Presentation Information
[AAS08-P53]Photo-induced OH Radical Formation in the Aqueous Phase of Limonene-Derived SOA
*Kota Mizuno1, Yuri Tabayashi1, Manato Sawabe1, Shuto Misasa1, Kouichi Matsumoto1, Nanase Kohno1 (1.Kindai University)
Keywords:
SOA,VOC,OH Radical,Limonene
Objective
OH radicals generated within atmospheric aerosols are suggested to potentially influence air quality, climate change, and human health by altering the chemical composition of aerosols through their high-oxidizability. Particularly, when OH radicals are generated upon aerosol dissolution in water, they may react with water within the body to produce OH radicals, potentially increasing health risks. Therefore, investigating the formation processes of OH radicals and their precursors within aerosols is a critical research topic. This study investigated OH radicals generated when secondary organic aerosols (SOA), formed by the oxidation reaction between limonene (a type of biogenic volatile organic compound, BVOC) and O3, dissolve in water. Since limonene is emitted not only by plants but also used in products like household goods, analyzing the composition of its oxidation products and evaluating their health impacts is a research theme that contributes not only to atmospheric chemistry but also to life sciences.
Experimental Methods
O3, zero gas, and vapor of limonene were introduced into a glass reaction cell (volume 7 L) to generate aerosols via the O3 oxidation reaction of the VOC. O3 was generated using a low-pressure mercury lamp and adjusted to a concentration of 80~90 ppb within the reaction cell. The flow rate within the reaction cell was set to 2 L/min. The aerosol generated within the reaction cell was collected on a quartz fiber filter (Tokyo Direk, Tissuquartz 2500QAT-UP). Furthermore, to prevent O3 oxidation reactions on the quartz filter, a diffusion filter containing activated charcoal was installed prior to aerosol collection. The collection filter was retrieved 24 hours after the start of the reaction and extracted for one hour in a shaker using a terephthalic acid (TA) aqueous solution (1.2 * 10-3 mol/L) to detect OH radicals. After filtering the extract solution, excitation light at 310 nm (Ocean Insight LSM-310A) was irradiated, and fluorescence intensity was recorded every 5 minutes for 30 minutes using a fiber spectrometer (BroLight Technology, BIM-6002). TA rapidly reacts with OH radicals to form stable 2-hydroxyterephthalic acid (HTA). When HTA is irradiated with 310 nm light, it transitions to an excited state and emits fluorescence at 420 nm. Thus, OH radical generation was confirmed by measuring the 420 nm fluorescence intensity. Additionally, detection of H2O2 in the extract solution was performed using a fluorescent reagent (Sigma-Aldrich, MAK165-1KT, Fluorimetric Hydrogen Peroxide Assay Kit). For other radical species, ESR measurements were performed using a multiband ESR system (JEOL RESONANCE, JES-FA200). For ESR measurements, DMPO (5,5-dimethyl-1-pyrrolidine-N-oxide) was used as the spin trap agent to stabilize the radicals. GC-MS was used to detect the organic compounds generated in the SOA extraction solution.
Results
In the detection of OH radicals using the chemical probe method, the fluorescence intensity at 420 nm was low immediately after HTA excitation light irradiation. However, continued irradiation resulted in an observable increase in fluorescence intensity (Figure 1). This indicates that the relative concentration of OH radicals in the extract solution increases with 310 nm light. Simultaneous detection of H2O2 in the extract solution suggested that aerosols generated by the O3 oxidation reaction of limonene reacted with water to produce H2O2, which then underwent photodissociation to generate OH radicals. Furthermore, ESR measurements revealed that oxygen and carbon-centered radicals are also generated (Figure 2). Particularly, oxygen-centered radicals exhibited light dependence similar to that of OH radicals, suggesting that OH radicals are generated not only from H2O2 but also from organic peroxides via photodissociation. Furthermore, we investigated the humidity dependence of the OH radical generation process and report the results.
OH radicals generated within atmospheric aerosols are suggested to potentially influence air quality, climate change, and human health by altering the chemical composition of aerosols through their high-oxidizability. Particularly, when OH radicals are generated upon aerosol dissolution in water, they may react with water within the body to produce OH radicals, potentially increasing health risks. Therefore, investigating the formation processes of OH radicals and their precursors within aerosols is a critical research topic. This study investigated OH radicals generated when secondary organic aerosols (SOA), formed by the oxidation reaction between limonene (a type of biogenic volatile organic compound, BVOC) and O3, dissolve in water. Since limonene is emitted not only by plants but also used in products like household goods, analyzing the composition of its oxidation products and evaluating their health impacts is a research theme that contributes not only to atmospheric chemistry but also to life sciences.
Experimental Methods
O3, zero gas, and vapor of limonene were introduced into a glass reaction cell (volume 7 L) to generate aerosols via the O3 oxidation reaction of the VOC. O3 was generated using a low-pressure mercury lamp and adjusted to a concentration of 80~90 ppb within the reaction cell. The flow rate within the reaction cell was set to 2 L/min. The aerosol generated within the reaction cell was collected on a quartz fiber filter (Tokyo Direk, Tissuquartz 2500QAT-UP). Furthermore, to prevent O3 oxidation reactions on the quartz filter, a diffusion filter containing activated charcoal was installed prior to aerosol collection. The collection filter was retrieved 24 hours after the start of the reaction and extracted for one hour in a shaker using a terephthalic acid (TA) aqueous solution (1.2 * 10-3 mol/L) to detect OH radicals. After filtering the extract solution, excitation light at 310 nm (Ocean Insight LSM-310A) was irradiated, and fluorescence intensity was recorded every 5 minutes for 30 minutes using a fiber spectrometer (BroLight Technology, BIM-6002). TA rapidly reacts with OH radicals to form stable 2-hydroxyterephthalic acid (HTA). When HTA is irradiated with 310 nm light, it transitions to an excited state and emits fluorescence at 420 nm. Thus, OH radical generation was confirmed by measuring the 420 nm fluorescence intensity. Additionally, detection of H2O2 in the extract solution was performed using a fluorescent reagent (Sigma-Aldrich, MAK165-1KT, Fluorimetric Hydrogen Peroxide Assay Kit). For other radical species, ESR measurements were performed using a multiband ESR system (JEOL RESONANCE, JES-FA200). For ESR measurements, DMPO (5,5-dimethyl-1-pyrrolidine-N-oxide) was used as the spin trap agent to stabilize the radicals. GC-MS was used to detect the organic compounds generated in the SOA extraction solution.
Results
In the detection of OH radicals using the chemical probe method, the fluorescence intensity at 420 nm was low immediately after HTA excitation light irradiation. However, continued irradiation resulted in an observable increase in fluorescence intensity (Figure 1). This indicates that the relative concentration of OH radicals in the extract solution increases with 310 nm light. Simultaneous detection of H2O2 in the extract solution suggested that aerosols generated by the O3 oxidation reaction of limonene reacted with water to produce H2O2, which then underwent photodissociation to generate OH radicals. Furthermore, ESR measurements revealed that oxygen and carbon-centered radicals are also generated (Figure 2). Particularly, oxygen-centered radicals exhibited light dependence similar to that of OH radicals, suggesting that OH radicals are generated not only from H2O2 but also from organic peroxides via photodissociation. Furthermore, we investigated the humidity dependence of the OH radical generation process and report the results.