© 2012 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 85, No. 10, 1155-1159 (2012) 1155
Water Aerosol Formation upon Irradiation of Air
Using KrF Laser at 248 nm
Keitaro Yoshihara,*1,2 Yoshiki Takatori,3 and Yoshizumi Kajii2,4
1Toyota Physical and Chemical Research Institute, Nagakute, Aichi 480-1192
2Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397
3Toyota Central R&D Laboratory, Nagakute, Aichi 480-1192
4Graduate School of Global Environmental Studies, Kyoto University, Kyoto 606-8501
Received May 16, 2012; E-mail: yoshihar@tmu.ac.jp
We report water aerosol formation upon irradiation of wet air with a KrF laser at 248 nm. It occurred at temperatures
ranging from 0 to 50 °C and at relative humidity values from ca. 10% to 100%. Aerosols were detected either by light
scattering or by differential mobility analyses, and particle growth from 10 nm to sub-micrometers was observed.
Photodissociation of oxygen was proposed as the initial step of the reactions leading to formation of the final stable
molecule of hydrogen peroxide as a nucleus with a low vapor pressure. Simulations of the chemical kinetics by
considering wet air revealed that the amount of the final product could be controlled by laser operational conditions such
as laser repetition rates and pulse energies. KrF laser light can reach a few kilometers in the atmosphere owing to a small
absorption cross section of oxygen and yet can produce water aerosol.
Water condensation is one of the most important issues
in atmospheric science and human life. Under atmospheric
conditions, a low-vapor pressure material as a nucleus for
condensation is often produced by complicated photochemical
and thermochemical reaction chains.1 Much effort has been
devoted to the artificial control of precipitation through
techniques such as spraying chemicals (e.g., silver iodide) or
dropping coolant materials like dry ice into the air.
We previously reported the photochemical formation of
water droplets/aerosols in response to the irradiation of wet
air by UV light using either a low pressure Hg lamp (185 nm)
or an ArF laser (193 nm).2,3 Additionally, we proposed a
reaction mechanism for this water condensation. Specifically,
the chemical reaction started by photodissociation of oxygen
and further sequential reactions produced hydrogen peroxide
(H2O2) as a final stable product with a low vapor pressure, and
nucleation occurred. The chemical intermediates hydroperoxy
radical (HO2) and ozone (O3) were observed in situ and were
successfully simulated by the chemical kinetic model.2
In the 1970s, photoinduced nucleation of water vapor with
helium or hydrogen as a carrier gas was reported following
continuous VUV or UV irradiation;4,5 however, this did not
receive considerable attention from atmospheric scientists.
More recently, water droplet formation in both the laboratory
and atmospheric air was reported following application of
ionized filaments generated by intense ultrafast laser pulses.6,7
Here, we report water-aerosol formation upon irradiation of
air with a KrF laser at 248 nm. It was observed in the laboratory
by light scattering and by differential mobility analyses under
very wide temperature and relative humidity (RH) conditions.
Simulation of the chemical kinetics revealed that formation of
the final product, hydrogen peroxide, could be controlled by the
operating conditions of the laser, namely pulse energies and
repetition rates. The wavelength of 248 nm is much longer than
that used in our previous study. It is worth noting that at this
wavelength, a laser beam can reach long distances into the air
owing to the small absorption cross section of oxygen and yet
can produce aerosols if there is no scattering by other materials
such as dust or clouds.
Experimental
Two types of reaction vessels were made; a flow vessel for
particle-velocity measurements and a batch cell for laser
scattering experiment. The former was a 55 cm © 31 cm ©
31 cm (53 L) box with two quartz windows for laser incident
and exit directions. The batch cell was a hollow quartz tube
with a diameter of 15 cm and height of 15 cm (2.6 L). Particle
formation was observed by light scattering with a laser pointer
(green light), which was stretched to 2D by a cylindrical lens.
For excitation, we used a pulsed KrF laser (Lambda Physik,
Complex 110F or Coherent, LPX Pro 210) (0.05-0.8 J/pulse
at 248 nm; laser cross section, 1.2 cm © 2.3 cm) at 5-100 Hz
without focusing.
Particle-size distributions (10-1000 nm in diameter) were
analyzed using a differential mobility analyzer (DMA) (MSP,
1000XP). The peak size-distribution values were given in the
dN/d log Dp value of size distribution (Dp is the particle
diameter and N is the particle number cm¹3). The laboratory
air before laser irradiation was much below 103 cm¹3 after pass-
ing through two water bubblers and a 10-cm tube filled with wet
fibers. Based on the measurement time of the particle-size distri-
bution histogram, the system has a time resolution of about 60 s.
To check for possible effects of laser ablation of the quartz
windows and formation of small particles, we removed them