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 located 1 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) (1 pt) Is the mass of A in balance? If not, is the total mass in the volume increasing or decreasing? (c) (1 pt) Visibility may become low 1 km) when dust aerosol density exceeds 3000 (1 g = 10-6 g). Suppose that the initial dust aerosol density is 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)

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

(c) For visibility to become low we need a concentration of 1000microgram/ m3

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

=10000 x 1000microgram

=107 microgram or 104 milligram  

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

=104/100

= 100 minutes

Therefore 100 minutes are required to surpass the threshold for low visibility.

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