In contrast, implementing a similar model description for monoterpenes or isoprene, largely emitted

10 in low-NOx environments, may not be as crucial for estimating their global contributions

to SOA.

Previously assumed to generate negligible amounts of SOA because of its low reactivity with OH, benzene is estimated in this work to be the most important aromatic

species with regards to formation of SOA owing to its low initial reactivity and the loca15 tion of its emissions. Its low initial reactivity allows benzene to be transported away from

source regions, where [NO]/[HO2] ratios are high, to more remote regions, where this

ratio is lower and, hence, the ultimate yield of SOA is higher. In total, nearly 75% of the

aromatic SOA is formed via the low-NOx pathway, though only 39% percent of the aromatic species react via this mechanism. Predicted SOA concentrations from aromatics

20 in the Eastern United States and Europe are actually largest during the summer, owing to both higher HO2

radical concentrations and lower NO concentrations. Influence

of NOx variability on SOA formation is particularly interesting as current models may

underestimate seasonal NOx cycles in many areas as indicated by observations from

GOME (van Noije et al., 2006; Wang et al., 2007), though further analysis is warranted.

25 Even though the predicted burden and production rate of aromatic SOA is twice

that of previous estimates (Tsigaridis and Kanakidou, 2003, 2007), the contribution

of these sources to global SOA is small relative to contributions from monoterpenes

and isoprene, which alone are estimated to comprise 82% of the global SOA burden.

However, owing to differences in spatial distributions of sources and seasons of peak

14583

production, there are regions in which aromatic SOA is predicted to contribute substantially to, and even dominate, the local SOA concentrations, such as outflow regions

from North America and South East Asia during the wintertime. The contribution of

aromatic SOA to carbonaceous aerosol may also affect interpretation of total organic

5 carbon aerosol concentrations from surface stations in which winter concentrations

have been assumed to be primary in origin (Liao et al., 2007).

Overall, comprehensive model treatment of SOA is still difficult owing to a large range

of effects not yet included, from interactions of aqueous and organic phases (Pun and

Seigneur, 2007) to effects of particle acidity on yields from isoprene (Surratt et al.,

10 2007). Uncertainties in biogenic emissions estimates are frequently cited; uncertainties of even the aromatic emissions are possibly as large as a factor of two or more

(Warneke et al., 2007). Recent works by Presto and Donahue (2006) and Pathak

et al. (2007) also emphasize the importance of accounting for other effects, such as

low aerosol mass loading, when parameterizing SOA yields from chamber data; all

15 of these topics will be important areas of future research and model development.

Additional mechanisms beyond those presented here may play important roles in production of SOA from aromatic species. However, the importance of NOx on estimating

yields of SOA from aromatics is clear. This work highlights the importance of additional

studies of the NOx dependence of SOA formation from other species, and the role that

20 such dependance plays on SOA yields of long lived hydrocarbons in general, as this

represents an important mechanism by which anthropogenic activity can affect global

SOA. This is of concern for not only interpretation of current measurements of organic

aerosol, but also for estimates of the effects of future climate and emissions changes

on global aerosol burdens (Tsigaridis and Kanakidou, 2007, Heald et al., 20072).

25 Acknowledgements. This work was supported by U.S. Environmental Protection Agency, grant

R832158, and the National Science Foundation, grant NSF ITR AP&IM 0205198, which provided access to the TeraGrid resources at the National Center for Supercomputing Applications.

14584
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14590
Table 1. Stoichiometric coefficients, αi ,j, and equilibrium partitioning coefficients, Ki ,j, for SOA

formation derived from high and low-NOx chamber experiments of reaction of aromatics with

OH (Ng et al., 2007). The reference temperature for the Ki ,j’s is 295 K.

Parent aromatic i αi ,H αi ,1 αi ,2 Ki ,1 Ki ,2

[m3 μ g

−1

] [m3 μ g

−1

]

benzene 0.2272 0.0442 0.5454 3.3150 0.0090

toluene 0.2349 0.0378 0.0737 0.4300 0.0470

m-xylene 0.2052 0.0212 0.0615 0.7610 0.0290

14591

Table 2. Reaction rate constants, k=AeB/T.

Reaction Rate constantak298 A B

[cm3 molec−1s

−1

] [cm3 molec−1s

−1

] [K]

(R1) kOH,B 1.22×10−12 2.33×10−12 −193

(R1) kOH,T 5.63×10−12 1.81×10−12 338

(R1) kOH,X 2.31×10−11 2.31×10−11 0

(R2) kH 1.5×10−11 1.4×10−12 700

(R3) kN 8.5×10−12 2.6×10−12 350

aB = benzene, T = toluene, X = xylene. Constants kH and kN assumed equal for each parent

aromatic.

14592
Table 3. Emission ratios for biofuel and biomass burning sources of aromatic species (mmoles

emitted per mole CO emitted) taken from Andreae and Merlet (2001) with updates from

M. O. Andreae (personal communication, 2006).

Aromatic species Biofuel burning Biomass burning

benzene 4.06 2.33

toluene 2.01 1.24

xylene 0.82 0.48

14593

Table 4. Emissions of each aromatic species broken down by source [Tg C /yr].

Aromatic species Industrial and fossil fuel Biofuel burning Biomass burning

benzene 1.3 1.8 2.5

toluene 4.3 1.0 1.6

xylene 3.5 0.5 0.7

14594
Table 5. Global SOA budgets.

Hydrocarbon Emission (Tg/yr) SOA Production (Tg/yr) Burden (Tg)

terpenes 121 8.2 0.21

alcohols 38.3 1.5 0.03

sesquiterpenes 14.8 2.0 0.03

isoprene 461 13.2 0.43

aromatics 18.8 3.5 0.08

total 654 28.4 0.78

14595

60oS

30oS

0o

30oN

60oN

180o 120oW 60oW 0o 60oE 120oE

60oS

30oS

0o

30oN

60oN

180o 120oW 60oW 0o 60oE 120oE

60oS

30oS

0o

30oN

60oN

180o 120oW 60oW 0o 60oE 120oE

60oS

30oS

0o

30oN

60oN

180o 120oW 60oW 0o 60oE 120oE

(a) Benzene, 5.6 Tg C/yr (b) Toluene, 6.9 Tg C/yr

(c) Xylene, 4.7 Tg C/yr (d) Total, 17.2 Tg C/yr

0.0 1.7 x 1011 3.3 x 1011 5.0 x 1011 [atoms C / cm2 / s]

Fig. 1. Emissions of aromatic compounds.

14596
51%

37%

26%

49%

63%

74%

0%

25%

50%

75%

100%

benzene toluene xylene

% of products react with HO2

% of products react with NO

32%

29%

11%

16%

8%

4%

0%

10%

20%

30%

40%

50%

benzene toluene xylene

% of total aromatic SOA formed via HO2 pathway

% of total aromatic SOA formed via NO pathway

Fig. 2. Percentages of peroxy radical from each parent aromatic that react with HO2radical (R2)

vs with NO (R3) and the eventual contributions from each pathway to the total SOA production

from all aromatic species.

14597

DJF MAM

JJA SNO

Fig. 3. Seasonal distributions of the total surface-level SOA concentrations from benzene,

toluene and xylene.

14598
(a) NO (DJF) (b) NO (JJA)

(c) HO2 (DJF) (d) HO2 (JJA)

(e) log10(r

NO/rHO2

) for toluene (DJF) (f) log10(r

NO/rHO2

) for toluene (JJA)

Fig. 4. Panels (a–d) are the seasonally averaged surface-level concentrations of NO and HO2.

Panels (e) and (f) show the log of the resulting branching ratio between reaction of the toluene

peroxy radical with NO vs reaction with HO2. Not shown are values where net reacted peroxy

radical is <1% of the surface-level maximum. Comparing results in the Eastern United States

and Europe, both decreased NO and increased HO2 during JJA lead to increased reaction via

the low-NOx(high-yield) pathway.

14599

DJF MAM

JJA SON

Fig. 5. The log of the ratio of seasonally averaged surface concentrations of anthropogenic

to biogenic SOA, log10 

[SOAarom]

[SOAbiogenic]



, omitting values where the total SOA concentration is less

than 0.05 μg/m3. For simplicity, biomass burning sources of aromatics are considered to be

anthropogenic.

14600
(a) SOA from aromatic species (b) SOA from non-aromatic species

(c) SOA from all species (d) OC (primary and secondary)

Fig. 6. A profile of SOA concentrations at 70◦ W (east of the United States) during DJF. Panels

(a) and (b) show the linear contributions from anthropogenic and biogenic sources, respectively.

Noting the change in scale, panels (c) and (d) show the total SOA from all sources and the total

organic carbon aerosol from primary and secondary sources.

14601