Sensitivity of NO₂ photolysis frequency to aerosol optical properties

Abstract. The photolysis frequency of nitrogen dioxide (NO₂), J(NO₂), links aerosol radiative effects with near-surface photochemistry and ozone formation. Its response to aerosol optical properties is difficult to quantify because scattering, absorption, hygroscopic growth, black carbon coating properties, aerosol vertical distribution, and surface reflection are coupled. We developed a framework combining Mie calculations, the Constrained Parameter (CP) method, and the Tropospheric Ultraviolet and Visible (TUV) model to quantify sensitivities and interactions controlling J(NO₂) using inputs representative of northern China. Two pathways were examined: direct perturbation of aerosol optical depth (AOD), single-scattering albedo (SSA), the asymmetry factor (g), and surface albedo in TUV, and a microphysical pathway in which AOD, SSA, and g were derived from aerosol microphysical parameters using Mie calculations. Among direct radiative inputs, SSA and surface albedo showed the largest CP sensitivities, approximately 12.0×10^-3 and 5.3×10^-3 dJ/(dx/x), respectively, indicating stronger control by scattering–absorption partitioning and the lower radiative boundary than by column extinction alone. In the microphysical analysis, coating thickness, black carbon density, black carbon absorption enhancement (BCAE), the imaginary part of the black carbon refractive index, hygroscopicity, and surface albedo were key factors. Interaction and fixed-factor analyses showed that sensitivity rankings were state dependent. The strongest interaction occurred between coating thickness and BCAE. Better constraints on SSA, surface albedo, coating thickness, BCAE, hygroscopicity, and aerosol vertical distribution are needed to improve photolysis-frequency calculations and assessments of coupled fine particulate matter and ozone pollution.