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The basic concepts of common atmospheric pollution

Jun Matsumoto
Associate Professor, Faculty of Human Sciences, Waseda University

1. Principles of “quantity” in atmospheric pollution

Atmospheric pollution can be attributed to a range of factors, including photochemical oxidants[1], PM2.5[2], various environmental pollution, acid rain and global warming. What is common among them is that all the pollutants are released into the air by human activities and adversely affect humans and/or ecosystems either directly or indirectly. Each pollutant is emitted through different mechanisms and in variable spatial and temporal scales, and problems arise when the quantity of the pollutant exceeds the tolerable level. That’s when the principles of “chemistry” need to come into play in order to characterize pollutants and other atmospheric constituents on a molecular level. In atmospheric pollution issues, the principal components of air such as nitrogen N2 and oxygen O2, comprising 78 and 21 molecules, respectively, out of 100 atmospheric molecules, are minor; it is the trace components that play leading roles. Carbon dioxide (CO2), for example, which is known to be associated with global warming, is a trace component of air. Although CO2 is a relatively abundant trace component, atmospheric CO2 levels have recently been reported as around 400 ppmv. In other words, 400 in 1 million atmospheric molecules, or only one in 2,500, are CO2 (ppmv is a unit of volume mixing ratio in one over 1 million gaseous molecules).

2. Factors that determine the quantity of atmospheric trace components

Figure 1 shows the general behavior of trace components in the air. Once a component is emitted from the source into the atmosphere, its concentration becomes diluted as it mixes with the surrounding air (diffusion) and travels long distances with the wind (transport). In parallel, the component can react with other atmospheric constituents, which can reduce the component concentration but also possibly form secondary products. Some components are absorbed into the surface soil or seawater and disappear from the air (deposition). Spatial distribution and temporal changes of each component are determined by its movements through diffusion and transport, increases due to emissions as well as reactions, and decreases due to depositions as well as reactions with other components. For example, CO2 is not significant as it reacts with other components very slowly. In contrast , as for tropospheric ozone (O3), which is the dominant component of photochemical oxidants, and nitrogen oxides (NOx = NO + NO2), which are major precursors of ozone, reactions of these components are significant. Additionally, reactions of “atmospheric radicals” are critical in the atmosphere.

Figure 1 Schematic representation of the behavior of atmospheric trace components. Emissions, transport, diffusion, reactions, and depositions primarily control spatial distribution and temporal trends of each component.

3. A perspective on PM2.5 and features of my research activities

When we face an atmospheric pollution issue, it is essential to identify its causal substances, their permissible levels and conditions of applications, and mechanisms and requirements of their formation, as well as to accurately understand the current situation. If you are worried about PM2.5, for instance, you should be aware of some details such as, “what exactly PM2.5 is, and what its properties are,” as well as, “which parameters indicate the danger of PM2.5, under what conditions and at what levels, and how dangerous those can be.” You can then assess the possible exposure levels relative to the maximum permissible levels and make your own decision as to, “it’s safe enough,” “it’s better not to stay outside for a long time,” or, “it may harm my health even with a brief outing.” In reality, however, it is hard to make your own judgment, and so you need to rely on other sources of information like warnings from the local government, news reports and the internet. But even so, understanding the basic concepts can guide you to appropriate answers to such questions as, “What does this warning mean?” “What does this news mean?” “What are the facts, and what are the author’s views?” and “What position is the author in?” I’m aware such concepts are complex and not easy for many of you to comprehend, but I do hope you become interested in atmospheric pollution that is commonplace, and start learning about it even little by little.

Before I conclude, let me mention a part of my research activities. One of the most important approaches to the identification of as-yet-unknown reaction mechanisms and understanding of the true circumstances is to monitor the levels of atmospheric components. I have developed novel techniques for the measurement of O3, NOx, and related compounds using highly sensitive NO2 analyzers, and have been monitoring these tracers in the air. My aim is to accumulate actual monitoring data that will help devise new measures to resolve pollution issues.

[1] Photochemical oxidants:
In the lower atmosphere (troposphere), oxidants (primarily ozone O3) are formed as the secondary products from precursors such as NOx and volatile organic compounds (VOCs) when atmospheric radicals cause photochemical reactions under sufficient sunlight (UV rays). The products are referred to as photochemical oxidants and their levels are constantly monitored at various observation points to detect if they exceed the specified environmental standard. They used to be termed photochemical smog, and were the subject of concern in large cities where the precursors were present in high concentrations. The photochemical smog issue was later resolved through a range of actions taken to address emissions of precursors. Recently, however, the national average photochemical oxidant concentration has been increasing by about 1% each year despite the decreasing precursor levels. Photochemical oxidants therefore remain one of the important research subjects in atmospheric chemistry. The ozone in the troposphere is chemically the same as the ozone in the stratosphere that is declining in abundance, but its increase in the troposphere is not desirable as it can harm our health and it also has greenhouse effects.

[2] PM2.5:
Besides gaseous components, the atmosphere contains suspended particulate matter (SPM; also known as aerosols). PM2.5 is one type of SPM. SPM is classified according to the physical size (particle diameter) and chemical components (composition) of the particle. Thus, PM2.5 stands for fine particles that are 2.5 µm or smaller in diameter (concentrations are measured in units of µg/m3). Fine particles are of concern due to their risk of respiratory effects in humans (respiratory exposure), and PM2.5 has been used as an index of fine particle abundance.

Jun Matsumoto
Associate Professor, Faculty of Human Sciences, Waseda University

[Profile]
Born in Kawaguchi city, Saitama prefecture. Graduated from the Department of Chemistry, School of Science, at the University of Tokyo in 1996. Completed a Doctor of Science at the Department of Chemistry, Graduate School of Science, at the University of Tokyo in 2001. Holds a Doctor of Science degree. Served as the Postdoctoral Fellow at the Japan Science and Technology Agency; Postdoctoral researcher at the Solar-Terrestrial Environment Laboratory, Nagoya University; Assistant Professor at the Integrated Research Institute, Tokyo Institute of Technology; and Associate Professor at the Center for Priority Areas, Tokyo Metropolitan University; before taking up the current position in 2012.

[Related key publications]
Matsumoto J, et al. (2001) Direct measurement of NO2 in the marine atmosphere by laser-induced fluorescence technique. Atmos. Environ. 35: 2803-2814.
Kosugi N, Matsumoto J, et al. (2005) Development of an NO_3/N_2O_5 analyzer utilizing a laser-induced-fluorescence technique and evaluation of winter nocturnal oxidation by nitrogen Oxides. Journal of Japan Society for Atmospheric Environment 40: 95-103 (in Japanese).
Matsumoto J, et al. (2006) Examination on photostationary state of NOx in the urban atmosphere in Japan. Atmos. Environ. 40: 3230-3239.
Matsumoto J, et al. (2006) Nocturnal sink of NOx via NO3 and N2O5 in the outflow from a source area in Japan. Atmos. Environ. 40: 6294-6302.
Matsumoto J, et al. (2010) Real-time monitoring of individual pollutants and evaluation of OH reactivity in exhaust gas of a mode-driving vehicle utilizing the laser multi-photon ionization technique. Journal of Japan Society for Atmospheric Environment 45: 205-211 (in Japanese).
Matsumoto J. (2013) Measurement of atmospheric potential ozone based on the laser-induced fluorescence technique. Journal of Japan Society for Atmospheric Environment 48: 35-42 (in Japanese).
Matsumoto, J. (2014) Measuring biogenic volatile organic compounds (BVOCs) from vegetation in terms of ozone reactivity. Aerosol and Air Quality Research, in press.
Matsumoto J. (2014) Measurements of Total Radical Reactivity of Volatile Organic Compounds (VOCs) and Total Organic Nitrates to Evaluate Secondary Organic Aerosol (SOA) Formation. [SOA seisei hyoka no tameno zenkutai VOCs no rajikaru hannotai to zenyukishosanryo no keisoku] Earozoru Kenkyu (in Japanese; in press).