Environmental Chemistry Letters

, Volume 12, Issue 1, pp 109–116

Water spray geoengineering to clean air pollution for mitigating haze in China’s cities

Authors

    • College of Environment and ResourcesZhejiang University
    • Department of Marine, Earth and Atmospheric SciencesNorth Carolina State University
Review

DOI: 10.1007/s10311-013-0444-0

Cite this article as:
Yu, S. Environ Chem Lett (2014) 12: 109. doi:10.1007/s10311-013-0444-0

Abstract

In the past 30 years, China has suffered from air pollution and heavy haze created by fast industrial growth and economic expansion. This article reviews the techniques for remediation of air pollution. Then, I propose a geoengineering method for mitigating air pollution and haze in China’s cities by using water to scavenge air pollution. Here, water should be sprayed into the atmosphere like watering garden. The scientific rationale and mechanism for the geoengineering scheme are explained. It is found that precipitation scavenging coefficients are very sensitive to the size distributions of both aerosol and raindrops, and rain intensity. I found that the water spray geoengineering method can reduce the PM2.5 pollution in the atmosphere very efficiently to 35 μg m−3 level in a very short time period from few minutes to hours or days, depending on the precipitation characteristics. In addition, the water spray geoengineering method has excellent advantages such as rapidity, already available technology, low cost, and a nature-like process. This proposed geoengineering scheme can be one of the answers for fighting air pollution in the cities globally.

Keywords

Air pollutionHazePrecipitation scavengingGeoengineering approachMegacities

Introduction

Over the past three decades, China, especially in the megacity areas, has suffered from air pollution and heavy haze because of its decades-long burst of economic growth and rapidly expanding clout as an industrial giant. According to new data from the International Energy Agency (www.iea.org/publications/), China will pass the USA to become the world’s biggest energy consumer in 2013. China is currently the world’s second-largest economy after USA, and many economists predict that China will overtake the USA as the world’s largest in the future. On the other hand, China’s rapid expansion has caused a tremendous increase in the emissions of air pollutants and also in the number of heavy haze days [with visibility <10 km under the conditions of 80 % relative humidity (RH)] in the megacities and their vicinities (Chan and Yao 2008; Shao et al. 2006). Nearly 75 % of the urban population has been regularly exposed to air quality that does not meet China’s national ambient air quality standards according to the studies for 360 cities in China in 2004 (Shao et al. 2006).

Fine particle (PM2.5) refers to particulate matter with aerodynamic diameter <2.5 μm and is mainly responsible for the formation of the regional haze in China (Yu et al. 2001; Chan and Yao 2008; Shao et al. 2006). More recently, in January 12 of 2013, PM2.5 concentrations in Beijing rose to a record with 993 μg m3, while concentrations of PM2.5 rose to 297 μg m3 at 12 p.m. near the Tiananmen Square from 195 μg m3 in the past 24 h, according to the Beijing Municipal Environmental Monitoring Center (http://www.bjmemc.com.cn/). Figure 1 shows the examples of the heavy pollution over the China Central Television (CCTV) headquarter building, Shanghai Oriental Pearl Tower (Dongfang Mingzhu), and Tiananmen Square in January of 2013 (http://www.theatlantic.com/infocus/2013/01/chinas-toxic-sky/100449/). In the week of October 24 of 2013, air pollution levels in the northeastern city of Harbin surpassed the previous record levels in Beijing with the PM2.5 concentration levels of 1,000 μg m3. This is 40 times the safety level recommended by the World Health Organization (http://www.huffingtonpost.com/2013/10/21/china-smog-photos-pollution_n_4137675.html). Air pollution recently has become China’ biggest environmental problem.
https://static-content.springer.com/image/art%3A10.1007%2Fs10311-013-0444-0/MediaObjects/10311_2013_444_Fig1_HTML.jpg
Fig. 1

Conceptual pictures of the geoengineering scheme by spraying water into the atmosphere like watering gardens to scavenge air pollution at the top of high buildings and towers (like a CCTV building and b Shanghai Dongfang Tower) and square (like c Tiananmen square). The background is haze pollution in Beijing and Shanghai in January of 2013 based on the pictures from (http://www.theatlantic.com/infocus/2013/01/chinas-toxic-sky/100449/). An example for spraying water into the atmosphere at the top of US Embassy Beijing building to scavenge air pollution is shown in (d)

To mitigate the air pollution in China, intensive efforts have been made to reduce air pollutant emissions in China. To manage air pollution in the megacities of China is not easy because their typical pollution composition reveals fumes from coal burning as well as traffic and heating in the megacities themselves. Technological solutions include development of clean energy resources such as solar and wind energies, nuclear power, and hydropower, promotion of clean coal technologies, and enhancement of vehicle emission control such as public transport and alternative-fuel and advanced vehicles (Shao et al. 2006; Chan and Yao 2008; Wang and Hao 2012). This is because energy consumption, coal-combustion, and car emissions are major sources of air pollutions in China (Shao et al. 2006; Chan and Yao 2008; Wang and Hao 2012). There is a need for implement of a comprehensive control strategy on multiple pollutants and emission sources at both regional and local scales on the basis of air quality observations, modeling, and regulation (Shao et al. 2006; Chan and Yao 2008; Wang and Hao 2012). Some good experiences in North American megacities for mitigating air pollutions with urban and industrial development can be used for other megacities, especially in developing countries like China (Parrish et al. 2011).

In this study, we propose a geoengineering approach for mitigating the air pollution and haze problems in the megacities of China by using water to scavenge air pollution through spraying water into the atmosphere like watering gardens. The scientific rationale and mechanism for this geoengineering approach are described. The arguments in favor of this geoengineering scheme such as nature-like process, speed of action, technological feasibility, cost, efficacy, and its low potential problem and risk are discussed as well.

Scientific rationale (logic) and mechanism for this geoengineering approach

Wet deposition processes refer to the natural processes by which aerosols are scavenged by atmospheric hydrometeors (cloud and fog drops, rain and snow) (Pruppacher and Klett 1997; Seinfeld and Pandis 2006). They are highly efficient in the removal of aerosols from the atmosphere. It is well known that precipitation scavenging is the single most efficient way of removing aerosol pollution in the atmosphere. This is because rain droplets can simply collect the pollutants with them by interception, Brownian diffusion, and inertial impaction with water droplets when they fall through the atmosphere—the mechanism behind acid rain which refers to precipitation with pH < 5.0 (Pruppacher and Klett 1997; Yu et al. 1988, 1990, 1991, 1992a, b, 1994a, b, 1998; Yu 2000). The precipitation scavenging coefficient (μ) is defined as the rate of loss of aerosol particles from the atmosphere by their incorporation into rain drops. The precipitation scavenging coefficient (μ) is a function of location, time, rain characteristics (rain intensity and size distribution), and aerosol characteristics (chemical composition and size distribution) (Pruppacher and Klett 1997; Seinfeld and Pandis 2006).

The precipitation scavenging coefficient (μ) has units of inverse time and can be estimated as follows for mono-disperse aerosols and raindrops (Pruppacher and Klett 1997; Seinfeld and Pandis 2006):
$$ \mu \left( {d_{\text{a}} } \right) = \frac{3}{2} \frac{{E\left( {D_{\text{r}} ,d_{\text{a}} } \right)P}}{{D_{\text{r}} }} $$
(1)
where da and Dr are diameters of aerosol particles and raindrops, respectively, E(Dr, da) is collection efficiency, and P is precipitation rate (rainfall intensity). There are many studies about the collection efficiencies between submicron aerosols and raindrops (Slinn and Shen 1970; Pruppacher and Klett 1997; Andronache et al. 2006; Seinfeld and Pandis 2006; Ladino et al. 2011). The collection efficiency is very sensitive to the sizes of both aerosols and raindrops and relative humidity (RH) (Seinfeld and Pandis 2006; Ladino et al. 2011). For example, the collection efficiency for particle diameter of 0.2 μm can change from ~0.0002 to ~0.5 when the raindrop diameter changes from 1,000 to 25.6 μm at RH = 90 % (Ladino et al. 2011). The collection efficiency for particle diameter of 0.2 μm can change from ~0.04 to ~2.0 when the RH values change from 99 to 50 % at Dr = 25. 6 μm (Ladino et al. 2011). The conditions with smaller raindrops and lower RH favor higher collection efficiencies. According to the calculation for a simplified scenario with precipitation rate of 1 mm h−1 and drop diameters of 0.2 and 2 mm, Fig. 2 shows that precipitation scavenging coefficients for particle diameter of 1 μm can change by a factor of 100 from ~0.0001 to ~0.01 h−1 when raindrop diameter changes from 2 to 0.2 mm (Seinfeld and Pandis 2006). This indicates that in order to obtain useful estimates for precipitation scavenging coefficient, realistic size distributions for both aerosols and raindrops must be known.
https://static-content.springer.com/image/art%3A10.1007%2Fs10311-013-0444-0/MediaObjects/10311_2013_444_Fig2_HTML.gif
Fig. 2

The scavenging coefficients (μ) as a function of aerosol particle diameters for two different raindrop diameters (0.2 and 2.0 mm) under the conditions of a simplified scenario: mono-disperse aerosols and raindrops, and precipitation rate of 1 mm hr−1 (created by modifying and redrawing figure 20.7 of Seinfeld and Pandis 2006). The precipitation scavenging coefficients for particle diameter of 1 μm can change by a factor of 100 from ~0.0001 to ~0.01 h−1 when raindrop diameter changes from 2 to 0.2 mm

On the other hand, precipitation can also efficiently scavenge the gaseous air pollutants such as HNO3 and SO2, strongly depending on the raindrop diameter, because the scavenging rate can decrease by eight orders of magnitude for the three order increase in the raindrop diameter (Seinfeld and Pandis 2006). It is found that for irreversibly soluble gases such as HNO3, typical scavenging rates are in the range of 1–3 % min−1, and these very soluble gases can be significantly depleted within a 30-min rainfall (Seinfeld and Pandis 2006). It is found that raindrops smaller than 2 mm are responsible for most of the scavenging processes for the very soluble gases because very small raindrops fall more slowly with more time to clean the atmosphere and are more efficient for mass transfer (Seinfeld and Pandis 2006).

The so-called geoengineering scheme refers to using scientific technologies to modify Earth’s environment deliberately (IPCC 2007; Crutzen 2006). Nature already offers many examples about the results of precipitation scavenging in cleaning air pollution. For example, the observations of the regional severe haze in megacity Beijing at a Beijing urban atmospheric environmental monitoring station showed that the PM2.5 concentrations decreased from ~220 to ~30 μg m−3 on September 26, 2011 because of heavy precipitation (Liu et al. 2013). Liu et al. (2013) also found that the PM2.5 concentrations increased gradually by the accumulation of air pollutions from September 20 to September 26 and reached at 220 μg m−3 at midnight of September 26 before rain. When we have rain, especially heavy rain, rainwater can clean the air pollution in a very short time period from few minutes to hours or days, depending on the precipitation rates. Figure 3 shows the changes in PM2.5 concentrations (CPM2.5) with precipitation scavenging time for different scavenging coefficients from 0.01 to 3.0 h−1 at three different original PM2.5 concentrations (C0,PM2.5) calculated by the following simple equation:
$$ C_{{{\text{PM}}2.5}} = C_{{{\text{o}},{\text{PM}}2.5}} e^{ - \mu t} $$
(2)
https://static-content.springer.com/image/art%3A10.1007%2Fs10311-013-0444-0/MediaObjects/10311_2013_444_Fig3_HTML.gif
Fig. 3

PM2.5 concentrations as a function of scavenging time for different scavenging coefficients (0.01, 0.1, 1.0, 2.0, and 3.0 hr−1) at three different original PM2.5 concentrations a 100 μg m−3, b 200 μg m−3 and c 1000 μg m−3. The reference lines for 35 μg m−3 (National Ambient Air Quality Standard (NAAQS)) are also shown. PM2.5 concentration removal is very sensitive to scavenging coefficients. To reduce the PM2.5 concentrations to 35 μg m−3 in the atmosphere within 30 minutes, the scavenging coefficients need to be 2.1, 3.5, and 6.7 hr−1 for the original PM2.5 concentrations of 100, 200, and 1,000 μg m−3, respectively

Note that in the above Eq. (2), possible effects from emissions and chemical reactions are not considered (Seinfeld and Pandis 2006).
As can be seen, for the case with the original PM2.5 concentrations of 100 μg m−3, PM2.5 concentrations after 0.5 h scavenging time will be reduced to 99.5, 95.1, 77.9, 47.2, 36.8, 28.7, and 22.3 μg m−3 for the scavenging coefficients of 0.01, 0.1, 0.5, 1.5, 2.0, 2.5, and 3.0 h−1, respectively. For the case with the original PM2.5 concentrations of 1,000 μg m−3, PM2.5 concentrations after 0.5 h scavenging time will be reduced to 995.0, 951.2, 778.8, 472.4, 367.8, 286.5, and 223.1 μg m−3 for the scavenging coefficients of 0.01, 0.1, 0.5, 1.5, 2.0, 2.5 and 3.0 h−1, respectively. Table 1 summarizes the scavenging time needed to decrease the PM2.5 concentrations from three different original PM2.5 concentrations to 35 μg m−3 [National Ambient Air Quality Standard (NAAQS)] level in the atmosphere for different scavenging coefficients on the basis of the calculations from the Eq. (2). The simple calculations in Fig. 3 and Table 1 indicate the very sensitivity of PM2.5 concentration removal to scavenging coefficients. To reduce the PM2.5 concentrations to 35 μg m−3 in the atmosphere within 30 min, the scavenging coefficients need to be 2.1, 3.5, and 6.7 h−1 for the original PM2.5 concentrations of 100, 200, and 1,000 μg m−3, respectively. This suggests the need for realistic scavenging coefficient in order to obtain useful and significant precipitation scavenging for PM2.5 in the atmosphere when the water is sprayed into the atmosphere for this geoengineering approach.
Table 1

The scavenging time needed (h) to decrease the PM2.5 concentrations from three different original PM2.5 concentrations (100, 200, and 1,000 μg m−3) to 35 μg m−3 (National Ambient Air Quality Standard (NAAQS) in the atmosphere for different scavenging coefficients

Scavenging coefficient (h−1)

C0,PM2.5 (μg m−3)

100

200

1,000

0.01

105.0

174.3

335.2

0.05

21.0

34.9

67.0

0.1

10.5

17.4

33.5

0.5

2.1

3.5

6.7

1

1.0

1.7

3.4

1.5

0.7

1.2

2.2

2

0.5

0.9

1.7

2.5

0.4

0.7

1.3

3

0.3

0.6

1.1

The features of the geoengineering scheme and its examples

The arguments in favor of this geoengineering approach include its nature-like process, speed of action, technological feasibility, efficacy, low cost, and side benefits as discussed below.

Precipitation scavenging processes are a natural process, and their behaviors have been studied by models and observations for many years (Pruppacher and Klett 1997; Yu et al. 1988, 1990, 1991, 1992a, b, 1994a, b, 1998; Yu 2000). Several different names are used for the wet deposition including wet deposition, wet removal, rainout (typically used for in-cloud scavenging processes), and washout (typically used for below-cloud scavenging processes). The geoengineering approach we proposed here by spraying the water into the atmosphere to scavenge the air pollutions is related to the scavenging of aerosol particles by the falling raindrops and belonged to the mechanism of below-cloud scavenging processes (washout) theoretically.

In terms of speed of action for this geoengineering approach, precipitation scavenging management can work quickly. This is because the precipitation scavenging can remove air pollution within half hour if enough water is spayed into the atmosphere in the correct manners according to the above study in Fig. 3 and Table 1, and other observations (Yu et al. 1988, 1990, 1991, 1992a, b, 1994a, b, 1998, Liu et al. 2013).

Technological feasibility for this geoengineering approach is that all technologies and material required are preexisting: high building, high tower, water, aircraft, weather modification, automatic sprinkler head to spray water, etc. It should not be a problem to society about deploying such geoengineering scheme with careful and considered evaluation beforehand for each area in the megacities. Spraying water into the atmosphere can result in scavenging air pollution quickly. This is analogous to creating the equivalent of below-cloud scavenging processes in nature. As can be expected, this can be one of the permanent solutions to air pollution in megacities globally, in addition to other solutions such as emission controls. In order to engage in such an approach, some sorts of policies have to be in place such as Haze Action Day. During the Haze Action Day, the water will be sprayed from all possible high locations (high buildings, high towers, etc.) into the atmosphere to scavenge the air pollution simultaneously. Since the air pollution is produced every time and the precipitation scavenging can only remove the air pollution during the precipitating period, there is a need to do the geoengineering scheme on a daily basis to clean the atmosphere daily to avoid the accumulation of air pollution in the atmosphere. This is because most of the regional severe haze problems are results of the accumulation of air pollutions.

For efficacy of the geoengineering approach, certain efficacy issues specific to the use of this geoengineering approach include how, which location, which height, and when to spray water into the atmosphere to have efficient removal of air pollutions. The different ways to spray water into the atmosphere will definitely affect efficiency in mitigating the haze problem. To make the geoengineering approach more efficient in scavenging air pollution, air quality models such as the Weather Research and Forecasting (WRF)–Community Multiscale Air Quality (CMAQ) model (WRF–CMAQ) (Yu et al. 2003, 2004, 2005, 2007, 2008, 2012a, b, 2013) are needed. The models can be used to study the spatial and temporal distributions of the air pollution for each city and forecast the air pollution to prepare for the possible pollution events. Nowcasting air pollution events with models (Yu et al. 2003, 2004, 2005, 2007, 2008, 2012a, b, 2013) can be a good method to get the air quality situation for next few hours at high spatial resolutions and identify the areas and times to carry out the geoengineering approach. The simultaneous monitoring of air pollutions is also needed to see the change in aerosol concentrations and decide when you can stop spraying water into the atmosphere to increase efficacy and low the cost. It has been observed that fine particle concentrations in the 0.3–1.0 μm size range could increase during or just after the light thunderstorm rain because of the increase in RH from <70 to >90 % and not enough precipitation scavenging (Sisterson et al. 1985). The precipitation scavenging coefficients are very sensitive to the raindrop sizes, and we must avoid the complete evaporation of the falling raindrops before reaching the ground. The water delivery system, which should include water pressure system and spraying system with automatic sprinkler head, must be specifically designed to spray specific raindrops with favorable sizes and rain intensity for different heights under the different meteorological conditions. During the severe regional haze episode period, some specific methods should be used to create precipitation scavenging to reduce air pollution. This includes weather modification to produce some precipitation and using aircrafts to spray the water into the air pollution layer to create precipitation scavenging. Definitely, these will be more expensive than spraying water into the atmosphere from the preexisting high buildings and towers.

For low-cost issue, the low-tech nature of this geoengineering approach has led us to believe that it will cost much less than many other interventions such as cutting emissions. If the water spraying system is installed at the top of the buildings in the cities and water can be obtained from the rivers and lakes or any water system easily, the cost for deployment of this geoengineering approach will be very low. The square areas will need to build the high towers to deliver the water to high attitudes and will cost a little more. Besides, the water after falling to ground by this geoengineering approach can still be recollected on the ground and reused for the next precipitation scavenging processes. The geoengineering approach can be used on a massive scale with low cost as expected. If you can offer a half-hour watering your garden, then you can offer a half-hour watering your ambient atmosphere to keep air clean by this geoengineering approach.

There are some side benefits for this geoengineering approach too. Since the precipitation scavenging can also remove other gaseous pollutants such as SO2 and NOx from the atmosphere, the geoengineering approach can serve to mitigate the root causes of the other air pollution problems such as O3 as expected. Although water with some alkaline may increase scavenging efficiencies for acid gaseous precursors such as SO2 and NOx, it is not recommended to add some chemical agents to water, which will be sprayed. This can keep the geoengineering approach as a natural process without some side problems to the environment caused by the added chemical agents. On the other hand, the falling raindrops from the geoengineering approach can help clean the street of the cities as well.

For possible potential problems for this geoengineering approach, geoengineering approach in general is a controversial technique and carries some problems and risks (Crutzen 2006; IPCC 2007). The certain problems are specific to, or more pronounced with this geoengineering approach include possible flood, the humidification of the low atmosphere and slipping grounds in the cold seasons. If too much water is sprayed into the atmosphere at one time and one place, this will cause flood as we can expect. If water droplets do not reach ground but evaporate completely during the falling, this will increase the RH in the atmosphere and increase PM2.5 concentration as mentioned before, leading to much worse regional haze problems (increase foggy days) in the cities. If water is sprayed into the atmosphere in the cold days such as winter time, this will cause slipping grounds because of freezing rain. However, all these potential problems for this geoengineering approach can be avoided with careful and considered evaluation and preparation beforehand for each city area. As we can expect, this technique will not have the potential to create more problems than it solves because all water for scavenging air pollution will go back to the rivers and lakes where it originally comes from although the water chemical composition will have some changes.

Figure 1 shows conceptual pictures of the geoengineering approach by spraying water into the atmosphere like watering gardens to scavenge air pollution at the top of high buildings (e.g., China Central Television (CCTV) headquarters building, Fig. 1a), towers (e.g., Shanghai Oriental Pearl Tower, Fig. 1b), and flat areas (e.g., Tiananmen Square, Fig. 1c). The CCTV headquarters building is 234 m and 44-story skyscraper. Shanghai Oriental Pearl Tower is 468 m high, and Tiananmen Square is the third largest city square in the world (440,000 m− 880 × 500 m). You can imagine how the PM2.5 pollutions in the air will be scavenged if the water is sprayed into the atmosphere like those in Fig. 1 during the hazy days. Another simple and good example which can be done is to spray water from the top of US Embassy building in Beijing (see Fig. 1d) to scavenge the PM2.5 pollution in the atmosphere. This can immediately know how the geoengineering approach works locally in mitigating PM2.5 pollution. This is because the PM2.5 concentrations are already measured hourly and released to public on their Website as an indication of the air quality on the Embassy compound located in Chaoyang district of megacity Beijing (e.g., http://beijing.usembassy-china.org.cn/070109air.html). Since heights of the most buildings in megacities are from ~50 to ~200 m and if 100 or 200 m towers are also builded over the flat areas (such as gardens and squares), then the geoengineering approach proposed here can remove most of PM2.5 concentrations and keep the air clean below these heights. This should be good enough for human life and health because most of the people in the cities are living below these heights. Besides, if most of PM2.5 pollutions are already removed locally for each area in the cities, the long-range transport of air pollutions from the cities to other regions will be significantly reduced. This can lead to clean air regionally, even globally. This proposed geoengineering scheme can be one of the major answers for fighting air pollution in megacities locally, regionally, and globally as expected.

Conclusion

A geoengineering scheme for mitigating the haze problems in the cities of China by using water to scavenge air pollution through spraying water into the atmosphere like watering gardens is proposed in this paper. The analysis of the scientific rationale and mechanism indicates that the geoengineering approach can reduce the PM2.5 pollution in the atmosphere very efficiently to 35 μg m−3 level in a very short time period from few minutes to hours or days, depending on the precipitation characteristics. Nature already provides many examples about the results of precipitation scavenging in efficiently cleaning air pollution. The features of this geoengineering approach such as nature-like processes, quick speed of action, technological feasibility, efficacy, low cost, and some side benefits make it to be an excellent approach for mitigating air pollution in the cities of China. With careful and considered evaluation beforehand for each area in the cities, the geoengineering approach is environmentally safe without significant side effects and can be deployed easily by the society. This geoengineering scheme should be able to help solve the dilemma facing policy makers in China, who are confronted simultaneously with the task to both urban and industrial development and air quality improvement. The megacities without air pollutions will be the best and wonderful places for human being to live. If you can offer a half-hour watering your garden, then you can offer a half-hour watering your ambient atmosphere to keep air clean by this geoengineering approach. Definitely, there are still significant challenges in designing a specific delivery system that is capable of delivering the water in the right manner, right place, and right time to encourage most effective precipitation scavenging for air pollution. These types of scientific research and experiments are currently under way.

Acknowledgments

The author would like to thank Prof. Weiping Liu from College of Environment and Resources at Zhejiang University for his help and support. The author would like to thank Dr. Eric Lichtfouse for his comments and supports, and Dr. Theresa Foley for her help. This work is supported by the “Zhejiang 1000 Talent Plan.”

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