Trinh Thai Ha

称号及び氏名博士（工学）

学位授与の日付

平成 29 年 9 月 25 日

論A Study of Gaseous Nitrous Acid: Exhaust-Induced
Emission and Variations in the Ambient Atmosphere ofJapan and Vietnam Urban Sites

文名論文審査委員

主査

竹中

規訓

副査

長岡勉

副査

林晃敏

論文要旨
Due to its significant role to the atmospheric processes as one dominant source of hydroxyl (OH)radical, nitrous acid (HONO) catches more and more attention from the scientific community. Previousstudies have reported that HONO can contribute up to 33 - 56 % of total OH radical production in urban,
rural and forest areas. In recent years, numerous field campaigns and simulation studies have been
conducted in order to enrich scientific understandings on HONO chemistry, its sources and sinks under
different conditions. In general, scientists have proposed five formation pathways of HONO, including directemission, homogeneous gas phase reactions, heterogeneous reactions, photolysis / photo -enhanced reactions,and biological processes.
Reactive nitrogen species emission from the exhausts of gasoline and diesel vehicles, including
nitrogen oxides (NO x) and HONO, contributes as a significant source of photochemical oxidant precursors
in the ambient air. The HONO / NO x ratio is commonly used to quantify the HONO emission from vehicle
exhaust and traffic. A profound traffic-induced HONO / NO x ratio of approximately 0.8 %, established by
Kurtenbach et al. (2001), is recently under controversial discussion because it possibly linked to theunderestimation of OH radical budget and ozone level in the morning.

Most of the published studies on HONO were performed in America, Europe and China. Up to thepresent, only a few HONO measurement studies have been conducted in Japan; therefore, understanding ofHONO chemistry in the region is limited. Meanwhile, continuous measurement of HONO has never been
conducted in Vietnam, particularly in Hanoi (Vietnam). The works presented in this thesis focused on themeasurement of HONO emission in vehicle exhaust and monitoring of HONO in the ambient atmospherein Sakai (Japan) and Hanoi. It is believed that the results will contribute significantly to the understandingof atmospheric HONO production and loss, its emission factors from diesel- and gasoline-fueled vehicles
and the expansion of the global HONO mixing ratios map into two under-reported urban sites.
In Chapter 1, an introduction of HONO’s role in atmospheric chemistry and its measurement
methods, HONO emission from vehicle exhaust, and the current state of global and regional researches onambient HONO, were presented. The content of the thesis was also rationalized.
In Chapter 2, the chassis dynamometer measurement of exhaust gases from two diesel andgasoline vehicles equipped with the updated emission reduction technologies was reported. The results
comprised of (1) integrated emission as well as temporary profiles of HONO and NO x emission from thetested vehicles in compliance with the Japanese new long-term emission regulation; (2) the HONO / NOx
mixing ratios in the exhausts from these vehicles and the comparison of our results with previous studies.
The emission of regulated gaseous compounds was also measured and reported in this work. For the tested
gasoline vehicles equipped with three-way catalytic converter (TWC) after-treatment device, HONO /NO x ratios ranged from 0 to 0.95 % with very low average HONO concentrations. For the tested dieselvehicle equipped with diesel particulate active reduction (DPR) device, HONO / NO x ratios varied from
0.16 to 1.00 %. Under the JC08 real-world driving cycles, the average HONO emission from the tested
diesel vehicle (approximately 868.8 ppb) was dramatically greater than that from the tested gasolinevehicle (1.2 ppb). Our study suggested that the unexpected exceedance of NO 2 concentration for sootoxidation process in diesel particulate filter was a significant factor which affected the production ofHONO. The results also suggested a minor contribution of TWC-installed gasoline vehicle emission to thetotal HONO budget while such contribution of DPR-installed diesel vehicle emission is dramaticallyhigher.
Chapter 3 focused on seasonal variations of gaseous HONO in the urban ambient atmosphere in
Sakai and Hanoi. It provided the major results on (1) modification of the air-dragged aqua-membrane
denuder / fluorescence detector system for online measurement of gaseous HONO, which was free of NO 2
interferences with high sensitivity (limit of detection = 0.04 ppb); (2) observation of trace gases andmeteorological parameters in Sakai during spring / autumn (2014 - 2015) and in Hanoi during spring(2015); and (3) discussion of diurnal variations of gaseous species in the two mentioned cities. A samesystem was used to measure HONO in Sakai and Hanoi. The obtained results well-represented theoccurrences of HONO in association with variations of other trace gases and meteorological parameters.
Correlation analysis was employed to justify the influences of different factors on diurnal HONO
variations in Japan and Vietnam. The average concentrations of HONO in Sakai during spring / autumn
were 0.71 ± 0.35, 0.86 ± 0.50 ppb, respectively. The average concentration of HONO in Hanoi during
spring was 1.73 ± 0.76 ppb.
In Chapter 4, vertical behaviors of gaseous nitrous acid in the urban atmosphere of Osaka (Japan)
were presented. The study investigated the vertical profiles of HONO at 4 different heights (0, 5, 16, and25 m) above the ground level (AGL) in an urban site in Osaka, Japan during two measurement periods(January 21st - 22nd and February 3 rd - 4th, 2016). The correlations of HONO and various factors, including
nitrogen oxides levels, PM 2.5 and meteorological parameters, were analyzed. Average diurnal HONO
concentrations at 0, 5, 16, and 25 m were 1.7, 1.5, 2.2, and 1.2 ppb, respectively. Even though, nocturnal
HONO concentrations presented a slightly increasing trend with the increase of heights; daytime anddiurnal HONO mixing ratios showed no specific trend. The results of correlation analysis suggested thatheterogeneous conversion NO 2 on wet surface had a great influence on the total HONO production.
Besides, the HONO / NO x ratios were 0.06 ± 0.01, 0.11 ± 0.02, and 0.04 ± 0.05 for 5, 16, and 25 m AGL,respectively. It implied that direct emission from traffic positively affected the total HONO mixing ratiosof the ambient air and this influence was lower at higher altitude, where the air was diluted by active
turbulence. Meanwhile heterogeneous formation of HONO on ground surface was likely a significant
source after sunrise. Furthermore, high correlations of HONO, NO and solar radiation levels suggested
that photolysis of HONO was dominated and contributed to the loss of day-time HONO.
In Chapter 5, general conclusions and future researches were presented.

審査結果の要旨

大気中の亜硝酸ガス(HONO)は大気の浄化に最も重要であると考えられている OH ラジカルの発生源であるが、HONO の生成メカニズムはまだ解明されていない。また、オゾン濃度にも影響を与えるにもかかわらず、その測定は欧米や中国など限られた地域だけで日本やベトナムではほとんど行われていない。本論文は、自動車排ガスや日本やベトナムでの HONO の測定に関する報告であり、次のような重要な結果を得ている。
１）新長期規制のディーゼル車およびガソリン車から直接排出される HONO 濃度を測定し、HONO／NOx 比がガソリン車で 0～0.95%、ディーゼル車で 0.16～1.0％であり、この値は従来報告されている値と同程度であることを報告した。さらにディーゼル車のスピードと HONO 濃度の関係より、HONO の生成がディーゼル粒子フィルターで粒子が除去されるまでに生成している可能性が高いことを推測し、排ガスからの HONO は、エンジンからテールパイプより排気されるまでの間に粒子上で NO2 から二次生成したものであることを指摘した。
２）日本やベトナムで HONO を測定するために所属する研究室で開発された従来の HONO 測定装置の改良を行った。HONO 吸収部のガラス管の内面を磨りガラス状にし、さらに 2 つの吸収管を直列に並べることで、HONO 以外の物質の干渉を低減した。この装置により、ハノイ
市の HONO 濃度および HONO／NOx 比が堺市に比べて非常に高いことを報告した。ベトナ
ムでの HONO の連続測定はこれまでに報告がなく非常に貴重なデータを報告した。
３）HONO の発生源を明らかにする目的で高度別の HONO 濃度の測定を行い、他の物質との相関、気象要素との関係を調べた。その結果、自動車からの排出が HONO 濃度に大きな影響を与えていること、日の出後の HONO は地面での NO2 の不均一反応による生成の可能性が高いこと、日中の HONO は光分解による消失過程が大きいことを示した。
以上の諸成果は、従来データがなかった新長期規制車排ガス、ベトナムハノイの HONO 濃度のデータを初めて報告し、まだ解明されていない HONO の発生源や消失過程解明に対して重要な知見を示したもので、大気化学の進展に貢献するところ大である。また、申請者が自立して研究活動を行うのに十分な能力と学識を示したものである。