Heterogeneous reactions of isoprene and ozone on α-AlO: The suppression effect of relative humidity.

The heterogeneous reactions of α-AlO particles with a mixture of ozone (∼50 ppm) and isoprene (∼50 ppm) were studied as a function of relative humidities (RHs). The reactions were monitored in real time through the microscopic Fourier transform infrared (micro-FTIR) spectrometer. The results show that the presence of ozone leads to the rapid conversion of isoprene to carboxylate (COO) ions on the surfaces of α-AlO particles in the initial stage. The water significantly suppresses the formation of the carboxylate ions. For the isoprene ozonolysis reaction on the α-AlO particles, the reactive uptake coefficient is strongly suppressed by over a factor of 8 when the RH increases from 8% to 89%. The negative correlation between RH with the secondary organic aerosol (SOA) produced by isoprene ozonolysis plays a key role in the actual atmospheric environment under high humidity. Our results may provide insight into the ozonolysis process of biogenic alkenes over mineral aerosol surfaces with the influence of RHs.