Reaction Kinetics,Mechanism And Secondary Organic Aerosol Formation Potential Of Three Typical Ethers

Ethers are being increasingly used as motor fuel additives to increase the octane number and to reduce CO emissions.Previous studies related to aliphatic ethers mainly focused on some symmetric aliphatic ethers,such as methyl ether and diethyl ether,while there were few studies on some asymmetric ethers.The present work reports three selected asymmetric ethers,methyl n-propyl ether(MnPE,CH3OCH2CH2CH3),methyl n-butyl ether(MnBE,CH3OCH2CH2CH2CH3)and methyl s-butyl ether(MsBE,CH3OCH(CH3)CH2CH3).The gas phase reaction kinetics,reaction product,SOA yield,and particle size distribution were studied using the smog chamber.Rate constants at room temperature(293±2)K and atmospheric pressure for the reaction of three selected ethers with OH and NO3 radicals and Cl atom have been determined in a 100 L FEP-Teflon reaction chamber in conjunction with gas chromatography-flame ionization detector(GC-FID)as the detection technique.The obtained rate constants for MnPE reaction with OH,NO3,and Cl are(9.91±2.30)×10-12,(1.67±0.32)×l0-15,and(2.52±0.14)×10-10 cm3 molecule-1 s-1,respectively.The obtained rate constants for MnBE reaction with OH,NO3,and Cl are(1.45± 0.59)×10-11,(1.88±0.15)×10-15 and(2.89± 1.16)×10-10 cm3 molecule-1 s-1,respectively The obtained rate constants for MsBE reaction with OH,NO3,and Cl are(10.35 ±3.74)×10-12,(2.73±0.34)×10-15 and(2.18±0.87)×10-10 cm3 molecule-1 s-1,respectively.The gas-phase products were detected using gas chromatography-mass spectrometry(GC-MS).For the reaction of MnPE with OH radical,seven main products were identified,among which,1-methoxypropanal,methyl propanoate,formaldehyde and methyl formate.Methyl propanoate was detected for the MnPE reaction with both Cl and NO3 while propyl formate was also detected for the reaction of MnPE with Cl.Four main products(methyl acetate,2-butanone,s-butyl formate,and methyl propionate)were observed for MsBE reaction with OH and two main products(methyl formate and methyl butyrate)were observed for MnBE reaction with OH.Based on the products detected,the reaction mechanisms for the gas phase reaction were proposed.The products formed by the OH-initiated oxidation of the selected ethers in this work are aldehydes,ketones and esters.The SOA formation experiments have been investigated in a 1 m3 smog chamber.The SOA yield for the reaction of MnPE,MnBE,MsBE,ethyl n-butyl ether(EnBE),methyl t-butyl ether(MtBE)and di-n-propyl ether(DnPE)with OH radicals were measured as 0.45%,0.39%,0.70%,0.82%,0.24%and 0.65%,respectively.When the seed aerosols(ammonium sulfate,AS)were added in the system,the SOA yield for the reaction of MnPE and MnBE with OH increased significantly,eventually reaching 11.04%and 27.00%.In the presence of NOx,the SOA yield was 11.07%for the MnBE reaction system.This indicates that the SOA yield will decrease at high NOx condition.The 1,3,5-trioxane and 2,2-dimethoxybutane were detected as the particle phase products for the reaction of MnBE with OH using GC-MS.The spectra measured with ATR-FTIR indicated that the products contained hydroxyl groups,aldehyde groups and carboxylic acid groups.Atmospheric lifetimes of the three typical ethers were calculated from the respective measured rate constants.The lifetimes of MnPE,MnBE and MsBE with OH radical were 28.0,19.2 and 26.8 hours,respectively.These short lifetimes mean that the main degradation pathway of the three ethers in the atmosphere is the reaction with OH and that the three ethers will alter the tropospheric chemistry in areas close to their emission sources.The measurement of optical properties shows that MnBE SOA can absorb at 280 nm.When the NOx was added in the system,the MnBE SOA could also absorb in the visible wavelengths.The excitation-emission matrices displayed a distinct peak at ex/em = 470/480 nm.The absorption of MnBE SOA increased at ex/em = 400/410 and 350/400-500 nm when NOx existed.