The O 3 for reaction R 1 was generated upstream of the flow reactor by continuously passing zeroair through an ozone generator (Jelight, model: 600). NO 2 was continuously added to the flow reactor from a cylinder (806 ppm NO 2 in nitrogen gas). The concentration of NO 2 and O 3 in the flow reactor were measured with a NO x analyzer (Thermo Scientific, 42i NO x ) and an O 3 analyzer (Thermo Scientific, 49i), respectively. The ratio of NO 2 : O 3 before reaction in the flow reactor was ∼2:1. The gas-phase rate constants for these reactions are shown in Table .