19. Tropospheric ozone. Tropospheric ozone is a pollutant that has significant n
ID: 119758 • Letter: 1
Question
19. Tropospheric ozone. Tropospheric ozone is a pollutant that has significant negative human health effects. The net equation for the photochemical reaction governing tropospheric ozone production is given by CO + 202 + hv CO2 + 03 where hv represents the energy associated with a photon of wavenumber v. (Note: this process is catalyzed (i.e. accelerated) by the presence of NO and NO2.) Consider the budget for Os i the volume bounding an urban city center in the lowest 1 km of the atmosphere on a sunny summer afternoon, assuming the following sources and sinks within the volume: photochemical production of Os at a rate of 7 ppbv/hr photochemical loss (photolysis) of Os at a rate of 1 ppbv/hr, zero emission, zero de- position, transport of Os downward from the stratosphere into the volume at a rate of 0.2 ppbv/hr, net horizontal transport of Os out of the volume at a rate of 1.5 ppbv/hr (a) (1 pt) Write an equation for the budget of [Os (b) (1 pt) Is the concentration of Os in balance? If not, is the concentration increasing or decreasing? (c) (1 pt) Ozone concentrations exceeding 70 parts per billion by volume (ppbv) are typically considered haul to human health. Suppose at noon the local ozone concentration is measured to be 45 ppbv. Given your answer to parts (a) and (b) above, by what time of day will the ozone concentration exceed the threshold for harmful health effects?Explanation / Answer
(a)
Photochemical production of ozone = 7ppbv/hr, loss due to photolysis = 1ppbv/hr, transport from stratosphere = 0.2ppbv/hr, horizontal transport from the system = 1.5ppbv/hr
Budget of O3 = Number of hours x 7 + Number of hours x 0.2 - Number of hours x 1 - Number of hours x 1.5
= Number of hours x 7.2 - Number of hours x 2.5
Budget of O3 = Number of hours x 4.7
(b) No, the budget of O3 is not balanced as we'll have an increase of 4.7 ppbv per hour.
(c) At noon the concentration of O3 is 45 ppbv, O3harmful for human health = 70 ppbv
Therefore extra O3 concentration needed = (70-45)ppbv = 25ppbv
Time in hours required for 25 ppbv = 25/4.7 = 5.319 hours
Therefore, time required for concentration to exceed threshold for harmful effects = 5.319 hours
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