Aerosol Liquid Water Driven by Anthropogenic Inorganic Salts: Implying Its Key Role in Haze Formation over the North China Plain
Zhijun Wu, Yu Wang, Tianyi Tan, Yishu Zhu, Mengren Li, Dongjie Shang, Haichao Wang, Keding Lu, Song Guo, Limin Zeng, and Yuanhang Zhang. Environmental Science & Technology Letters Article ASAP.
Air Pollution: a historical problem
Air pollution has been a
problem for Beijing since the county’s rise of industrialization. Haze, especially severe haze events, can affect the health of millions of people, killing an estimated 1.1 million per year. Yet the factors affect haze formation and haze density specifically are still not well understood.
Figure 1. A severe haze event in Beijing. (Left) Beijing on a clear day. (Right) Beijing in February, 2013 from the same view. Source: Wikimedia.
We’ve known since the early 2000s that secondary aerosols, particulates that form from gas in the atmosphere, play an important role in haze formation. These include sulfates, nitrates, and ammonium, among others. But it’s not just these inorganic compounds and the chemical composition that determine the characteristics of haze: it’s also ambient relative humidity and the primary particulate pollution, or more simply, the amount of water in the atmosphere and the amount of dust and other solid particles directly emitted into the atmosphere.
These factors together influence what’s called the aerosol liquid water content (ALWC). Since the haze itself is made up of solid and liquid particles, the actual amount of water in the haze droplets can vary widely, from 2%- 74%. Greater water content means more severe haze: more serious health impacts and lower visibility.
Mitigating these impacts requires understanding how haze forms, and how different factors (like ALWC and inorganic composition) influence the process. Wu and his team set off to study just that.
The Science: methods and findings
The scientists used a variety of methods to calculate the ALWC, including aerosol mass spectrometry, physical filter samples, weather station measurements such as wind speed and direction, ambient humidity, and temperature, and several thermodynamic computer simulations. They also used a three-year data set to confirm that their thermodynamic modeling of a few haze incidents was generally true for a range of conditions in the Beijing atmosphere.
Their data shows some surprising findings, and some not so surprising. For instance, heavy haze events, as depicted in “Episode 1” and “Episode 2” below, occurred after strong northerly winds, which carry in a clean air mass, died down. Following the northerlies are the southerlies: slow-moving air masses from the highly industrialized areas in south and southwest Beijing. The PM2.5 concentration – the mass concentration of solid particulates with diameters of 2.5 um and below – increases and lingers in the area, without any wind to push it out. This generates the heaviest smog days, seen on December 1 and 9, 2015.
Figure 2. Time series of the PM2.5 mass concentrations and chemical composition (left y-axis) and of the ambient relative humidity (right y-axis). The wind speed is represented by the size of the gray circles. Source: Wu et al. 2018.
The figure above shows us something unexpected: a positive correlation between the secondary inorganic aerosol (SIA) mass fraction (the blue, red, and yellow bars corresponding to sulfates, nitrate, and ammonium) and the relative humidity. As humidity increases, the aerosol concentration increases.
We used to think that the concentration of these aerosols in the atmosphere was independent of weather conditions like relative humidity; that the concentration only depended on the amount of pollutants emitted from the ground below. In other words, the more nitrates emitted by power plants, the more nitrates we would expect in the atmosphere. Same amount of nitrates emitted on the ground, same amount of nitrates in the atmosphere.
Wu and his team turned this upside down: given the same amount of nitrates emitted, if humidity increased, we find more nitrates in the atmosphere (see figure 3).
Figure 3. The inorganic fraction increases as humidity increases, leading to more severe haze. Source: Wu et al. 2018.
Positive Feedback: the gift that keeps on giving
Consider a water droplet in the atmosphere. Just as solid table salt dissolves into water, gases dissolve into water, and our airborne droplet is no exception. A thousand feet up in the atmosphere about Beijing, some nitrate gas dissolves into a water droplet.
Figure 4. An innocent water droplet. Source: Wikimedia.
This water droplet can do more than just absorb gases, however; it’s a scaffold for more water or organic matter to stick to, and for water-based reactions to occur on and in. These reactions include transforming pollutants into other compounds (like nitrates, ammonium, and sulfates).
There are rules, however. The droplet must conform to certain thermodynamic laws: it must be in equilibrium with the water vapor in the atmosphere, as well as with Henry’s Law, which defines the maximum amount of a gas that will dissolve into a volume of water- even a droplet-sized volume.
Together, these two laws cause the droplet to grow. As the concentration of gaseous inorganic matter increases in the air, Henry’s Law causes that gas to dissolve into the droplet. The droplet concentration of that compound increases- the droplet is now saltier. Since it must be in equilibrium with the water vapor, it is forced to absorb more water (just like the classic middle school carrot/egg/potato in freshwater osmosis experiment).
The larger water droplet can absorb more gas, and we see a positive feedback loop emerge: larger particles absorb more gas, then must grow in water content, then absorb more gas, and so on.
Implications: where do we go from here?
Haze is bad for many reasons, including decreased visibility and adverse health impacts. Larger water particles can cause more severe haze events, but there are other, more subtle impacts as well.
Larger water droplets serve as scaffolds for other chemical reactions. Having more surface area and more volume essentially means that there are more “reactors”, speeding up transformation reactions of other chemicals in the atmosphere. Chemistry, in this scenario, happens faster, and generates more kinds of chemicals, ultimately leading to faster aerosol mass accumulation over stagnant air periods.
This doesn’t only matter for the North High Plains of China, where coal burning for power generation causes most of the pollution, or for the regions east of there where the wind eventually sweeps the haze (see Figure 5 below). Research in the eastern US and in Italy has found similar results, differing mostly on whether nitrates or sulfates were the primary chemical culprit in ALWC. In the US, sulfate emission regulations have largely limited their impact on ALWC; nitrate emissions, however, have risen to the forefront.
Figure 5. Haze over China. Source: NASA.
Controlling emissions is the primary way to address haze and other air quality issues, whether it’s controlling automobiles, coal combustion, or other manufacturing emissions. This research illustrates a previously unknown relationship between our emissions and local weather conditions, and how they influence each other and ultimately affect human health.