ARTICLES
PUBLISHED ONLINE: 15 DECEMBER 2014 | DOI: 10.1038/NGEO2324
Significant contribution of combustion-related
emissions to the atmospheric phosphorus budget
Rong Wang1,2,3*, Yves Balkanski1,3, Olivier Boucher4, Philippe Ciais1,3, Josep Peñuelas5,6 and Shu Tao2,3
Atmospheric phosphorus fertilizes plants and contributes to Earth’s biogeochemical phosphorus cycle. However, calculations
of the global budget of atmospheric phosphorus have been unbalanced, with global deposition exceeding estimated emissions
from dust and sea-salt transport, volcanic eruptions, biogenic sources and combustion of fossil fuels, biofuels and biomass,
the latter of which thought to contribute about 5% of total emissions. Here we use measurements of the phosphorus
content of various fuels and estimates of the partitioning of phosphorus during combustion to calculate phosphorus
emissions to the atmosphere from all combustion sources. We estimate combustion-related emissions of 1.8 Tg P yr−1 , which
represent over 50% of global atmospheric sources of phosphorus. Using these estimates in atmospheric transport model
simulations, we find that the total global emissions of atmospheric phosphorus (3.5 Tg P yr−1 ) translate to a depositional
sink of 2.7 Tg P yr−1 over land and 0.8 Tg P yr−1 over the oceans. The modelled spatial patterns of phosphorus deposition
agree with observations from globally distributed measurement stations, and indicate a near balance of the phosphorus
budget. Our finding suggests that the perturbation of the global phosphorus cycle by anthropogenic emissions is larger than
previously thought.
P
hosphorus (P) limits plant productivity and influences carbon
(C) storage and nitrogen (N) fixation in terrestrial and
aquatic ecosystems1,2 . For instance, by enhancing the marine
biological pump, a long-term increase of P deposition from dust
over the oceans is thought to have contributed to maintain the
CO2 content of the atmosphere lower during glaciations3 . Over
even longer time scales, a sustained increase of P input into
the oceans, exceeding 20% of the natural background weathering
from rocks during the mid-Cretaceous, is likely to have triggered
large-scale oceanic anoxic events4 . Since the beginning of the
Industrial Era, human-caused perturbation of the P cycle has
been listed as one of the ten critical ‘planetary boundaries’ of the
Earth system5 .
Application of P fertilizers from mined rocks is at present the
main anthropogenic driver perturbing the terrestrial P cycle2,6 ;
although this impact is limited to cultivated ecosystems, yet with
P applied in excess being transferred to freshwaters and oceans.
The second human-caused perturbation is the emission of P into
the atmosphere from the combustion of fuels and its subsequent
deposition. This deposition can increase primary productivity in
regions with P deficits, including inland lakes7 , ocean ecosystems8
and over P-limited tropical forests9 . Only a few studies have
attempted to estimate sources of atmospheric P (refs 10–12) and
large uncertainties remain, manifested by the poor agreement
between modelled and observed concentrations of P in atmospheric
aerosols12 . The total emission into the atmosphere of 1.39 Tg P yr−1
estimated in ref. 12 is less than 50% of the global deposition sink
(3–4 Tg P yr−1 ) interpolated from sparse in situ observations6,10,13 .
This lack of balance of the atmospheric P budget hampers our
understanding of the biogeochemical cycle of P.
We develop an inventory of P emissions from anthropogenic
and natural combustion sources covering 222 countries/territories
for the period 1960–2007. Unlike the previous estimate12 , our
inventory is based on the P content of fuels and on data showing
the partitioning of P into that released to the atmosphere and that
retained in combustion residues. The uncertainties are quantified
using a Monte Carlo approach in which the parameters of the
emission inventory model are varied randomly within plausible
bounds (Methods). The results of this inventory can be used to assess
the ecosystem-fertilizing effect of P deposition in aerosols.
Phosphorus content of fuels
The P content of fuels was collected for hard coal, lignite, petroleum,
municipal waste, biodiesel, dung cake, tree biomass, crop residues
and grass from 31 published studies (Supplementary Information).
These data included coal samples from 13 countries (Armenia,
Australia, Canada, China, France, India, Indonesia, New Zealand,
South Africa, Russia, Turkey, UK and USA), tree biomass from
11 major species (Abies balsamea, Acer rubrum, Betula papyrifera,
Picea glauca, Picea mariana, Pinus banksiana, Pinus contorta,
Populus tremuloides, Pseudotsuga menziesii, Thuja occidentalis and
Tsuga heterophylla) and crop residues from 9 species (bean,
cotton, maize, peanut, potato, rice, sugar, tobacco and wheat). The
frequency distributions of the P content of these fuels are shown
in Fig. 1. The reported P contents of different fuels are normally
distributed (Lilliefors test, p > 0.05), except for hard coal, which is
better fitted by a log-normal distribution (p > 0.1). The P content
compiled for the 225 hard-coal samples varies from 0.00044 to
0.77%, with arithmetic and geometric means of 0.036 and 0.016%,
respectively. This P content is, on average, lower than a previous
estimate (0.05%; ref. 14). The P content of hard coal produced by
each country was estimated, and the content of hard coal actually
burnt in each country was then calculated, taking into account the
origin of coal imports15 .
1 Laboratoire
des Sciences du Climat et de l’Environnement, CEA CNRS UVSQ, 91191 Gif-sur-Yvette, France. 2 Laboratory for Earth Surface Processes,
College of Urban and Environmental Sciences, Peking University, 100871 Beijing, China. 3 Sino-French Institute for Earth System Science, College of Urban
and Environmental Sciences, Peking University, 100871 Beijing, China. 4 Laboratoire de Météorologie Dynamique, IPSL/CNRS, Université Pierre et Marie
Curie, 75252 Paris Cedex 05, France. 5 CSIC, Global Ecology Unit CREAF-CEAB-UAB, Cerdanyola del Vallès, 08193 Catalonia, Spain. 6 CREAF, Cerdanyola
del Vallès, 08193 Catalonia, Spain. *e-mail: [email protected]
48
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NATURE GEOSCIENCE DOI: 10.1038/NGEO2324
a
1.2
n = 225
China
b
Mean = −1.79
s.d. = 0.57
USA
20
0.8
20
All
10
0.4
Mean = 0.079
s.d. = 0.076
Bark
c
4
n=9
Foliage
0.1
Mean = 0.150
s.d. = 0.063
3
20
2
4
10
1
2
0
0.3
0.2
e 10
Mean = 0.653
s.d. = 0.310
n = 77
0
0.0
0.1
0.2
6
6
0
f 6
n = 28
0.12
Mean = 7.46
s.d. = 4.20 0.09
4
6
0.3
P content in crop residues (%)
Mean = 0.213
s.d. = 0.091
8
1.0
4
4
0.5
2
2
0.03
2
0.5
1.0
0.0
1.5
0
0.0
P content in dung cake (%)
PDF
0.06
2
0
0.0
8
n = 23
30
P content in wood (%)
1.5
4
40
Branches
0
0.0
P content in hard coal, log (%)
Frequency of samples
n = 102
Stem
5
0.0
0
−3.5 −3.0 −2.5 −2.0 −1.5 −1.0 −0.5 0.0
d
82
15
Australia
10
25
PDF
Frequency of samples
30
ARTICLES
0.1
0.2
0.3
0.4
0
0.5
P content in grass (%)
0
−5
0
5
10
15
0.00
20
P content in biodiesel (PPM)
Figure 1 | Frequency distributions of phosphorus contents of different fuels. a, Hard coal. b, Tree biomass. c, Crop residues. d, Dung. e, Grass. f, Biodiesel.
The frequencies of samples are shown as red bars, with the curves for the probability density functions (PDFs) shown as black lines. The PDF curves are
also shown for three representative countries (USA, China and Australia) in a and four wood components (stem, bark, branches and foliage) in b. Sample
size (n), mean and standard deviation (s.d.) are indicated in each panel. P content is fitted with a log-normal distribution for coal (a), with the horizontal
axis displayed on a log scale, and with normal distributions for the other fuels.
Budget of phosphorus combustion products
On the basis of the P content for each type of fuel and the
consumption of fuel for each country, we estimated an average
global emission of 6.7 Tg P yr−1 (90% confidence interval from 2.1
to 14.4) for 1960–2007 from anthropogenic and natural combustion
sources. Only a fraction of these P products was emitted into the
atmosphere as fly ash (26%), with the rest either left as residual ash
(66%) or removed by control devices (8%) (Supplementary Table 1).
The ratio of C to P in emissions is an indicator of the relative impact
of combustion on the C and P cycles. The average C:P emission ratio
is 15,000:1, with a large variation by source (Supplementary Table 1),
depending on the content of C and P in fuels, as well as on the
fraction of P going into the fly ash. The spread between the highest
C:P ratio (20,000,000:1 in the combustion of residue fuel oil in power
plants) and the lowest ratio (1,000:1 in the domestic combustion of
dung cake) spans several orders of magnitude.
Figure 2 shows the historical trends of total P emissions from
all combustion sources, with details broken down for sectors and
regions. The total P emission has increased from 1.4 to 2.2 Tg P yr−1
over the past five decades. This emission increase equals 60% of the
total sources to the atmosphere from natural fires and dust input
(1.3 Tg P yr−1 ) in ref. 12. Most of the increase in combustion-related
emissions is attributed to Asia, where emissions from fossil fuel
burning have increased rapidly by 2.5% per year (Supplementary
Fig. 1). Today, this region contributes 43% of the global total
emissions. A fraction of P emitted from East Asian sources is
transported to the North Pacific, and can reach the Arctic after being
transported by mid-latitude westerlies16 . By contrast, P emissions
from fossil fuel combustion began to decline in Europe and North
America in 1990 (Supplementary Fig. 1) and P emissions from
wildfires and fossil fuels have remained rather stable over time
in Africa, South America and Oceania. The spatial distributions
of P emissions from combustion during 1960–2007 are shown in
Supplementary Fig. 2. This illustrates that the emission centres
are moving from Europe to East Asia and South Asia, implying a
significant biological impact regionally.
Comparison with previous studies
Our estimates of P emissions from combustion sources are higher
than the earlier commonly used values found for year 199612 ,
which were 0.024, 0.021 and 0.025 Tg P yr−1 , for coal, biofuels and
biomass burning in the field, respectively. We found values of 0.55,
0.54 and 0.96 Tg P yr−1 for the same sources and year. Ref. 12
derived the fossil fuel emissions from data on the P content of fine
particulate matter, and scaled up emissions from the P content in
particulate matter and particulate matter emission factors. However,
P is volatile in combustion (unlike, for example, calcium)17,18 , and
there is gaseous P at the emission sources, in particular in the
form of phosphorus pentoxide or phosphorus trioxide when oxygen
is limiting19 , which may condenses on particles after entering the
atmosphere. Thus, the emission of P could be underestimated if P in
the gaseous phase, which has been evidenced in few measurement
studies20 , is not accounted for. There were only three peer-reviewed
studies (refs 21–23) available when ref. 12 estimated the P content in
particulate matter from the combustion of coal and, unfortunately,
the contribution of P emitted in the gas phase was not assessed
in these studies. For the burning of biofuels and biomass, ref. 12
assumed a globally uniform ratio of P to black carbon, derived
from the slope of P and black-carbon concentrations observed
over the Amazon, and assumed that P was associated with black
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49
NATURE GEOSCIENCE DOI: 10.1038/NGEO2324
ARTICLES
1.6
Table 1 | Global budget of phosphorus in the atmosphere.
Deposition sinks
Total sinks from the model
Over land
Over the oceans
Ref. 10
Ref. 13
1.8 (0.5–4.4)
1.1 (0.3–3.1)
0.7 (0.2–1.3)
0.93 (0.23–2.1)
0.58 (0.16–1.0)
0.006 (0.003–0.009)
0.16 (0.0049–0.33)
0.00020 (0.000038–0.00036)
3.5 (0.9–7.8)
2.7 (0.7–6.2)
0.8 (0.2–1.6)
4.5
3.7
All uncertainties are given as the 90% confidence intervals in brackets. P emissions from
combustion and the uncertainties are derived from the Monte Carlo simulation in this study.
carbon (refs 11,12). This assumption, however, is not fully justified
because the atmospheric lifetime of P concentrated in coarse fly
ash is probably shorter than that of fine black-carbon particles. In
addition, ref. 12 focused on P in particulate matter with sizes less
than 10 µm; thus P emitted in larger particles was not included.
Alternatively, all the P that can be transported to deposition
measurement stations is included in our P budget, although our
simulations have accounted for the fact that larger particles are not
transported as far as smaller particles.
In addition, another study (ref. 14) used a approach similar
to ours, and estimated a P emission of 0.07 Tg P yr−1 from the
combustion of fossil fuels (mainly coal) for year 1967. By our
estimation, P emissions from fossil fuel combustion today amounts
to 0.62 Tg P yr−1 . Ref. 14 assumed, however, that 10% of all trace
elements in fossil fuels were emitted into the atmosphere, without
accounting for the volatilized P in the source. In fact, the authors
suggested that if the element was volatile, the emission should be
higher14 , which, together with higher emissions today than in the
1960s, is consistent with our estimate.
Our estimates are based on the budget of P during the
combustion process, but the mechanisms of P emission are still
elusive. The existence of substantial volatile P emissions should be
readily testable by future field measurements. Clearly, more studies
should be conducted to address the combustion emissions of the
different phases of P in power plants and domestic stoves, with direct
measurements of P in the gas phase, to elucidate the contribution by
combustion sources.
Global budget of phosphorus in the atmosphere
We estimated the global total emission sources and deposition sinks
of P in the atmosphere with the uncertainties (Table 1). With our
new estimate of combustion-related sources completed by noncombustion sources (mineral dust input, primary biogenic aerosol
particles (PBAP), volcanoes, sea salt and phosphine; Methods),
we derived a global total P emission of 3.5 Tg P yr−1 with a 90%
confidence interval of 0.87–7.8 Tg P yr−1 . Thus, the anthropogenic
sources from fossil fuels, biofuels and deforestation contribute 13%,
13% and 5% of the total P emission, respectively. Note that refs 24,25
suggested that in some regions dust emission is increasing as a result
of human-induced land-use change. Therefore, we expect that the
50
Emission of P (Tg P yr−1)
Fluxes, Tg P yr−1
Sources
Combustion (1960–2007)
Anthropogenic (including
deforestation fires)
Natural
Mineral dust input
PBAP
Volcanoes
Sea salt
Phosphine from freshwater
wetlands and rice paddies
Total sources
Global fossil fuels +
biofuels +
deforestation fires
Global natural fires
1.4
1.2
1.0
Asia
0.8
Africa
Europe
0.6
South America
0.4
Oceania
0.2
North America
0.0
1960
1970
1980
1990
2000
2010
Year
Figure 2 | Historical trends of phosphorus emissions from fossil fuels,
biofuels, deforestation fires and natural fires. Total emissions of P from all
combustion sources for each region are indicated by different symbols. The
activity data of fire-identified deforestation were taken from the Global Fire
Emissions Database version 3 data set (ref. 32) for the period from 1997 to
2007, and interpolated to the period from 1960 to 1996 using the data of
fires in the RETRO (REanalysis of the TROpospheric chemical composition
over the past 40 years) data set (ref. 33).
anthropogenic contribution of the total P emissions (31%, including
fossil fuels, biofuels and deforestation) presented here—far higher
than the 5% previously reported12 —could be simply a lower bound.
Similarly, the contribution by natural fires is 20% in our estimate,
higher than the 2% in ref. 12.
Based on a three-dimensional atmospheric transport model
(Methods), we simulated a total P deposition sink of 2.7 (90%
confidence interval from 0.7 to 6.2) and 0.8 (0.2 to 1.6) Tg P yr−1 ,
respectively, over land and the oceans. The uncertainty of modelled
P deposition was quantified by accounting for the uncertainties in
emission sources and size distributions. Our predicted global total
deposition sink of P is close to that reported in refs 10,13 (Table 1).
However, these studies estimated the sink using observations which
are sparsely distributed in space. To further evaluate the model,
we compared the predicted P deposition rates against observations
in ref. 13 after excluding several outliers and including more data
in Asia (Supplementary Information and Table 2). Figure 3 shows
the modelled and observed spatial patterns of P deposition, as
well as that from individual sources. High-P deposition rates are
observed at stations in Western and Central Europe, eastern USA
and the savannas of Central Africa, which are far away from
non-combustion sources. Globally, 73% of the stations located in
top 50th percentile of P deposition measurements are associated
with a contribution from combustion larger than 60% (Fig. 3f).
The estimated P emissions from combustion sources in ref. 12
account for only 3% relative to our new estimates. It is therefore
sound to compare modelled and observed P deposition rates with
or without including our new estimates of P emissions from
combustion sources (Fig. 4). At most stations, the model better
predicts the observed P deposition rate when combustion sources
are included. The geometric mean of P deposition rate observed
at all stations is 0.029 g P m−2 yr−1 , compared with the modelled
0.013 g P m−2 yr−1 (90% confidence interval from 0.004 to 0.030)
or 0.003 (0.001–0.006) g P m−2 yr−1 with or without including
combustion sources, respectively. In particular, P in particulate
matter with sizes larger than 10 µm contributes 35% to total P
deposited over all measurement stations. The largest improvement
can be found in Asia and Europe, where the ratio of observed
and modelled P deposition rates as a geometric mean is reduced
from 8.1 and 7.3 to 1.1 and 1.3, respectively. Therefore, our
modelling result confirms the improvement induced by a higher
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a
ARTICLES
b
Modelled P dep. (mg m−2 yr−1)
Observed P dep. (mg m−2 yr−1)
P dep. (Tg yr−1)
Total: 3.5
Land: 2.7
Ocean: 0.8
0.1
c
0.5
1
5
10
20
40
60
0.1
100
d
P dep. from combustion (mg m−2 yr−1)
e
0.5
1
5
10
20
40
60
100
60
100
80
90
P dep. from BGV (mg m−2 yr−1)
P dep. (Tg yr−1)
Total: 0.6
Land: 0.4
Ocean: 0.2
P dep. (Tg yr−1)
Total: 1.8
Land: 1.6
Ocean: 0.2
0.1
0.5
1
5
10
20
40
60
0.1
100
0.5
f
P dep. from dust and sea salt (mg m−2 yr−1)
1
5
10
20
40
Fraction of combustion (%)
P dep. (Tg yr−1)
Total: 1.1
Land: 0.7
Ocean: 0.4
0.1
0.5
1
5
10
20
40
60
100
10
20
30
40
50
60
70
Figure 3 | Spatial distributions of phosphorus deposition. a, Modelled total deposition of P. b, Observed deposition of P at 121 stations. c–e, Modelled
deposition of P from combustion (c), primary biogenic aerosol particles and volcanoes (BGV) (d), mineral dust sources and sea salt (e). f, Fraction of
P deposition from combustion sources. Combustion sources are for the 1960–2007 average. Dust and sea salt are for the 2000–2011 average. BGV are for
year 2005. The average deposition rates of P over land and the oceans are listed in a, c–e.
estimate of P emissions from combustion, to approach atmospheric
budget balance. Furthermore, it lends credence to these independent
observational estimates of the global P sink10,13 . However, there is
still underestimation of modelled P deposition at some stations
in Fig. 4a, which may confirm rather than disprove the sizeable
contribution from combustion sources. In addition to uncertainties
associated with the atmospheric emissions and transport of P,
sample contamination is another source of error. For example,
the modelled deposition rate is one tenth of the observed rate
at a cropland station in eastern USA, possibly contaminated by
local pollen and vegetation materials26 . However, contamination
should not influence deposition rates averaged over large areas, as
many studies have made efforts to minimize contamination13 . For
instance, contaminated data were removed from the observational
data set in ref. 13 according to the content of nitrogen and
potassium. A global observational network monitoring P deposition
in contrasted environments with additional information on the
chemical speciation of P (that is, ratio of organic and inorganic P)
would help to better understand the sources of atmospheric
P deposition.
N:P emission ratios
An imbalance of the N:P ratio in material emitted by human
activities, transported in the atmosphere and recorded in
atmospheric deposition is expected to impact the structure,
functioning and diversity of terrestrial and aquatic ecosystems1 .
Our upward revision of P emissions from combustion implies
that the input of P to the atmosphere from human activities has
increased over time (Fig. 5a). In parallel, global N emissions also
increased strongly before 199027 . The increase of N emissions
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Modelled deposition (mg m−2 yr−1)
b
With combustion
1,000
Asia (1.1)
Africa (2.2)
Europe (1.3)
North America (3.9)
Other (6.6)
100
10
1
1
10
100
Without combustion
1,000
Modelled deposition (mg m−2 yr−1)
a
Asia (8.1)
Africa (8.7)
Europe (7.3)
North America (9.8)
Other (18.4)
100
10
1
1,000
Observed deposition (mg m−2 yr−1)
1
10
100
1,000
Observed deposition (mg m−2 yr−1)
Figure 4 | Comparison between modelled and observed phosphorus deposition with or without accounting for phosphorus emissions from combustion.
a, Deposition accounting for combustion. b, Deposition not accounting for combustion. The error bars are derived by using the 90% confidence intervals of
emissions from different sources as well as applying different size distributions for P in combustion. Numbers in brackets represent the ratio of modelled
and observed P deposition rates as a geometric mean at all stations in each region. The solid, dashed, and dotted lines show the 1:1, 1:2 and 2:1, and 1:5 and
5:1 lines, respectively.
N
60
P
4
3
40
All combustion sources
2
20
1
Anthropogenic
combustion sources
1970
1980
1990
2000
0
50
200
Europe
North America
45
160
120
40
Global
80
35
South America
30
25
1960
Asia
Oceania
Africa
1970
Year
1980
1990
2000
40
Regional N:P emission ratios
5
0
1960
b
80
Nitrogen emission (Tg N yr−1)
P emissions (Tg P yr−1)
6
Global N:P emission ratios
a
0
Year
Figure 5 | Historical evolution of atmospheric sources of phosphorus and nitrogen. a, Atmospheric emissions of total P (black line) and N (red line) from
all sources. P emissions from all combustion sources (grey line) and anthropogenic combustion sources (dotted line) are also shown. Emissions of P from
combustion sources are from the median estimates of Monte Carlo simulation. Emissions of N are from ref. 46 for natural lightning and the MACCity
inventory27,32,47 for all other sources. b, Variations in the global (left vertical axis) and regional (right vertical axis) N:P emission ratios (molar basis). The
trends are shown for the globe (thick black line) and six different regions (thin coloured lines). The global N emissions from lightning (ref. 46) are allocated
to each region based on the spatial distribution of NOx emissions from lightning (ref. 48).
in Asia was partly offset after 1990 by their decline in Europe
and North America28 . Furthermore, the emissions of ammonium
decreased in India in the 1990s (Supplementary Fig. 3) owing to
a reduction in livestock numbers27 . The stalled N emissions after
1990 led to a stabilization of N:P emission ratios (Fig. 5b). From
1980 in Europe and North America, the use of low-P fuels such
as oil and gas has resulted in regional N:P emission ratios being
as high as 200:1. N:P emission ratios increased before 1990 for all
regions except Africa and Oceania and then stabilized after 2000
over most regions. Note that the consumption of coal in developing
countries may go down in the coming decade, as a result of policies
targeted at improving air quality29 . The input of P to the atmosphere
will thereby be reduced. The future trends of N:P ratios are thus
fairly uncertain.
Implications for the global phosphorous budget
The major finding of our study is that combustion is an important
source of P in the atmosphere owing to a high content of P in
coal and biomass. According to our estimation, the combustion
sources account for more than half of the total emissions of P to the
52
atmosphere. By running a global atmospheric transport model, it is
found that observed deposition of P on the ground can be captured
well by the model with our new inventory of combustion sources
considered. As a result, the global budget of P in the atmosphere is
better balanced with uncertainties in the sources and sinks quantified. Ref. 30 estimated that the fertilizing effects of aerosols on
marine and terrestrial productivity is responsible for a reduction
of the increase of CO2 above pre-industrial levels by 7–50 ppm in
the atmosphere, which translates into a negative climate forcing of
−0.5 ± 0.4 W m−2 . To reach this estimate, the author assumed that
half of the additional carbon uptake by the Amazon is attributed to
the fertilizing effect of increased P deposition due to deforestation
fires, which alone contributes to a negative but uncertain climate
forcing in the range 0 to –0.12 W m−2 . In this study, we significantly revise upwards P emissions from fossil fuels (0.46 Tg P yr−1 )
and biofuels (0.47 Tg P yr−1 ), which are altogether a factor of five
higher than P emissions from deforestation fires (0.17 Tg P yr−1 ).
This suggests a larger anthropogenic perturbation of the P cycle than
thought, with more ecosystems being potentially fertilized today by
increased P deposition, especially tropical and subtropical forests in
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Asia and Africa. Therefore, the present-day effect of aerosol-borne P
on the carbon cycle may be even larger than previously estimated in
ref. 30. Also, the amount of P needed by terrestrial plants to sustain
increased carbon storage seems to exceed the expected supply from
soil mineralization under all scenarios by the end of this century1 .
Policies aiming to reduce the emissions of combustion aerosols
represent a win–win option for improving air quality while reducing
black-carbon warming29 , but evaluation of their net climate effects
should also account for their impacts on the terrestrial and oceanic
sinks of CO2 .
Methods
Phosphorus emissions from combustion. Emissions of P from 1960 to 2007
were estimated for 64 types of fuel defined in the global fuel-consumption
inventory PKU-FUEL (ref. 31; available at
http://inventory.pku.edu.cn/home.html), covering fossil fuels, biofuels and
biomass burning in fires32,33 . Based on the mass balance of P in the combustion,
the emission (E) of P to the atmosphere was calculated from the P content of
each type of fuel and the partitioning of P into the fly ash for each combustion
process, using the equation:
E=
64
X
x=1

Ax bx cx
n
X

Fx,y (1 − rx,y )(1 − Mx,y )
(1)
y=1
where x represents a fuel type in PKU-FUEL, y represents a specific combustion
type, n is the number of combustion types, Ax is the consumption of fuel, bx is
the fraction of fuel burned (combustion rate), cx is the P content of the fuel, Fx,y
is the fraction of combustion type y in the combustion process, rx,y is the fraction
of P retained in the residual ash and Mx,y is the removal rate specified for a given
combustion type. The fuel-consumption data from 1960 to 2007 for 222
countries/territories were compiled in refs 15,31,34–36. Determination of the
combustion rate (bx ), fraction of P retained in the residual ash (rx,y ), P content of
fuel (cx ), fraction attributed to each source (Fx,y ), removal rate (Mx,y ) and spatial
allocation of P emissions from different combustion sources are described in the
Supplementary Information.
Uncertainty analysis. A Monte Carlo ensemble simulation was run 1,000 times
to quantify the variability of the input parameters. Normal distributions were
fitted for the P content of all fuels except for coal (log-normally distributed), with
the standard deviation (s.d.) derived from all measured data (geometric s.d. for
coal; Fig. 1). The fraction of P retained in the residual ash was assumed to be
uniformly distributed with the ranges listed in the Supplementary Information.
The relative s.d. of combustion rate was set at 20%, with a uniform distribution.
The uncertainties in fuel-consumption data were assessed by assuming uniform
distributions with fixed relative s.d. (Supplementary Information).
Phosphorus emissions from non-combustion sources. A detailed description of
methods to estimate P emissions from non-combustion sources is presented in
the Supplementary Information. In brief, P emission from mineral dust was
derived from the global transport model LMDZ-INCA (ref. 37) run from 2000 to
2011, with a constant P content in dust (0.072%; ref. 12). For PBAP, we
considered the estimate of P emissions from PBAP (ref. 12) and the spread of
PBAP emissions12,38 . For P emission from volcanoes, we considered the estimate
in ref. 12 and the variations of volcanic sulphur emissions39 and fraction of P
converted to aerosols40,41 . For P emission from sea salt, we took the average of
two estimates10,12 . P emission as phosphine was estimated by the phosphine
emission rates over freshwater wetlands42,43 or rice paddies42,44 and the
corresponding global areas45 .
A model of the phosphorus atmospheric cycle. A global transport model
LMDZ-INCA (ref. 37), at a horizontal resolution of 0.94◦ latitude by 1.28◦
longitude with 39 vertical layers spanning from the surface to 4.3 Pa, was used to
simulate the atmospheric transport and deposition of P. A detailed description of
the model is provided in the Supplementary Information. We analysed 12 years of
model simulations from 2000 to 2011 to derive P from mineral dust and sea salt
with a constant P content of 720 ppm in dust12 and 6 ppm in sea salt according to
the total P emission from sea salt in Table 1. For P emissions from combustion,
PBAP and volcanoes, we ran the model for year 2005. The 0.5◦ × 0.5◦ gridded
combustion P emission was generated as an average for 1960–2007 and used as
an input to the model. The simulated P deposition rate at a station from any year
between 1960 and 2007 was obtained by scaling the modelled P deposition rate
from combustion by the ratio of the national P emission in the year, considered to
the 1960–2007 average for the country where the station is located. P emissions
from PBAP, volcanoes, dust and sea salt were assumed to remain constant in time.
In the model, P emissions from PBAP and volcanoes were treated like dust (that
is, as a coarse-mode aerosol), whereas P emission from combustion was divided
into one fine-mode and two coarse-mode aerosols. We applied two different
fractions of P from combustion in different modes (Supplementary Information),
and the variation of simulated P deposition rate was derived as the uncertainty.
In addition, uncertainty in the simulated P deposition rate was assumed to be
proportional to that in the corresponding emission sources (Table 1). Finally, the
uncertainties in the size distributions and emission sources were added in
quadrature. As the fate of phosphine is not very clear and its contribution is not
significant (0.005% in total emissions), this source was not included in the model.
Received 14 May 2014; accepted 13 November 2014;
published online 15 December 2014
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Acknowledgements
The authors thank Ether/ECCAD for distribution of emission data used in this study. We
thank B. G. Li, F. Zhou, W. M. Hao, Y. Ying and M. McGrath for discussions, and J. Gash
for editing the English. R.W. was supported by the ‘FABIO’ project, a Marie Curie
International Incoming Fellowship from the European Commission. This work was also
conducted as part of the ‘IMBALANCE-P’ project of the European Research Council
(ERC-2013-SyG-610028). S.T. was supported by the National Nature Science Foundation
of China (41390240, 41130754) and the 111 Program (B14001). Some of the
computations were performed using HPC resources from GENCI-TGCC (grant
2014-t2014012201).
Author contributions
R.W. designed the research, performed all calculations and analysed the uncertainties. All
authors took part in interpreting the results and writing the paper.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to R.W.
Competing financial interests
The authors declare no competing financial interests.
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