Ⅱ-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 51 (2018)

Measurements of nitric acid formation in humidified airby proton irradiation
Y. Nakai,∗1 S. Tomita,∗2 R. Suganuma,∗2 S. Hattori,∗3 K. Kobayashi,∗3 Y. Shiina,∗2 S. Ishino,∗3 N. Yamamoto,∗2and K. Sasa∗4
Gas-phase particle formation by the irradiation ofionizing particles (ionizing radiations) is an important
process involving condensable chemical species, such asa water, nitric acid, and sulfuric acid, under conditions of atmospheric pressure and temperature. Thisphenomenon, the so-called “ion-induced nucleation,”
has attracted attention as one of the important processes leading to cloud particle formation in the atmosphere.1,2) Sulfuric and nitric acids are thought to beimportant chemical species for the nucleation processes
in the atmosphere. However, the detailed processes ofsuch acid formations by the irradiation of ionizing particles have not been identiﬁed experimentally. Furthermore, large injection of ionizing particles into the atmosphere, such as solar proton events, causes an increaseof the atmospheric nitric acid.3) For understanding theinﬂuences of such injection events on the atmosphericenvironment, laboratory experiments on nitric acid formation by ionizing particle irradiation potentially canprovide helpful information. Therefore, we investigated
the formation processes of sulfuric and nitric acids by
the irradiation of protons. As the ﬁrst step, we performed experiments for nitric acid formation in humidiﬁed air by the irradiation of a proton beam.
The setup for the present experiment consists of a humidiﬁed sample-gas generator, an irradiation chamber,a monitor for the sample gas pressure, and a ﬁlter-packholder used for collecting the nitric acid that forms inthe sample gas. Wet air was generated by bubbling pure
air in ultra-pure water. The sample gas was producedby mixing the wet air with pure dry air. The humidityof the sample gas was adjusted by controlling the ﬂow
rates of the wet and dry gases. The total ﬂow rate of
the sample gas was maintained at 2.5 SLM (Standard
Liters per Minute) using mass ﬂow controllers. Thehumidity of the sample gas was monitored using a dewpoint meter. The sample gas was introduced into theirradiation chamber consisting of a borosilicate glass
nipple tube with a length of 25 cm. The 6 MeV proton
beam delivered from the 6 MV Pelletron accelerator at
Tandem accelerator complex in University of Tsukuba
was introduced into the irradiation chamber through apolyimide-ﬁlm window. The proton beam penetrates
along the glass tube, as well as the sample gas ﬂow.
After irradiation by protons, the sample gas was transported into the ﬁlter holder in which three kinds of
∗1
∗2
∗3
∗4

RIKEN Nishina Center
Institute of Applied Physics, University of Tsukuba
School of Materials and Chemical Technology, Tokyo Institute of Technology
Tandem Accelerator Complex, University of Tsukuba

Fig. 1. Concentrations of gas-phase nitric acid in the
proton-irradiated gas versus beam intensities (preliminary).

ﬁlters were stacked along the gas ﬂow to collect thegas-phase nitric acid and other chemical species in theirradiated gas (Filter-pack method4) ). A polyamide
membrane ﬁlter was used for collecting the gas-phase
nitric acid. The collected nitric acid on the polyamide
ﬁlter was dissolved in 40 mL ultra- pure water. The
amount of nitric anion, NO3 − , in the solution was measured using ion chromatography analysis. The amountof nitric acid in the irradiated gas was estimated fromthose of NO3 − ions in the solution under the assumption that the collection eﬃciency of the gas-phase nitricacid with the polyamide ﬁlter was 100%.
Figure 1 shows the preliminarily results for the concentrations of the gas-phase nitric acid produced in theproton-irradiated gas under the condition of 1 atm pressure, ∼42% relative humidity, and ∼296 K temperature; the concentration was estimated by dividing theamount of collected nitric acid by the total volume ofgas passing through the ﬁlters. It is found that the production of nitric acid is almost proportional to the beam
intensity. Certain factors, such as collection eﬃciencyof the gas-phase nitric acid, are yet to be considered.
Further analysis and consideration are now proceeding.
References
1) J. Curtius, E. R. Lovejoy, K. D. Froyd, Space Sci. Rev.
125, 159 (2006).
2) F. Arnold, Space Sci. Rev. 137, 225 (2008).
3) B. Funke et al., Atmos. Chem. Phys. 11, 9089 (2011).
4) Network Center for EANET, Technical Manual for AirConcentration Monitoring in East Asia, (2013).

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