3.5 Conclusions
4.1.1 The AIDA cloud chamber
The AIDA aerosol and cloud simulation chamber is a cylindrical steel vessel (in-ner diameter 4 m, height 7.5 m, volume 84.5 m3) located in a insulating thermally controlled housing. It is well suited for investigating chemical kinetics, aerosol chemistry, aerosol physics, and cloud microphysics in a wide range of atmospheric pressure (1 to 1000 hPa), temperature (60◦C to −90 ◦C), and relative humidity
4.1 Experimental setup
(from almost 0 % to 100 % in the ’static’ mode, and exceeding 100 % in the ’dy-namic mode’).
The chamber can be filled with different gases, such as synthetic air (99.9990%, hydrocarbons<0.5 ppmv, Basi), nitrogen (99.9990%, hydrocarbons<0.2 ppmv, Air Liquide) or argon (99.9990%, hydrocarbons <0.1 ppmv, Air Liquide), both alone or in a mixture with oxygen (99.998%, hydrocarbons<0.2 ppmv, Air Liq-uide). Precisely controlled amounts of trace gases like sulfur dioxide (1% SO2 in nitrogen, Linde) or ammonia (3% NH3 in nitrogen, Linde), can be injected from gas cylinders via a gas handling system with mass flow control. Addition-ally, in order to assess their impact on the aerosol dynamics, a variety of common volatile organic compounds (VOCs), generated by evaporation of a well defined pressure into a 1 l glass bulb, can be mixed with synthetic air and flushed into the AIDA vessel with a 10 l/min flow. Such chemicals are, for example, Toluene (99%, Merck, an anthropogenic aromatic hydrocarbon derivative of benzene), or α-pinene (99%, Aldrich), belonging to the terpene class, representative of bio-genic VOCs. Alternatively, outdoor ambient air, sampled about 10m above the roof of the AIDA building and filtered by a PM10 inlet for particles larger than 10µm, can be injected into the vessel.
Before each experiment the chamber was evacuated to about 1 Pa and flushed several times with synthetic air. Then the desired pressure of water vapor was evaporated from a stainless steel reservoir containing purified water (Nanopure, Barnstead) at 30◦C into the chamber, which was subsequently filled with the de-sired gas at 1 atm pressure.
Artificially prepared aerosols, whose size distribution is previously characterized, can also be sprayed into the chamber, e.g. to serve as cloud condensation nuclei.
For example, in some experiments ammonium sulphate has been used, generated by an atomizer (TSI) from an aqueous solution and added into the vessel via a diffusion dryer.
The AIDA chamber can be operated in either ’static’, meaning constant temper-ature, pressure and relative humidity, and ’dynamic’ mode. The latter simulates the rising of an air parcel in the natural atmosphere, which cools down and sets up favourable conditions for spontaneous water vapor condensation, thus repro-ducing the cloud formation dynamics. During these so called ’cloud activations’
(or ’cloud processing’), transient water and ice super-saturations are obtained for time periods from minutes up to about 1 hour with peak values of∼120%. This operating mode is achieved by a progressive depressurization of the chamber, at typical pumping rates of 50 m3/h, which yields adiabatic expansion and cooling of the air. The condensation of water vapor on residual particles inside the cham-ber during the depressurization cycle and the subsequent flushing of the vessel with fresh synthetic air or other gases result in a very clean, virtually aerosol-free atmosphere; this extremely low background (<1 cm−3) was extremely relevant
controlled atmosphere for our investigations, since it provided the chance to observe the laser-induced production of new particles out of the gas phase.
Optionally, a mixing fan (31 m3/min) located approximately 10 cm below the laser beam homogenized the atmosphere inside the chamber within 1-2 minutes, diluting by a factor of 3×108the species produced in the filament volume. This avoids local inhomogeneities in the particle distribution, which can potentially al-ter the atmospheric chemistry and/or cause artifacts in the detection. Such dilution of the laser-generated plasma by the mixing fan simulated a lateral wind speed of about 1.5 m/s, thus introducing a realistic atmospheric perturbation.
Besides its large volume, what makes AIDA a unique facility for air pollution, weather, and climate research is the combination of its well-controlled atmosphere with the large variety of cloud instrumentation to monitor and characterize atmo-spheric conditions as well as particle concentration, size and shape. A short description of the devices that were employed for our experiments is presented here.
The water vapor concentration, the total water and the condensed water or ice water concentration inside the chamber were simultaneously monitored via ’di-rect Tunable Diode Laser Absorption Spectroscopy’ (dTDLAS), measuring the optical absorption line of water at 1.37 µm along different optical paths. This techniques features a lower detection limit of 100 ppb and a time resolution of 1 s. Alternatively, the total water concentration could be measured from outside the chamber with a dew point mirror, which detects a loss of reflectance from its surface due to scattering from condensed particles. The temperature at which the mirror surface must be kept in order to form such droplets is strictly related to the total water concentration of the ambient air.
Trace gases were sampled via teflon tubes (FEP, 4 mm inner diameter) located right above the mixing fan, about 5 cm away from the laser beam, and monitored by optical and/or photochemical analysers. Ozone was measured by UV absorp-tion with 20 s time resoluabsorp-tion and 1 ppb detecabsorp-tion limit (O341M, Environment SA). SO2 was measured by UV fluorescence with 20 s time resolution and a de-tection limit of 1 ppb (AF22M, Environment SA). Nitrogen oxides were measured by reduced pressure chemiluminescence with 30 s time resolution and 2 ppb de-tection limit (APNA-300E, Horiba).
Aerosols were sampled via stainless steel tubes located above the laser beam, 60 cm away from the AIDA central axis towards the exit window, and their number concentration was monitored with Condensation Particle Counters (3010, 3022 &
3775, 3025 & 3776, TSI) for particles larger than 10, 4 and 2.5 nm respectively.
The particle size distributions were measured with scanning mobility particle siz-ers (DMA 3071 with CPC 3010 and DMA 3085 with CPC 3776, TSI), with a time resolution of 300 s in the size ranges 14−820 nm and 3−63 nm, respectively.
Larger particles, i.e. in the 0.5−20µm size range, were monitored with an
aero-4.1 Experimental setup
dynamic particle sizer (APS 3321, TSI).
CCN particles and cloud hydrometeors were characterized by optical scattering at 488 nm, both forward (θ =2◦) and backward (θ =178◦); the backward chan-nel included a depolarization measurement distinguishing liquid particles from ice crystals or highly aspheric particles. The same discrimination was performed by a ’small ice detector’ probe (SID, Univ. of Herfortshire), which analysed the 2D angular scattering patterns from the particles in order to detect deviations from cylindrical symmetry and infer non-sphericity of the particles. The particles were sampled via a vertical stainless steel tube, as the optical scattering devices were located under the bottom of the AIDA vessel.
Cloud particles (either supercooled droplets and/or ice crystals) were also counted and sized by optical particle counters (OPC, WELAS2000, Palas) in the size range of 0.7−40 µm (OPC1) and 2.3−104 µm (OPC2). Size distributions were scribed by their geometric mean diameter (GMD) and the geometric standard de-viation (GSD) of the log-normal size distribution that fitted the measured one.
The Teramobile laser
The Teramobile container was placed next to the AIDA housing, as shown in the schematic of Fig. 4.1. Two existing ports with 1 m length extension steel tubes
Figure 4.1: Top-view of the relative position between the Teramobile laser and the AIDA building.
controlled atmosphere ensured the optical access for the laser beam into the chamber, through optical glass windows mounted at the end of the tubes.
The requirement to simultaneously generate a filament channel exactly at the cen-ter of the chamber and to avoid premature filamentation along the beam access path or through the glass meant the beam had to be expanded as much as possible and focused it with a lens located just before the input window. Therefore, the magnification factor of the sending telescope of the Teramobile was set in order to increase the beam diameter to 10 cm; the expanded beam was directed upward on the roof of the container, where a periscope reflected it through the housing wall towards the chamber entrance port. A f =4 m lens focused it into the AIDA vessel, close to its base. Facing the input port, an equivalent access was used as the exit for the laser beam, which was blocked downstream by a beam dump.
The laser was operated at 10 Hz, delivering pulses at a central wavelength of 800 nm, with two settings of energy and pulse duration: during one set of experi-ments (’weak’ filamentation), energy and pulse duration were 140 mJ and 140 fs respectively, while for the ’strong’ filamentation set of experiments the energy was 170 mJ and the pulse duration 60 fs. The resulting filament length and number were respectively 1 m, 10 filaments and 0.5 m, 20 filaments, both corresponding to a total filament volume of typically 80 mm3. In each case the filaments diame-ter was 100µm, and the shot-to-shot energy stability was 3% RMS.
A schematic of the AIDA vessel and its detection equipment is displayed in Fig. 4.2.
4.2 Filament-induced particle formation in an arti-ficial lower troposphere
The first experimental campaign at the AIDA facility was aimed at investigat-ing the laser-assisted particle production from a background-free atmosphere, as well as extending our knowledge on the photochemistry which is at the root of the filament-induced water condensation. We therefore focused first on the role of hu-mid nitrogen and oxygen, since they both are key ingredients in the H2O−HNO3 binary condensation pathway [19]. We subsequently evaluated the efficiency of some trace gases (ammonia and sulfates) and organic compounds, which can be typically found in the real atmosphere, in assisting particles formation. The re-sults obtained with different artificial atmospheres, in terms of particle production rates, were compared with the outcome of the same experiments run in the cham-ber filled with outside ambient air.
Most of the measurements were performed in the static mode of the chamber, under atmospheric conditions which were favourable to the laser-induced
wa-4.2 Filament-induced particle formation in an artificial lower troposphere
Figure 4.2: Schematic of the experimental setup displaying the Teramobile laser, the AIDA cloud chamber and the employed detection devices [23].
No. Figure T[K] %RH Main gases Trace
com-pounds
Production rate
[cm−3s−1]
1 4.3-4.4 283.6 62 synthetic air - 3.2×108
2 4.3-4.4 283.6 60 synthetic air NH3 3.7×109
3 4.3-4.4 283.6 58 synthetic air (NH4)2SO4 4.4×109
4 4.5-4.6 283.6 97 synthetic air - 4.4×109
5 4.7-4.8 283.6 64 100% Ar - 4×109
6 4.7-4.8 283.6 97 85% Ar, 15%O2 - 4×108
7 4.9-4.10 283 92 synthetic air 21 ppbα−pin. 3.6×108 8 4.9-4.10 283 90 synthetic air 63 ppbα−pin. 1×109 9 4.9-4.10 283.6 89 synthetic air α−pinene
+7 ppb SO2
4.8×109
10 4.11-4.12 283.6 76 ambient air 1×109
11 4.11-4.12 279.3 90 ambient air 1×109
Table 4.1: Summary of experimental conditions.
controlled atmosphere ter condensation, as it resulted from field experiments in the lower atmosphere (Sec.3.2), i.e. a temperature of approximately 10◦C and relative humidity in the range 70−90%.
In order to assess whether the laser-generated particles can efficiently serve as cloud condensation nuclei, we adiabatically expanded the air in the chamber, to monitor their activation, i.e. their growth to cloud droplets.
4.2.1 Results and discussion
A total of 11 experiments of laser-induced particle generation were conducted (both static and dynamic), as summarized in Table 4.1 together with the corre-sponding atmospheric parameters and the air composition.
Humid synthetic air atmosphere
Humid synthetic air has a simple and perfectly known chemical composition, i.e.
only nitrogen and oxygen. Therefore, it features the same main gas constituents as the real atmosphere, but is free from organic and inorganic contaminants that may also take part to the photo-initiated chemistry.
Two representative experiments are shown for relative humidities of 60 % (Exp.
1-3, Fig. 4.3 and 4.4) and 97 % (Exp. 4, Fig. 4.5 and 4.6). Fig. 4.3 displays the evolution of the air mass in the cloud chamber during the experiment, as char-acterized by several detection devices: particle counters and sizers in the upper panel, atmospheric parameters and trace gas concentrations in the middle panel.
Optical scattering, mixing fan and laser operation are in the bottom one.
Illuminating the chamber atmosphere with laser filaments resulted in the forma-tion of new particles at a producforma-tion rate as high as 4.4×109 cm−3 s−1 in the plasma volume1, at 97 % RH. Hereinafter, the particle production rate is calcu-lated from the increase in measured particle number concentration (particles of di-ameter>3 or 2.5 nm if not stated otherwise), divided by the time during which the laser was propagating into the chamber, and re-scaled by considering the volume ratio between the vessel and the plasmaR=84.5/0.08 m3·cm−3=1.056×109. Such observed production rates exceed, by several orders of magnitude, the typical formation rate of atmospheric nanoparticles. In their review, Kulmala et al. [11]
collected existing data from literature and data banks, covering more than 100 investigations both from campaigns and continuous measurements, and reported formation rates for 3−nm particles ranging from 0.01 cm−3s−1in the boundary layer to 105cm−3s−1in coastal areas and industrial plumes.
1From now on this quantity will be simply denoted as cm3s−1.
4.2 Filament-induced particle formation in an artificial lower troposphere
Figure 4.3: Particle and trace gas formation by laser filaments in the synthetic air for experiments 1-3 at 283 K and∼60% RH. The panels show (a) aerosol particle number (black and red lines) and mass concentrations (blue circles); (b) relative humidity (black line), ozone (blue line), NO (dark green line), and NOx concen-trations (light green line); (c) as well as near-forward scattering (black line), laser power (red line) and mixing fan operation (shaded grey area). Dashed vertical lines indicate the time of events like switching on/off the laser, the fan, or a pump.
Adapted from [23]
The large concentration of new particles in the chamber induced a significant in-crease of the forward scattering signal intensity (Fig. 4.3c).
Considering the extremely low background prior to laser ignition, i.e. less than 1 cm−3as checked by condensation particle counters (CPC), these fresh particles were formed directly out of the gas phase, showing that pre-existing particles are not a fundamental requirement for the laser-induced aerosol production.
The particle production was only visible during the periods when the laser was firing and the mixing fan was not diluting the plasma. Indeed, for all the ex-periments in the AIDA chamber, the mixing fan had a strong influence on the measurements, both qualitatively and quantitatively. When it was switched off,
controlled atmosphere
Figure 4.4: Particle size distributions for selected times during experiments 1-3 of Fig. 4.3. Adapted from [23]
strong local fluctuations (e.g. like moving plumes) of trace gases and particles with high peak concentrations were detected. Homogeneous concentrations could only be observed after turning on the mixing fan; therefore, the amount of trace gases or particles formed by the laser plasma has been quantified exclusively on data acquisition from the periods with the fan operating.
Increasing the relative humidity from 60% to 97% yielded substantial differences in the number, size and mass of the laser-generated particles: while∼1200 cm−3 new stable particles in a monodisperse mode (1.8 nm GSD) of 18 nm diameter were generated at 60% RH (Fig. 4.4), a bimodal distribution of new particles is observed at 90% RH (Fig. 4.6), more precisely∼16000 cm−3 with a diameter of 6 nm, and∼2000 cm−3 with a diameter of 101 nm. The corresponding inte-grated mass increased by a factor of 6, reaching the 90 ng m−3 at 97 % RH and subsequently decaying to 35 ng m−3 over two hours, owing to the instability of such particles. The particle mass is calculated from the measured size distribu-tions assuming that particles are spherical and feature the same density of water, i.e. 1.0 g cm−3. Furthermore, the particle production rate increased by one order of magnitude, from 3.2×108 cm−3s−1at 60 % RH up to 4.4×109cm−3s−1 at 97 % RH.
4.2 Filament-induced particle formation in an artificial lower troposphere
Figure 4.5: Particle and trace gas formation by laser filaments in the synthetic air for experiment 4 at 283 K and∼97% RH. Adapted from [23]
Figure 4.6: Particle size distributions for selected times during experiment 4 of Fig. 4.5. Adapted from [23]
controlled atmosphere These observed differences may be due to the higher amounts of water vapour, consistent with the positive correlation between the absolute water vapour content and the particle increase (Fig. 3.10) observed in the field experiments [19]. Water, indeed, can assist the particle formation e.g. by potentially higher OH radicals yields in the plasma [127, 129], or simply by providing condensing molecules.
Role of nitrogen and oxygen on particle formation
We have interpreted the surprising results on laser-induced water condensation (see Chapter 3) suggesting a photo-chemical pathway relying on HNO3, supported by the observation of ppm-levels of this trace gas inside the filaments [19]. In or-der to address the question whether this is the unique pathway leading to particle generation, we varied the composition of the air inside the chamber, depriving it from O2and/or N2, in order to assess to which extent they contribute to the laser-induced water condensation.
For this purpose, the AIDA chamber was filled with pure humid argon. Surpris-ingly, the particles can still be formed in this nitrogen-free atmosphere; indeed the laser generated particles with diameters of 6 to 20 nm (see Fig. 4.8) with a concen-tration of about 2×104 cm−3in the 84.5 m3AIDA volume. These particles are produced at a rate of 4×109cm−3plasma s−1, with total condensed mass limited to below 20 ng m−3. For a comparable humidity, the experiments in pure argon featured one order of magnitude higher production rates than the one performed in humid synthetic air, but smaller particles (cf. Fig. 4.8 and 4.6), yielding a lower condensed mass.
The particle number concentration measured by CPC3025 was more sensitive to the smaller freshly formed particles than that measured by CPC3022 and showed therefore higher concentrations especially when new particles are formed and the mixing fan was off.
When 15% of oxygen was added into the chamber, a 72 minutes firing period (see Fig. 4.7, 14:40-15:52) resulted in a one order of magnitude lower particle produc-tion rate, as compared with pure argon, and no significant particle mass increase.
On the contrary, as expected, a substantial increase of the ozone concentration was observed, even when the mixing fan was diluting the plasma.
Concluding, we observed that, although with a lower rate, particle production still persisted in argon atmospheres free from elemental nitrogen. These findings illus-trate the need to identify other particle formation channels, based e.g. on the OH radicals, as well as on ions and excited species produced in the filament plasma.
The absence of nitrogen, in fact, inhibited the HNO3pathway [19], which is thus not the exclusive process leading to the formation of new particles.
4.2 Filament-induced particle formation in an artificial lower troposphere
Figure 4.7: Particle and trace gas formation by laser filaments for experiments 5-6 in pure argon and in an argon-oxygen mixture (15% O2) at 283 K and 64 % as well as 55 % relative humidity, respectively. Adapted from [23]
Figure 4.8: Particle size distributions for selected times during experiments 5-6 of Fig. 4.7. [23]
controlled atmosphere Yoshihara et al. [111], for instance, suggested that efficient laser-induced conden-sation can occur on stable H2O2 clusters, although it appears that, at the vapour concentrations considered in their work, corresponding to a molar fraction of 10−7, the stability of such nuclei is not ensured.
As the constituents of the atmosphere under investigation, i.e. hydrogen, oxygen and argon, do not interact to form any other species at low vapour pressure, we may infer that the observed particles originated from contaminants in the simula-tion chamber. Hydrocarbons impurities from argon and oxygen supplies appeared to be present at too low concentrations, according to what is declared by the sup-plier, since their contribution to the global effect could be quantified in a few ng m−3h−1, i.e. 20% of the global yield. The majority of the particles observed in these experiment may thus stem from the laser-induced oxidation of other back-ground VOCs, that could not be completely washed out.
The reduced efficiency in producing particles in the argon-oxygen mixture may be only partially be due to the lower humidity, and it’s unlike to stem from a lower plasma density which is expected to be similar in both gases [45]. Rather, the electron scavenging effect of oxygen may play an important role.
The experiments in a humid Argon atmosphere showed that the VOCs oxidation channel is virtually impossible to inhibit, owing to the unavoidable contamination
The experiments in a humid Argon atmosphere showed that the VOCs oxidation channel is virtually impossible to inhibit, owing to the unavoidable contamination