Incremental Aerosol Reactivity:  Application to Aromatic and Biogenic Hydrocarbons

The concept of incremental aerosol reactivity is introduced, and the incremental aerosol reactivities of a number of important anthropogenic and biogenic hydrocarbons are investigated for four ambient scenarios. The incremental aerosol reactivity, defined as a change in the secondary organic aerosol mass produced (in μg m-3) per unit change of parent organic reacted (in ppb), is a measure of the aerosol-forming capability of a given parent organic in a prescribed mixture of other organic compounds. The base-case scenario is a mixture of both aromatic and biogenic organics. Reactivity values depend on the choice of the initial organic mixture, so cases are also examined in which all biogenic hydrocarbon concentrations are set to zero and all aromatic concentrations are set to zero. The influence of additional organic aerosol is also investigated. For the compounds studied, incremental aerosol reactivities range from 0.228 μg m-3 ppb-1 (m-xylene) to 10.352 μg m-3 ppb-1 (α-humulene) for the base case, from 0.234 μg m-3 ppb-1 (m-xylene) to 9.446 μg m-3 ppb-1 (α-humulene) for the base case in the presence of initial organic aerosol, from 0.133 μg m-3 ppb-1 (m-xylene) to 0.801 μg m-3 ppb-1 (diethylbenzene) for the zero-biogenic case, and from 0.456 μg m-3 ppb-1 (linalool) to 6.923 μg m-3 ppb-1 (α-humulene) for the zero-aromatic case. Using m-xylene as a basis, relative incremental aerosol reactivities range from 1.153 (low-yield aromatics) to 3.338 (diethylbenzene) for aromatics in the base case and from 1.349 (low-yield aromatics) to 6.032 (diethylbenzene) in the zero-biogenic case. Using α-pinene as a basis, relative incremental aerosol reactivities range from 0.569 (terpinolene) to 12.843 (α-humulene) for biogenics in the base case and from 0.882 (linalool) to 13.385 (α-humulene) in the zero-aromatic case.