Multi-Generation Chemical Aging of Secondary Organic Aerosol Components Under Initial High/Low NOx Conditions
Secondary organic aerosol (SOA) formation from volatile organic compounds (VOCs) in the atmosphere can be thought of as a succession of oxidation steps. The production of later-generation SOA via continued oxidation of the first-generation products is defined as chemical aging. Zeroth order parameterizations have been developed to model the chemical aging of semi-volatile and intermediate volatility organic compounds in chemical transport models. Models using these schemes show improved performance in urban areas. However, they tend to over-predict OA in areas such as the southeastern United States where biogenic VOCs dominate if chemical aging is assumed to be a major source of additional SOA. As a result, the importance of aging of biogenic SOA as a source of SOA mass concentration remains an issue of debate. In the first part of this work, we investigated aging in the α-pinene ozonolysis system with hydroxyl radicals (OH) through smog chamber experiments. The first-generation α-pinene ozonolysis products were allowed to react further with OH formed via HONO photolysis. After an equivalent of 2-4 days’ of atmospheric oxidation, homogeneous OH oxidation of the α-pinene ozonolysis products resulted in a 20-40 % net increase of the SOA for the experimental conditions used in this work. A more oxygenated product distribution was observed after aging based on the increase in aerosol atomic oxygen to carbon ratio (O:C) by up to 0.04. Experiments performed at intermediate relative humidity (RH) of 50 % showed no significant difference in additional SOA formation during aging compared to those performed at low RH of less than 20 %. The interaction of particles with the chamber walls has been a significant source of uncertainty when analyzing results of experiments performed in Teflon chambers. For complex experiments such as chemical aging of secondary organic aerosol, the results of the SOA quantification analysis can be quite sensitive to the adopted correction method due to the evolution of the particle size distribution and the duration of these experiments. In the second part of this work, we evaluated the performance of several particle wall-loss correction methods for aging experiments of α-pinene ozonolysis products. Determining the loss rates from seed loss periods is necessary for this system because it is not clear when chemistry is over. Results from the organic to sulfate ratio and the size-independent correction methods can be influenced significantly by the size-dependence of the particle wall-loss process. Coagulation can also affect the particle size distribution, especially for particles with diameter less than 100 nm, thus introducing errors in the results of the wall-loss correction. The corresponding loss rate constants may vary from experiment to experiment, and even during a specific experiment. Friction between the Teflon chamber walls and non-conductive surfaces can significantly increase particle wall-loss rates and the chamber may require weeks to recover to its original condition. Experimental procedures are proposed for the characterization of particle losses during different stages of these experiments and the evaluation of corresponding particle wall-loss correction. NOx (NO+NO2) can affect secondary organic aerosol formation by introducing a critical branching point in the fate of organo-peroxy radicals (RO2) produced from the oxidation of organic compounds. The formation of organonitrates (R-ONO2) from this pathway can potentially shift the product distribution and thus affect SOA yields and composition. In the third part of this work, we investigated the oxidation of the first-generation products of α-pinene ozonolysis from the RO2+NO pathway with additional OH radicals by performing aging experiments of α-pinene ozonolysis products formed under high NOx conditions in a smog chamber. The fate of RO2 radicals during these experiments was determined using a kinetics box model. The α-pinene in our experiments reacted with O3, NO3 and OH. The reduction in the first-generation yields under the high NOx levels used in this study was mainly attributed to α-pinene reacting with NO3. Our zeroth order estimate suggests that the ozonolysis RO2+NO pathway yielded similar amount of SOA compared to the RO2+RO2 alternative. After an equivalent of 1-2 days’ of typical atmospheric exposure to OH, the chemical aging processes of the α-pinene ozonolysis products formed under high NOx resulted in a net increase of up to 25 % in the SOA for the experimental conditions used in this work. A more oxygenated product distribution was observed after aging based on the increase in aerosol atomic oxygen to carbon ratio (O:C) by up to 0.06. Organonitrates accounted for around one quarter of the first-generation SOA. Their fractional contribution to SOA increased to around 40 % after aging.