Indirect Photolysis in the Atmosphere Troposphere — Reactions with Hydroxyl Radical HO

The presence of relatively low levels of 03 in the troposphere is important because photolysis of 03 in the troposphere occurs in the wavelength region of 290-335 nm 

At 298 K and atmospheric pressure with 50% relative humidity, about 0.2 HO are produced per O( D) atom formed. Photolysis of 03 in the presence of water vapor is the major tropospheric source of HO , particularly in the lower troposphere where water vapor mixing ratios are high (for an explanation of the term mixing ratio see below). Other sources of HO in the troposphere include the photolysis of nitrous acid (HONO), the photolysis of formaldehyde and other carbonyls in the presence of NO, and the dark reactions of 03 with alkanes. Note that all these processes involve quite complicated reaction schemes. For a discussion of these reaction schemes we refer to the literature (e.g., Atkinson, 2000). 

Rate Constants and Tropospheric Half-Lives for Reactions with HO  

Second-order rate constants, k HO, allow us to estimate tropospheric half-lives jf1/2 ho. (Eq. 16-23) for environmentally relevant chemicals (see Fig. 16.7 for some examples)  

Note that the units of kH0. are (molecule cm 3) 1 s l and that tm,w has been calculated for an average global tropospheric HO concentration of 1 x 106 molecule cm-3 at 25°C. We should also note that, as a first approximation, we neglect the effect of temperature on k H0., because over the temperature range of the lower troposphere (-40° to 30°C), this effect is less than a factor of 2 to 3 for many compounds of interest to us (Atkinson 1989 and 1994). 

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Indirect Photolysis in the Atmosphere (Troposphere)—Reactions with Hydroxyl Radical (HO )... 

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