Critical load
Encyclopedia
In the study of air pollution
, a critical load is defined as ”A quantitative estimate of an exposure to one or more pollutant
s below which significant harmful effects on specified sensitive elements of the environment
do not occur according to present knowledge”. (Nilsson and Grennfelt 1988)
Critical loads and the similar concept of critical levels have been used extensively within the 1979 UN-ECE
Convention on Long-Range Transboundary Air Pollution
. As an example the 1999 Gothenburg protocol
to the LRTAP convention takes into account acid
ification (of surface waters and soils
), eutrophication
of soils and ground-level ozone
and the emissions of sulfur dioxide
, ammonia
, nitrogen oxide
and non-methane volatile organic compounds (NMVOC
). For acidification and eutrophication the critical loads concept was used, whereas for ground-level ozone
the critical levels were used instead.
To calculate a critical load, the target ecosystem
must first be defined and in that ecosystem (e.g. a forest
) a sensitive "element" must be identified (e.g. forest growth rate). The next step is to link the status of that element to some chemical criterion (e.g. the base cation to aluminium
ratio, Bc/Al) and a critical limit (e.g. Bc/Al=1) which should not be violated. Finally, a mathematical model (e.g. the Simple Mass Balance model, SMB) needs to be created so that the deposition levels that result in the chemical criterion reaching exactly the critical limit can be calculated. That deposition level is called the critical load and the difference between the current deposition level and the critical load is called exceedance.
In the early days, critical loads were often calculated as a single value, e.g. critical load of acidity. Today a two-dimensional critical load function is often calculated, with the x-axis
as N-deposition and the y-axis as S-deposition. The critical loads concept is a steady-state
concept and that it therefore includes no information whatsoever regarding how long it takes before effects are visible. A simplified illustration of dynamic aspects is the target load function, which is the load at which the chemical criterion recovers before a chosen year, the target year. Thus, for target years in the near future the target load function is lower than the critical load and for target years in the distant future the target load function approaches the critical load function.
Calculating critical load functions and target load functions include several simplifications and thus can be viewed as a risk concept: The higher the exceedance the higher the risk for adverse effects and there is a certain risk that zero exceedance will still lead to adverse effects.
Air pollution
Air pollution is the introduction of chemicals, particulate matter, or biological materials that cause harm or discomfort to humans or other living organisms, or cause damage to the natural environment or built environment, into the atmosphere....
, a critical load is defined as ”A quantitative estimate of an exposure to one or more pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...
s below which significant harmful effects on specified sensitive elements of the environment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
do not occur according to present knowledge”. (Nilsson and Grennfelt 1988)
Critical loads and the similar concept of critical levels have been used extensively within the 1979 UN-ECE
United Nations Economic Commission for Europe
The United Nations Economic Commission for Europe was established in 1947 to encourage economic cooperation among its member states. It is one of five regional commissions under the administrative direction of United Nations headquarters. It has 56 member states, and reports to the UN Economic and...
Convention on Long-Range Transboundary Air Pollution
Convention on Long-Range Transboundary Air Pollution
The Convention on Long-Range Transboundary Air Pollution, often abbreviated as Air Pollution or CLRTAP, is intended to protect the human environment against air pollution and to gradually reduce and prevent air pollution, including long-range transboundary air pollution.-Overview:The convention...
. As an example the 1999 Gothenburg protocol
Multi-effect Protocol
The 1999 Multi-effect Protocol is a multi-pollutant protocol designed to reduce acidification, eutrophication and ground-level ozone by setting emissions ceilings for sulphur dioxide, nitrogen oxides, volatile organic compounds and ammonia to be met by 2010.The Protocol is part of the Convention...
to the LRTAP convention takes into account acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
ification (of surface waters and soils
Soil acidification
Soil acidification is the buildup of hydrogen cations, also called protons, reducing the soil pH. This happens when a proton donor is added to the soil. The donor can be an acid, such as nitric acid and sulfuric acid . It can also be a compound such as aluminium sulfate, which reacts in the soil to...
), eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
of soils and ground-level ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
and the emissions of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...
and non-methane volatile organic compounds (NMVOC
NMVOC
NMVOC is the abbreviation for non-methane volatile organic compounds.It is a generic term for a large variety of chemically different compounds, like for example, benzene, ethanol, formaldehyde, cyclohexane, 1,1,1-trichloroethane or acetone....
). For acidification and eutrophication the critical loads concept was used, whereas for ground-level ozone
Tropospheric ozone
Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...
the critical levels were used instead.
To calculate a critical load, the target ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
must first be defined and in that ecosystem (e.g. a forest
Forest
A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...
) a sensitive "element" must be identified (e.g. forest growth rate). The next step is to link the status of that element to some chemical criterion (e.g. the base cation to aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
ratio, Bc/Al) and a critical limit (e.g. Bc/Al=1) which should not be violated. Finally, a mathematical model (e.g. the Simple Mass Balance model, SMB) needs to be created so that the deposition levels that result in the chemical criterion reaching exactly the critical limit can be calculated. That deposition level is called the critical load and the difference between the current deposition level and the critical load is called exceedance.
In the early days, critical loads were often calculated as a single value, e.g. critical load of acidity. Today a two-dimensional critical load function is often calculated, with the x-axis
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
as N-deposition and the y-axis as S-deposition. The critical loads concept is a steady-state
Steady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
concept and that it therefore includes no information whatsoever regarding how long it takes before effects are visible. A simplified illustration of dynamic aspects is the target load function, which is the load at which the chemical criterion recovers before a chosen year, the target year. Thus, for target years in the near future the target load function is lower than the critical load and for target years in the distant future the target load function approaches the critical load function.
Calculating critical load functions and target load functions include several simplifications and thus can be viewed as a risk concept: The higher the exceedance the higher the risk for adverse effects and there is a certain risk that zero exceedance will still lead to adverse effects.