Secondary organic aerosol formation from reactions of linear, branched and cyclic alkanes with hydroxyl radicals in the presence of nitrogen oxides

Secondary organic aerosol (SOA) formation from reactions of linear, branched and cyclic alkanes with OH radicals in the presence of NO was investigated in the environmental chamber. Chemical compositions of SOA were investigated using a thermal desorption particle beam mass spectrometer. SOA products were all organic nitrates, such as first-generation 1,4-hydroxynitrates and higher-generation substituted tetrahydrofuran hydroxynitrates. Although the 1,4-hydroxycarbonyls are the major first-generation products based on studies of gas-phase reactions, they rapidly form cyclic hemiacetals by acid catalyzation, and dehydrate to dihydrofurans. Through this heterogeneous process, dihydrofurans react fast with OH radicals to form carbonylesters, which react further with OH to form multi-generation products. OH reactions with all of linear, branched and cyclic alkanes form similar products, thus they undergo the same chemistry.;SOA yield was investigated using a scanning mobility particle sizer for monitoring size distribution of SOA, and gas chromatography with flame ionization detection for quantify the alkane concentration before and after chamber reactions. For the linear alkanes, the SOA yield increases with linear fashion, as the carbon number increases. The SOA yield for the branched alkanes were lower than the corresponding carbon number of the linear alkanes. The potential lowering factor for the branched alkanes is the enhancement of decomposition rate once the OH-initiated reaction reached formation of alkoxy radicals. Unlike the branched alkanes, the SOA yields for OH reactions with the cyclic alkanes were higher than the corresponding carbon number of the linear. This is possibly due to the further reaction of ring-opening products from the alkoxy radicals, or oligomer formation during the chamber reaction since the ring-opening products contain carbonyl groups.;Finally, the conversion of 1,4-hydroxycarbonyls to cyclic hemiacetals and dihydrofurans was modeled based on reaction mechanisms, gas-particle partitioning and acid-catalyzing heterogeneous reactions. Results showed that in the conversion of 1,4-hydroxycarbonyls to the dihydrofurans, formation of cyclic hemiacetals was fast for all carbon numbers. Simulations performed with model parameters obtained from the experiments indicate that conversion of 1,4-hydroxycarbonyls to dihydrofurans will be important in polluted areas with low ammonia concentrations.