Article pubs.acs.org/JPCA
Identification of Nitration Products during Heterogeneous Reaction of NO2 on Soot in the Dark and under Simulated Sunlight Chun Guan, Xinling Li,* Wugao Zhang, and Zhen Huang* Key Laboratory for Power Machinery and Engineering of M. O. E, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: Author: The present work, involving the formation of NO and nitrous acid (HONO) and the nitration of polycyclic aromatic hydrocarbon (PAHs) to nitro-PAHs as well as the uptake coefficients of NO2, has been performed on a normal-pressure flow reactor to identify the nitration products during the heterogeneous reaction of soot toward NO2 in the dark and under simulated sunlight. Two types of soot particles, namely the commercial black carbon (BC) and the diesel engine soot (ES), were selected as the studied soot to compare the impacts of soot properties on heterogeneous nitration. During the whole reaction on either of the two studied soots in the dark, a fast reversible physical adsorption is observed at the very beginning, followed by a slow irreversible chemical conversion from NO2 to HONO and NO, in good agreement with the “reduction−oxidation” mechanism. HONO is the most abundant product during the nitration reaction on the two studied soots, contributing to 70−90% of consumed NO2 after 50 min exposure. Reaction orders of NO2 for HONO are determined as 1.20 ± 0.07 and 1.31 ± 0.04 for BC and ES, respectively, which are both close to first-order. Moreover, four sorts of PAHs compounds and their five nitro-derivatives have been identified and quantified during the reaction. About 40% and 20% of the total four measured PAHs are consumed on BC and ES, respectively, resulting in an increase in the total five measured nitro-PAHs by 21-fold on BC and 2.8-fold on ES. Finally, the impacts of light on gaseous and organics products have been investigated and the results confirm that simulated sunlight can enhance the reactivity of PAHs toward NO2 and cause the photolysis of newly formed nitro-compounds with more HONO formation, strongly suggesting that photochemistry of soot in the presence of NO2 is of great importance to be a photochemical source of HONO and would also influence the fates of PAHs and nitro-PAHs on soot.
1. INTRODUCTION Soot particles only account for less than 10−15% of the total atmospheric aerosol mass concentration, however, when spreading in the atmosphere, soot particles with condensed organic aerosols would undergo aging processes through a series of heterogeneous reactions with atmospheric oxidation gases. These reactions can cause strong impacts on atmospheric composition, especially involving the formation of nitrous acid (HONO).1−3 HONO is considered as a major night-time reservoir of hydroxyl radical (OH)4,5 as well as an important contributor to the production of O3.6 Since the porous surface and the fractal morphology of soot particles can provide a significant fraction of available reaction sites for atmospheric chemistry reactions, the heterogeneous reaction of NO2 on soot has been proposed to account for the high level of HONO concentration during the night time, especially in the polluted urban regions where high levels of soot particles from the motor vehicles are observed.1,6,7 To date, a number of previous researches have observed the formation of HONO and NO during the nitration reaction of NO2 on different types of soot aerosols, including hydrocarbon flame soot,8−10 spark discharge soot,11 and diesel soot12 as well as commercial black carbon.13,14 The nitration reaction here is a wide concept involving the formation of nitrated derivatives, such as the formation of HONO, NO, or nitro-organics. © XXXX American Chemical Society
However, the uptake coefficients of NO2 on soot particles vary 7 orders of magnitude in different research groups ranging from 10−1 to 10−8, which is highly depended on the properties of soot particles, NO2 concentration, reaction system, measurement time scale as well as assumed surface area. Aubin and Abbatt15 have pointed out that the uptake coefficient of NO2 based on geometric surface area would be orders of magnitude higher than that based on BET surface area and have suggested that the BET surface area should be more reasonable to be used for the calculation of uptake coefficients on soot because it could represent the total amount of surface area available to interact with NO2. In the meanwhile, organics and inorganics on the soot would also affect the nitration reactions, involving consumption of NO2, consumption of OH as well as formation of HONO. Khalizov et al.9 have studied the heterogeneous reaction of NO2 on fresh and coated soot by coating pyrene, sulfuric acid, and glutaric acid and have concluded that the soot reactivity and formation rate of HONO would rely on the chemical composition of the coating materials. Although, in Khalizov’s work,9 the coating pyrene on soot did not show significant Received: September 6, 2016 Revised: November 10, 2016 Published: December 22, 2016 A
DOI: 10.1021/acs.jpca.6b08982 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
the soot samples, involving Brunauer−Emmett−Teller (BET) surface area, water fraction, volatile fraction as well as ash fraction were characterized using a multipoint surface area and pore size analyzer (NOVA 2000e, Quantachrome instruments) and a thermogravimetric analysis (Discovery, TA Instruments). The results are summarized in Table 1. Experimental gases
effects on neither reaction rate nor HONO yield, the nitration of polycyclic aromatic hydrocarbon (PAH) on soot has been observed by other researchers,16−18 giving rise to the formation of nitro-PAHs, which are often more toxic than their parent PAHs and would highly increase the potential of mutagenicity19 and carcinogenicity20,21 of the soot particles. However, the consumption of PAHs and the formation of relative nitro-PAHs during the heterogeneous nitration of NO2 on soot particles are still not well understood, therefore, more independent investigations are needed to identify the mechanism and products during the reaction so as to assess the atmospheric lifetime of soot particles as well as their impacts on air quality and climate. More recently, Monge and his co-workers22 have investigated the effects of UV-A radiation on reactivity of soot particles and have implied that the consumption of NO2 and the formation of HONO were dramatically enhanced under light. The reactivity regeneration and enhancement of NO2-exposed soot particles upon UV-A radiation would lead to persistent reactivity of soot particles to hours, which further implied the key role of soot particles by photochemistry on the atmospheric heterogeneous reactions. Subsequently, Han et al.2 have investigated the heterogeneous photochemistry of NO2 on soot by using in situ attenuated total internal reflection infrared spectroscopy, and have observed that sunlight could enhance the aging of soot in the presence of NO2. The formation of nitro-compounds were observed in their work, but the specific identification of the nitro-compounds were not conducted. Therefore, the effects of visible light on reactivity of soot, formation of gaseous products and nitration of PAHs to nitroPAHs as well as photolysis of nitro-PAHs on the soot are still not clearly clarified. Overall, the present work has been performed on a normalpressure flow reactor to identify the nitration products during the heterogeneous reaction of soot in the presence of NO2 in the dark and under simulated sunlight. Our work involved the formation of NO and HONO as well as the nitration of PAHs to nitro-PAHs. NO2 concentration varied from 5.01 × 1014 to 3.75 × 10 15 molecule/cm 3 , which was close to the concentration near the vehicle tailpipe, especially when the vehicle was equipped with a diesel oxidation catalyst (DOC). Two types of soot particles, namely the commercial black carbon (BC) and the diesel engine soot (ES), were selected as the studied soot. Moreover, the uptake coefficients of NO2 as well as the reaction mechanisms of the major reactions on the two studied soots were also investigated and discussed in this study.
Table 1. Major Properties of the Soot Samples soot type
BC
ES
mass (mg) BET (m2/g) water fraction (wt %) volatile fraction (wt %) ash fraction (wt %)
2 ± 0.1 100 ± 4 0.49 ± 0.05 2.00 ± 0.05
