3.3. Natural and Artificial Corona Discharge Including Thunderstorms and Lightning
The atmosphere surrounding the earth is subjected to a natural electric field and its intensity
is continuously fluctuating under both local and global influences [9]. The local influences include
geographical location and weather conditions such as thunderstorms, rain, fog, mist, and so on; the
global facts refer to classical daily electric field variations [4]. When leaf points or branches of trees
have a high potential difference from their surroundings in their electric fields, corona discharge
(also called point discharge) occurs and NAIs may be released [5,10]. Generally, corona discharge
Int. J. Mol. Sci. 2018, 19, 2966 4 of 19
occurs at the atmosphere conditions under high average electric fields [5]. For example, in a mountain
area, high electric fields and low atmospheric pressure promote the onset of corona discharge [11].
Thunderstorms and lightning will generate very high electric field conditions and corona discharge
subsequently occurs. Therefore, NAIs will be released at a huge amount after thunderstorms and
lightning. However, released NAIs will be gradually decayed with the discontinuous thunderstorms.
In addition to thunderstorms and lightning, mist may also contribute to NAI generation. In a forest,
electric field variations were observed during the mist formation and dissipation, which may trigger
corona discharge and NAI generation [9].
Artificial corona discharge is an efficient way to generate NAIs. When a high negative voltage
is applied to a conductor/electrode and generated electric field is high enough, corona discharge
occurred [12,13]. If a charged conductor/electrode has a needle-type with a sharp point, the electric
field around the tip will be significantly higher than other parts and air near the electrode can become
ionized and NAIs are generated [14]. Intensity of corona discharge depends on the shape and size of
the conductors as well as applied voltage. Irregular conductor, especially with a sharp point, gives rise
to more corona than a smooth conductor and large-diameter conductors produce lower corona than
small-diameter conductors; the higher the voltage applied, more NAIs are generated [14,15]. The closer
the distance to corona point, the higher NAI concentration is detected as continuous generation of
NAIs by corona discharge is related to a chain reaction process called an electron avalanche [16].
The application of artificial electric field and corona discharge on plants was carried out as early as the
1960s [17,18]. Bachman and Hademenos (1971) showed that under high voltage, artificially applied
electrical fields near the pointed barley leaf tips were intensified [19] and as a result, corona discharge
occurred and air ions and ozone were generated. Studies mainly focused on biological effects such as
growth response, evaporation, and plant damage as well as the effects of generated ozone and NAIs
on plant growth [11,18–22].
3.4. The Shearing Forces of Water (Lenard Effect)
The considerable numbers of NAIs are found under waterfalls or in the seashores. These NAIs are
generated by Lenard effect. Lenard effect was also called spray electrification or waterfall effect and
was first systematically studied by Philipp Lenard [23], who won the Nobel Prize for Physics in 1905
for his research on cathode rays and the discovery of many of their properties. The study showed that
NAIs were generated from the surrounding air molecules by charging themselves negatively when
water droplets collide with each other or with a wetted solid to form fine spray of drops. The study
also showed that several factors may affect the degree of charge separation in spray processes and,
therefore, may affect the generation and concentration of NAIs. These factors include water drop
temperature, dissolved impurities, speed of the impinging air blast, and foreign impinging surfaces
of droplets. Based on the “Lenard effect”, water shearing appliance has been designed to generate
NAIs [24]. Water shearing produced only superoxide ions (O2
−) which was bound to clusters of water
molecules to form the structure O2
−(H2O)n [25], and was essentially regarded as a natural source of
NAIs [24]. NAIs generated by the “Lenard effect” might improve erythrocyte deformability, thereby
aerobic metabolism [24].
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3.5. Plant-Based NAI Release under Normal Growth Conditions and by Pulse Electric Stimulation
Plants were reported to have the ability to generate NAIs under normal growth conditions
and have been regarded as natural resources for NAI generation [26,27]. Different plants released
different amounts of NAIs under natural growth conditions (Table 1). However, under normal growth
conditions, plants released very low concentration of NAIs (<200 ions/cm3
, Table 1). Bachman and
Hademenos (1971) reported the NAI generation by applying high-voltage electric field to plants [19].
Later, Tikhonov et al. (2004) showed that plants could release huge amounts of NAIs under pulse
electric field (PEF) stimulation [28]. Since then, several other studies were carried out to investigate
the effect of PEF stimulation on plant NAI generation (Table 2). Generally, under natural growth
Int. J. Mol. Sci. 2018, 19, 2966 5 of 19
conditions, plants release less than 200 ions/cm3
(Table 1). However, after PEF stimulation, more than
3.5 × 106
ions/cm3 were detected (Table 2). Several parameters may affect the NAI release under
PEF stimulation including plant species and output voltages in PEF stimulation (Table 2) as well
as light intensity, temperature, pulse interval, and the pulse width of PEF [28–30]. Studies of plant
morphology on NAI release showed that species with blade shapes generated higher concentration
of NAIs [27]. All these studies may provide an alternative to artificially generate NAIs through a
plant-based method.
Besides the abovementioned 5 different sources of NAIs, some other ways also may generate
NAIs. For example, the friction by rapidly moving great volumes of air over land may generate NAIs
and storms have been also regarded as an energy source for NAI generation [31].
