11:15 AM - 11:30 AM
[HTT16-09] Changes in sulfur isotope ratios during the formation of acid clouds in the central mountain regions of Japan
Keywords:cloud water, sulfate aerosol, sulfur dioxide, sulfur isotope ratio, fractionation factor
1. Introduction
Sulfur compounds play an important role in environmental chemistry and climate through their effect on both chemical and microphysical properties of aerosols, for example, acidification of clouds and precipitation, the increase in the reflectance of sunlight due to an increase in aerosols (cooling effect). Sulfate aerosols act as cloud condensation nuclei. Dissolution of the aerosols into droplets results in both SO42− production and acid cloud formation. Similarly, gas-phase SO2 is highly dissolved into droplets and the subsequent oxidation brings formation of SO42-, thereby generating acid rain. Thus, quantifying the sulfate production in droplets is of importance in understanding the physicochemical properties of clouds quantitatively. Harris et al. (2012) reported that the measured fractionation factor for 34S/32S during SO2 aqueous oxidation by H2O2 (αH2O2) or O3 (αO3) is (1.0167±0.0019)−((8.7±3.5)×10−5)T(℃) and the isotopic fractionation factor from a radical chain reaction in solution catalysed by iron (αFe, ) is 0.9894±0.0043 at 19 C° for 34S/32S. We will report that (1) the characteristics of δ34S in cloud water, aerosol and SO2 sampled during summers from 2022 to 2024 near the summit of Mt. Norikura, and (2) the fraction of SO42- produced through aqueous oxidation of SO2 to the total SO42- in cloud water using the fractionation factor for 34S/32S (αobs), to estimate newly produced SO42- in cloud water.
2. Methods for Observations and Analyses
The observations were conducted at the Norikura Observatory, Institute for Cosmic Ray Observatory, University of Tokyo during the summers from 2022 to 2024. The observatory is located at an elevation of 2770 m.a.s.l near the summit of Mt. Norikura (36° 06′ N, 137° 33′ E) in central Japan. Cloud water was collected using an active string-cloud collector (FWG-400F, Usui Kogyo Kenkyusho, Tokyo, Japan). For ion analyses, atmospheric aerosols were sampled onto a 0.2 µm PTFE filter at an 8–10 L min-1 flow rate with a vacuum pump. Using a high-volume air sampler at a flow rate of 100−400 L min-1 (HV-RW, Sibata Scientific Technology Ltd., Japan), aerosols were collected on the quartz fiber filter and SO2 was collected on the alkaline cellulose acetate filter. After the samples filtered through membrane filter, they were subjected to quantitative analysis for concentrations using ion chromatography (ICS-1100, Thermo Fisher Scientific Inc, USA. installed in the Gifu Univ. ICS-6000; Thermo Fisher Scientific Inc, USA. installed in the RIHN) for anions and cation, and ICP-AES (ULTIMA2, Horiba Ltd.) for transition metal (Fe and Mn). For the δ34S was precipitate SO4²- in the sample as BaSO4. It was analyzed using S-IRMS (EA IsoLink CN IRMS System; Flash IRMS Elemental Analyzer+Delta V plus +Conflo IV; Thermo Fisher Scientific Inc, USA.) and Selenium was analyzed ICP-MS (Agilent Technologies 8900, Agilent, USA) installed in the RIHN.
3. Results
(1) δ34S for SO42- in cloud water, aerosols, and SO2 were 3.9−8.4 ‰ (n=35), 1.6−5.3 ‰ (n=13), and -2.6−3.5 ‰ (n=7), respectively. δ34S in cloud water was higher than those in the aerosols and SO2. As inferred from the results of the laboratory experiment by Harris et al. (2012), it was thought that isotope fractionation occurred when SO2 was oxidized in cloud water, resulting in an increase in δ34S.
(2) pH of cloud water was mostly less than 5.5, and the concentration of Fe and Mn in cloud water were low (<0.01 mg L-1). Thus, it is thought that main oxidant of SO2 in cloud water was H2O2 rather than O3 and O2. When only H2O2 acts as an oxidant of SO2 in cloud water, the fractionation factor αH2O2 should be 1.0167 using the formula by Harris et al. (2012). However, αobs calculated from the results of our observation was 1.0050 and lower than αH2O2 reported by Harris et al. (2012). It can be said that αobs being lower than αH2O2 was presumably due to the contribution of δ34S in sulfate aerosols taken in cloud water as condensation nuclei.
Sulfur compounds play an important role in environmental chemistry and climate through their effect on both chemical and microphysical properties of aerosols, for example, acidification of clouds and precipitation, the increase in the reflectance of sunlight due to an increase in aerosols (cooling effect). Sulfate aerosols act as cloud condensation nuclei. Dissolution of the aerosols into droplets results in both SO42− production and acid cloud formation. Similarly, gas-phase SO2 is highly dissolved into droplets and the subsequent oxidation brings formation of SO42-, thereby generating acid rain. Thus, quantifying the sulfate production in droplets is of importance in understanding the physicochemical properties of clouds quantitatively. Harris et al. (2012) reported that the measured fractionation factor for 34S/32S during SO2 aqueous oxidation by H2O2 (αH2O2) or O3 (αO3) is (1.0167±0.0019)−((8.7±3.5)×10−5)T(℃) and the isotopic fractionation factor from a radical chain reaction in solution catalysed by iron (αFe, ) is 0.9894±0.0043 at 19 C° for 34S/32S. We will report that (1) the characteristics of δ34S in cloud water, aerosol and SO2 sampled during summers from 2022 to 2024 near the summit of Mt. Norikura, and (2) the fraction of SO42- produced through aqueous oxidation of SO2 to the total SO42- in cloud water using the fractionation factor for 34S/32S (αobs), to estimate newly produced SO42- in cloud water.
2. Methods for Observations and Analyses
The observations were conducted at the Norikura Observatory, Institute for Cosmic Ray Observatory, University of Tokyo during the summers from 2022 to 2024. The observatory is located at an elevation of 2770 m.a.s.l near the summit of Mt. Norikura (36° 06′ N, 137° 33′ E) in central Japan. Cloud water was collected using an active string-cloud collector (FWG-400F, Usui Kogyo Kenkyusho, Tokyo, Japan). For ion analyses, atmospheric aerosols were sampled onto a 0.2 µm PTFE filter at an 8–10 L min-1 flow rate with a vacuum pump. Using a high-volume air sampler at a flow rate of 100−400 L min-1 (HV-RW, Sibata Scientific Technology Ltd., Japan), aerosols were collected on the quartz fiber filter and SO2 was collected on the alkaline cellulose acetate filter. After the samples filtered through membrane filter, they were subjected to quantitative analysis for concentrations using ion chromatography (ICS-1100, Thermo Fisher Scientific Inc, USA. installed in the Gifu Univ. ICS-6000; Thermo Fisher Scientific Inc, USA. installed in the RIHN) for anions and cation, and ICP-AES (ULTIMA2, Horiba Ltd.) for transition metal (Fe and Mn). For the δ34S was precipitate SO4²- in the sample as BaSO4. It was analyzed using S-IRMS (EA IsoLink CN IRMS System; Flash IRMS Elemental Analyzer+Delta V plus +Conflo IV; Thermo Fisher Scientific Inc, USA.) and Selenium was analyzed ICP-MS (Agilent Technologies 8900, Agilent, USA) installed in the RIHN.
3. Results
(1) δ34S for SO42- in cloud water, aerosols, and SO2 were 3.9−8.4 ‰ (n=35), 1.6−5.3 ‰ (n=13), and -2.6−3.5 ‰ (n=7), respectively. δ34S in cloud water was higher than those in the aerosols and SO2. As inferred from the results of the laboratory experiment by Harris et al. (2012), it was thought that isotope fractionation occurred when SO2 was oxidized in cloud water, resulting in an increase in δ34S.
(2) pH of cloud water was mostly less than 5.5, and the concentration of Fe and Mn in cloud water were low (<0.01 mg L-1). Thus, it is thought that main oxidant of SO2 in cloud water was H2O2 rather than O3 and O2. When only H2O2 acts as an oxidant of SO2 in cloud water, the fractionation factor αH2O2 should be 1.0167 using the formula by Harris et al. (2012). However, αobs calculated from the results of our observation was 1.0050 and lower than αH2O2 reported by Harris et al. (2012). It can be said that αobs being lower than αH2O2 was presumably due to the contribution of δ34S in sulfate aerosols taken in cloud water as condensation nuclei.