Identification and Quantification of Secondary Organic Aerosol Compounds Using Improved Spectroscopic Methods for Functional Group Analysis
Spectrophotometric functional group derivatization methods were developed, validated, and utilized to study the products and mechanisms of secondary organic aerosol (SOA) formation from reactions of VOC systems with atmospheric oxidants in environmental chamber studies. These methods utilize derivatizing agents that form characteristic chromophores which can be detected at wavelengths specific to each functional group and can be used to determine SOA composition.
In chapter 2, the derivatizing methods were adapted for use with an Implen nanophotometer, a UV-Vis spectrophotometer with at 1 or 0.2 mm fixed pathlength, which requires ~10-100 µg of SOA sample. The detection limits are approximately 0.03, 0.3, 1, 0.02, 1, and 0.07 nmoles for carbonyl, carboxyl, ester, hydroxyl, peroxide, and nitrate groups. The method was then validated for linearity, repeatability, and accuracy by comparing results from analysis of standards and SOA formed from the reaction of α-pinene and O3 with results using previously developed macroscale methods.
In chapter 3, the derivatizing methods for carbonyl, hydroxyl, carboxyl, and ester were adapted for use with high performance liquid chromatography (HPLC) in conjunction with chemical ionization ion trap mass spectrometry (CI ITMS) with ammonia and/or isobutane as the reagent ion gas. The methods were tested for linearity, ability to detect molecular ion, and for characteristic fragmentation patterns with derivatized standard. In chapter 4, these methods were used to identify and quantify SOA products formed from the reaction of n-hexadecane with OH radicals in the presence of NOx. These results will be used to evaluate and improve the GECKO-A atmospheric model for predicting fate and transport of atmospheric emissions.
In chapter 5, the rate constant for the dehydration of particulate cyclic hemiacetal to dihydrofuran was determined with respect to the acidity of the particle as 0.07 ± 0.00, 0.11 ± 0.00, 0.25 ± 0.01, and 0.57 ± 0.02 per h-1 for 0.25, 0.5, 1.0, and 2.0 ppmv of gas-phase HNO3, respectively, which was proportional the amount of HNO3 measured in SOA particles. These quantities will be used to model the behavior of SOA as this constant is necessary for understanding the ageing process in alkane-type reactions.