INTRODUCTION

The first laboratory experiments to study the generation of aerosols from precursor gases were carried out by Aitken [1]. As shown in works [2–5], new nanoparticles are generated from aerosol precursor gases in the atmosphere as a result of photochemical and chemical transformations. During evolution, these particles grow to sizes on the order of 100–300 nm and can be considered as atmospheric condensation nuclei for cloud droplets. The main processes of the generation of new aerosol particles are summarized in [6]. Based on observation data in natural conditions, the predominant role of solar radiation (photochemical reactions) in the generation of new particles [7] and of atmospheric ions in their stabilization was noted.

The important role of atmospheric ions in generation of new particles was confirmed by laboratory studies [8, 9] in the CLOUD (Cosmics Leaving Outdoor Droplets) chamber of 26.1 m3 in volume built at CERN with walls made of electropolished stainless steel [10]. No atmospheric air was pumped into the chamber; the air was formed through evaporation of liquid nitrogen and oxygen. Aerosol precursor gases, SO2, NH3 [9], and other organic and inorganic gas components [8], were added to the chamber. The ion composition in the chamber was formed due to additional ionization of air by a beam of π+ mesons.

Before transiting to the aerosol phase, SO2 and organic volatile biogenic compounds should pass oxidation stages, generating a sufficient amount of vapors of low-volatile H2SO4 and highly oxidized molecules of organic compounds. They are first oxidized during reactions with ozone (O3), hydroxyl (OH*), and nitrate radical \({\text{(NO}}_{3}^{*})\) (* means excited state) [11, 12]. The latter rapidly photolytically decays in the daytime; hence, it plays a significant role in the chemistry of the troposphere only in the evening and at night [13–15]. The reactivity of the OH radical* is one order of magnitude higher than of \({\text{NO}}_{3}^{*}\) and five orders of magnitude higher than of O3 [15]. The main mechanism of HO* generation in the troposphere is the interaction of water molecules with metastable oxygen O(1D) generated during photolysis of O3 [11, 15]:

$$\begin{gathered} {{{\text{O}}}_{3}}\xrightarrow{{h\nu }}{{{\text{O}}}_{2}} + {\text{O}}{{(}^{1}}{\text{D}}){\text{,}}\,\,\,\,\lambda < 319\,\,{\text{nm}}, \\ {\text{O}}{{(}^{1}}{\text{D)}} + {{{\text{H}}}_{2}}{\text{O}} \to 2{\text{HO}}{\kern 1pt} {\text{*}}. \\ \end{gathered} $$

In accordance with this, before pumping mixtures of precursor gases into the chamber in [7], α-pinene was oxidized by ozone and by hydroxyl radical (OH*) resulted from ozone photolysis and secondary reactions. In [9], to stimulate photolytic reactions, in particular, oxidation of SO2 to H2SO4 in the presence of O3 and H2O, the content of the chamber was UV irradiated in the wavelength range from 250 to 400 nm using 250 vacuum fiber optic inputs at the top of the chamber. In [7–9], in addition to the size distributions of the resulting particles and air ions, their chemical composition was determined using mass spectrometers, as well as SO2, O3, and NH3 concentrations.

The presence of ions increases the rate of generation of critical nuclei due to their stabilization (ion-induced nucleation), provided that the nucleation rate does not exceed the ionization rate limit [16]. Ion-induced nucleation in the atmospheric boundary layer is limited by the ion pairing rate with a maximum of about 4 cm−3 s−1 [10]. The influence of the chamber walls limited the aerosol generation observation time to five hours.

All of the above indicates the topicality of studying the generation and evolution of new particles in purified atmospheric air in a large-volume chamber.

The aim of the study is to experimentally confirm the generation of new aerosol particles from gas components of atmospheric air inside the Large Aerosol Chamber (LAC).

EXPERIMENTAL STUDIES AND DISCUSSION OF RESULTS

Experimental Setup

The experiments were carried out in the Large Aerosol Chamber of SRA Typhoon, where the conditions are as close as possible to natural atmospheric conditions (Fig. 1).

Fig. 1.
figure 1

Interior of the Large Aerosol Chamber of RPA Typhoon: entrance vestibule 60 × 160 cm2 in size (1); rail for equipment installation (2); suspended platform with temperature sensors (for measuring dry and humid air), an optical transparency sensor, and a photoelectric sensor for measuring the cloud droplet size (3); internal HEPA13 filter with brushless fan (4); tube 18 mm internal diameter and 2 m long for air sampling from LAC to the spectrometer (5); Sapphire-3M ion counter (6).

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Chamber volume is 3200 m3. It is equipped with two (external and internal) aerosol HEPA13 filters to remove aerosols from atmospheric air pumped into the chamber, as well as air inside the chamber isolated from the outside environment. Its design and thermodynamic characteristics are described in detail in [17]; the measuring equipment is described in [18].

The air in the chamber is completely isolated from sunlight and is exposed only to ionizing radiation.

Experiment to Confirm the Absence of Technical Sources of Aerosol Generation in LAC

Aerosol particles were detected by a mobility spectrometer SMPS (Scanning Mobility Particle Sizer) model 3936L88-N (TSI Company). The spectrometer measures the mobility of precharged particles of different size in an electric field. To separate particles by size, a differential mobility analyzer DMA model 3081, included in the spectrometer, was used, which made it possible to measure the particle size distribution in the diameter range from 10 to 1000 nm in 115 measuring channels. In experimental studies, the DMA was adjusted to the range from 15 to 1000 nm. The air was sampled from LAC through a galvanized steel tube 18 mm inner diameter and 2 m long passed through the chamber wall. A similar tube is passed through a window located near the device to take in outside air. The loss of nanoparticles in the air intake tube has not been assessed.

The purification of atmospheric air pumped into LAC and air inside the chamber from aerosols was carried out with HEPA13 filters fixed at the inlet and inside the chamber, respectively.

To confirm the hypothesis that there were no sources of aerosol particles inside LAC, the air contained in the chamber was purified to almost zero concentration in two stages (Fig. 2). During the first stage (starts at t = t1 and lasts approximately 1.5 h), the air pumping into the chamber passed through the external filter. The air in the chamber was almost completely replaced by purified atmospheric air, where the concentration of aerosol particle of >15 nm in size (the lower limit of particle size in the SMPS), was on the order of several tens of particles per cm3. Then the internal cavity of the chamber was isolated from external air inflow. Since newly generated aerosol particles begin to be detected in the chamber some time after filling it with purified air, the second stage of purification was provided (t2). It was performed after the increase in the number concentration of small (∼15 nm) aerosol particles ceased. During the second stage, aerosols generated from the gas components of the atmospheric air pumped into the chamber was removed with the use of the internal air filter for about 2 h (indicated as t2 t3). After the second stage of purification, the aerosol concentration in the chamber is nearly zero (no more than a few particles per cm3) and holds for more than 300 h. Turning on the equipment located in the chamber, including the fan which mixes the air in the chamber, does not affect the aerosol concentration. All of the above described confirms that the walls of the chamber and the equipment inside it do not generate aerosol particles. Therefore, the experimental results can be used to estimate the rate of generation of new particles.

Fig. 2.
figure 2

Time dependence of the number concentration of aerosols (N) in LAC during their evolution after filling with outdoor air passed through external and then internal filters [19].

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During the initial stage of the generation of new particles, their number concentration is low. It can be assumed that coagulation processes are insignificant compared to the generation of new particles. Using the dependence suggested in [6], we have estimates the rate of generation of new particles in air containing aerosol precursor gases characteristic of the air pumped into the chamber: JA ∼ (0.7−0.8) cm3 s−1.

We associate the appearance of new particles in a completely dark room with the formation of ions inside the chamber, which are involved in the chain of chemical transformations of aerosol precursor gases which compose atmospheric air.

Measurements of Ion Concentrations in LAC

At the beginning of the experiment, outside air with the moisture content a = 8 g/m3 and the concentration of aerosol particles Na = 7200 cm−3 was pumped into LAC without filtration. After filling the chamber, the air temperature became equal to the temperature of the chamber walls, 25°C. During the experiment, air was filtered from aerosols. Filtration began after stabilization of the particle number concentration and was performed four times for 45 min. After the filtration, the number concentration of aerosols was measured with the SMPS spectrometer. Concentration of ions of both polarities with the mobility above 0.4 cm2/V s was determined using the Sapphire-3M air ion counter. The average exposure gamma radiation dose in LAC remained almost unchanged within 9.4 µR/h (measured with a DGDM dosimeter) through the experiment. Experimental results are shown in Fig. 3. The arrows show the values of atmospheric aerosol concentration (Na, cm−3) after each 45 min of operation of the internal filter. Note that at aerosol concentrations Na < 100 cm−3 in LAC, the concentrations of negative and positive ions attained maxima (3 × 103 and 4 × 103 cm−3, respectively), i.e., at a given exposure gamma radiation dose rate, the rate of ion generation became equal to the rate of their recombination.

Fig. 3.
figure 3

Variation in the concentration of air ions (n) in LAC with a change in the aerosol concentration.

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The ion concentration after turning off the internal filter first sharply increases, then the increase slows down until the next filtering session. Apparently, the sharp increase in the ion concentration is associated with a sharp decrease in the ion sink due to a decrease in the aerosol concentration after filtering at a constant intensity of their generation, since the average radiation dose in LAC remained almost unchanged throughout the experiment. The ion concentration then stabilized at a higher level. The reduced content of aerosol particles in the air in LAC leads to an increase in the ion lifetime and, hence, to an increase in their concentration.

Dynamics of Aerosol Particle Generation and Their Evolution

Fifteen experiments were performed in summer and fall and four experiments in winter in 2018–2019. In all the experiments (except one in winter), the generation of new particles was noted. This confirms the opinion [2–5] about generation of new aerosols in nature. Experimental measurements of ion concentration in LAC confirm the dependence of generation of particles on the time of day and season when air is pumped into LAC other conditions being the same (in the absence of light).

We did not determine the composition of air pumped into LAC, as well as the concentration of trace gases. At a qualitative level, we noted that the mass concentration of particles newly formed in LAC in filtered atmospheric air is more variable and the spectra of their size distribution are narrower in summer and autumn.

Let us consider the case of August 30, 2018, when air with a temperature of 20°C and moisture content of 12 g/m3 began to be pumped into the chamber through the external filter at 12:17 local time. The particle generation and evolution of their size spectrum were studied during six days. The results are shown in Fig. 4 [19].

Fig. 4.
figure 4

Generation and evolution of secondary aerosol in LAC after filling it with air purified from aerosol by the external HEPA13 filter (August 30, 2018); the curves correspond to the beginning of 10-min measurement cycles in hours counted from the time when the chamber has been filled; the vertical line shows the lower SMPS measurement threshold [19].

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To compare the characteristics of newly generated aerosol particles and aerosol of atmospheric air, we use the spectrum of aerosol in outdoor air (curve with the index “external”) with the concentration Naext ≈ 4 × 103 cm−3. The time t = 0 corresponds to the beginning of recording the aerosol spectra in LAC; it coincides with the time of completion of filling the chamber with purified air. The aerosol concentration measured at this time Na0 = 27 cm−3. Since particles were recorded at that time only in separate channels, the curve “t = 0” is represented by individual dots. Other time points correspond to the time in hours from this time point to the beginning of the corresponding measurements. One can see the appearance of additional particles with sizes above the lower threshold of 15 nm already after 1/3 h against the background of the residual aerosol. An almost complete distribution function is formed in 3 h, and particles smaller than 15 nm are absent in the function in 20 hours.

The experimental results in summer and autumn show that the concentration of gases transformable into the aerosol phase, which can be estimated by the mass concentration of newly generated particles, is maximal in the daytime air (0.6 μg/m3); it decreases to 0.2 μg/m3 in the evening and is 0.09 μg/m3 at night.

The results of winter experiments are qualitatively different from summer and autumn experiments.

The first difference is that the power of sources of organic volatile biogenic compounds emitted by vegetation is extremely low in winter. The same was observed in Antarctica [20], where appearance of new aerosol particles in winds from the open ocean and the absence of new particles in winds from the continent or ice fields were noted.

The second difference is that the number of hydroxyl groups, which presumably can participate in generation of new aerosol particles in the purified air in LAC, is usually smaller in winter at negative air temperatures and low absolute humidity.

We examined four cases where the surrounding area was completely covered with snow, wind was weak, and air temperature was negative. One of the experiments was carried out at a relatively low temperature (T = −11°C). The meteorological conditions under which air was pumped into the chamber corresponded to the edge of an anticyclone with an isothermal temperature profile in the layer up to 300 m, after a light snowfall. The number and mass concentrations of outdoor air aerosol were small (Na = 1400 cm−3, Ma = 1.2 µg/m3). No new particles were observed during 22 h of observations in that experiment. In other winter experiments, which were carried out at moderate negative temperatures of air pumped into the chamber (from −1 to −7°C), new aerosol particles were detected in the chamber. Moreover, their mass concentration was lower than in the summer and autumn experiments and ranged from 0.08 to 0.095 μg/m3.

CONCLUSIONS

The results of our experiments enable us to draw the following conclusions.

(1) New aerosol particles with diameters >15 nm are detected in atmospheric air in the Large Aerosol Chamber isolated from the environment in the dark in 20 min after air filtering from aerosols. The particles become larger within 20 min. The process of generation of new particles in the air lasts no more than 20 h.

(2) Since the mass concentration of newly generated aerosol is proportional to its density, and its total mass is equal to the total mass of condensing components, it is possible to estimate (based on measurements of the size spectra of aerosol particles) the mass concentration of condensing components at 0.09 μg/m3 (night) to 0.6 µg/m3 (day) assuming an aerosol substance density of 1 g/cm3. If we assume the density of the aerosol substance to be, for example, 2 g/cm3, then the mass concentration is multiplied by 2, and so on.

(3) Experiments have shown that the number concentration of atmospheric ions in absolutely clean air (in the absence of aerosols) in LAC under a radiation doze of ∼9.4 µR/h can reach limit values of 3 × 103 cm−3 for negative and 4 × 103 cm−3 for positive ions.

(4) Since neither the walls of LAC nor equipment located inside it generate new particles, LAC is suitable for experimental studies of generation and evolution of aerosol particles in atmospheric air in the absence of sunlight.