Article pubs.acs.org/est
Effects of NOx on the Volatility of Secondary Organic Aerosol from Isoprene Photooxidation Lu Xu,† Matthew S. Kollman,† Chen Song,‡ John E. Shilling,‡ and Nga L. Ng*,†,§ †
School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡
S Supporting Information *
ABSTRACT: The effects of NOx on the volatility of the secondary organic aerosol (SOA) formed from isoprene photooxidation are investigated in environmental chamber experiments. Two types of experiments are performed. In HO2-dominant experiments, organic peroxy radicals (RO2) primarily react with HO2. In mixed experiments, RO2 reacts through multiple pathways, including with NO, NO2, and HO2. The volatility and oxidation state of isoprene SOA are sensitive to and exhibit a nonlinear dependence on NOx levels. Depending on the NOx levels, the SOA formed in mixed experiments can be of similar or lower volatility compared to that formed in HO2-dominant experiments. The dependence of SOA yield, volatility, and oxidation state on the NOx level likely arises from gas-phase RO2 chemistry and succeeding particlephase oligomerization reactions. The NOx level also plays a strong role in SOA aging. While the volatility of SOA in mixed experiments does not change substantially over time, SOA becomes less volatile and more oxidized as oxidation progresses in HO2-dominant experiments.
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INTRODUCTION Isoprene is the most abundant nonmethane hydrocarbon (NMHC) emitted into the atmosphere with a global emission of ∼500 Tg/year.1 Isoprene oxidation by OH radicals plays a critical role in tropospheric ozone chemistry and secondary organic aerosol (SOA) formation.2−5 Recent studies have shown that nitrogen oxides (NOxNO + NO2) are highly influential in SOA formation from various hydrocarbon precursors, including isoprene.6−10 The effects of NOx on SOA formation have been attributed to the chemistry of organic peroxy radicals (RO2). While the reaction of RO2 with HO2 and NO2 produces low volatile species, the RO2 + NO reaction could form volatile species via fragmentation of the resultant RO radical.11−13 Despite the fact that SOA formation from isoprene has been intensively studied, many observations in both laboratory studies and field measurements cannot be well explained based on our current knowledge of isoprene oxidation chemistry and yield.14−18 For example, recent aircraft and ground-based studies during the CARES field mission suggested that SOA formation was enhanced when NOx was mixed with isoprene-rich air masses, though the mechanism for the enhancement remains unclear.19,20 Volatility is a key property of organic aerosol because it determines the partitioning between the gas and particle phases, and thus SOA formation. Most of the aerosol volatility studies focused on SOA generated from monoterpene ozonolysis.21−24 © 2014 American Chemical Society
Limited studies have been conducted on the volatility of isoprene SOA, and the results from these studies are inconclusive. King et al. observed that isoprene SOA volatility was not affected by NOx.25 However, Kleindienst et al. reported that the effective −1 enthalpies of vaporization (ΔHeff vap) were 38.4 and 43.2 kJ mol for SOA from isoprene photooxidation in the absence and presence of NOx, respectively, indicating that isoprene SOA formed in the presence of NOx was less volatile.26,27 Thus, the effects of NOx on the volatility of isoprene SOA are highly uncertain. In this study, we systematically investigate the effects of NOx on the volatility of SOA from isoprene photooxidation in a series of chamber experiments. With a thermal denuder (TD) set at predefined temperatures, we focus on the measurement of the volatility of both the bulk organic aerosol and key chemical properties of that aerosol. Two types of experiments are conducted with a wide range of NOx conditions, in which the fate of RO2 radicals varies and leads to differences in SOA yield, volatility, and oxidation state. Received: Revised: Accepted: Published: 2253
October 30, 2013 January 17, 2014 January 28, 2014 January 28, 2014 dx.doi.org/10.1021/es404842g | Environ. Sci. Technol. 2014, 48, 2253−2262
Environmental Science & Technology
Article
Table 1. Experimental Conditions and Results expt.
[isoprene]0 (ppb)
[NO]0 (ppb)
[OH] (106 molecule−1 cm3)
SOA (μg/m3)a,c
SOA yield (%)a,c
1 2 3 4 5 6 7 8
45.5 78.4 144.7 97.7 91.4 114.6 105.5 100.6
