The present study is the first report of utilizing $TiO_2$ photocatalyst to analytically calibrate the hydroperoxyl radical ($HO_2;^{cdot}$). An in-situ calibration method of $HO_2;^{cdot}$ is proposed for air monitoring by using an 2-methyl-6-(pmethoxyphenyl)- 3,7-dihydroimidazo-[1,2-a]pyrazin-3-one (MCLA)-chemiluminescence (CL) technique. In this method, $HO_2;^{cdot}$($pK_a$ = 4.80) is produced by the ultraviolet (UV) photolysis of immobilized $TiO_2$ using a constant flow rate of air equilibrated water, in which $HO_2;^{cdot}$ is controlled by using various lengths of knotted tubing reactor (KTR). The principle of the proposed calibration is based on the experimentally determined halflife ($t_{1/2}$) of $HO_2;^{cdot}$ and its empirically observed pH-dependent rate constant, $k_{obs}$, at a given pH. The concentration of $HO_2;^{cdot}$/$O_2;^{cdot}$? is increased as pH increases. This pH dependence is due to the different disproportionative reactivities between $HO_2;^{cdot}$/$O_2;^{cdot}$? and $HO_2;^{cdot}$/$O_2;^{cdot}$?. Experimental results indicate the practical feasibility of the approach, producing very promising method.