Theoretical, actinometric, and radiometric determinations of the photolysis rate coefficient of NO₂ during the Mauna Loa Observatory Photochemistry Experiment 2

Abstract

Measurements of the photolysis rate coefficient of NO2 (jNO2) were made using a chemical actinometer during the fall (September 15 to October 23, 1991), winter (January 15 to February 15, 1992), spring (April 15 to May 15, 1992), and summer (July 15 to August 15, 1992) intensives during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2), Hawaii. The measured clear sky jNO2 values of MLOPEX 2 for all four intensives have substantially increased over the measured jNO2 values of MLOPEX 1 (May, 1988); a 37 % increase occurs in midday clear sky jNO2 from the spring intensive of MLOPEX 1 to the spring intensive of MLOPEX 2. The ratio of actinometer measurements to theoretical calculations ofjNO2 are 1.44+_0.03, 1.45+_0.02, 1.43+0.03, and 1.34+-0.02 for the fall, winter, spring, and summer intensives, respectively. Inclusion of stratospheric aerosols or aerosols of any reasonable optical parameters in a derailed discrete ordinate model cannot account for the discrepancy between measurements and model. The photolysis rate coefficient of NO2 is also derived from simultaneous measurements of the ultraviolet irradiance measured with an Eppley radiometer using a semiempirical relationship for each of the four intensives. A simplified cloud model is proposed to explain jNO2 values that exceed clear sky values during days that have partial cloud coverage. (3) where c•0•,T) is the wavelength and temperature dependent absorption cross section of molecule a, (p()•,T) is the quantum yield of molecule a, and F(/•,z,Z) is the solar actinic flux where z is the altitude and Z the solar zenith angle. The absorption cross section and quantum yield are usually measured experimentally in the laboratory and the actinic flux is calculated theoretically using a radiative transfer model. There are several possible methods for determining the photolysis rate coefficient of NO 2 (jNO2); these include theoretical calculations using (3), direct measurements such as with a chemical actinometer, or semiempirical calculations from UV filter radiometer measurements [Jackson et al., 197 6; Harvey etal., 1977; Bahe et al., 1980; Dickerson et al., 1982; Parrish et al., 1983; Madronich, 1987a; Shetter et al., 1992; Brauers and Holfxumahaus, 1992, Kelley et al., 1995, A. Lerner et al., Airborne measurements of the photolysis of NO2 aboard the C-130: Instrumentation and clear sky data, submitted to the Journal of Geophysical Research, 1995]. The photolysis rate coefficient of NO 2 at the Mauna Loa Observatory (MLO) during the second Mauna Loa Photochemistry Experiment (MLOPEX 2) is investigated with these three methods. The photolysis of ozone, reaction (2), is discussed by Shetter et al., [this issue]. The photolysis rate coefficient of NO2 and the UV irradiance were measured with a chemical actinometer and

Measurements of the HONO photodissociation constant

“…These values are in good agreement with literature (Lantz et al, 1996;Shetter et al, 1999;Kylling et al, 2005). For all below-cloud data, the experimental transmission factors, r, ranged from 0.2-1.7 for SZÃ 6SO.…”

“…Near solar noon, the observed optical thickness of the clouds above the measurement site decreased considerably, and the photolysis frequency was enhanced by as much as 16%. This compares well with that of Lantz et al (1996) who reported a 24% enhancement in the actinic flux over clear sky values around local noon for cloud covers of 50% and a cloud optical depth of 4.…”

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

“…Clouds generally reduce the actinic flux reaching Earth's surface, although temporary or local enhancements can occur with broken clouds that, as is often the case, appear brighter than the blue sky sector they obscure (Lantz et al, 1996;Crawford et al, 2003). At high altitudes above the clouds, actinic fluxes are generally enhanced by the strong up-welling radiation component from cloud reflections, and within clouds both increases and decreases of actinic fluxes can occur.…”

Airborne analysis of the Los Angeles aerosol

“…Alternatively, vertical aerosol pro"les derived from airborne measurements or vertically resolved models can be used to determine the potential impact of scattering and absorption on photolysis rates. Several investigations of this type have been reported for di!erent regions with varying cloud cover, surface albedo, and solar zenith angle (Demerjian et al, 1980;Ruggaber et al, 1994;Lantz et al, 1996;Castro et al, 1997;Dickerson et al, 1997;Jacobson, 1998;Liao et al, 1999). The extensive aerosol measurements made with the Pelican provide much of the data necessary for such an analysis, thereby reducing uncertainty in the role of the aerosol in atmospheric photochemistry.…”