Negative air ions

Negative air ions (NAI) are mainly composed of oxygen-containing negative ions in the air combined with several water molecules to form atomic groups. According to the theory adopted by the Joint Atmospheric Commission of the International Union of Geophysics and Geodesy, NAI is O2−(H2O)n, or OH−(H2O)n, CO4(H2O)2, which is a general term for negatively charged single gas molecules and their light ion groups. Since oxygen molecules are more electrophilic than CO2, N2 and other molecules, oxygen molecules will preferentially obtain electrons to form negative ions, so NAI is mainly composed of negative oxygen ions, so it is often called air negative oxygen ions.

Research history
In 1889, German scientists Elster and Geitel first discovered the existence of NAI. At the end of the 19th century, German physicist Dr. Philip Leonard first proved the efficacy of negative oxygen ions on the human body in academic research. Scholars such as Aschkinass and Caspari further confirmed the biological significance of NAI in 1902. In 1932, the world's first medical NAI generator was born in the United States. In July 2020, the Advanced Manufacturing R&D Center of the Tianjin Institute of Advanced Equipment, Tsinghua University, successfully developed a medical and healthcare-grade high-concentration negative oxygen ion generator. It only needs to be sprayed on the walls of the room to form a uniform and dense nanoparticle layer on the wall, so that the indoor walls can stably and long-term release high-concentration small-particle negative oxygen ions.

Generation mechanism
In addition to nitrogen, oxygen, carbon dioxide, water vapor and various aerosol particles, there is also some ionized air in the atmosphere. Ionized air includes negative and positive ions. Air molecules are composed of atoms, which are made up of nuclei and electrons. The nuclei carry positive charges and the electrons carry negative charges. When air molecules are ionized and receive enough energy, the outer electrons that are freed from the nuclei become free electrons, and the neutral molecules or nuclei that lose electrons become positive air ions. When neutral molecules or atoms in the air capture the free electrons that escape, they become negative air ions.

NAIs include single gas molecules and light ion clusters with negative charges. NAIs and positive air ions exist simultaneously in the atmosphere. According to the size of the atmospheric ions, they can be divided into large ions, medium ions and small ions. Small ions are ions of molecular size; large ions are ions that are adsorbed by aerosol particles and carry positive or negative charges; and when several neutral molecules gather around air ions, they become medium ions, which are between small ions and large ions. Most of the NAIs discussed in current research refer to small negatively charged air ions.

Generation methods
There are various ways in which negative ions are generated in the air, and here are some of the main methods:

Natural environments

 * Solar radiation : Solar radiation can generate a significant amount of ions in the air, especially in high-altitude areas and coastal regions.
 * Areas with lower air pressure and humidity : The concentration of ions is higher in areas with lower air pressure and humidity.
 * Forests, waterfalls, and beaches : Forests, waterfalls, and beaches are places with very high concentrations of negative oxygen ions. Trees release negative ions, and a large number of negative ions are also generated when water droplets break.
 * Thunderstorms: Oxygen and nitrogen undergo ionization when they are subjected to high-speed motion or an electric field, resulting in the production of both negative and positive ions. For example, thunderstorms and lightning during electrical storms release a large number of ions into the atmosphere.

Artificial ionization
There are several ways to produce artificial ions in the air, including corona discharge, thermal electron emission from hot metal electrodes or photoelectrodes, radiation from radioactive isotopes, ultraviolet light, etc.

Air Purification
When it comes to eliminating smoke and dust, both large-sized negative ion and small-sized negative ion can directly remove smoke. The difference lies in the fact that small-sized ions have higher reactivity, so they can eliminate smoke quickly, while large-sized ions have lower reactivity, leading to a slower removal of smoke. The World Health Organization and the China Meteorological Administration both define the concentration of negative oxygen ions in the air as an important condition for measuring air quality. They both agree that only when the concentration of negative oxygen ions in the air reaches 1000-1500/cm³ or above can it be considered fresh air. Research by the EPA laboratory of the United States Environmental Protection Agency and many experts in the field of air environmental protection of the United Nations have shown that high-concentration negative oxygen ions can effectively eliminate the following pollution sources:

Ecological-grade negative oxygen ions can actively capture various small dust particles in the air, causing the pollution sources to condense and precipitate, effectively removing dust particles of 2.5 microns (pm2.5) and below in the air, and even 1 micron particles, thereby reducing the harm of pm2.5 to human health. Ecological-grade negative oxygen ions can actively remove indoor air pollution, react chemically with carcinogens such as formaldehyde and benzene, and decompose into non-toxic and odorless carbon dioxide and water.
 * Dust removal:
 * Decomposition of toxic gases :



△ Chemical formula of formaldehyde decomposition by negative oxygen ions Ecological-grade air-negative oxygen ions have high activity and strong redox effects. According to tests, in a high-concentration negative oxygen ion environment, the number of mold and bacteria in the air can be reduced by more than 90%.
 * Antibacterial, sterilization, and virus elimination :

Promoting health
Negative oxygen ions have high reactivity and strong redox properties. They can disrupt the cell membranes of bacteria or the activity of cytoplasmic enzymes, thus achieving antibacterial and sterilization effects. Research has found that negative oxygen ions can combine with positively charged particles such as bacteria, dust, and smoke, causing them to aggregate into balls that fall to the ground. This helps in killing germs and eliminating odors (such as those from cigarette smoke and harmful gases released from construction materials).

Determination method
NAI determination is divided into NAI determination and NAI identification. NAI determination can be achieved by measuring the change in atmospheric conductivity when NAI passes through a conductive tube. NAI identification is achieved by identifying ions produced by mass spectrometry using a corona source, which can effectively measure the characteristics of a single molecule. This method has been used to identify a variety of negative ions, including O−,O2−,O3−,CO4−,NO2−, and NO3−.

Evaluation method
There is no unified standard for the evaluation of negative ions in the air at home and abroad. The main evaluation indexes include the monopolar coefficient, the ratio of heavy ions to light ions, the Abe air quality evaluation coefficient (CI), and the relative density of air ions. Among them, the monopolar coefficient and the Abe air quality evaluation coefficient are the most widely used evaluation indicators.

Monopole coefficient (q)
In a normal atmosphere, the concentrations of positive and negative ions in the air are generally not equal. This characteristic is called the monopolarity of the atmosphere. Monopolarity is expressed by the monopolar coefficient, which is the ratio of positive ions to negative ions in the air, that is, q=n+/n-. The smaller the monopolar coefficient, the higher the concentration of negative ions in the air is than the concentration of positive ions, and the more beneficial it is to the human body. Research by Japanese scholars shows that when n- is greater than 1000 cm-3 and the q value is less than 1, the air is clean and comfortable. When q>1, the air is not clean, and when the q value increases to more than 3, people will feel irritable and uneasy. Generally, the q value in the lower atmosphere is between 1 and 1.2; the q value in areas with more vegetation is less than 1; and the q value on high mountains can be as low as 0.53.

Abe Air Quality Evaluation Index (CI)
Japanese scholar Abe established the Abe Air Ion Evaluation Index through research on air ions in urban residential areas. The Abe Air Quality Evaluation Index reflects the degree to which the ion concentration in the air is close to the natural air ionization level, CI=n-/1000q.

CI is the air quality evaluation index, n- is the concentration of negative ions in the air (pcs·cm-3), q is the monopolar coefficient, and the number of negative ions of 1000 pcs/cm-3 is the most basic requirement standard for the human body. The air quality evaluation index takes negative ions in the air as an indicator, while also taking into account the composition ratio of positive and negative ions. It is relatively comprehensive and objective. Therefore, it has been widely used in the evaluation of urban air ions abroad.

The larger the CI value, the better the air quality. It can be divided into the following five levels: when CI>1.00, the air quality is Class A, the cleanest; when CI is 1.00~0.70, the air quality is Class B, clean; when CI is 0.69~0.50, the air quality is Class C, medium; when CI is 0.49~0.30, the air quality is Class D, acceptable; when CI<0.29, the air quality is Class E, 0.29 is the critical value, and air below 0.29 is polluted air.

Plant community pattern
The oxygen released by the photosynthesis of plants is easily photoelectrically affected by short-wave ultraviolet radiation to form oxygen-negative ions, thereby increasing the level of negative air ions in a small area. The concentration of negative air ions in areas covered by green vegetation is much higher than in other areas. Different vegetation types and plant communities, different forest ages and canopy densities have very different effects on the concentration of negative air ions.

Environmental meteorological factors
There are many reports on the correlation between negative air ion concentration and meteorological factors. Studies generally believe that the concentration of negative air ions is positively correlated with humidity and negatively correlated with temperature. There is controversy over the effect of wind speed on the concentration of negative air ions. The presence of water bodies has a great influence on the concentration of negative ions. For example, the content of negative ions in the air in waterfalls, fountains, coastal areas and after thunderstorms will increase significantly. In addition, suspended matter, carbon dioxide, sulfur dioxide and nitrogen oxides in the air are significantly negatively correlated with air ions.

Seasonal dynamics and diurnal dynamics
The change in negative air ion concentration has significant seasonal dynamics and diurnal dynamics. Generally speaking, the seasonal dynamics of negative air ion concentration are highest in summer, followed by spring and autumn, and lowest in winter. The daily dynamics are that the average daytime negative air ion concentration is higher than the average nighttime negative air ion concentration, with the highest concentration in the early morning to morning, followed by noon to afternoon, and a certain rise in the evening, while the lowest concentration is at night.