Article pubs.acs.org/est
Measurements of Secondary Organic Aerosol Formed from OHinitiated Photo-oxidation of Isoprene Using Online Photoionization Aerosol Mass Spectrometry Wenzheng Fang,* Lei Gong, Qiang Zhang, Maoqi Cao, Yuquan Li, and Liusi Sheng* †
National Synchrotron Radiation Laboratory, School of Nuclear Science and Technology, University of Science and Technology of China, Hefei 230029, China S Supporting Information *
ABSTRACT: Isoprene is a significant source of atmospheric organic aerosol; however, the secondary organic aerosol (SOA) formation and involved chemical reaction pathways have remained to be elucidated. Recent works have shown that the photo-oxidation of isoprene leads to form SOA. In this study, the chemical composition of SOA from the OHinitiated photo-oxidation of isoprene, in the absence of seed aerosols, was investigated through the controlled laboratory chamber experiments. Thermal desorption/tunable vacuum-ultraviolet photoionization time-offlight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) was used in conjunction with the environmental chamber to study SOA formation. The mass spectra obtained at different photon energies and the photoionization efficiency (PIE) spectra of the SOA products can be obtained in real time. Aided by the ionization energies (IE) either from the ab initio calculations or the literatures, a number of SOA products were proposed. In addition to methacrolein, methyl vinyl ketone, and 3-methyl-furan, carbonyls, hydroxycarbonyls, nitrates, hydroxynitrates, and other oxygenated compounds in SOA formed in laboratory photo-oxiadation experiments were identified, some of them were investigated for the first time. Detailed chemical identification of SOA is crucial for understanding the photo-oxidation mechanisms of VOCs and the eventual formation of SOA. Possible reaction mechanisms will be discussed.
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O3 formation and NOx removal as well as SOA formation.7 Furthermore, a number of multifunctional products formed from isoprene photo-oxidation have been identified in the gas phase by chemical ionization mass spectrometry.13 As some of these organic nitrates are large (C4−C5) and multifunctional,25 they may be semivolatile and can enter the particle phase through gas/particle partitioning; hence they can play a role in SOA formation.3,12 Measurements of these chemical products of isoprene SOA provide substantial insight into the SOA formation process. However, the detailed chemical mechanism of isoprene SOA remains highly uncertain and not fully understood or characterized due to the chemical and physical processes associated with isoprene SOA formation are complex and large number of difficult-to-measure molecular products.1 Thus, further studies are necessary to better elucidate the chemical mechanism and SOA formation pathways. In this study, we use thermal desorption/tunable vacuumultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) to investigate the nonvolatile and semivolatile components of SOA generated from the photo-oxidation of isoprene in real time. TD-VUV-TOF-
INTRODUCTION Secondary organic aerosol (SOA) formed by the oxidation of biogenic volatile organic compounds (VOCs) is a major global contributor to atmospheric aerosol mass.1 Isoprene (2-methyl1,3-butadiene, C5H8) is an abundant biogenic VOC emitted into the atmosphere, estimated to be 440−660 TgCyr−1.2 Because of its two highly reactive double bonds, isoprene can quickly oxidize in the atmosphere by hydroxyl radicals (OH), nitrate radicals (NO3), and ozone (O3) to form numerous oxidation products. Recently, the field and laboratory studies indicated that isoprene oxidation indeed contribute to the formation of SOA in the atmosphere.3 Formation of SOA during isoprene oxidation has been estimated to be the single largest source of atmospheric organic aerosol and contribute approximately 50% to the global secondary organic aerosol budget, with OH being the primary oxidant.3−12 The SOA formation from isoprene photo-oxidation has been extensively investigated over the last several years.3,6−9,12−24 The chemical identification of SOA is crucial for understanding the photo-oxidation mechanisms of VOCs and the eventual formation of SOA. Tetrols with the same carbon backbone as isoprene were first identified in aerosols from the Amazonian forest and proposed to be photo-oxidation products of isoprene.4 Isoprene-derived organonitrates12,13,25 and organosulfates11,15,16,26 formed from OH radical-initiated reactions of isoprene have also been detected in the atmosphere and impact © 2012 American Chemical Society
Received: Revised: Accepted: Published: 3898
December 27, 2011 February 26, 2012 March 8, 2012 March 8, 2012 dx.doi.org/10.1021/es204669d | Environ. Sci. Technol. 2012, 46, 3898−3904
Environmental Science & Technology
Article
type aerodynamic lens30 assembly and a three stage differential pumping system was used to sample them at atmospheric pressure. Once entered the ionization region of the detection chamber, the aerosols impacted on a heater tip that inserted between the TOF optics. The particle spot deposited on the heater tip is about 1 mm diameter. The nascent vapor expanded back into the source region was ionized by the tunable VUV SR. SR from an undulator-based Atomic and Molecular Physics Beamline (U14-A) of 800 MeV electron storage ring at the National Synchrotron Radiation Laboratory (NSRL, Hefei, China), is dispersed with a 6 m length monochromator, which covers the photon energy from 7.5 to 22.5 eV for 370 grooves mm−1.31,32 Thermal desorption photoionization mass spectra of SOA particles can be directly measured by a reflectron TOF mass spectrometer at different photon energies. The IEs of the proposed SOA products are helpful and supportive for identifying particulate chemicals. Further information concerning the experiments, the data collection and analysis, and the theoretical calculations of IEs is provided in the Supporting Information.
PIAMS provides real-time photoionization mass spectra for online analysis particle-phase organic products. The online measurement is important for SOA identification because the semivolatile nature and great chemical diversity of SOA implies that, for offline methods, particle-phase composition can change during sample collection, storage, and analysis. Environmental chamber has also been used as an approach for simulating the complexity of atmospheric SOA in a laboratory setting providing a controlled and repeatable condition to study and characterize the complex SOA. Moreover, the tunability of synchrotron radiation (SR) allows for multidimensional peak analysis. Tunable VUV ionization can suppress fragmentation greatly by tuning the wavelength of the VUV radiation to achieve near threshold ionization. The photoionization mass spectra of particle-phase products as a function of photon energy can be obtained by ionizing at different energies. The molecular level analysis is aided by the online mass spectra analysis as well as the ionization energies (IE) of the proposed products formed from the photooxidation of isoprene. The particulate species identified from isoprene SOA and the possible mechanisms leading to these products are discussed. And thus provide insights into the chemical mechanism for SOA formation from isoprene.
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RESULTS AND DISCUSSION Online Mass Spectra of Secondary Organic Aerosols. In this work, we present a mass spectrometric study of SOA formed from the OH-initiated photo-oxidation of isoprene in an environmental chamber. CH3ONO was used as the OH radical precursor. The SOA particles were sampled directly from the environmental chamber. Particle-phase photooxidation products were measured directly with TD-VUVTOF-PIAMS. The mass spectra can be collected at different time points using different photon energies for ionization in the isoprene SOA-forming experiments. In addition, there were no ion signals in the mass spectra when tuned off the heater, further ensures that the ion signals are generated from the SOA particles. TD-VUV-TOF-PIAMS is a good online analytical tool for monitoring complex products in SOA formation process because of its soft ionization method and facile desorption method.27,29 In this experiment, the heated tip in the TD-VUVTOF-PIAMS instrument was operated at ∼393 K to lessen the thermal fragmentation of the isoprene SOA products. The photoionization at low energy (≤10.5 eV) was shown that it can not bring fragmentations from dissociative photoionization.27,29 Some mass spectra of pure samples taken at ∼393 K by low VUV photon energies were shown in our previous publications.27,29Additionally, a mass spectra of cholesterol particles measured at 423 K by 9.5 eV photon energy is shown in Figure S1 of the Supporting Information. In all of these mass spectra, only a single peak corresponding to the molecular ion could be observed. Thus, the peaks in the mass spectra indeed originate from the molecular ions and that they are not fragments of the high mass peaks. Representative online thermal desorption photoionization mass spectra of SOA particles in the photo-oxidation experiments of isoprene are shown in the panels of Figure 1. In panels A−H of Figure 1, SOA particles were vaporized at 393 K and ionized at 10.50, 10.00, 9.80, 9.60, 9.45, 9.20, 9.00, and 8.80 eV photon energies, respectively. Numerous ions corresponding to the photooxidation products were observed in panel A of Figure 1. Some of them were also detected in previous works using PTR-MS or other measurements.7,13,21 The photon energy in the range of 10.50−9.60 eV (shown in Panels A−D of Figure 1) does not have a substantial effect on the ion signals observed. Similar ion signals of the isoprene SOA products can be detected, however,
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EXPERIMENTAL SECTION Photo-oxidation of isoprene was performed in a ∼1500-L flexible Teflon bag suspended in the 4-m3 photochemical environmental chamber, described in detail elsewhere.27 The overall experimental apparatus consists of a sampling system, an environmental chamber system and the detection system. The cylinder bags are surrounded by twelve fluorescent blacklamps (320−400 nm, Philips, R-UVA) and aluminum sheets for maximum reflectivity. Reactions were initiated by turning on the black lights that generate OH radicals by photolyzing methyl nitrite (CH3ONO).27,28 Before all experiments, the evacuated bag chambers were repeatedly flushed with clean air and irradiated with UV light for cleaning, until the particle number concentration is
