17. Deposition of Nutrients and Pollutants to Ecosystems
scapes (Lovett 1994; Lovett et al. 1997), and local
emission sources, such as farm operations (local
enhancement of NH3 deposition) or unpaved roads
(local enhancement of base cation deposition) (Hedin and Likens 1996). In addition, different methodologies sample over different spatial scales. For
example, a wet-deposition collector samples nutrient fluxes in rain and snow over a small (the diameter of the collector) and fixed location in a
landscape. In contrast, micrometeorological measures of trace gas flux integrate fluxes over a larger
area of a landscape (termed "footprint"); the exact
size and location of this footprint varies over time
as a function of local meteorological conditions.
Such effects must be considered during the selection of appropriate sites and methods for studying
atmospheric deposition. The aim is generally to
select locations that are representative of the surrounding landscape, and thus are subject to as few
local effects as possible. The aim is also to select
methods in which the sampling footprint is well
known.
Considerable information is needed to extrapolate from single locations to regional-scale patterns
of atmospheric deposition. Spatial deposition patterns are typically derived from networks of sampling stations, either based on relatively straightforward isopleth mapping routines or from more
complex geostatistical approaches that permit examination of underlying patterns of spatial variance. Such networks have been successful at describing regional patterns of wet deposition in
North America and Europe, including the National
Atmospheric Deposition Program/National Trends
Network (NADPINTN; over 200 stations in North
America; URL address: http://nadp.sws.uiuc.edu)
and the Cooperative Programme for the Monitoring and Evaluation of Long Range Air Pollutants
In Europe (EMEP; 106 stations in 25 European
countries). Much less is known, however, about
regional patterns of dry or cloud deposition,
mainly due to the inherent difficulties of estimating these vectors (e.g., Ollinger et al. 1993; Weathers et al. 1988).
Also temporal scales influence how we interpret
atmospheric deposition. While rare, long-term records from single sites or networks have been valuable for understanding changes in deposition at
scales of decades (e.g., Hedin et al. 1994; Likens
et al. 1990).
267
Wet Deposition
Wet deposition occurs as rain or snow, and is estimated by combining measures of the chemistry and
volume of individual precipitation events:
(17.1)
where D is wet deposition for event E (in kg ha ~ 1),
[X]E is concentration of element X in event E (in
mg liter ~ 1), and V E is the precipitation volume of
event E (in cm of precipitation depth). Wet deposition can then be integrated over time, as the sum
of individual deposition events over a period:
where Dtotal is the total wet deposition during the
period of events 1 to n.
Precipitation volumes are typically measured by
rain gauges that integrate precipitation depth over
time, such as the mechanical Belfort®-type gauge,
or the increasingly popular electronic tippingbucket gauges. Precipitation chemistry is generally
sampled by specialized collectors that minimize
any risks for contamination of chemically dilute
rain or snow. So-called "bulk precipitation collectors" are now rarely used, as they remain open at
all times and therefore are sensitive to dust contamination during periods between precipitation
events. Instead, most studies now employ so-called
"wet-only" collectors (Fig. 17.1), which use a
heated sensor to detect rain or snow events, and
automatically open only during these events.
It is critically important to collect precipitation
samples with minimal or no contamination from
the collector itself, from any vegetation near the
collector, or from the handling or shipping of water samples. The field collection vessel, and any
bottles that hold the precipitation sample, must be
composed of chemically inert materials (e.g., highdensity polyethylene or Teflon) and must be thoroughly cleaned by washing in dilute acid followed
by copious rinsing and leaching by ultrapure
deionized water. Sample bottles should be closed
tightly to prohibit any gaseous exchanges, refrigerated, and shipped to the analytical laboratory as
soon as possible. It may be necessary to add a
preservative (e.g., chloroform) to sample bottles to
stabilize solutes that are subject to bacterial up-
scapes (Lovett 1994; Lovett et al. 1997), and local
emission sources, such as farm operations (local
enhancement of NH3 deposition) or unpaved roads
(local enhancement of base cation deposition) (Hedin and Likens 1996). In addition, different methodologies sample over different spatial scales. For
example, a wet-deposition collector samples nutrient fluxes in rain and snow over a small (the diameter of the collector) and fixed location in a
landscape. In contrast, micrometeorological measures of trace gas flux integrate fluxes over a larger
area of a landscape (termed "footprint"); the exact
size and location of this footprint varies over time
as a function of local meteorological conditions.
Such effects must be considered during the selection of appropriate sites and methods for studying
atmospheric deposition. The aim is generally to
select locations that are representative of the surrounding landscape, and thus are subject to as few
local effects as possible. The aim is also to select
methods in which the sampling footprint is well
known.
Considerable information is needed to extrapolate from single locations to regional-scale patterns
of atmospheric deposition. Spatial deposition patterns are typically derived from networks of sampling stations, either based on relatively straightforward isopleth mapping routines or from more
complex geostatistical approaches that permit examination of underlying patterns of spatial variance. Such networks have been successful at describing regional patterns of wet deposition in
North America and Europe, including the National
Atmospheric Deposition Program/National Trends
Network (NADPINTN; over 200 stations in North
America; URL address: http://nadp.sws.uiuc.edu)
and the Cooperative Programme for the Monitoring and Evaluation of Long Range Air Pollutants
In Europe (EMEP; 106 stations in 25 European
countries). Much less is known, however, about
regional patterns of dry or cloud deposition,
mainly due to the inherent difficulties of estimating these vectors (e.g., Ollinger et al. 1993; Weathers et al. 1988).
Also temporal scales influence how we interpret
atmospheric deposition. While rare, long-term records from single sites or networks have been valuable for understanding changes in deposition at
scales of decades (e.g., Hedin et al. 1994; Likens
et al. 1990).
267
Wet Deposition
Wet deposition occurs as rain or snow, and is estimated by combining measures of the chemistry and
volume of individual precipitation events:
(17.1)
where D is wet deposition for event E (in kg ha ~ 1),
[X]E is concentration of element X in event E (in
mg liter ~ 1), and V E is the precipitation volume of
event E (in cm of precipitation depth). Wet deposition can then be integrated over time, as the sum
of individual deposition events over a period:
where Dtotal is the total wet deposition during the
period of events 1 to n.
Precipitation volumes are typically measured by
rain gauges that integrate precipitation depth over
time, such as the mechanical Belfort®-type gauge,
or the increasingly popular electronic tippingbucket gauges. Precipitation chemistry is generally
sampled by specialized collectors that minimize
any risks for contamination of chemically dilute
rain or snow. So-called "bulk precipitation collectors" are now rarely used, as they remain open at
all times and therefore are sensitive to dust contamination during periods between precipitation
events. Instead, most studies now employ so-called
"wet-only" collectors (Fig. 17.1), which use a
heated sensor to detect rain or snow events, and
automatically open only during these events.
It is critically important to collect precipitation
samples with minimal or no contamination from
the collector itself, from any vegetation near the
collector, or from the handling or shipping of water samples. The field collection vessel, and any
bottles that hold the precipitation sample, must be
composed of chemically inert materials (e.g., highdensity polyethylene or Teflon) and must be thoroughly cleaned by washing in dilute acid followed
by copious rinsing and leaching by ultrapure
deionized water. Sample bottles should be closed
tightly to prohibit any gaseous exchanges, refrigerated, and shipped to the analytical laboratory as
soon as possible. It may be necessary to add a
preservative (e.g., chloroform) to sample bottles to
stabilize solutes that are subject to bacterial up-