Comparison of airborne measured and calculated spectral actinic flux and derived photolysis frequencies during the PEM Tropics B mission

Abstract

[1] Scanning actinic flux spectroradiometer (SAFS) instruments were deployed on the National Aeronautics and Space Administration (NASA) DC-8 and P-3B research aircraft for the Pacific Exploratory Mission in the Tropical Pacific (PEM Tropics B mission). The SAFS instruments measured the wavelength-dependent downwelling and upwelling actinic flux from 282 to 420 nm. From the actinic flux spectra, photolysis frequencies of a number of molecules were calculated. Flight tracks during the PEM Tropics B mission varied in altitude from the surface up to 12 km, with much time spent above, below, and among broken and scattered clouds. Comparisons of the measured photolysis frequencies to the photolysis frequencies calculated with the National Center for Atmospheric Research (NCAR) tropospheric ultraviolet visible (TUV) radiative transfer model, assuming clear skies, demonstrate some expected differences in this complex radiation environment, while in general the agreement between the measurements and the model is within 15%. Differences between the measurements and the model are expected in and around clouds, because the radiation model is not equipped to input broken and scattered cloud effects, except on an individual case-by-case basis. The differences in the composite mean ratio of SAFS measurement to TUV model for total photolysis frequencies vary from 6% to 18% for jNO 2 and from 6% to 11% for jO( 1 D) for both aircraft. The differences between the measurements and the models represent the accuracy that can be expected when using photolysis frequencies from clear-sky radiative transfer modeling in regional and global atmospheric chemistry models for regions with broken and scattered clouds in the troposphere.

Ultraviolet actinic flux in clear and cloudy atmospheres: model calculations and aircraft-based measurements

“…assuming homogeneous cloudiness horizontally (reducing the problem from three dimensions to one), vertical discretization into uniform layers, and analytic approximations of the scattering phase functions. Actinic flux measurements confirm the high variability in the presence of clouds (Junkermann, 1994;Pfister et al, 2000;Lefer et al, 2003;Shetter et al, 2003;Thiel et al, 2008) but in most cases the detailed information needed as input to radiative transfer models is not available.…”

Chemical processes related to net ozone tendencies in the free troposphere

“…This method is not ideal, since it does not take into account the wavelength dependency of either transmission or reflection by clouds. Shetter et al (2003) indicate that the TUV simulation of J (NO 2 ) and J (O 1 D) compared to observations are accurate to within 6-18 and 6-11 %, respectively.…”

Measurements of the HONO photodissociation constant

“…Spectroradiometers capable of recording actinic flux spectra in both the UV and visible regions only started being developed in the late 1990's (Junkermann et aI., 1989(Junkermann et aI., , 2002Van der Hage et aI., 1994;Cotte, 1995;Miiller et al, 1995;Hofzumahaus, 1998,2000;Hofzumahaus et aI., 1999Hofzumahaus et aI., ,2002Hofzumahaus et aI., ,2004Shetter and Miiller, 1999;Balis et aI., 2002;Shetter et al, 2002;Bais et al, 2003;Cantrell et aI., 2003;Crawford et al, 2003;Holland et al, 2003;Kylling et aI., 2005). Another approach to obtain the spectral actinic flux is to evaluate it theoretically using a radiative transfer model.…”