Evaluating the impact of new observational constraints on P-S/IVOC emissions, multi-generation oxidation, and chamber wall losses on SOA modeling for Los Angeles, CA

摘要

Secondary organic aerosol (SOA) is an important contributor to fineparticulate matter (PM) mass in polluted regions, and its modeling remains poorly constrained. A box model is developed that uses recently publishedliterature parameterizations and data sets to better constrain andevaluate the formation pathways and precursors of urban SOA duringthe CalNex 2010 campaign in Los Angeles. When using the measurementsof intermediate-volatility organic compounds (IVOCs) reported in Zhao et al. (2014) and of semi-volatile organic compounds (SVOCs) reported inWorton et al. (2014) the model is biased high at longerphotochemical ages, whereas at shorter photochemical ages it isbiased low, if the yields for VOC oxidation are not updated. Theparameterizations using an updated version of the yields, whichtakes into account the effect of gas-phase wall losses inenvironmental chambers, show model–measurement agreement at longerphotochemical ages, even though some low bias at short photochemicalages still remains. Furthermore, the fossil and non-fossil carbon splitof urban SOA simulated by the model is consistent with measurementsat the Pasadena ground site.Multi-generation oxidation mechanisms are often employed in SOAmodels to increase the SOA yields derived from environmental chamberexperiments in order to obtain better model–measurementagreement. However, there are many uncertainties associated withthese aging mechanisms. Thus, SOA formation in the model iscompared to data from an oxidation flow reactor (OFR) in orderto constrain SOA formation at longer photochemical ages thanobserved in urban air. The model predicts similar SOA mass at shortto moderate photochemical ages when the aging mechanisms or theupdated version of the yields for VOC oxidation are implemented. Thelatter case has SOA formation rates that are more consistentwith observations from the OFR though. Aging mechanisms may still play animportant role in SOA chemistry, but the additional mass formed byfunctionalization reactions during aging would need to be offset bygas-phase fragmentation of SVOCs.All the model cases evaluated in this work show a large majority ofthe urban SOA (70–83 %) at Pasadena coming from the oxidationof primary SVOCs (P-SVOCs) and primary IVOCs (P-IVOCs). The importance of these two types of precursors is further supported by analyzing the percentage of SOAformed at long photochemical ages (1.5 days) as a function of theprecursor rate constant. The P-SVOCs and P-IVOCs have rate constantsthat are similar to highly reactive VOCs that have been previouslyfound to strongly correlate with SOA formation potential measured bythe OFR.Finally, the volatility distribution of the total organic mass (gasand particle phase) in the model is compared againstmeasurements. The total SVOC mass simulated is similar to themeasurements, but there are important differences in the measuredand modeled volatility distributions. A likely reason for thedifference is the lack of particle-phase reactions in the model thatcan oligomerize and/or continue to oxidize organic compounds evenafter they partition to the particle phase.