Atmospheric Biogeochemistry Final Project: Effects of Aerosols on Biogeochemical Cycles
Massive emission of aerosols, especially from anthropogenic sources with multiple pollutant particles has long been considered as a major influential factor of ecosystem. Several air pollutants (e.g. sulfur, nitrogen and ozone) are studied to figure out effects on earth’s ecosystems (e.g. terrestrial and aquatic systems) especially the long-term ones. Four iconic pollutants: N, S, O3 and mercury have various interlocking effects in the ecosystem, affecting the living environment and lifestyle of animal plants. Recent studies suggest the indirect effects of aerosol in the atmosphere could be much more important on the temperature and precipitation changes. On the other hand, nutrient cycles could also be influenced by aerosols and human activities has greatly changed the cycle patterns in the last 150 years. Human beings have a conscious or unconscious effect on the climate in production and life, and long-term accumulation can have huge consequences. Furthermore, because these effects are generally “long-termed”, analyzing the historical data and future predictions on anthropogenic aerosol emissions is favorable for works on reducing aerosol emissions in the future global environmental policies and measures.
Key words: Aerosols, ecosystem, climate change, nutrient biogeochemical cycle
Aerosols are liquid or solid particles suspended in the atmosphere, ranging 3nm-100μm. Anthropogenic activities, such as power generation, combustion of fossil fuels and biomass burning has become large sources of aerosols in the last 150 years, which brings issues like air pollution and respiratory diseases. Recent studies show air pollution is responsible for about 3.3 million of premature death per year globally, which are related closely to the aerosols such as PM2.5 (Lelieveld et al., 2015).
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Earth’s climate has also been influenced by aerosol effects. The direct effect is considered as blocking and reflecting of solar radiation by aerosol particles, resulting cooling of the temperature. More recent studies indicate that the aerosol indirect effects, which interacts with cloud droplets and modifies the precipitation patterns, is important for long-term climate changes (Mahowald, 2011). However, as the aerosol effects are regionally dependent, it is hard to precisely generate a globally model to provide insights into future outcomes (NASA, 2017).
This paper focuses on following questions by reviewing related studies: 1) What are the aerosol effects on Earth’s terrestrial and aquatic ecosystems (e.g. N, S, O3 and Hg)? 2) How the aerosols change the climate in terms of the direct and indirect effect? 3) How the nutrient cycles influenced by the aerosols and how anthropogenic emissions changed that?
2 Ecosystem Impacts
We will mainly focus on the role of N, S, O3 and mercury in ecosystems.In popular cognition, because a large number of media reports such as acid rain damage water quality and aquatic life, people are generally aware of the impact of air pollution on aquatic ecosystems, but much less is known about the impacts of terrestrial ecosystems (Fig. 1). In terrestrial ecosystems, the biogeochemical effects of S and N on ecosystems depend on the mobility of these contaminants in the canopy and the soil they deposit. If the anions (sulfates and nitrates) they form are leached through the canopy and soil rather than being retained, they can strip the leaves and the soil of valuable nutrient cations such as calcium and magnesium. Leaching of sulfates and nitrates leads to acidification of soil and surface water and affects aluminum to some extent. Aluminum is a natural component of the soil, but under acidic conditions, it becomes more soluble, exhibits high concentrations in soil water, is toxic to many plants and animals, and can penetrate into surface waters that are toxic to fish and other aquatic organisms. Both S and N accumulate in vegetation and soil, leading to delayed effects. The accumulation of N in terrestrial ecosystems may lead to changes in species composition. Affinity N species will survive more easily, and species that are not compatible with N will face survival tests (Gough et al. 2000).
The effects of pollutants on aquatic ecosystems and their impact on terrestrial ecosystems vary widely. The main effect of S is to acidify the water. Nitrogen can promote the occurrence of acidification, but it may also be due to eutrophication (over-enrichment of nutrients) in aquatic ecosystems due to excessive supply of N (Weathers and Lovett 1998). The effect of acid deposition on water quality includes a decrease in pH (increased acidity), a decrease in acid neutralization capacity, and an increase in aluminum concentration. The main variables of concern to organisms are pH and aluminum concentrations. In organisms with sputum breathing, Al interrupts the gas and ion transport on the respiratory membrane, causing the five major functions of cockroaches to be destroyed: (1) ion transport, (2) osmotic adjustment, (3) acid-base balance, (4) N excretion, and (5) breathing. Other physiological effects also occur in aquatic organisms, including altered hormones and behavioral responses. The sensitivity of aquatic organisms to acidification varies widely. When the pH drops below about 6, the most sensitive organisms are adversely affected, and some acid-tolerant organisms can survive in water at pH 4. However, the acidification of streams and lakes can be sporadic or chronic, often occurring occasionally during large water events such as heavy storms or snowmelt periods. In addition to the direct toxicity of pH and aluminum to aquatic organisms, the indirect effects of lake and stream ecosystems are also important. For example, lowering the pH lowers the concentration of dissolved organic carbon (DOC) in the lake, allowing light to penetrate further into the lake (Monteith et al. 2007). This increases the available light for large plants and benthic algae growing at the bottom of the lake. In addition, increased visibility alters the efficiency of predators in the lake. Dissolved organic carbon is another important reason: it combines aluminum complexes to reduce its toxicity, so a decrease in DOC increases the toxicity of Al. Indirect food web effects can also occur; for example, if predators are highly tolerant to acidic environments but their prey does not adapt to acidic environments, predators will not be able to survive in acidified lakes.
Ozone is a well-studied contaminant known to be toxic to plants and animals. In plants, O3 appears to affect membrane function, resulting in reduced photosynthesis, slow growth, and, in severe cases, death. In animals, the O3 effect is primarily studied in humans, where it damages lung tissue and exacerbates breathing problems such as asthma. Ozone has little effect on water, but may have an impact on emerging aquatic plants or aspirated animals that are part of the aquatic ecosystem. Mercury accumulates in the soil, but studies of its effects have focused on aquatic ecosystems, where anaerobic conditions contribute to the production of methylmercury. Human and animal exposure is primarily related to methylmercury(Driscoll et al. 2007).Previously, terrestrial organisms believed to be at risk of mercury poisoning were animals of other animals that feed on aquatic food webs, such as birds that feed on aquatic insects or raccoons that eat aquatic invertebrates(Chen et al. 2005). The methylation and bioaccumulation of mercury in terrestrial food webs can affect the health of humans at the top of the food chain. At the same time, mercury, a potent neurotoxin that accumulates in aquatic food webs, alters the behavior and reproduction of high-nutrient organisms.
3 Climate Changes
Levy et al (2013) used models to simulate historical and future aerosol effects in terms of global surface temperature and precipitation changes. They also compared new models (CM3, which simulates aerosols interaction with liquid clouds as cloud condensation nuclei (CCN)) with their previous models (CM2.1) to indicate the importance of indirect effects of aerosols. These two models are forced by changing concentrations (CM2.1) or emissions (CM3) of sulfate, Black Carbon (BC) and Organic Carbon (OC) aerosols while the other forcing agents are set as constant at 1860 (1861 for CM2.1) levels.
For their future scenarios, RCP4.5 and RCP4.5** are applied to predict global annual mean surface temperature and annual mean precipitation changes. RCP4.5 models multiple types of greenhouse gases as well as anthropogenic aerosols and treats them as integrated mixture. RCP4.5** generally shares the same properties with RCP4.5, while in this model aerosol emissions are fixed at year 2005 levels based on simulation results in CM3.
3.1 Historical Changes Modelling
The global annual mean surface temperature and annual mean precipitation levels are simulated since 1860 (Fig. ). About 1.0 ℃ cooling in total is observed in the CM3 simulation, and the dashed blue line is mean surface air temperature change in which only aerosol-cloud interaction (aerosol indirect effect) is considered to influence the aerosol emissions. Meanwhile, only about 0.1 ℃ drop of surface temperature in CM2.1, which does not include the aerosol indirect effect (Fig. 2).
Two influential factors can be summarized to the large difference between these two models: First, the aerosol indirect effect is essential for the cooling, which is predominantly involved with sulfate emissions. According to the historical emission curve simulated by Levy et al (2013), at ~1940, the emission reached maximum and it was at that time the temperature dropped dramatically at CM3 model. Besides, additional simulation with only SO2 and BC emissions are increasing through time shows the SO2 increase solely result in 1.1 ℃ cooling of temperature, which corresponds with the total aerosol changes result, while no significant changes for historical BC emissions solely. Second, although SO2 emission increases strongly, the aerosol direct effect gives modest cooling through time in CM2.1. That is because the cooling effect has been neutralized by the BC absorption, which net effect warms the climate.
In the precipitation change curves, two models show the dropdown of 1% and 3%,respectively. Note that indirect aerosol effect only accounts for 1.5% decrease in the precipitation. As previous studies suggest (Ming et al., 2010), the first reason for precipitation decrease is the cooling of surface and troposphere resulted from aerosol emissions, which is compensated by reducing of precipitation. This explains the discrepancy between CM2.1 and CM3 results as different surface cooling levels. Second reason is the absorbing aerosol particles (e.g. BC) heat the atmosphere, which is also balanced by the decrease of condensation heating and precipitation. This could explain the difference between indirect-only effect and the overall effect, as the total aerosol yields more drying in terms of atmospheric heating (Allen and Ingram, 2002).
3.2 Future Prediction
Two prediction models, RCP4.5 and RCP4.5** are curved to show the results (Fig. 3). About 2.2 ℃ mean surface temperatures increase during the 21st century, with ~40% of the increase due to the decrease in aerosol emissions according to the model in RCP4.5. This is basically the result of the decreasing of aerosol and its indirect cooling effect on the surface temperature.
Precipitation patterns indicate more increases in RCP4.5 compared with RCP4.5** the curves separate at ~2050. This difference can be explained by both a surface temperature warming due to the reduction in sulfate aerosol emissions as well as the weakened atmospheric heating effect because of the reduction in BC, both of which result in increased condensation heating (precipitation) as energy compensation.
4 Nutrient Cycles
Every year, various nutrient particles in aerosols are emitted into the atmosphere and biogeochemical cycles, including natural and anthropogenic sources. Natural emissions including soil/dust, volcanoes, ocean chemistry and wildfires. Anthropogenic emissions involves fossil fuel combustions, energy production and biomass burning, etc. As Fe, P and N are the main nutrients which controls the primary productivity in the ocean, their biogeochemistry cycle and transformation are concerned in previous works (Moore et al., 2013; Kanakidou et al., 2018).
After emitted into the atmosphere, nutrients in the aerosols such as dust are mostly insoluble, which need to transform into soluble form to become available for ecosystems. Therefore this section discusses three major nutrients and their transformation of solubilities (Fig. 4).
Although Fe is well enriched in the Earth’s crust, most of them are insoluble iron oxides, while soluble Fe is limited for living organisms. Three major mechanisms have been summarized for Fe solubilization: proton-driven, ligand-driven (Paris and Desboeufs, 2013) and photo-induced reductive Fe solubilization (Chen and Grassian, 2013). The particle size also has a reversed relationship with Fe solubility, which is partly because smaller particles are enriched in acidity compared with coarser ones (Ito and Feng, 2010).
The solubilization of P depends mostly on pH as well and the concentration of soluble PO43- derived from dust aerosols also increases as H+ is concentrated (Nenes et al., 2011). Besides, the HNO3/NO3– transformation is also affected by pH and ions in dust (e.g. Ca2+, K+), resulting the different distribution patterns in N deposition globally.
4.2 Biogeochemical Cycles
Atmospheric aerosols are important components in global biogeochemical cycles. The nutrient elements, such as N, P and Fe in the aerosols are all essential for biochemical reactions, especially primary-productivity-related ones. Thus the biogeochemical cycle of nutrients are worth focusing in terms of energy cycles.
The present-day global influx of soluble N patterns (Fig. 5) show larger N influx amount on major continents, with high values in east Asia and central Africa, which results from agricultural or industrial emissions as well as biomass burning. Anthropogenic N emission has risen from ~50 TgN yr−1 in 1850 to ~130 TgN yr−1 in 2005, and future simulation shows the increase amount will be less than 5 TgN yr−1 due to the effect of emission limitations.
Unlike nitrogen, nearly 40% of soluble P deposition is distributed in the ocean (Fig. 5). P is a limiting element of primary productivity in specific regions (e.g. the Mediterranean and the North Pacific Ocean), where N and P are controlling the productivity rate together by their ratio. In the eastern Mediterranean, the phytoplankton biomass has increased by 16% over the past 150 years, which corresponds with the rising of atmospheric deposition of anthropogenic N over P in the area (Christodoulaki et al 2016). Similar results found in North Pacific Ocean by Brahney et al. (2015), which suggest when N influx is much larger than P, the influence of P to the primary productivity may be greater.
Dust emissions has increased by 40% and the soluble Fe deposition has doubled in the last century (Mahowald et al., 2017). However, as more environmental measures being effective, the decrease of Fe emission in the future could cause the decrease of soluble Fe deposition, which will affect primary productivity and other biochemical processes in the ocean, as summarized by Tagliabue et al. (2017).
5 Human Activity Effects
The conscious or unconscious effects of human beings on climate during production and life, including changes in atmospheric composition and moisture content, release of heat into the atmosphere, and changes in the climatic effects of the physical and biological properties of the underlying surface (Jackson, et al 2006).
Following facts indicate that these effects are happening: 1) The massive burning of fossil fuels leads to an increase in CO2, SO2 and NO2 in the atmosphere, resulting in global warming. Moreover, due to the large consumption of energy, carbon dioxide increases at a rate of 0.7 ppm per year. By the middle of the 21st century, the earth’s ice and snow will melt more than half, resulting in Sea levels rise, inundating coastal cities and destroying the natural environment and ecosystems of mankind. (e.g. acid rain, photochemical smog.) 2) Excessive use of air conditioners leads to an increase in fluorocarbons in the atmosphere, leading to destruction of the ozone layer. Therefore, the ability of the ozone layer to absorb ultraviolet radiation is greatly weakened, resulting in a significant increase in ultraviolet rays reaching the earth’s surface, Bring many harms to human health and the ecological environment (At present, there are major concerned on affects which can damage human health, terrestrial plants, aquatic ecosystems, biogeochemical cycles, materials, and tropospheric atmospheric composition and air quality.). 3) Excessive deforestation reduces the photosynthesis of vegetation, increases CO2 in the atmosphere, and global warming (Fig. 6).
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Among the atmospheric pollutants composed of aerosols, sulfur oxides are the main concern. Now global environmental laws have strictly controlled this aspect, and have imposed restrictions and total control on corporate emissions (Cunningham et al., 1999). Companies have also gained a new understanding of sulphide. The main reason is that sulfur oxides are substances that are relatively polluted in the production process of industrial enterprises. The characteristics of sulfur oxides are toxic and corrosive. When industrial and enterprise smoke emissions are large, the emission of sulfur oxides will increase, which will have a great impact on people’s health and life. It is well known that fossil fuels need to be burned in both industrial and kiln furnaces. Fossil fuels have a relatively high sulphur content. Although their sources are different and their sulphur content is different, they also cause certain harm to the ambient air. With the burning of fossil fuels, the sulfur contained in them will produce sulfur oxides. To make people’s respiratory system and so on affected. Sulfur oxides have been reduced with the use of sulfur-containing fuels and by the environmental protection sector’s control of sulfur oxides and advances in their control strategies. Sulfur oxides are being greatly reduced worldwide, and many countries have met the ambient air quality standards for sulfur oxides. Other indicators of national sulfur oxides that do not meet the standards are also being reduced. After the sulfur oxides are released into the atmosphere, they return to the ground in the form of acid rain. Sulfur oxides are harmful to humans, animals and plants, and many plants are particularly sensitive to sulfur oxides. Heart failure caused by air pollution caused by respiratory diseases such as bronchitis and emphysema is thought to be mainly caused by sulfur dioxide and sulfuric acid particles (Biosphere:The Sulfur cycle, from Encyclopedia Britannica: 2006.).
The study said that farmland fertilization (including nitrogen fertilizer or organic fertilizer) is unreasonable, farm animal manure management is not good, coal burning, automobile exhaust emissions, etc. will increase the release of artificial nitrogen into the atmosphere, these gases and the gas formed by the secondary reaction Sol/fine particles (such as PM2.5) cause air quality degradation or air pollution. The amount and shape of reactive nitrogen from atmospheric deposition to terrestrial and aquatic ecosystems will also affect the stability and function of ecosystems (Solange et al, 2007). Nitrous oxide (N2O), which is one of the trace gases in the atmosphere, has received global attention in recent years. This is because N2O has the property of absorbing infrared rays, which can reduce the heat radiation from the surface to the outside air, which leads to the greenhouse effect. It also shows that N2O in the stratosphere reacts with the oxygen atoms of the D ionosphere to form NO. And further react with the stratospheric ozone (O3), which consumes O3 and destroys the ozone layer, resulting in enhanced ultraviolet radiation reaching the Earth’s surface, which affects human survival and health (EAS Jan 2019). At present, the concentration of N2O in the atmosphere has risen from 0.288 μmol·mol-1 before the Industrial Revolution to 0.310 μmol·mol-1, and has increased at an annual rate of 0.25%. In the past 100 years, the contribution of N2O to the greenhouse effect is about 5%, although the concentration and annual growth rate of N2O in the atmosphere is lower than that of CO2, its potential warming effect is about 190 to 270 times that of CO2, which is 4 to 21 times that of CH4, and N2O remains in the atmosphere. The longer time (average life is 150 years), therefore, the increase in the concentration of N2O in the atmosphere and its factors affecting emissions have received much attention.
Multiple aerosol components, such as N, S, O3 and Hg, are harmful in different time periods for both terrestrial and aquatic ecosystems. Aerosol indirect effect, as the modelling of historical and future trends, could be important in controlling global temperature as well as the precipitation levels. Furthermore, the simulation results implies that although the reduction of air pollution preserves health of ecosystems, we may also lose much of the inhibition for global warming. N, P and Fe are essential elements for living organisms as well as the biogeochemical cycles. Modified greatly by anthropogenic emissions, distribution of these nutrients has changed and will be changing in the future, which will influence the primary productivity in the ocean. Again, the reduction of aerosol emissions in the future would decrease the oceanic productivity, which indicates the complexity of aerosol issues.
Figure 1. Schematic of key interactions of acidic deposition with soils. Biological (uptake), soil (exchange and adsorption) and geological (weathering) factors interact to determine the effects of the acidic deposition in the soil (Lovett et al., 2009).
Figure 2. a) Simulation of global mean surface temperature time series by CM2.1 and CM3 models. In CM2.1, natural aerosol concentrations are held constant; for CM3, the emissions are calculated interactively. Red and blue shaded area are three-member (sulfate, BC and OC) ensemble envelope, while red and blue solid lines are the mean values. The dashed blue line shows the result of aerosol indirect effect (aerosol-cloud interaction) only in CM3. b) Historical global mean precipitation changes with the same setup as a) (Levy et al., 2013).
Figure 3. a) Global annual mean surface temperature simulated by CM3 model, which is based on the historical temperature changes from 1860 to 2005; the series from 2006 to 2100 is simulated by either RCP4.5 or by RCP4.5** (BC, OC, and SO2 levels held fixed at 2005 values). Gray shading: the three-member ensemble envelope; green, gray and red circles: annual mean values. b) Global annual mean precipitation time series with the same setup as a) (Levy et al., 2013).
Figure 4. The biogeochemical cycles of nutrients (N, P and Fe) including the solubilization process in the atmosphere (Kanakidou et al., 2018).
Figure 5. Present-day deposition fluxes of soluble N (DN, at 2005), soluble P (DP, at 2008) and soluble Fe (DFe, at 2008) (Kanakidou et al., 2018).
Figure 6. Example of human effect on Nitrogen and Sulfur cycle (USDA Forest Service).
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