Critical load

Last updated

In the study of air pollution, a critical load is defined as "a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge". [1]

Contents

Air pollution research in relation to critical loads has focused on nitrogen and sulfur pollutants. After these pollutants are emitted into the atmosphere, they are subsequently deposited into ecosystems. Both sulfur and nitrogen deposition can acidify surface waters and soils. As added acidity lowers the pH of water, fish and invertebrate health are negatively impacted. [2] Sulfur and nitrogen, as acidifying agents, may change soil nutrient content by removing calcium and releasing toxic aluminum, further impacting plants and animals. [3] Nitrogen deposition can also act as a fertilizer in the environment and alter the competitive interactions of plants, thereby favoring the growth of some plant species and inhibiting others, potentially leading to changes in species composition and abundance. The deposition of nitrogen contributes to nutrient enrichment in freshwater, coastal, and estuarine ecosystems, which may cause toxic algal blooms, fish kills, and loss of biodiversity. [4] [5] Air pollutants impact essential ecosystem services such as air and water purification, decomposition and detoxification of waste materials, and climate regulation.

When deposition is greater than the critical load of a pollutant for a particular location, it is considered a critical load exceedance, meaning the biota are at increased risk of ecological harm. Some components of an ecosystem are more sensitive to deposition than others; therefore, critical loads can be developed for a variety of ecosystem components and responses, including (but not limited to) shifts in diatoms, increases in invasive grass species, changes in soil chemistry, decreased forest health, altered and reduced biodiversity, and lake and stream acidification.

The history, terminology, and approach used to calculate critical loads differ by region and country. The differences between approaches used by European countries and in the U.S. are discussed below.

Europe

In European countries, 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 acidification (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 (NMVOCs). 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.

United States

In the U.S., while various entities were discussing critical loads prior to 2000, efforts were independent and disjointed. However, in 2010, following a series of critical loads workshops from 2003 to 2005 and an ad hoc committee established in 2006, national efforts were unified through the development of the Critical Loads of Atmospheric Deposition (CLAD) Science Committee as part of the National Atmospheric Deposition Program (NADP). CLAD is a multi-agency group consisting of federal and state government agencies, non-governmental organizations, environmental research organizations, and universities. The goals of CLAD are to: facilitate sharing of technical information on critical loads topics within a broad multi-agency/entity audience, fill gaps in critical loads development in the U.S., provide consistency in development and use of critical loads in the U.S., and promote understanding of critical loads approaches through development of outreach and communications materials.

Federal Land Managers, such as the National Park Service, U.S. Forest Service, and U.S. Fish and Wildlife Service, use critical loads to: identify resources at risk, focus research and monitoring efforts, inform planning and other land management activities, evaluate potential impacts of emission increases, and develop pollution reduction strategies. The U.S. Environmental Protection Agency is expanding use of critical loads for assessments and policy development, including consideration of critical loads when setting National Ambient Air Quality Standards.

The U.S. has adopted two approaches for creating critical loads: empirical and steady-state mass balance critical loads. Empirical critical loads are derived based on observations of ecosystem responses (such as changes in plant diversity, soil nutrient levels, or fish health) to specific deposition levels. These relationships are created using dose-response studies or by measuring ecosystem responses to increasing gradients of deposition over space or time. Steady-state mass balance critical loads are derived from mathematical mass-balance models under assumed or modeled equilibrium conditions. A steady-state condition may be achieved far into the future. The models used to determine steady-state critical loads vary in complexity with regard to process representation but can include water and soil chemistry, mineral soil weathering rates, deposition data, and ecological response data.

Asia

In Asia, both empirical and steady-state mass balance approaches have been used to estimate critical loads. [6] [7] Empirical critical loads were simply determined as the deposition levels with reported field occurrence of detrimental ecological effects. The steady-state mass balance model calculates the critical load of an ecosystem over the long-term by defining acceptable values for elements leaching out of the ecosystem.

Although empirical nitrogen critical loads have been well summarized for Europe and the United States, [4] [5] [2] large uncertainties still exist in Asia due to very limited and short-term experimental studies by using relatively high levels of nitrogen application. [6] In regions (e.g., eastern and southern China) where historical nitrogen deposition has already been very high and perhaps even higher than the actual critical load, experimental studies may fail to quantify the critical loads because substantial ecosystem changes had already occurred. Moreover, the values of the critical loads can vary remarkably when based on different biological or chemical response of an ecosystem, such as physiological variation, reduced biodiversity, elevated nitrate leaching, and changes in soil microorganisms. Empirical critical loads have been assessed for some forests and grasslands in China, [6] but the values for many other ecosystems remain unassessed. With more emerging field experiments, critical loads will be better estimated in the near future.

In South and East Asia, comprising China, Korea, Japan, the Philippines, Indo-China, Indonesia, and the Indian subcontinent, critical loads were first computed and mapped as part of the impact module of the Asian version of the Regional Air pollution INformation and Simulation model (RAINS-Asia) based on the steady-state mass balance approach. [8] Thereafter, critical loads with higher resolution were calculated in many Asian countries such as Japan, Russia, South Korea, India, and China. [7] Although similar methods were applied in Asia as in Europe, the steady state mass balance approach has been improved by considering base cation deposition. Steady-state mass balance critical loads have been used to designate Acid Rain Control Zones and Sulphur Dioxide Pollution Control Zones in China. In the near future, critical loads will be more widely applied to guide emission abatement strategies.

Related Research Articles

<span class="mw-page-title-main">Acid rain</span> Rain that is unusually acidic

Acid rain is rain or any other form of precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions. Most water, including drinking water, has a neutral pH that exists between 6.5 and 8.5, but acid rain has a pH level lower than this and ranges from 4–5 on average. The more acidic the acid rain is, the lower its pH is. Acid rain can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids.

<span class="mw-page-title-main">Ecosystem</span> Community of living organisms together with the nonliving components of their environment

An ecosystem consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and microbes.

<span class="mw-page-title-main">Eutrophication</span> Excessive plant growth in response to excess nutrient availability

Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytoplankton productivity". Water bodies with very low nutrient levels are termed oligotrophic and those with moderate nutrient levels are termed mesotrophic. Advanced eutrophication may also be referred to as dystrophic and hypertrophic conditions. Eutrophication can affect freshwater or salt water systems. In freshwater ecosystems it is almost always caused by excess phosphorus. In coastal waters on the other hand, the main contributing nutrient is more likely to be nitrogen, or nitrogen and phosphorus together. This depends on the location and other factors.

<span class="mw-page-title-main">Nitrogen cycle</span> Biogeochemical cycle by which nitrogen is converted into various chemical forms

The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biological and physical processes. Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen, making it the largest source of nitrogen. However, atmospheric nitrogen has limited availability for biological use, leading to a scarcity of usable nitrogen in many types of ecosystems.

<span class="mw-page-title-main">Water pollution</span> Contamination of water bodies

Water pollution is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water pollution results when contaminants mix with these water bodies. Contaminants can come from one of four main sources: sewage discharges, industrial activities, agricultural activities, and urban runoff including stormwater. Water pollution is either surface water pollution or groundwater pollution. This form of pollution can lead to many problems, such as the degradation of aquatic ecosystems or spreading water-borne diseases when people use polluted water for drinking or irrigation. Another problem is that water pollution reduces the ecosystem services that the water resource would otherwise provide.

<span class="mw-page-title-main">Air quality index</span> Measure of air pollution

An air quality index (AQI) is used by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. AQI information is obtained by averaging readings from an air quality sensor, which can increase due to vehicle traffic, forest fires, or anything that can increase air pollution. Pollutants tested include ozone, nitrogen dioxide, sulphur dioxide, among others.

<span class="mw-page-title-main">Marine pollution</span> Pollution of oceans from substances discarded by humans

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, although marine transportation significantly contributes as well. Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean. The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans. Pathways of pollution include direct discharge, land runoff, ship pollution, atmospheric pollution and, potentially, deep sea mining.

<span class="mw-page-title-main">Nonpoint source pollution</span> Pollution resulting from multiple sources

Nonpoint source (NPS) pollution refers to diffuse contamination of water or air that does not originate from a single discrete source. This type of pollution is often the cumulative effect of small amounts of contaminants gathered from a large area. It is in contrast to point source pollution which results from a single source. Nonpoint source pollution generally results from land runoff, precipitation, atmospheric deposition, drainage, seepage, or hydrological modification where tracing pollution back to a single source is difficult. Nonpoint source water pollution affects a water body from sources such as polluted runoff from agricultural areas draining into a river, or wind-borne debris blowing out to sea. Nonpoint source air pollution affects air quality, from sources such as smokestacks or car tailpipes. Although these pollutants have originated from a point source, the long-range transport ability and multiple sources of the pollutant make it a nonpoint source of pollution; if the discharges were to occur to a body of water or into the atmosphere at a single location, the pollution would be single-point.

<span class="mw-page-title-main">Surface runoff</span> Flow of excess rainwater not infiltrating in the ground over its surface

Surface runoff is the unconfined flow of water over the ground surface, in contrast to channel runoff. It occurs when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the soil is saturated by water to its full capacity, and the rain arrives more quickly than the soil can absorb it. Surface runoff often occurs because impervious areas do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or man-made processes. Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water. The land area producing runoff that drains to a common point is called a drainage basin.

Soil acidification is the buildup of hydrogen cations, which reduces the soil pH. Chemically, this happens when a proton donor gets added to the soil. The donor can be an acid, such as nitric acid, sulfuric acid, or carbonic acid. It can also be a compound such as aluminium sulfate, which reacts in the soil to release protons. Acidification also occurs when base cations such as calcium, magnesium, potassium and sodium are leached from the soil.

<span class="mw-page-title-main">Air Pollution Index</span> Air quality measurement in Malaysia

The Air Pollution Index is a simple and generalized way to describe the air quality, which is used in Malaysia. It is calculated from several sets of air pollution data and was formerly used in mainland China and Hong Kong. In mainland China the API was replaced by an updated air quality index in early 2012 and on 30 December 2013 Hong Kong moved to a health based index.

<span class="mw-page-title-main">Human impact on the nitrogen cycle</span>

Human impact on the nitrogen cycle is diverse. Agricultural and industrial nitrogen (N) inputs to the environment currently exceed inputs from natural N fixation. As a consequence of anthropogenic inputs, the global nitrogen cycle (Fig. 1) has been significantly altered over the past century. Global atmospheric nitrous oxide (N2O) mole fractions have increased from a pre-industrial value of ~270 nmol/mol to ~319 nmol/mol in 2005. Human activities account for over one-third of N2O emissions, most of which are due to the agricultural sector. This article is intended to give a brief review of the history of anthropogenic N inputs, and reported impacts of nitrogen inputs on selected terrestrial and aquatic ecosystems.

<span class="mw-page-title-main">Wild fisheries</span> Area containing fish that are harvested commercially

A wild fishery is a natural body of water with a sizeable free-ranging fish or other aquatic animal population that can be harvested for its commercial value. Wild fisheries can be marine (saltwater) or lacustrine/riverine (freshwater), and rely heavily on the carrying capacity of the local aquatic ecosystem.

The 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone 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. As of August 2014, the Protocol had been ratified by 26 parties, which includes 25 states and the European Union.

<span class="mw-page-title-main">Agricultural pollution</span> Type of pollution caused by agriculture

Agricultural pollution refers to biotic and abiotic byproducts of farming practices that result in contamination or degradation of the environment and surrounding ecosystems, and/or cause injury to humans and their economic interests. The pollution may come from a variety of sources, ranging from point source water pollution to more diffuse, landscape-level causes, also known as non-point source pollution and air pollution. Once in the environment these pollutants can have both direct effects in surrounding ecosystems, i.e. killing local wildlife or contaminating drinking water, and downstream effects such as dead zones caused by agricultural runoff is concentrated in large water bodies.

<span class="mw-page-title-main">Nutrient pollution</span> Contamination of water by excessive inputs of nutrients

Nutrient pollution, a form of water pollution, refers to contamination by excessive inputs of nutrients. It is a primary cause of eutrophication of surface waters, in which excess nutrients, usually nitrogen or phosphorus, stimulate algal growth. Sources of nutrient pollution include surface runoff from farm fields and pastures, discharges from septic tanks and feedlots, and emissions from combustion. Raw sewage is a large contributor to cultural eutrophication since sewage is high in nutrients. Releasing raw sewage into a large water body is referred to as sewage dumping, and still occurs all over the world. Excess reactive nitrogen compounds in the environment are associated with many large-scale environmental concerns. These include eutrophication of surface waters, harmful algal blooms, hypoxia, acid rain, nitrogen saturation in forests, and climate change.

<span class="mw-page-title-main">Pollution in Canada</span> Overview of pollution in Canada

Pollution is an environmental issue in Canada. It has posed health risks to the Canadian population and is an area of concern for Canadian lawmakers. Air, water and soil pollution as well as the associated health effects are prominent points of contention in modern Canadian society.

<span class="mw-page-title-main">Freshwater acidification</span>

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas, or by the reduction of acid anions, like sulfate and nitrate within a lake. Freshwater acidification is primarily caused by sulfur oxides (SOx) and nitrogen oxides (NOx) entering the water from atmospheric depositions and soil leaching. Carbonic acid and dissolved carbon dioxide can also enter freshwaters in a similar manner associated with runoff through carbon dioxide-rich soils. Runoff that contains these compounds may incorporate acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms. Acid rain is also a contributor to freshwater acidification. It is created when SOx and NOx react with water, oxygen, and other oxidants within the clouds.

<span class="mw-page-title-main">Biodiversity loss</span> Extinction of species and local ecosystem loss reduction or loss of species in a given habitat

Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, depending on whether the environmental degradation that leads to the loss is reversible through ecological restoration/ecological resilience or effectively permanent. The current global extinction, has resulted in a biodiversity crisis being driven by human activities which push beyond the planetary boundaries and so far has proven irreversible.

<span class="mw-page-title-main">Human impact on marine life</span>

Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.

References

  1. ↑ Nilsson, J. and P. Grennfelt. 1988. Critical loads for sulphur and nitrogen. UNECE/Nordic Council workshop report, Skokloster, Sweden. March 1988.
  2. 1 2 Greaver, T. L., T. J. Sullivan, J. D. Herrick, M. C. Barber, J. S. Baron, B. J. Cosby, M. E. Deerhake, R. L. Dennis, J.-J. B. Dubois, C. L. Goodale, A. T. Herlihy, G. B. Lawrence, L. Liu, J. A. Lynch, and K. J. Novak. 2012. Ecological effects of nitrogen and sulfur air pollution in the US: what do we know? Frontiers in Ecology and the Environment 10:365-372.
  3. ↑ Driscoll, C. T., G. B. Lawrence, A. J. Bulger, T. J. Butler, C. S. Cronan, C. Eagar, K. F. Lambert, G. E. Likens, J. L. Stoddard, and K. C. Weathers. 2001. Acidic Deposition in the Northeastern United States: Sources and Inputs, Ecosystem Effects, and Management Strategies. BioScience 51:180-198.
  4. 1 2 Bobbink, R., K. Hicks, J. Galloway, T. Spranger, R. Alkemade, M. Ashmore, M. Bustamante, S. Cinderby, E. Davidson, F. Dentener, B. Emmett, J.-W. Erisman, M. Fenn, F. Gilliam, A. Nordin, L. Pardo, and W. De Vries. 2010. Global assessment of nitrogen deposition effects on terrestrial plant diversity: a synthesis. Ecological Applications 20:30-59.
  5. 1 2 Pardo, L. H., M. E. Fenn, C. L. Goodale, L. H. Geiser, C. T. Driscoll, E. B. Allen, J. S. Baron, R. Bobbink, W. D. Bowman, C. M. Clark, B. Emmett, F. S. Gilliam, T. L. Greaver, S. J. Hall, E. A. Lilleskov, L. Liu, J. A. Lynch, K. J. Nadelhoffer, S. S. Perakis, M. J. Robin-Abbott, J. L. Stoddard, K. C. Weathers, and R. L. Dennis. 2011. Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States. Ecological Applications 21:3049-3082.
  6. 1 2 3 Liu, X.J., L. Duan, J.M. Mo, E.Z. Du, J.L. Shen, X.K. Lu, Y. Zhang, X.B. Zhou, C.E. He, and F.S. Zhang. 2011. Nitrogen deposition and its ecological impact in China: an overview. Environmental Pollution 159:2251-2264.
  7. 1 2 Duan, L., Q. Yu, Q. Zhang, Z. Wang, Y. Pan, T. Larssen, J. Tang, and J. Mulder. 2016. Acid deposition in Asia: emissions, deposition, and ecosystem effects. Atmospheric Environment 146:55-69.
  8. ↑ Hettelingh, J.P., H. Sverdrup, and D. Zhao. 1995. Deriving critical loads for Asia. Water, Air, and Soil Pollution 85(4):2565-2570.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.