RSC Advances PAPER View Article Online View Journal | View Issue VUV photoionization aerosol mass spectrometric study on the iodine oxide particles formed from O3- Cite this: RSC Adv.,2017,7,56779 initiated photooxidation of diiodomethane (CH2I2) Nana Wei,ab Changjin Hu, *a Shanshan Zhou,ab Qiao Ma,ab Pavel Mikuˇska,c Zbynekˇ Veceˇ ˇra,c Yanbo Gai,a Xiaoxiao Lin,a Xuejun Gu,a Weixiong Zhao,a Bo Fang,a Weijun Zhang,*ad Jun Chen,e Fuyi Liu,e Xiaobin Shane and Liusi Shenge Iodine oxide particles (IOPs) formed from O3-initiated photooxidation of diiodomethane have been investigated based on the combination of a thermal desorption/tunable vacuum ultraviolet time-of-flight photoionization aerosol mass spectrometer (TD-VUV-TOF-PIAMS) with a flow reactor for the first time. Characterization of the home-made flow reactor was performed, which indicates the applicability of its combination with TD-VUV-TOF-PIAMS. Based on that, aerosol mass spectra of IOP formation from photooxidation of CH2I2/O3 were studied on-line taking full advantage of both the virtues of the flow Creative Commons Attribution-NonCommercial 3.0 Unported Licence. reactor and TD-VUV-TOF-PIAMS. The main chemical components of IOPs, including atomic and molecular iodine (I, I2), iodine oxides (IO, OIO, I2O and I2O3) and hydrogen-containing iodine species (HI, HIO and HIO3), were observed and identified based on the corresponding photoionization energy (PIE) curves, and the probable chemical composition and formation mechanism of IOPs were proposed. The Received 16th October 2017 work has not only improved the understanding of the formation mechanism of IOPs, but also Accepted 12th December 2017 demonstrated the capability of TD-VUV-TOF-PIAMS for direct molecular characterization of aerosols in DOI: 10.1039/c7ra11413c flow reactor experiments, whose potential application in mass spectrometric studies of atmospheric rsc.li/rsc-advances aerosols is anticipated. This article is licensed under a 1. Introduction Nucleation and growth of IOPs, originating from iodine photochemistry, has been investigated in numerous laboratory 10–16 Atmospheric iodine chemistry has received increasing attention studies. Generally, it is believed that I atoms produced from Open Access Article. Published on 15 December 2017. Downloaded 9/26/2021 7:34:16 PM. 1 during the last few decades owing to its important role in the the photolysis of iodocarbons or I2 will be oxidized by O3 1 catalytic destruction of ozone2,3 and new particle formation leading to the production of IO: 4 (NPF) in the marine boundary layer (MBL). Although having hn 5 I2; CH3I; CH2I2; etc: ! I þ products (R1) been known for more than one century, NPF in the costal atmosphere was postulated to be related with classical binary I+O / IO + O (H2SO4/H2O) or ternary (H2SO4/NH3/H2O) nucleation for a long 3 2 (R2) time. Only recently, has it been identied that iodine Then OIO will be formed from the IO self-reaction:17,18 compounds (I2 or alkyl iodides) play a key role in ultra ne particle bursts at the coastal zones, and NPF is formed mainly 6–9 IO + IO / OIO + I (R3) during formation of iodine oxide particles (IOPs). The following recombination of OIO with IO or another OIO, followed by oxidation by O3, will result in the formation of 11,18,19 a ¼ ¼ Laboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics and Fine condensable iodine oxides (IxOy, x 2, 3, 4; y 3, 4, 5): Mechanics, Chinese Academy of Sciences, Hefei, 230031 Anhui, China. E-mail: [email protected]; [email protected] OIO + IO / I2O3 (R4) bGraduate School, University of Science and Technology of China, Hefei, 230026 Anhui, China OIO + OIO / I2O4 (R5) cInstitute of Analytical Chemistry of the Czech Academy of Sciences, Veveˇri 97, CZ- 60200 Brno, Czech Republic I2O3 +O3 / I2O4 +O2 (R6) dSchool of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei, 230026 Anhui, China I2O4 +O3 / I2O5 +O2 (R7) eNational Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7, 56779–56787 | 56779 View Article Online RSC Advances Paper At last, condensation or further polymerization of IxOy will (aerosol particles or gaseous reagents and products) in smog lead to formation of IOPs. So IOPs formation was traditionally chamber is always changing according to the reaction progress. believed to occur only when the precursor of iodine, O3, and UV So it is inevitable that the observed PIE curves are the super- radiation are present simultaneously. Although a number of imposition of the evolution curve of the compounds to be studies have tried to rationalize the mechanism of IOPs measured with the inherent PIE curves just depending on the formation, it is still controversial about which is the most energy of the exciting SR light, which inhabits the accurate probable IOPs chemical composition, I2O4 or I2O5. While both identication of the compounds especially for the weak signals of transmission electron microscope analysis and modelling of mass spectra. In contrast to that in smog chamber, the calculations of IOPs generated from photochemical reaction of concentration of the compounds to be measured in a xed I2 and O3 in dry conditions indicated that IOPs are essentially sampling site only depends on the total ow rate for a xed ow 11 33,34 I2O5, mass spectrometric study complemented with ab initio reactor, which is particularly suitable for PIE curve quantum calculations showed that I2O4 may be the most plau- measurement as aforementioned. sible candidate to initiate nucleation of IOPs,14 and experi- In this work, our motivation was to make full advantages of mental studies on IOPs formation from the two different IO both the virtues of ow reactor and thermal desorption/tunable generating ways (I + O3 and O + I2) based on ow tube also vacuum ultraviolet time-of-ight photoionization aerosol mass showed that polymerization of I2O4 play a pivotal role in IOPs spectrometry (TD-VUV-TOF-PIAMS) to study the formation 13 formation. Therefore unambiguous evidence of chemical mechanism of IOPs. As CH2I2 is the most abundant iodine- compositions of IOPs is still a major research challenge in containing compounds from biogenic emissions35 and plays understanding the mechanism of IOPs formation. an important role in the formation of iodine oxides in marine 36 The application of mass spectrometric techniques to the boundary layer, photooxidation of CH2I2/O3 was chosen as the measurement and characterization of aerosols represents reaction system to produce IOPs. Firstly, characterization of a signicant advance in the eld of atmospheric science. Over home-made ow reactor was performed. Then aerosol mass Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the past decades, aerosol mass spectrometry techniques have spectra of IOPs forming from photooxidation of CH2I2/O3 were been developed considerably, offering off-line or on-line investigated based on the combination of the ow reactor and chemical analysis for individual particles or on ensembles.20,21 the TD-VUV-TOF-PIAMS, and the main chemical components of Although their application in laboratory and eld has resulted IOPs were identied based on the PIE curves. Finally, the in many important improvements in understanding of atmo- probable chemical composition and formation mechanism of spheric aerosols and their processing,22–24 these traditional IOPs, as well as the potential application of the combination of aerosol mass spectrometry techniques suffered for extensive the ow reactor and the TD-VUV-TOF-PIAMS based on SR were fragmentation of organic constituents due to “hard ionization” discussed. (for example, electron ionization or multiphoton ionization), This article is licensed under a which prevents specication of chemical components of 2. Experimental methods atmospheric aerosols. Inspiringly, aerosol mass spectrometry coupled with vacuum ultraviolet (VUV) photoionization based Chemical analysis of IOPs formation from photooxidation of on synchrotron radiation (SR) sources has been proved to be an CH2I2 at the presence of O3 was performed based on the Open Access Article. Published on 15 December 2017. Downloaded 9/26/2021 7:34:16 PM. effective “so” ionization method for real-time, molecular combination of the TD-VUV-TOF-PIAMS and the home-made component analysis of organic particles.25–27 SR light sources is ow reactor (Fig. 1) at the Atomic and Molecular Physics featured with high photon ux and tunability. High photon ux Beamline of the National Synchrotron Radiation Laboratory makes it has better detection sensitivity than traditional VUV (NSRL) in Hefei (China).37 lamp for ultrane particle analysis. Tunability makes it capable As shown in Fig. 1, the ow reactor used in this study is of analyzing broad range of constituents of atmospheric aero- similar in spirit to the setup in Kamens's group in University of sols in two ways. One is it can offer fragment-free or less frag- North Carolina.33 Briey, it consisted of two 1 m straight ment mass spectra by single-photon ionization. The second is it sections of a 2.5 cm diameter quartz glass tube and one U- can identify different chemical composition ambiguously by shaped connection tube. While there are 16 sampling ports their ionization energies (IE) or appearance energies (AE) while distributed every 10 cm along the straight sections, only the exit not only by m/z values through tuning ionization energy. of the tube acted as the sampling ports in this study. Generally, Making the advantage of these features, AMS coupled with a SR the total ow rates through the tube were in the range of 1.2–2.0 À source has been successfully applied to laboratory studies of l min 1 corresponding to a Reynolds number of 146–243, which secondary organic aerosols (SOAs) formation and reaction indicates the laminar ow conditions in the ow reactor.