Relationship between the NO₂ photolysis frequency and the solar global irradiance

Abstract

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. A linear relationship between j(NO2) and G was reported in previous studies and has been used to estimate j(NO2) from G in the past 30 years. 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 (including uncertainty ranges): 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 (95.45% confidence level). The relationship is valid for sites below 800 m a.s.l. and under low background albedo conditions. It is not valid in alpine regions, above snow or ice and sandy or dry soil surfaces. Our function can be applied to estimate chemical life times of the NO2 molecule with respect to photolysis, and is useful for surface-atmosphere exchange and photochemistry studies close to the ground, e.g., above fields with short vegetation and above forest canopies.