The influence of OH concentration on SOA formation from isoprene photooxidation.

The formation of secondary organic aerosol (SOA) from isoprene photooxidation was investigated to reveal the influence of OH concentration on SOA formation through varying the concentrations of isoprene and H2O2 in a smog chamber. The results indicated that the higher the OH concentration was, the less the isoprene consumed for the detectable SOA mass concentration, for example, the lowest isoprene consumption for the detectable SOA was about 14.4 ppb under the OH concentration of about 1.65 × 107 molecules cm−3, whereas tens ppb of isoprene consumption were needed under the OH concentrations <1.0 × 107 molecules cm−3, and even no detectable SOA was observed with isoprene consumption of 75.1 ppb under OH concentration of 7.2 × 105 molecules cm−3. SOA yield was also found to increase with increasing OH concentration for a given aerosol loading (M0) at atmospherically relevant conditions, confirming that OH concentration played important role in SOA formation from isoprene photooxidation. The maximal SOA yields (5.8–42.8%) obtained by this study were a factor of 1.5–3.1 greater than those reported by previous study for the almost the same initial reactant concentrations of isoprene and H2O2, and the difference was mainly ascribed to the higher OH concentrations in the reaction systems of this study than those of previous study. The OH concentrations adopted in this study closed to those in the real atmosphere around noontime, and hence the SOA yield obtained from the isoprene photooxidation might be representative.