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UNC-Chapel Hill ENVR 890 - Predicted Secondary Organic Aerosol Concentrations from the Oxidation of Isoprene in the Eastern US

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Predicted Secondary OrganicAerosol Concentrations from theOxidation of Isoprene in the EasternUnited StatesTIMOTHY E. LANE†ANDSPYROS N. PANDIS*,†,‡Department of Chemical Engineering, Carnegie MellonUniversity, Pittsburgh, Pennsylvania 15213, and Departmentof Chemical Engineering, University of Patras, Patra,26500, GreeceIsoprene, the most abundant non-methane hydrocarbonemitted into the troposphere, has generally not beenconsidered a major source of SOA due to the relativelyhigh volatility of its oxidation products. In this study, the SOAformed from the oxidation of isoprene is predicted usinga three-dimensional chemical transport model, PMCAMx,across the eastern U.S. for July, October, January, and April2001-2002. The variability of the measured SOA yields inthe available smog chamber studies is captured by combiningthe base case scenario with upper and lower boundestimates of the measurements. For the base case simulation,the predicted annual average isoprene SOA concentrationin the southeast is 0.09 µgm-3(bounds 0.04-0.23 µgm-3). Isoprene is predicted to produce 70% less SOA acrossthe entire domain for spring and fall than during thesummer and negligible amounts of SOA during the winter.During the summer, the average concentrations in thenortheast are predicted to be 0.11 µgm-3(bounds 0.04-0.31 µgm-3) and in the southeast 0.19 µgm-3(bounds 0.11-0.58 µgm-3). PMCAMx predictions are compared toavailable measurements of some isoprene SOA componentsin North Carolina and New York State. These modelingresults suggest that on an annual basis isoprene oxidationis a small but non-negligible organic aerosol source inthe eastern U.S. Its contribution is relatively more importantduring the summer and in the southeast U.S.1. IntroductionSeveral recent studies have suggested that isoprene may bean important source of SOA (1-3). Claeys et al. (1) measuredsignificant concentrations of tetrols in the Amazon andprovided strong evidence that these low vapor pressurecompounds were formed during the atmospheric oxidationof isoprene (2, 3). With a global emission rate of 500 Tg yr-1,isoprene may produce appreciable amounts of SOA evenwith low SOA yields (4). Clayes et al. (1) estimated an annualSOA global source strength of 2 Tg yr-1from the oxidationof isoprene. Recent studies suggest that isoprene has SOAmass yields ranging between 0 and 5%, depending on theamount of isoprene reacted (3, 5-10).The chamber studies of Pandis et al. (5), Edney et al. (3),Kroll et al. (6), Kroll et al. (7), Ng et al. (8), Kliendeinst et al.(9), and Dommen et al. (10) have investigated the SOA formeddirectly during the photooxidation of isoprene. Due todiffering experimental conditions, such as temperature,relative humidity, seed loading, NOxand SO2levels, etc.,varying SOA yields were measured for different amounts ofisoprene reacted. Edney et al. (3) and Kliendeinst et al. (9)suggested that the isoprene SOA yields are higher whenH2SO4is present in the particulate phase. The SOA massyields from the oxidation of isoprene increased by a factorof 14, from 0.2% to 2.8%, in the presence of a strong acid (3).However, the ambient aerosol in continental areas rarelycontains pure H2SO4due to the presence of NH3(11). Krollet al. (6, 7) reported that the SOA mass yields from thephotooxidation of isoprene range from 0.9 to 5.5% in highNOxconditions. Under low (no) NOxconditions, Kroll et al.(7) measured SOA mass yields ranging from 0.9 to 3.6%. Nget al. (8) measured the real-time SOA formation during thephotooxidation of 500 ppb of isoprene at a VOC/NOxratioof 1.8. The final mass yield at the end of the experiment was2%, assuming a density of 1.25 g cm-3. Dommen et al. (10)measured SOA mass yields as high as 5% and observed theformation of oligomers in the aerosol from isoprene/NOxphotooxidation.Other studies suggest that isoprene can contribute toambient SOA by heterogeneous reactions under acidicconditions (12) or through polymerization of second genera-tion oxidation products (13). Through model simulations,Ervens et al. (14) and Lim et al. (15) suggested that cloudprocessing of water-soluble isoprene oxidation products mayalso contribute to the formation of SOA. According to Limet al. (15), isoprene can contribute at least 1.6 Tg yr-1of SOAon a global scale through cloud processing.Isoprene may form organic aerosol through a number ofpathways. In this study, the base case parametrization forthe oxidation of isoprene uses the SOA mass yields andsaturation concentrations proposed by Pandis et al. (5)together with the yield equation developed by Odum et al.(16). Temperature dependence is added to the correspondingsaturation concentrations to estimate the isoprene SOAconcentration. Two simulations are performed using the basecase parametrization to predict the isoprene SOA concentra-tions dependent on whether the primary organic aerosol(POA) is included in the organic solution (16-19). Given theuncertainty surrounding the processes leading to SOAformation, lower and upper bound parametrizations wereused to estimate the range of expected isoprene SOAconcentrations. The lower bound parametrization uses thebase case parametrization at 30 °C without a temperaturedependence and without the POA included in the organicsolution, while the upper bound is calculated using a constant2% SOA mass yield.For each scenario, a three-dimensional chemical transportmodel, PMCAMx, is used to predict the isoprene SOAconcentrations across the eastern United States for fourdifferent simulation periods: July 12-28, 2001; October 1-17,2001; January 1-17, 2002; and April 1-17, 2002. The predictedbase case isoprene SOA concentrations with the POA includedin the organic solution are compared to available ambienttetrol concentrations measured in Research Triangle Park,NC (20) and Potsdam, NY (21). The predicted upper andlower bound isoprene SOA concentrations give an estimateof the range of expected isoprene SOA concentrations acrossthe eastern U.S. for each season. Although there are significantuncertainties associated with the predicted base case isopreneSOA concentrations, the seasonal variations for isoprene SOAacross the eastern U.S. are quantified and discussed.* Corresponding author phone: (412)268-3531; fax: (412)268-7139;e-mail: [email protected].†Carnegie Mellon University.‡University of Patras.Environ. Sci. Technol.2007,41,3984-399039849ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 11, 2007 10.1021/es061312q CCC: $37.00


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UNC-Chapel Hill ENVR 890 - Predicted Secondary Organic Aerosol Concentrations from the Oxidation of Isoprene in the Eastern US

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