Aerosol-radiation interaction in the cloudless atmosphere during LACE 98 1. Measured and calculated broadband solar and spectral surface insolations

Vertical profile measurements of aerosol particle size distributions and of meteorological parameters (obtained from aircraft, radiosondes, and lidar) are used as input to a spectral radiative transfer model to calculate broadband solar and spectral surface insolations. The calculated values are compared to measured ones gathered with broadband solar pyranometers and pyrheliometers, and a fixed‐grating photodiode array spectroradiometer with 512 spectral channels between 500 and 920 nm wavelength. The measurements were obtained during the joint field campaign Lindenberg Aerosol Characterization Experiment (LACE) 98 near Berlin/Germany in the summer of 1998. Two cases (days with high and low aerosol loading, respectively) are investigated in detail. Furthermore, a measurement‐based sensitivity analysis was carried out focusing on the influence of particle composition (complex refractive index) and of microphysical and humidity growth uncertainties on the calculated surface insolations. Assuming a spectral refractive index of ammonium sulfate for the aerosol particles, on average the global component of the broadband solar surface insolations is 11–20 W m−2 (2–3%) greater than the measured values; the direct portion is 17–28 W m−2 (4–5%) higher, and its diffuse component is 6–7 W m−2 (4–10%) lower in comparison to the measurements. The measured and calculated spectral surface insolations (global portion) agree well in the central visible spectral region (500–600 nm wavelength). Toward larger wavelengths (near infrared) the calculated spectral surface insolations are increasingly higher than the measured ones.