Chapter 8 Effect of Highly Concentrated Dry (NH )2SO Seed Aerosols on Ozone and Secondary Organic Aerosol Formation in Aromatic Hydrocarbon/NO Photooxidation Systems 4
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Zifeng Lu , Kiming Hao *, Junhua Li , and Hideto Takekawa
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Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Engine Combustion and Environmental Laboratory, Mechanical Engineering Department, Toyota Central Research and Development Laboratory, Nagakute, Aichi 480-1192, Japan 2
Ozone and secondary organic aerosol (SOA) formation from toluene, m-xylene and 1,2,4-trimethylbenzene/NO photooxidation are studied in a 2 m temperature-controlled smog chamber in the presence of highly concentrated (>20 μm cm ) dry (NH ) SO seed aerosols. The results indicate that the presence of highly concentrated dry (NH ) SO aerosols has neither observable effect on ozone formation nor gas-phase reactions, but it does enhance SOA generation and increase SOA yield, which is found to be positively related with the (NH ) SO surface concentration. It is proposed that the heterogeneous reactions occurring at the (NH ) SO particle surface can cause the incondensable compounds (ICs) to oligomerize to condensable compounds (CCs), which could explain the dry (NH ) SO seeds effect on SOA formation. x
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© 2009 American Chemical Society
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112 Aromatic hydrocarbons, which reflect anthropogenic activities, are an important class o f volatile organic compounds ( V O C s ) in the atmosphere (/, 2). In urban areas, aromatic hydrocarbons are the second largest contributor (next to alkanes) to total non-methane V O C s , and the largest contributor to maximum incremental reactivity ( M I R ) , which approximates the ozone formation potential (2). The chemical oxidation o f aromatic hydrocarbons through atmospheric reactions can also lead to the formation o f secondary organic aerosols ( S O A s ) , which are a major contributor to fine particulate matter ( P M , P M with aerodynamic diameter less than 2.5 μχή) (3, 4\ and raises several environmental and geophysical concerns (5, 6). 2 5
U s i n g smog chambers, a number o f laboratory experiments have been conducted to study S O A formation in the past two decades, and most o f these experiments were conducted with inorganic seed aerosols to facilitate initial condensation ( 7 - / 2 ) . Cocker et al. (7, 8) investigated the effect o f water on S O A gas-particle partitioning in both α - p i n e n e / o z o n e and aromatic photooxidation systems, and found that the measured S O A yield was not affected by the presence o f dry inorganic seed aerosols, even at elevated relative humidity ( R H ) . Recent studies (13, 14) showed that seed particle acidity enhances S O A yield by accelerating the formation o f larger oligomers, and a composite seed aerosol o f ( N H ) S 0 and H S 0 was usually used as an acid catalyst to catalyze the heterogeneous reactions o f carbonyl compounds (15, 16). 4
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However, most o f these previous studies were conducted with relatively l o w concentrations o f inorganic seed aerosols. The initial concentration o f inorganic seed particles was typically 1000-20000 particles cm" with a number mean diameter o f approximately 50-100 nm (7-/2). Assuming the initial seed aerosol had a log-normal size distribution with a geometric standard deviation (a ) o f 1.6, the volume concentration o f inorganic seed aerosols in these studies was no more than 20 μτη cm" . However, highly concentrated aerosols always exist in the ambient atmosphere o f developing cities. F o r example, Beijing, the capital o f China, is experiencing serious P M pollution. The annual average concentration of P M in Beijing was about 100 μ% m" , and during smog episode days it w o u l d exceed 300 μ% m" , in which ionic species account for one third (17, 18). A new 2 m temperature controlled smog chamber system was constructed in Tsinghua University to study photochemical reactions under high P M contaminated condition specified for Beijing. In this work, the effect o f highly concentrated (>20 μτη cm" ) dry ( N H ) S 0 seed aerosols on ozone and S O A formation in aromatic hydrocarbon/NO photooxidation systems are investigated. ( N H ) S 0 is chosen as the seed aerosol because it is a major (sometimes the most abundant) ionic species in Beijing during dust and haze days (18). Toluene, /w-xylene and 1,2,4-trimethylbenzene are selected as the surrogate o f aromatic hydrocarbons because they are the three most abundant aromatics in the urban air (2) and have been widely studied previously (8-12). 3
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Experimental Section The experiments were performed in a smog chamber which is described in detail elsewhere (19). T h e schematic o f the chamber system is shown in Figure 1. The cuboid reactor, which has a volume o f 2 m and a surface-to-volume ratio 3
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o f 5 m ' , is constructed with 50 μιη-thick F E P - T e f l o n film. The reactor is situated i n a temperature controlled room (Escpec S E W T - Z - 1 2 0 ) where the temperature can be controlled in the range o f 10 to 60 ° C with an accuracy o f ± 0.5 ° C . The reactor is irradiated by 40 blacklights ( G E F 4 0 T 1 2 / B L B ) , and the N 0 photolysis rate was calculated to be 0.21 min" . 1
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Temperature Controlled Room GC/FID NO Analyzer x
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Smog Chamber
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SMPS Exhaust
