2.2.3. Hydroxyl Radical Consumption Rate

Hydroxyl radical is the most significant oxidant in the troposphere. The photooxidation reactions of various VOCs often start with their reaction with hydroxyl radicals, during which new hydroxyl radicals are constantly formed in O2 and NOx, with light degradation leading to chain reactions. All VOC species are compared on the same standard to evaluate the reactivity of hydroxyl radical. The hydroxyl radical consumption rate is often used to measure the ability of VOCs to participate in atmospheric autooxidation reactions to form peroxanoxy radicals, which can also be used to estimate the impact of a specific VOC species on the ozone formation rate in a polluted atmosphere [29]. Its calculation formula is as follows:

$$\rm L^{\rm OH}{}\_{\rm i} = \rm C\_{i} \times K^{\rm OH}{}\_{\rm i} \tag{5}$$

In the formula, LOH <sup>i</sup> is the hydroxyl radical consumption rate of the ith VOC with the unit of s–1. Ci is the observed concentration of the ith VOCs with the unit of ppbv. KOH <sup>i</sup> is the hydroxyl radical consumption rate constant corresponding to the ith VOC at the temperature of 298 K [30]. However, it should be pointed out that their reaction with hydroxyl radical is not the only way VOCs can react in the atmosphere, since many VOCs are also consumed to a significant extent by reaction with ozone or (at night) with NO3, and some also with photolyze.
