Recent studies have shown that the photolysis of particulate nitrate may be one of the important sources of daytime atmospheric nitrous acid (HONO). However, the existing laboratory studies have reported a wide range of measured photolysis frequency of nitrate, spanning 1 to 2 orders of magnitude, which significantly affects the accurate assessment of the contribution of nitrate photolysis to daytime HONO formation. A common practice to determine the photolysis rates of nitrates is to use a flow tube reactor, where artificial light-source are commonly used to simulate solar radiation, so how to accurately obtain the radiation intensity of light sources is a critical step for this kind of experiments and may induce a significant uncertainty in measuring photolysis rates of nitrates. In this study, nitrogen dioxide (NO₂) actinometry was used to determine the radiation intensity inside a flow tube. The principle is that NO₂ will be photolyzed into NO and swiftly form ozone (O₃) at the present of oxygen (O₂). At the same time, O₃ reacts with NO rapidly to form NO₂. Eventually, a dynamic equilibrium will be reached. Therefore, the actinic flux and photolysis rates of NO₂J(NO₂) can be deduced from the measured concentrations of NO_x and O₃, and then the photolysis frequency of nitrate J(HNO₃) can be obtained by using the empirical formula. Different from the traditional nitrate solution actinometry, which uses nitrate actinometry to measure the absorbed radiation, the method proposed in this work avoids the bias of light absorption caused by water since no aqueous solution is present. Meanwhile, the NO₂ actinometry does not depend on the physical configuration of the flow tube in this method and thus can provide more accurate measurement results. When a xenon light source (500 W) was set directly above the flow tube (i.e., zenith angle θ = 0°) and the gas passage time was 61.7 s under 1 standard atmospheric pressure and at 25 ℃, the J(NO₂) measured by the NO₂ photolysis method was 6.78 × 10^-3 s^-1, and J(HNO₃) = 3.40 × 10^-7 s^-1 was finally obtained by using the empirical formula of previous studies.