VISCOUS SECONDARY ORGANIC AEROSOLS ELEVATE GLOBAL LONG-RANGE
TRANSPORT AND LUNG-CANCER RISK
M. SHRIVASTAVA1, S. LOU1, A. ZELENYUK1, R.C.EASTER1, R. CORLEY1, B.D. THRALL1, P.J.
RASCH1, J.D. FAST1, S.L.M. SIMONICH2, H. SHEN3 and S. TAO4
1
Pacific Northwest National Laboratory, Richland, WA, USA 99352
Department of Chemistry and Environmental and Molecular Toxicology, Oregon State University,
Corvallis, OR 97331
3
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332
4
Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking
University, Beijing, 100871, China
2
Keywords: VISCOUS ORGANIC SHIELD, ORGANIC AEROSOLS, HETEROGENEOUS
CHEMISTRY, POLYCYCLIC AROMATIC HYDROCARBONS.
INTRODUCTION
Polycyclic aromatic hydrocarbons (PAHs) have toxic impacts on humans and ecosystems. One of the most
carcinogenic PAHs, benzo (a) pyrene (BaP), is efficiently partitioned to and transported with atmospheric
particles. Laboratory measurements show that adsorbed BaP degrades in a few hours by heterogeneous
reaction with ozone, yet field observations indicate BaP persists much longer in the atmosphere, and some
previous chemical-transport modelling studies have ignored heterogeneous oxidation of BaP to bring
model predictions into better agreement with field observations. We attribute this unexplained discrepancy
to the shielding of BaP from oxidation by coatings of viscous organic aerosol (OA). Accounting for this
OA viscosity-dependent shielding, which varies with temperature and humidity, in a global
climate/chemistry model brings model predictions into much better agreement with BaP measurements,
and demonstrates stronger long-range transport, greater deposition fluxes, and substantially elevated lungcancer risk from PAHs. Model results indicate that the OA coating is more effective in shielding BaP in
the mid/high latitudes compared to the tropics because of differences in OA properties (semi-solid when
cool/dry vs. liquid-like when warm/humid). Faster chemical degradation of BaP in the tropics leads to
higher concentrations of BaP oxidation products over the tropics compared to higher latitudes. This study
has profound implications demonstrating that OA strongly modulates the atmospheric persistence of PAHs
and their cancer risks.
METHODS
We incorporate the two formulations (new, where OA coatings shield BaP from heterogeneous oxidation,
and default, where OA does not shield BaP) within the global Community Atmosphere Model version 5.2
(CAM5). Simulations are performed for 2007 to 2010. A major fraction (generally ≥90%) of freshly
emitted gaseous BaP is absorbed within co-emitted primary organic aerosol (POA) or subsequently
formed secondary organic aerosol (SOA), or adsorbed onto co-emitted soot/black carbon (BC). At
warm/humid conditions, BaP heterogeneous oxidation is assumed to be relative humidity (RH)-dependent
based on laboratory measurements of BaP coated by SOA [Zhou et al., 2013]. At cool and/or dry
conditions however, we assume that highly viscous OA effectively shields BaP from heterogeneous
oxidation, similar to that observed for eicosane (a highly viscous solid organic) coatings [Zhou et al.,
2012]. We use a global BaP emissions inventory from 2008 [Shen et al., 2013], and assign temporal and
vertical profiles to BaP emissions in relevant source categories. Both secondary organic aerosol (SOA)
and directly emitted primary organic aerosol contribute to the OA coating thickness, but SOA dominates
the global budget of OA [Shrivastava et al., 2015]. While SOA is treated as liquid-like and semi-volatile
in the default formulation, the new formulation treats SOA as a semi-solid and effectively non-volatile,
which has been shown to agree with a suite of global OA measurements [Shrivastava et al., 2015]. The
simulated coating thickness of OA around the BC core is calculated in every model grid and time step and
often exceeds the threshold of 20 nm for being classified as a thick coating. Model simulated coating
thickness is within the range of reported measurements of OA coating thicknesses [Forrister et al., 2015].
We examined one PAH in particular as a representative of all PAH mixtures. Called BaP, this is one of the
most carcinogenic PAHs.
RESULTS
We compared the simulation results to BaP field measurements from 69 rural sites and 294 urban sites
worldwide. Each site included hundreds of measurements. Fig. 1 shows that BaP predictions from the new
shielded modeling formulation were far more accurate than the default unshielded modeling formulation .
Fig. 1. Evaluation of 2008-2010 near-surface BaP concentration predicted by the default unshielded (red)
and new shielded (blue) modelling formulations against field measurements of BaP. Model results for
each site are averages over observation days. Top row shows scatter plots of simulated and observed
concentrations at (a) 69 background/remote sites and (b) 294 non-background sites around the world
[Borůvková J. et al., 2015; EMEP; IADN; Shen et al., 2014]. Modified normalized mean biases (MNMB)
are calculated as in Wagner et al. [2015]. Areas of circles are proportional to the number of days sampled
at each site.
To demonstrate the consequences of atmospheric long-range transport of BaP, we conduct several
additional simulations in which BaP emissions from different source regions are turned on in the model,
one at a time. Fig. 2 compares the long-range transport of BaP emitted from three major regions, East
Asia, Western Europe and Africa, which together comprise 63% of global BaP emissions. Fig. 2 shows
that the shielded PAHs (top rows) traveled much farther from their places of origin across oceans and
continents, whereas in the previous unshielded model, they barely moved from their country of origin. The
heterogeneous oxidation lifetime of PAHs was substantially increased from ~2 hours in the default
unshielded model to ~5 days when they were shielded by viscous organic aerosols.
Fig. 2. Simulated near-surface 2008-annual average concentrations of BaP from 3 major source regions:
East Asia (left), Western Europe (center) and Africa (right) for new shielded (top panels) and the default
unshielded modelling formulations (bottom panels), as indicators of long-range transport potential. BaP
emissions are only turned on for the respective source regions with its emissions over the rest of the globe
turned off. White areas are grid cells with BaP concentrations < 10-5 ng m-3.
We then investigated the impact of PAHs on human health, by combining a global climate model, running
either the shielded PAH scenario or the previous unshielded one, with a lifetime lung-cancer risk
assessment model. Globally, the previous model predicted half a cancer death out of every 100,000
people, which is halfway to the limit set forth by the World Health Organization for PAH exposure. But
taking into account that the shielded PAHs actually travel great distances, the new model found the risk
was four times that, or two cancer deaths per 100,000 people, exceeding WHO standards. With a world
population of about 7 billion people, that equates to a rise from about 35,000 lung cancer deaths to about
140,000.
Finally, we examined how the PAHs behaved within the coating of the aerosols at different locations
around the globe. We found that the extent of shielding was much lower over the tropics compared to the
mid and high latitudes. As the aerosols traversed the warm and humid tropics, ozone could get access to
the PAHs and oxidize them. This was because the coating was more liquid than solid, thanks to warmer
and more humid tropical weather, which allowed the PAHs to move freely in the particles. Future studies
are needed to better understand how the shielding of PAHs varies with the complexity of aerosol
composition, atmospheric chemical aging of aerosols, temperature and relative humidity.
ACKNOWLDEGEMENTS
The research described in this paper was conducted under the Laboratory Directed Research and
Development Program at Pacific Northwest National Laboratory, a multiprogram national laboratory
operated by Battelle for the U.S. Department of Energy. This research was also supported by the
Environmental Molecular Science Laboratory (EMSL), a U.S. Department of Energy (DOE) Office of
Science user facility sponsored by DOE's Office of Biological and Environmental Research and located at
PNNL. R.C. and S.L.M.S. were supported by the National Institute of Environmental Health Sciences
(NIEHS) through grant number P30ES00210 and P42ES016465. S.L.M.S. was also supported by National
Science Foundation (NSF) through grant number AGS-11411214. A.Z. was supported by the US DOE,
Office of Science, Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences
and Biosciences. P.J.R was supported by the U.S. Department of Energy, Office of Science, Biological
and Environmental Research program as part of the Earth System Modeling Program. Part of the work
was also carried out with the support of core facilities of RECETOX Research Infrastructure, project
number LM2015051, funded by the Ministry of Education, Youth and Sports of the Czech Republic and
L01214 (National Sustainability Programme). The PNNL Institutional Computing (PIC) program and
EMSL provided computational resources for the model simulations. The CESM project is supported by
the National Science Foundation and the Office of Science of the U.S. Department of Energy. The Pacific
Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial
Institute under contract DE-AC05-76RL01830.
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