Supporting Online Material
Atmospheric carbonyl sulfide sources from anthropogenic activity: Implications for carbon cycle constraints
Campbell, J. E., Sierra Nevada Research Institute, University of California, Merced.
Whelan, M. E., Sierra Nevada Research Institute, University of California, Merced.
Seibt, U., Dept. of Atmospheric and Oceanic Sciences, University of California, Los Angeles.
Smith, S. J., Joint Global Change Research Institute, PNNL, College Park, MD.
Berry, J. A., Department of Global Ecology, Carnegie Institution, Stanford, CA.
Hilton, T. W., Sierra Nevada Research Institute, University of California, Merced.
Indirect CS
2
The anthropogenic COS source is thought to be dominated by the indirect source from industrial
CS
2
emissions which are rapidly oxidized to COS in the troposphere [ Chin and Davis , 1993].
Industrial CS
2
emissions are primarily associated with the production of viscose rayon. Here we estimate the indirect CS
2
source from viscose rayon production (S rayon
) as follows:
S rayon
(m,t) = α ∙ E(m,t) ∙ A(m,t) (1) where α is the atmospheric oxidation of CS
2
to COS, E is the rayon emission factor (g CS
2 emitted / g rayon produced), A is the production activity (g rayon produced / year), m is the rayon material type (rayon staple or rayon yarn), and t is the year.
We obtain global production activity data for rayon staple and yarn materials beginning in the
first production year in 1890 through the year 2013 [ Fiber Economics Bureau , 1952 - 2014].

The two primary material types are rayon staple (discreet lengths of fiber that are often used to produce fabrics) and rayon yarn (long continuous filaments). The emission factor for rayon yarn has been estimated as 0.25 g CS
2
emitted per g of yarn produced [ USEPA , 2003]. Production of
yarn requires 0.32 g CS
2
as input per g yarn produced [ SRI , 2011], which is equivalent to 80% of
the required CS
2
input being lost to the atmosphere from the rayon factory during production.
Rayon staple material production has higher recycle rates and requires only 0.15 g CS
2
as input
per g yarn produced [ SRI , 2011]. We apply the 80% emissions rate from yarn to estimate a
staple emission factor of 0.12 g CS
2
emitted per g of staple produced. This staple emission rate is less than half of the yarn emission rate. Modern equipment has improved the recycle rate resulting in a reduced emission factor from staple production of 0.07 g CS
2
emitted per g of
staple rayon produced but no change in the emission factor for yarn [ Dodd et al.
, 2013]. These
modern technologies are deployed in developed countries as well as China, which is currently the dominant production country [Lenzing company, personal communication, 2014]. We apply the modern emission factors for all global capacity that was added after the year 2000. We use the conventional emission factors for earlier years and for the temporary decline in production in the year 2008.
Using material-specific emission rates is critical for two reasons. First, the relative production of rayon staple to rayon yarn has changed dramatically over the last three decades (see manuscript).
Second, the emission factor for rayon staple is less than half of the emission factor for rayon yarn. The most recently published EPA emission factor for the rayon industry assumes that the yarn emission factor is representative of the entire industry. This single emission factor was used
in an emission inventory for Asia [ Blake et al.
, 2004] which is the most recent review of
inventory methods. However, the EPA emission factor and its application in the Asian inventory overlook previously available information that indicates that the two dominant rayon materials
(rayon staple and rayon yarn) have very different emission factors.
We estimate CS
2
emissions from rayon information because there is no direct information on
CS
2
production and consumption. The industry reports that track production and consumption of
CS
2
also use rayon data for their CS
2
estimates [ SRI , 2011].

A smaller source of industrial CS
2
emissions is from solvent applications. Previous work reports that solvent applications account for 10% of industrial CS
2
use and that 80% of the CS
2
used for
solvents is emitted to the atmosphere [ Chin and Davis , 1993;
SRI , 2011]. We follow this same
approach here.
The atmospheric oxidation of CS
2
to COS has a molar conversion efficiency that has been
estimated to be 80% [ Chin , 1992; Chin and Davis , 1993]. The oxidation of CS
2
by OH involves multiple reactions which include products of COS and SO
2
and rate parameters that are
summarized in Atkinson et al. [2004].
We assume a 50% uncertainty in the COS indirect source from industrial CS
2
emissions based on half the range for the following emission factors and atmospheric oxidation rates. The atmospheric oxidation of CS
2
to COS has a molar conversion efficiency that has been estimated
at 80%, but a recent study has assumed 87% [ Chin and Davis , 1993;
Launois et al.
, 2015]. In
addition to our emission factor estimates discussed above, lower emission factors are reported in national regulations of 0.20 and 0.09 g CS
2
emitted per g of yarn and staple, respectively [ CPCB ,
2014]. We do not have uncertainty estimates for the industry activity (rayon production per
year) but we expect the activity uncertainty to be much lower than the emission factor uncertainty because rayon is an economically valuable commodity that is tracked by multiple industry groups. Because coal and aluminum are also economically valuable commodities, we also expect the emission factors to be the dominant source of uncertainty for the coal and aluminum emissions described below. However, for biomass burning, both the emission factor and the activity (biomass burning per year) are incorporated into the uncertainty estimates.
Aluminum
We model the global emissions of COS from primary aluminum production using emission factors for prebake and Søderberg smelters and annual productions levels for each technology.
We modeled aluminum emissions (S al
) as follows,
S
Al
(s,t) = E(s) ∙ A(s,t) (2)

where E is the aluminum emission factor (kg COS emitted per ton of primary aluminum production), A is the activity (primary aluminum production per year), s is the smelter type
(prebake or Søderberg), and t is the year.
Previous inventories have relied on a single emission factor estimate of 2.16 kg S emitted per ton
primary aluminum produced [ Harnisch et al.
, 1995]. This single study was based on
measurements from a prebake smelter with an anode sulfur content of 1.1% which required a linear extrapolation to a sulfur content of 2.5% which is more representative of the global mix of smelters. More recent experiments discovered that COS smelter emissions do not increase linearly with anode sulfur content and find a much smaller emission factor of 0.6 kg COS per ton
of primary aluminum production for globally representative sulfur contents [ Kimmerle et al.
,
1997]. Additional work has revealed that Søderberg smelters have twice the emission factor as
prebake smelters [ Utne et al.
, 1998]. Here we use emission factors of 0.6 and 1.2 kg COS per
ton of primary aluminum production for prebake and 0.6 kg Søderberg smelters, respectively.
We obtained annual aluminum activity data for primary aluminum production from global
industry statistics for the onset of production in year 1890 through 2012 [ Menzie et al.
, 2010].
Coal
Previous studies estimated global coal emissions using a single emission factor from a power plant in the western U.S. of 2.3 pmol COS per μmol CO
2
[Khalil and Rasmussen, 1984]. More
recent observations of three power plant plumes in the eastern U.S. find a larger emission factor of 6.0 ± 1.1 pmol COS per μmol CO
2
[ Blake et al.
, 2008]. We estimate a weighted average
emission factor for the U.S. of 5.5 pmol COS per μmol CO
2
based on the fraction of coal consumed in the western and eastern U.S. regions. Previous work has scaled COS coal emissions in time using coal SO
2
emissions [ Montzka et al.
, 2004], which is supported by more recent
airborne observations that show correlations between COS and SO
2
coal emissions [ Blake et al.
,
2008; Blake et al.
, 2004] and scrubber studies that show removal of both COS and SO
2
[ Blake et al.
, 2008;
Utne et al.
, 1998]. Here we follow this approach by scaling the emissions in space and
time using country-level SO
2
emission inventories [ Klimont et al.
, 2013;
Smith et al.
, 2011].

Biomass Burning
Previous estimates of biomass burning show a significant COS source [ Crutzen and Andreae ,
1990;
Montzka et al.
, 2007; Nguyen et al.
, 1995]. Nguyen et al. [1995] estimate a source of 70
Gg S yr
-1
and this estimate is applied in several subsequent budgets [ Kettle et al.
, 2002;
Watts ,
2000]. Montzka et al. [2007] estimate a source of 68 to 144 Gg S yr
-1 using an updated global
CO source, a broader range of COS to CO emission factors [ Andreae and Merlet , 2001;
Nguyen et al.
, 1995], and a broader range of combustion sources [ Andreae and Merlet , 2001]. Berry et al. [2013] assume a source from the high end of the previously reported range in order to test
model sensitivity to a high biomass burning estimate. Here we update these climatological estimates by incorporating new emission factors from airborne observations and histories of biomass burning rates. We estimate emissions for three categories of biomass burning including open burning, agriculture residue, and biofuels. While the open burning source is not entirely due to anthropogenic activities, it represents a small fraction of the total anthropogenic budget estimated here.
Our biomass burning estimates for these three categories are calculated as the products of the
COS-to-CO emission ratios and the global CO emissions. We use a range of COS-to-CO emission ratios and a range of global CO emissions from the literature which implicitly account for uncertainties in both the CO-to-C emission ratio and the C emissions.
Emission factors for open burning for COS to CO range from 8.5 10 -5 to 18 10 -5 mol COS/ mol
CO [ Andreae and Merlet , 2001; Blake et al.
, 2008; Blake et al.
, 2004;
Nguyen et al.
, 1995].
Estimates of global CO emissions from open burning for the year 2000 range from 337 to 467 Tg
CO yr -1
[ Duncan et al.
, 2003;
Lamarque et al.
, 2010; Mieville et al.
, 2010; van der Werf et al.
,
2010]. The product of the COS to CO emission factors and the CO open burning emissions
gives a range of COS open burning emissions of 15 to 44 Gg S yr -1 .
For the burning of agriculture residue we use an emission factor of 3.3 10
-4
mol COS / mol CO and a range of year 2000 global CO emissions from two alternative global inventories of 20 and

48 Tg CO yr
-1
[ Andreae and Merlet , 2001;
Lamarque et al.
, 2010; Smith et al.
, 2011]. The
product of the emission factor and CO inventory give a range of COS emissions from agriculture residue burning of 8 to 18 Gg S yr
-1
.
For the burning of biofuels we use a range of emission factors of 1.7 10 -4 to 4.0 10 -4 mol COS / mol CO and a range of year 2000 global CO emissions from biomass burning inventories of 187 and 240 Tg CO yr -1
[ Andreae and Merlet , 2001;
Smith et al.
, 2011]. This results in a range of
COS emissions from biofuels of 36 to 109 Gg S yr -1 . the three biomass burning sectors gives a total range of 60 to 172 Gg S yr
-1
. We estimate the time-varying emissions by scaling these year 2000 estimates in time using historical biomass burning histories for SO
2
emissions from open burning, agriculture residue, and biofuels [ Smith et al.
, 2011].
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