Relationship between the NO2 photolysis frequency and the solar global irradiance
- 1Max Planck Institute for Chemistry, Biogeochemistry and Air Chemistry Department, P.O. Box 3060, 55020 Mainz, Germany
- 2Research Centre Jülich GmbH, Institute of Chemistry and Dynamics of the Geosphere 2: Troposphere, 52425 Jülich, Germany
- 3Agroscope ART, Air Pollution and Climate Group, 8046 Zürich, Switzerland
- 4Richard Assmann Observatory Lindenberg, German Meteorological Service, Germany
- 5Medical University, Division for Biomedical Physics, Müllerstr. 44, 6020 Innsbruck, Austria
- 6Institute of Environmental Meteorology, School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China
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.