In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at similar to 410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle of NO2. In the former case, the conical intersection between (B) over tilde B-2(1) and (A) over tilde B-2(2) (CI (B-2(2)/B-2(1)) first funnels NO2 out of the Franck-Condon region of (B) over tilde B-2(1) and relaxes to the (A) over tilde B-2(2) surface. Following the primary relaxation, the conical intersection between (A) over tilde B-2(2) and (X) over tilde (2)A(1) (CI(B-2(2)/(2)A(1))) drives NO2 to decay into highly vibrationally excited (X) over tilde (2)A(1) state that is more than 20 000 cm(-1) above zeroth-order vertical bar n(1),n(2),n(3) = 0 > vibrational level. In the latter case, increasing the O-N-O angle leads NO2 to relax to a minimum of (B) over tilde B-2(1) with a linear O-N-O arrangement. This minimum point is also funnel region between (B) over tilde B-2(1) and (X) over tilde (2)A(1) (CI(B-2(1)/(2)A(1))) and leads NO2 to relax into a highly vibrationally excited (X) over tilde (2)A(1) state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor, producing OH ((2)Pi(3/2)) radicals. The collision between NO2 and H2O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited (X) over tilde (2)A(1) states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the generation of OH radicals from NO2 and H2O in the troposphere.