Relationship between the NO₂ photolysis frequency and the solar global irradiance

Abstract. Representative values of the atmospheric NO₂ photolysis frequency j(NO₂) are required for the adequate calculation and interpretation of NO and NO₂ concentrations and exchange fluxes near the surface. Direct measurements of j(NO₂) at ground level are often not available in field studies. In most cases, modeling approaches involving complex radiative transfer calculations are used to estimate j(NO₂) and other photolysis frequencies for air chemistry studies. However, important input parameters for accurate modeling are often missing, most importantly with regard to the radiative effects of clouds. On the other hand, solar global irradiance ("global radiation", G) is nowadays measured as a standard parameter in most field experiments and in many meteorological observation networks around the world. Previous studies mainly reported linear relationships between j(NO₂) and G. We have measured j(NO₂) using spectro- or filter radiometers and G using pyranometers side-by-side at several field sites. Our results cover a solar zenith angle range of 0–90°, and are based on nine field campaigns in temperate, subtropical and tropical environments during the period 1994–2008. We show that a second-order polynomial function (intercept = 0): j(NO₂)=(1+α)× (B₁×G+B₂×G²), with α defined as the site-dependent UV-A surface albedo and the polynomial coefficients: B₁=(1.47± 0.03)×10^-5 W⁻¹ m² s⁻¹ and B₂=(-4.84±0.31)×10^-9 W⁻² m⁴ s⁻¹ can be used to estimate ground-level j(NO₂) directly from G, independent of solar zenith angle under all atmospheric conditions. The absolute j(NO₂) residual of the empirical function is ±6×10^-4 s⁻¹(2σ). The relationship is valid for sites below 800 m a.s.l. and with low surface albedo (α<0.2). It is not valid in high mountains, above snow or ice and sandy or dry soil surfaces.