Impact of Sulfur Dioxide Oxidation by Stabilized Criegee Intermediate on Sulfate 2 3 Golam Sarwar 1*, Heather Simon 2, Kathleen Fahey 1, Rohit Mathur 1,Wendy S

1 Impact of sulfur dioxide oxidation by Stabilized Criegee Intermediate on sulfate 2 3 Golam Sarwar 1*, Heather Simon 2, Kathleen Fahey 1, Rohit Mathur 1,Wendy S. Goliff 3,William R. 4 Stockwell 4 5 6 7 1Atmospheric Modeling and Analysis Division, National Exposure Research Laboratory, U.S. Environmental 8 Protection Agency, RTP, NC 27711, USA. 9 10 2Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, 11 North Carolina, USA. 12 13 3College of Engineering Center for Environmental Research and Technology, University of California at Riverside, 14 Riverside, CA 92507, USA. 15 16 4Department of Chemistry, Howard University, Washington, DC 20059, USA. 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 ______________________________ 45 46 *Corresponding author : Golam Sarwar, U.S. EPA, 109 T.W. Alexander Drive, Research 47 Triangle Park, NC 27711, USA; Tel.: 919-541-2669; fax: 919-541-1379; e-mail : 48 [email protected] 49 - 1 - 50 Abstract 51 We revise the Carbon Bond chemical mechanism to explicitly represent three Stabilized Criegee 52 Intermediates (SCIs) and their subsequent reactions with sulfur dioxide, water monomer, and 53 water dimer, and incorporate the reactions into the Community Multiscale Air Quality model. 54 The reaction of sulfur dioxide with SCI produces sulfuric acid which partitions into sulfate. We 55 examine the impact of sulfur dioxide oxidation by SCI on sulfate using two different measured 56 rate constants for the reaction of sulfur dioxide and SCI. When we use the higher rate constant 57 and emissions estimates from the Biogenic Emissions Inventory System, it enhances monthly 58 mean sulfate in summer by ~20% in biogenically active areas. Enhancements are driven 59 primarily by SCI produced from the reactions of biogenically derived alkenes and ozone. The 60 use of the lower rate constant only marginally enhances sulfate since it is 65 times lower than the 61 higher rate constant. We performed several sensitivity analyses to investigate the impacts of 62 uncertain biogenic emissions and SCI loss rates. When we use the higher rate constant and 63 emissions estimates from the Model of Emissions of Gases and Aerosols from Nature, it 64 enhances monthly mean sulfate by ~75%. A simulation using the lowest reported rate constant 65 for the reaction of SCI and water indicated the maximum enhancement of sulfate from this 66 chemistry was up to 4 µg/m 3 over a 24-hour period in some locations in the Southeastern U.S.. 67 Predictions without the SCI reaction are lower than observed sulfate while predictions with the 68 SCI reaction improve the agreements with observations. 69 70 Keywords: Stabilized Criegee Intermediate; alkene; ozone; sulfur dioxide; sulfate; water; 71 oxidation - 2 - 72 1. INTRODUCTION 73 Aerosols cause adverse health impacts (Pope et al., 2002), degrade atmospheric visibility (Malm 2- 74 et al., 1994), and alter Earth’s energy balance (Murphy et al., 2009). Sulfate (SO 4 ) is an 75 important component of atmospheric aerosols, comprising up to 60% of atmospheric aerosols in 2- 76 the U.S (Hand et al., 2012). SO 4 primarily forms from gas- and aqueous-phase oxidation of 77 sulfur dioxide (SO 2) by various atmospheric oxidants (Seinfeld and Pandis, 2006). Gas-phase 78 oxidation by hydroxyl radical (HO) and aqueous-phase oxidation by hydrogen peroxide are 2- 79 thought to produce the majority of atmospheric SO 4 (Sofen et al., 2011). 80 81 Calvert and Stockwell (1983) first proposed that the reactions of Stabilized Criegee Intermediate 82 (SCI) with SO 2 are important for production of organic acids and sulfate and incorporated the 83 reactions into the Regional Acid Deposition Mechanism (Stockwell, 1986). These studies found 84 that SCI intermediates could be important oxidants for SO 2 at high VOC to NO x ratios (VOC= 85 Volatile Organic Compounds and NO x = Oxides of Nitrogen) and at low relative humidity. 86 Recent laboratory and field experiments (Welz et al., 2012; Mauldin et al., 2012; Carlsson et al., 87 2012; Taatjes et al., 2013) more strongly show that Criegee Intermediates can oxidize SO 2 to 2- 88 SO 4 . Welz et al. (2012) studied the reaction of SO 2 and formaldehyde oxide (CH 2OO) using 89 direct measurement technique and measured a rate constant of 3.9x10 -11 cm 3 molecule -1 s-1 for 90 the reaction. This value is greater than any other previously reported rate constant. They 2- 91 suggested that SO 2 oxidation by Criegee Intermediates can enhance SO 4 by as much as SO 2 92 oxidation by the HO pathway. Mauldin et al. (2012) reported a lower rate constant of 6.0x10 -13 3 -1 -1 93 cm molecule s for the oxidation of SO 2 by Criegee Intermediate though they used different 94 Criegee Intermediates than the study of Welz et al. (2012). They suggested that oxidation of SO 2 2- 95 by HO alone could not explain the observed SO 4 in a boreal forest in Finland. However, the 2- 96 oxidation of SO 2 by HO and SCI was sufficient to explain the SO 4 observed in their 97 measurements in boreal forests. Carlsson et al. (2012) performed experiments in a static variable 98 pressure reaction chamber to study particle formation during ozonolysis of β-pinene and 2- 99 butene. They measured rate constant similar to the value reported by Welz et al. (2012) and 100 suggested that it provides a reliable estimate for substituted and larger SCI. Taatjes et al. (2013) 101 directly measured rate constants for the oxidation of SO 2 by two conformers of acetaldehyde 102 oxide (syn -CH 3CHOO and ant -CH 3CHOO). They also reported high rate constant (similar to the - 3 - 103 findings of Welz et al., 2012) and suggested that ant -CH 3CHOO is more reactive than syn - 104 CH 3CHOO. 105 106 The impact of SO 2 oxidation by SCI depends on the relative abundance of SCI (Vereecken et al., 107 2012). The main loss mechanism of SCI from the atmosphere is its reaction with H 2O. While 108 several recent studies directly measured the rate constant for the SCI + SO 2 reaction, only Welz 109 et al. (2012) and Taatjes et al. (2013) reported the direct measurement of the rate constant for the 110 SCI + H 2O reaction. However, they reported an upper limit, not the actual rate constant. Previous 111 studies determined the rate constant of the SCI + H2O reaction either by indirect measurements 112 or estimations. Indirect measurements quantified the relative rate to that of the SCI + SO 2 113 reaction and used it to derive the rate constant for the SCI + H 2O reaction. Since the rate constant 114 of the SCI + SO 2 reaction varied among different studies, the reported rate constant for the SCI + 115 H2O reaction also varied (Hatakeyama and Akimoto, 1994). 116 117 Anglada et al. (2011) and Vereecken et al. (2012) noted that the type of conformer as well as the 118 type of substitution in the SCI affects the rate constant for the reaction of SCI and H2O. Anglada 119 et al. (2011) used computational chemistry to study reactions of fifteen different SCIs with H 2O 120 and found that rate constants vary by ten orders of magnitude for reactions of substituent 121 carbonyl oxides and H 2O. They reported higher rate constants for the reactions for anti - 122 conformers with H2O than the reactions of syn -conformers with H2O. When both H molecules in -17 123 formaldehyde oxide were substituted by CH 3, the rate constant was calculated to be ~4 x 10 3 -1 -1 124 cm molecule s ; however, substituting one H molecule by CH 3 and one H molecule by -19 -18 3 125 CH=CH 2 resulted in significant smaller rate constants between ~3 x 10 - 8 x 10 cm 126 molecule -1 s-1. 127 128 Several studies suggest that the SCI loss cannot be fully accounted for with the SCI + H 2O 129 reaction alone and that reactions with water dimers are key to properly characterizing this loss. 130 Ryzkhov and Aryia (2004) suggested that reaction rates of SCI and water dimer [(H 2O) 2] are 131 greater than those of SCI and H2O. Ryzhkov and Ariya (2006) reported that SCI + (H 2O) 3 and 132 SCI + (H 2O) 4 reactions proceed at much slower rates than SCI + (H 2O) 2 reaction [(H 2O) 3 = water 133 trimer) and (H 2O) 4 = water tetramer]. Based on computational chemistry, Vereecken et al. (2012) - 4 - 134 calculated rate constants of the reactions of CH 2OO and CH 3C(H)OO with (H 2O) 2 to be five 135 orders of magnitude greater than the corresponding reactions with H2O. Finally, Long et al. 136 (2011) conducted a theoretical study and reported that experimental rate constant for SCI + H 2O 137 is inconsistent with the theoretical estimates since SCI also reacts with (H2O)2. 138 139 Several studies have incorporated this chemistry into various types of models in order to quantify 2- 140 its effects on atmospheric SO 4 concentrations. Boy et al. (2013) incorporated SCI into a zero- 141 dimensional model and compared predictions to the measurements in VOC-rich environments 142 from Finland and Germany. They employed reactions of SCI with other compounds [including 143 H2O but not (H 2O) 2] contained in the Master Chemical Mechanism (MCM). They used the 144 Mauldin et al. (2012) reported rate constant for the SCI + SO 2 reaction and suggested that the 145 SO 2 oxidation by SCI contributes up to 50% of the measured atmospheric sulfuric acid (H 2SO 4).

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