18. Dust aerosols. Consider the mass budget for dust aerosols (i.e. particles su

Question

18. Dust aerosols. Consider the mass budget for dust aerosols (i.e. particles suspended in air) A in a 10000 m3 volume that contains your house located1 km downwind from a sandy desert, assuming the following sources and sinks within the volume: 1 g/min net horizontal transport into the volume by the wind, zero chemical production, zero chemical loss, zero emission, and 100 mg/min deposition onto ground surfaces (a) (1 pt) Write an equation for the dust aerosol mass budget (b) ( pt) Is the mass of A in balance? I not, is the total mass in the volume increasing or decreasing? (c) (1 pt) Visibility may become low1 k) when dust aerosol density exceeds 3000 (1 9-10-6 g). Suppose that the initial dust aerosol density s zero. Given your answer to parts (a) and (b) above, how much time is required to surpass the threshold for low visibility?

Explanation / Answer

(a) Since we are getting only 100mg/min deposited which is 0.9 fraction or 90% of the input dust, hence mass budget for dust aerosol is:

Input dust aerosol in grams = time in minutes x 1 - (time in minutes x 0.9)

Input dust aerosol in grams = time in minutes x 0.1

(b) No, the total mass is not in balance as we are getting 1000 mg/min deposited and 900 mg/min moves out of the system and hence the deposition is 100mg/min. This means that the total mass in the volume in increasing.

(c) For visibility to become low we need a concentration of 3000microgram/ m3

Our system volume is 10000 m3, and in order to get low visibility we need a concentration of :

=10000 x 3000microgram

=3 x 107 microgram or 3 x 104 milligram (since, 1000 microgram = 1 milligram)

we are getting 100 mg/min deposited hence time needed for 104 milligram will be:

=3 x 104/100

= 300 minutes

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