GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 13, 1715, doi:10.1029/2003GL016892, 2003
A universal bias in inorganic rainwater chemical composition data
Gregory P. Ayers, Robert W. Gillett, and Paul W. Selleck
CSIRO Atmospheric Research, PMB 1, Aspendale, Australia
Received 8 January 2003; revised 3 March 2003; accepted 28 April 2003; published 12 July 2003.
[1] In the late 1970s, it was recognised that organic acids
contributed to the acidity and ionic content of rainwater, but
that these acids had not been detected because they were
consumed biologically in the period between rainwater
collection and subsequent laboratory analysis. Discussion
of consequences for measured rainwater composition has
been limited to assessment of pH gain that attends organic
acid loss. We show that biological effects on rainwater
ionic composition are not restricted to pH alone.
Ammonium, potassium, nitrate, sulfate, methanesulfonate,
and phosphate ions are also removed biologically, but
remain in the rainwater in biomass, implying that most
previous rainwater composition studies based on ionic
analyses will have systematically underestimated nutrient
INDEX TERMS: 0365 Atmospheric Composition
deposition.
and Structure: Troposphere—composition and chemistry; 0368
Atmospheric Composition and Structure: Troposphere—
constituent transport and chemistry; 0320 Atmospheric
Composition and Structure: Cloud physics and chemistry.
Citation: Ayers, G. P., R. W. Gillett, and P. W. Selleck, A
universal bias in inorganic rainwater chemical composition data,
Geophys. Res. Lett., 30(13), 1715, doi:10.1029/2003GL016892,
2003.
1. Introduction
[2] By the early 1980s the pioneering work of Likens,
Galloway and co-workers [Galloway et al., 1982; Likens et
al., 1983] at rainwater measurement sites across the globe
had revealed that organic acids made a ubiquitous contribution to rainwater acidity and ionic composition. The
dominant rainwater acids identified were the simplest C1
and C2 acids, formic and acetic, with formic about twice as
prevalent as acetic [Keene and Galloway, 1986]. In unpolluted rain these two organic acids were found at levels
comparable with, sometimes exceeding, the levels of the
two dominant, naturally-occurring mineral acids, nitric and
sulfuric [Galloway et al., 1982; Likens et al., 1983; Keene
and Galloway, 1986]. However organic acids were found to
be labile, disappearing from collected rainwater samples
within a matter of a day or so, unless the rainwater was
rendered sterile with a biocide at the time of collection. The
organic acids disappear from rain as neutral molecules (i.e.
the acid anion and its companion hydrogen ion are removed
in stoichiometric balance), thus previous rainwater composition studies had not detected the loss of organic acids
through the usual quality assurance checks based on lack of
ion or conductivity balance. The key advance made by
Galloway and co-workers was that they split samples into
two aliquots at collection, leaving one aliquot untreated and
Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL016892
48
adding chloroform as a biocide to the other. They found that
‘‘preserved’’ samples had systematically higher levels of
free acidity (hydrogen ion) than untreated samples, but an
insufficiency of inorganic anions to provide the required
anion balance, leading to the discovery of the organic acids.
[3] The circumstantial evidence that bacterial processes
were responsible for the decay of organic acidity in unpreserved rainwater was confirmed quantitatively in 1987 in a
laboratory study in which bacterial cell counts and microbial
uptake of the acids were measured in natural rainwater
[Herlihy et al., 1987]. Maximum measured uptake rates of
17% and 14% per hour for formic and acetic acids, respectively, suggested daily loss rates of 44 and 24 mmol l 1, rates
consistent with the mounting evidence that fresh rainwater
typically contained from a few to perhaps 20 mmol l 1 of
these acids, and that the acids were lost within about a day or
two. The question arises therefore: if organic acids are
consumed biologically in rainwater at significant levels
and rates, what of other nutrient species important to
biomass growth?
2. Approach
2.1. Hypothesis
[4] There are no implications for ecosystem acidification via ‘‘acid rain’’ containing organic acids, precisely
because the acids are consumed biologically as neutral
molecules, so do not release a proton to the environment.
Thus, there has been no significant motivation for adoption of rainwater preservation procedures by national and
international acid rain programs in Europe, Asia or North
America. The implicit assumption seems to have been that
loss of organic acids after rainwater collection has no
environmentally significant effect upon inorganic ionic
composition.
[5] We suggest that this is not the case, arguing here
that with few exceptions [e.g., Karlsson et al., 2000] a
significant effect upon ionic concentrations of several
nutrient species in rainwater largely has been overlooked.
Our suggestion is that bacterial consumption of organic
acidity must also involve assimilation of other rainwater
nutrients such as nitrogen, phosphorous, potassium and
sulfur, which energetically would be most easily assimilated in the form of ionic species. The absence of such
species from the ionic composition of rainwater via their
storage in biomass would go undetected, leading to an
underestimation of total nutrient deposition based solely on
ionic composition data.
[6] Support for this hypothesis is evident in ionic composition data from twelve rainwater samples collected at
Niigata in Japan recently in a small study carried out to
demonstrate the effectiveness of thymol as a rainwater
biocide [Gillett and Ayers, 1991; Ayers et al., 1998]. In that
- 1
48 - 2
AYERS ET AL.: UNIVERSAL BIAS IN CHEMICAL COMPOSITION DATA
Table 1. Matrix of Sample Number and Additive Levels of
Specific Nutrients Used to Explore the Role of the Various
Nutrients in Promoting Microbial Decomposition of a Standard
Rainwater Sample
was at 0.3 mmol l 1, and methansulfonate at 0.1 mmol l 1.
A small amount of fluoride was also present (2.0 mmol l 1).
Sample
HCOO
NH+4
PO34
MSA
3. Results
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.0
30.0
30.0
30.0
0.0
0.0
15.0
15.0
15.0
30.0
30.0
0.0
0.0
0.0
0.0
30.0
0.0
11.0
10.0
1.0
11.0
11.0
11.0
10.0
1.0
12.0
10.5
10.0
10.0
0.0
0.0
0.0
0.0
1.0
0.0
1.0
1.0
1.0
1.0
0.0
1.0
2.0
0.5
0.0
0.0
0.0
0.0
0.0
0.0
2.1
2.1
2.1
2.1
10.5
2.1
2.1
2.1
2.1
2.1
2.1
10.5
2.1
10.5
0.0
[8] Comparison of ion concentrations in the 15 pairs of
samples other than Sample 1 showed a very clear pattern of
ionic loss in the sample in each pair not preserved with
thymol, while the preserved samples showed no loss of any
ions in comparison with the control. Most spectacular were
losses for formate and phosphate, which were lost completely
in all samples to which these ions were added. The other ions
showing discernible, but partial loss were hydrogen ion,
ammonium, potassium, methanesulfonate, sulfate and nitrate
(in three samples). The controlling variable proved to be
phosphate, as shown in Figure 1, which shows proportionate
losses of ammonium, potassium, and methanesulfonate plus
sulfate, when plotted against phosphate loss. The N:P, K:P
and S:P mole ratios reflected by the regression slopes shown
in Figure 1 are quite consistent with literature values for the
composition of microbial cells [Prescott et al., 2000]. These
results confirm our hypothesis that losses of nutrient species
in addition to hydrogen ion and simple organic acids must
occur if the latter species are lost by microbial growth
processes in rainwater.
[9] It is notable that the sulfur loss (S:P = 1.6) is made
up by a slightly greater loss of methanesulfonate (MSA)
than sulfate, with respective regressions having slope
values (i.e. S:P ratios) of 0.87 and 0.74, with R2 values
of 0.87 and 0.76. Given that the starting sulfate level was
21.2 mmol l 1, but the MSA level even in the spiked
samples was well below this (see Table 1), it is evident
that a considerably larger faction of available MSA than
sulfate was assimilated. This is consistent with the lower
energy penalty paid by an organism cleaving the C-S bond
in the MSA ion compared with cleavage of the S = O
bonds in the sulfate ion.
[10] A similar situation was apparent in the case of N
assimilation from ammonium ion vs nitrate ion. In the 15 pairs
of samples nitrate was consumed in only three cases, those
being the cases in which ammonium levels were insufficient to
meet the average N:P requirement of 11.9 deduced from
Figure 1. Figure 2 shows nitrate change plotted against
ammonium/phosphate ratio: nitrate was not consumed in
samples in which the more easily assimilated ammonium
ion was sufficient to meet the required N:P ratio. However,
under ammonium-limited conditions, assimilation of nitrate
occurred. The Figure also shows evidence of a small nitrate
production in three samples. These three samples were those
with all other nutrients added, but zero or low levels of formate
added, suggesting that in the absence of other nutrient-limitations, C-limited conditions become more favourable for
nitrifying microbes.
MSA is methanesulfonate.
work, rainwater samples were collected from individual rain
events, and were split into two aliquots immediately after
the event, with one sample left untreated while the other
was treated with thymol, as had been done earlier by
Galloway and colleagues using chloroform. Subsequent
chemical analysis of the treated and untreated Niigata rain
samples showed not only the complete loss in untreated
samples of organic acids, but also of phosphate. Losses of
hydrogen ion, ammonium and methanesulfonate were also
found, but were not complete, leading to the suggestion that
either phosphate or the organic acids were the limiting
nutrients [Ayers et al., 1998].
2.2. Experimental
[7] We have carried out a laboratory experiment under
more controlled conditions to evaluate quantitatively the
effect of bacterial consumption upon levels of ionic rainwater nutrient species. Wet-only rainwater event samples
were collected using clean techniques over the first week of
October 2000 at CSIRO’s laboratories at Aspendale, a
suburb of Melbourne, located 20 km southeast of the city
centre. A composite sample of two litres volume was left to
stand at room temperature for one week in a capped
polyethylene bottle to allow biological processes to proceed
to completion. The laboratory experiment involved taking
16 aliquots of 45 ml volume, and adding to each 5ml of
solution having varying concentrations of additional nutrient species, as outlined in Table 1. Immediately upon
addition of the nutrient solution, the sample was mixed
thoroughly by vigorous shaking, then split into two 25 ml
sub-samples, into one of which was added 15 mg of thymol
as biocide [Gillett and Ayers, 1991]. The samples were then
left for one week at room temperature prior to analysis using
standard methods [Ayers et al., 1998]. The basic ionic
composition of all the samples is reflected by Sample 1,
the control sample, to which had been added only 5 ml of
deionised water, without additional nutrients. Sample 1 had
pH of 5.01, and concentrations of sodium, potassium,
magnesium, calcium and ammonium of 229.4, 11.9, 27.2,
7.1 and 10.3 mmol l 1. Concentrations of chloride, nitrate
and sulfate were 272.2, 2.9 and 21.2 mmol l 1, while
phosphate and acetate levels were undetectable. Formate
4. Discussion
[11] The purpose of this work was not, however, to
elaborate on the dynamics of the biological food web that
clearly exists in rainwater, as evidenced by the observed
nuances of behaviour that result from variations in formate, phosphate, ammonium, or other nutrient availabili-
AYERS ET AL.: UNIVERSAL BIAS IN CHEMICAL COMPOSITION DATA
48 - 3
definitively that changes to a range of other species
involved in microbial nutrition do occur in concert with
organic acid consumption.
[12] The question then remains: is the effect of any
significance outside the laboratory? This is a difficult
question to address given the fact that phosphate is
apparently the controlling rainwater nutrient, yet reports
of phosphate levels in biologically preserved rainwater
samples are virtually absent from the literature. However
we are able to report on two datasets for rainwater
sampled directly into wet-only samplers that contained
the preservative thymol and include analysis for phosphate. One represents a relatively unpolluted rural region
at a mid-latitude continental site in Australia, at which the
atmospheric burden of pollutants is low and rainwater is
commensurately low in dissolved salts [Ayers et al.,
1995]. Mean rainwater composition data from four sites
is available, averaged over the two years 1993 and 1994.
The other dataset is from a single, heavily polluted,
urban-industrial site in the equatorial tropics of Malaysia
[Ayers et al., 2000]. For this site annual mean rainwater
composition data are available for the five years from
1993 to 1997.
[13] The left-hand side of Table 2 shows for each
dataset the mean rainwater ionic concentrations of phosphate, ammonium, potassium, methanesulfonate and sulfate. The right-hand side shows the percentage loss in
concentration that would result if all the phosphate were
consumed biologically, and other species listed were lost
according to the N:P, K:P and S:P ratios discussed above
(see Figure 1). In the case of clean, rural rainfall in
Australia, the calculation suggests very significant losses
Figure 1. Losses of ammonium, potassium and methanesulfonate (MSA) plus sulfate in rainwater samples as a
function of phosphate loss.
ties. The key focus here was to determine whether widespread chemical changes to ionic rainwater composition,
hitherto unidentified, occurred during the biological consumption of rainwater organic acids. Our work shows
Figure 2. Nitrate change in rainwater samples to which
nutrients were added, as a function of the initial ammonium/
phosphate ratio. The cluster of points with no change in
nitrate clustered at the origin of the plot is for those samples
with zero added phosphate (see Table 1). The remaining
solid points show the transition to nitrate consumption when
available ammonium/phosphate ratio is at or less than the
required value of 11.9 (see Figure 1). The open points
showing a small nitrate production are for three samples
having zero or low levels of added formate.
48 - 4
AYERS ET AL.: UNIVERSAL BIAS IN CHEMICAL COMPOSITION DATA
Table 2. Left Hand Side: Volume-Weighted Mean Ion Concentrations From Four Sites in NSW [Ayers et al., 1995], Over the Years
1993 – 1994, and Annually at Petaling Jaya (PJ) in Malaysia [Ayers et al., 2000] From 1993 – 1997
Rainwater concentration (mmol l-1)
Calculated fractional loss
phosphate
ammonium
potassium
MSA
sulfate
phosphate
ammonium
potassium
MSA
sulfate
NSW
Site 1
Site 2
Site 3
Site 4
0.33
0.37
0.53
0.93
11.7
11.3
10.4
10.4
0.93
0.79
1.1
1.1
0.18
0.19
0.11
0.2
7.8
3.3
3.8
5.8
100%
100%
100%
100%
34%
39%
61%
100%
28%
37%
38%
66%
100%
100%
100%
100%
3%
8%
10%
12%
PJ
1993
1994
1995
1996
1997
0.05
0.19
0.19
0.27
0.20
16.8
25.8
16.8
19.6
32.1
1.2
1.6
1.5
1.4
2.3
0.43
0.08
0.12
0.30
0.19
23.4
24.6
18.5
24.1
33.7
100%
100%
100%
100%
100%
4%
9%
13%
16%
7%
3%
9%
10%
15%
7%
10%
100%
100%
78%
92%
0%
1%
1%
1%
0%
Right hand side: prospective % losses for the various ions calculated assuming 100% biological loss of phosphate and N:P, K:P and S:P relative losses as
discussed in the text (see also Figure 1).
in the tens of percent to 100% range for ammonium,
potassium, and methansulfonate, and even an average loss
of about 10% for sulfate. In the case of urban-industriallypolluted rainfall in Malaysia in which the dissolved ionic
content of rain is significantly elevated, losses are fractionally smaller, from a few percent to around 15% for
ammonium and potassium, and only around 1% for
sulfate. However, the phosphate loss would still be
100%, as would methanesulfonate loss. Moreover, the
calculated losses in this case are probably a lower bound,
as the thymol levels in a significant subset of the
Malaysian samples were found to be below the levels
required for unequivocal prevention of biological action
[Gillett and Ayers, 1991].
5. Conclusions
[14] Overall, this work suggests strongly that microbial
processes in rainwater sampled for chemical analysis, but
not preserved from biological action, cause transformation
of significant fractions of available nutrients from inorganic form into organic form. These nutrients remain in
the rainwater, and will be available to ecosystems on
which the rain deposits, however they will not be detected
by the standard ionic analysis procedures used by the
majority of the global rainwater composition networks.
This effect therefore constitutes a universal bias in the vast
majority of available rainwater composition datasets that
have been employed to estimate wet deposition fluxes of
nutrients.
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