Lecture 12: Atmospheric N pollution Flashcards
(26 cards)
Main anthropogenic sources of N in the atmosphere
Nitrous oxide from industrial combustion and ammonia (NH3) often from poultry farms
Nitrates can be used as fertiliser
Atmospheric pollution: N decomposition
- The pollutants that contribute to nitrogen deposition derive mainly from nitrogen
oxides (NOX) and ammonia (NH3) emissions. - In the atmosphere NOX is transformed to a range of secondary pollutants, including
nitric acid (HNO3), nitrates (NO3- ) and organic compounds, such as peroxyacetyl
nitrate (PAN), while NH3 is transformed to ammonium (NH4+). - Both the primary and secondary pollutants may be removed by wet deposition
(scavenging of gases and aerosols by precipitation) and by dry deposition (direct
turbulent deposition of gases and aerosols) (Fowler et al. 1989, Hornung et al. 1995)
(see earlier lecture). - Nitrogen deposition refers to the pollutant dose that may lead to nitrogen
eutrophication.
It should be noted that other nitrogen compounds occur in the atmosphere (N 2,
N2O), but these are not readily available for use by plants, and are therefore not
included in the assessment of nitrogen deposition
The layers of the atmosphere around the Earth
See figure in notes:
Exosphere 400 km altitude and above
Thermosphere 300 km
Mesosphere 50 km
Stratosphere 40 km
Troposphere 10 km
We will be concerned with the lower two layers in the coming three lectures, and
in the main, the lower layer – the troposphere – into which pollutants are emitted from the earth’s surface.
The atmosphere acts as a major channel for the transfer of pollutants from one place
to another, so that some harmful substances have been transported long distances from their points of emission
Historic atmospheric pollutant deposition:
sulphur (as sulphate) and nitrogen (as nitrate) deposition in Greenland:
see figures in notes
^historical deposition with increase during and after wartime now in decline as we have switched to nitrogenous oil and gas
^ Similar effects are seen in snow sulphate content, rising to the late 20th Century, plus the rather later nitrate increase in the latter half of the 20th Century (from the burning of sulphur-rich coal and the later transition to nitrogenous oil and gas prevalent today).
Note: the lowering of both nitrate and sulphate in recent years – see later…
Sources of N pollution
The majority of NOx - from road transport
and power stations but also Industry and Other (see figure in notes)
The majority of SO2 comes from power stations
The acidity status of precipitation is a result of a balance between acidifying compounds (oxides of sulphur and nitrogen) and alkaline compounds (ammonia and alkaline material in windblown soil dust).
Atmospheric nitrogen pathway:
See figure
^ Sources in red
Purple for transport/transform
Blue in removal – wet or dry deposition
Sinks in black
see also figure for nitrogen cycle recap
^ nitrogen fixed by soil bacteria and nitrogen fixing bacteria in legume roots
^ nitrites can then be used by plants for growth
^nitrogen fixing bacteria can reduce the need for fertiliser
The orographic effect can be a very important one in delivery of pollutants to uplands:
1 Cloud water droplets absorb gaseous pollutants from air currents. These clouds are transported by wind forming orographic clouds form over mountaintops
2 Precipitation enriched in pollutant
content falls on sensitive rainfall fed environments that are naturally
3 The excess nitrogen can disrupt these environments
(see figure in notes)
Factors determining the deposition of atmospheric pollutants
See figure in notes
^bedrock directly determines the impact of pollution
Limestone can buffer acids whereas granite cannot negate as much acid so graphite bedrock environments are more impacted by acid rain
see also:
https://www.fs.usda.gov/psw/publications/fenn/psw_2019_fenn002_du.pdf
^ Taller fume chimneys on factories removed pollution problems over britain this pollution now travelled over the north sea and began falling as acid rain in Norway and Scandinavia which are already acidic systems causing ecosystem disruption. You can see these high levels on the map.
^ This can inhibit tree growth
Communities most at risk from N eutrophication
are those inherently nutrient-poor communities rich in bryophytes and where species richness is comprised of slow-growing species.
e.g. Sphagnum spp. (‘bog mosses’) – no cuticle – exposed directly to ions in rainwater. No barrier to uptake.
^ Highly susceptible to acid rain/pollution damage
Some plant species particularly among the lower plants Sphagnum spp. And some lichens are characterised by large surface area to mass and wet acidic surfaces, which may make them especially susceptible to ammonia deposition
Increased N deposition – perturbs the N cycle
Denitrification reduces nitrates to nitrogen gas, thus replenishing the atmosphere. Once again, bacteria are the agents.
They live deep in soil and in aquatic sediments where conditions are anaerobic.
They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration. Thus they close the nitrogen cycle.
However, are denitrifiers able to keep up with increased N loading?
Production of N fertilisers for use in agriculture is now responsible for ca. 50% of all N fixation on earth – leading to N-enrichment of ecosystems. Massive unnatural addition of N into the environment and into the atmosphere
Nitrogen pollution/ acidification
Mosses are among the most sensitive components of the vegetation with respect to pollutant deposition, and can be sensitive to both acidity and N, which dominate today’s anthropogenic deposition. Because they have no waxy cuticle to protect them – membrane integrity is damaged and they can die
Like lichens, many mosses have become extinct from urban/ industrial environments, e.g. in the Lower Tyne valley.
Too much N can change morphology, often leading to sparser mats that are desiccation prone and less efficient at suppressing competitors.
Photosynthesis can be compromised along with membrane integrity and sexual reproduction may also be suppressed.
Mosses play an important role in nutrient cycling;
immobilization of N in some habitats, e.g. bogs and heathlands, effectively traps reactive N deposition preventing it from leaching into the pore water, making it unavailable to the roots of higher plants.
However, the ability to sequester N depends on the N load and systems can soon be saturated (see later).
Some mosses including those growing on bogs, Sphagnum spp., have evolved ‘liaisons’ with N fixing microbes to supplement their N supply, reciprocating through the provision of carbon.
In pristine environments where N deposition is very low, N fixation is a key source of N (NH4+).
Studies have identified both N sensitive and N tolerant mosses.
Studies around ammonia impacted woodland have found Eurynchium praelongum and Brachythecium rutabulum tolerate very high tissue N concentrations at least to 4% (Leith et al. 2005).
Pleurozium schreberi has been shown to be N sensitive in several studies (Solga et al. 2005; Sheppard et al. 2014) and appears to have a threshold N concentration, benefiting from modest inputs that do not raise the N concentration above the threshold.
Species within the important peatland genus Sphagnum show a range of tolerances to N; pool species appear to be the most tolerant, probably reflecting the lower ionic concentrations in these wet environments, whereas hummock formers are the most N sensitive.
In situ N manipulation studies on S. capillifolium indicate that wet N doses above 24 kg N ha-1y-1 can damage this species and reduce its cover.
Key points
1.The pollutants that contribute to nitrogen deposition derive mainly from nitrogen oxides
(NOX) and ammonia (NH3) emissions.
2.Nitrogen deposition refers to the pollutant dose that may lead to nitrogen eutrophication.
3.Wet, dry and occult (mists/fogs) deposition are all important methods of deposition of N
from the atmosphere to vegetation and soils.
4.Surface geology and landforms are key determinants of type and amount of deposition.
5.Moss-dominated upland mires and lowland fens are particularly sensitive to atmospheric
pollutant deposition.
6.Critical loads and levels of pollutant deposition are being developed to try to combat the
degradation of ecosystems.
Measurable aspects
See figure in notes
Animals remove nutrients from one area and deposit them in another
Humans cut back grass removing plant material and nutrients with them
See also N deposition impacts:
https://doi.org/10.1016/j.scitotenv.2017.08.307
and figure in notes providing an example of vegetation composition change over time due to N load
^ species composition change as you move towards city environments (higher nitrogen environments)
^pollution index
The concept of critical and total loads of a pollutant
Impacts of eutrophication in terrestrial ecosystems are associated with changes in floristic composition and in ecosystem function and stability.
Critical load is a term used to denote atmospheric loads of particular air pollutants. It indicates the amount of a given substance per defined unit of area and time which can be introduced into an ecosystem without bringing about environmental damage in the long-term, according to present knowledge.
See: J. C. I. Kuylenstierna et al, Environmental Pollution Volume 102, Issue 1, Supplement 1,
As scientists we identify an estimate for the critical load and then hand this figure over to policy makers to regulate for environmental safety.
So what’s all the fuss about
Critical loads are best viewed as highly uncertain estimates of relative risk, which themselves incorporate political choices, rather than precise damage thresholds determined by an objective, scientific process – even though much research is highly objective and peer-reviewed.
European policy makers proposed that critical loads for acid deposition should not be exceeded anywhere in Europe by the year 2015. (not achieved!!!)
A target load is the amount of pollution that is deemed achievable and politically acceptable when other factors (such as ethics, scientific uncertainties, and social and economic effects) are balanced with environmental considerations.
See: Species change points showing 5% and 95% bootstrap percentiles; symbols are sized in proportion to z score. *Species that also show a change point at the equivalent position on the precipitation gradient.
^ https://doi.org/10.1073/pnas.1214299109
see also:
Community change for species reduced in abundance [sum (z−)] showing critical load, inferred community threshold (dotted line at 14.2 kg N ha−1 y−1), and 5–95% bootstrap percentile range.
https://doi.org/10.1073/pnas.1214299109
The United Kingdom National Focal Centre (UK NFC)
http://www.cldm.ceh.ac.uk/uk-national-focal-centre
*The United Kingdom National Focal Centre (UK NFC) for critical loads modelling and mapping activities is based at CEH Bangor, North Wales.
*The centre is responsible for co-ordinating the critical loads mapping activities in the UK and compiling national critical loads datasets and maps from data supplied by UK experts.
*Empirical critical load for nutrient nitrogen:
*Empirical nutrient nitrogen critical loads have been set for different ecosystem types. They are based on observed changes in the structure or function of ecosystems as reported in the refereed literature from the results of experimental or field studies, or in a few cases dynamic ecosystem modelling.
Critical loads
In 1997, critical loads for acidification were exceeded in 71% of UK ecosystems.
As sulphur deposition continues to fall, this value fell to below half by 2010, when nitrogen deposition dominated as much of the UK switches from coal to oil/gas burning.
Critical loads for eutrophication (nutrient enrichment) in 1997 were exceeded in about a quarter of UK 1km by 1km squares with sensitive grasslands and a little over half with heathland. Again, this is expected to decline over the next 10 to 15 years.
Maps of nutrient nitrogen critical load for a selection of ecosystems in the UK:
See figures showing montane grassland
from: http://www.cldm.ceh.ac.uk/exceedances/maps
Blue where pH safe levels not exceeded
Green exceeded and in upland areas high excess N
pH ranges of survival for a range of fauna
see figure in notes fauna cannot survive:
below ~5.5 pH
crustaceans and molluscs
Salmon, Char and Roach
sensitive insect and plant+animal plankton
below ~5pH
Whitefish, Greyling, Rainbow Trout
below ~4.5pH
Perch, Pike and Brown Trout
below ~4pH
Eel and Brook trout
Insensitive insects can survive at v. low pH
as can Waterlily but below 5.5pH only white lily persists
Current definition of critical levels (CLEs)
(CLEs) for N-containing air pollutants:
Summarized definition
Concentration above which effects may occur
Exposure duration
Short term (1 year or less)
Effect of peak exposures
Included
Agent
Separate CLE for each N compound
Object of interest
Individual plant species
No effect concentration
Generally: the lowest statistically significant
response observed in experiments
Goal
Protection of sensitive plant species
Combination effects
Possibility of synergism is considered
The distinction between the definitions is important; it is much more difficult to demonstrate the absence of an effect (to test the CLO) than the presence of an effect (to challenge the CLE).
Current definition of critical loads
(CLOs) for N-containing air pollutants
Summarized definition;
Deposition below which effects do not occur
Exposure duration
Long term (+10 years)
Effect of peak exposures
Neglected
Agent
All N compounds added
Object of interest
Natural vegetation or forests; soils and freshwaters (ecosystems)
No effect concentration
Generally: estimate of a ‘safe’ deposition level derived from empirical evidence or modelling
Goal
Protecting proper functioning of ecosystems
Combination effects
Additivity is presumed (i.e. all forms of N have same effects)
The distinction between the definitions is important; it is much more difficult to demonstrate the absence of an effect (to test the CLO) than the presence of an effect (to challenge the CLE).
Aluminium (Al) Concentration as a function of pH :
Al3+ binding to fish gills inhibits Na+ and Ca2+ transport - affects gas transport
Al also precipitates in the gills as Al(OH)3 (s), causing suffocation.
Cause of death high conc. Aluminium (Al)
