Partitioning of Organonitrates in the Production of Secondary Organic Aerosols from?-Pinene Photo-Oxidation

The chemical pathways for the production of secondary organicaerosols (SOA) are influenced by the concentration of nitrogen oxides (NOx),including the production of organonitrates (ON). Herein, a series of experimentsconducted in an environmental chamber investigated the production andpartitioning of total organonitrates from alpha-pinene photo-oxidation from <1 to24 ppb NOx. Gas-phase and particle-phase organonitrates (gON and pON,respectively) were measured by laser-inducedfluorescence (LIF). Thecomposition of the particle phase and the particle mass concentration weresimultaneously characterized by online aerosol mass spectrometry. The LIF andMS measurements of pON concentrations had a Pearson correlation coefficient of0.91 from 0.3 to 1.1 mu gm-3. For 1-6 ppb NOx, the yield of SOA particle massconcentration increased from 0.02 to 0.044 with NOxconcentration. For >6 ppbNOx, the yield steadily dropped, reaching 0.034 at 24 ppb NOx. By comparison,the yield of pON steadily increased from 0.002 to 0.022 across the range ofinvestigated NOxconcentrations. The yield of gON likewise increased from 0.005 to 0.148. The gas-to-particle partitioning ratio(pON/(pON + gON)) depended strongly on the NOxconcentration, changing from 0.27 to 0.13 as the NOxincreased from <1 to24 ppb. In the atmosphere, there is typically a cross-over point between clean and polluted conditions that strongly affects SOAproduction, and the results herein quantitatively identify 6 ppb NOxas that point for alpha-pinene photo-oxidation under these studyconditions, including the production and partitioning of organonitrates. The trends in SOA yield and partitioning ratio as a functionof NOx occur because of the changes in pON volatility