Surface UV radiation is also affected by atmospheric NO2, which absorbs at UV wavelengths. Atmospheric NO2 typically absorbs a small percentage of global radiation at the earth's surface. Sources of atmospheric NO2 are both manmade and natural. Pollution from sources such as automobiles and aircraft account for NO2 near the ground, in addition to regions in the troposphere and lower stratosphere. NO2 occurs naturally in the stratosphere and plays an important role in the photochemistry of stratospheric ozone. Measurements of NO2 are therefore important for both pollution studies, as well as in the study of the stratosphere.
NO2 absorbs radiation in the visible as well as the UV region of the spectrum. Its absorption in the UV has little structure and is nearly linear with wavelength, so it does not interfere with UV ozone measurements. However, at visible wavelengths between 420 nm and 450 nm, NO2 absorption is highly structured with wavelength allowing its measurement (Brewer et al., 1973). Several researchers now monitor NO2 routinely using DOAS techniques with the visible absorption features (Hofmann et al., 1995, and references therein). The Brewer Mark IV instrument uses the set of five wavelengths positioned at wavelengths in the visible and an algorithm weighted to optimize NO2 absorption as described in detail by Kerr (1989b).
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