Initial concentration, c₁ = 100 μg/m³Final concentration, c₂ = 10 μg/m³Diameter of the raindrop, d = 2 mm Target efficiency, η = 0.1Rain rate, R = 0.1 in./h, The time required for the particle concentration to reduce to 10 g/m3 is approximately 707.22 hours.
The concentration of particles in air, after some time (let's say t hours), is 10 μg/m³. The rainstorm deposits 0.1 in./h of rain over a large area. The drops have an average diameter of 2 mm for which the target efficiency for the particles in air is estimated to be 0.1.To find the time required for the particle concentration to reduce to 10 μg/m³, we use the below formula:
$$\frac{dc}{dt} = -Rη\frac{c}{V_d}$$
Where, c is the concentration of the particles in air,
Vd is the volume of air in which the particles are present.
The above formula is a general equation for the rate of change of concentration of any substance in any medium.
Here, it applies to the particles in air. The negative sign signifies that the concentration of particles decreases with time.
$$ \Right arrow \frac{dc}{c} = -Rη\frac{dt}{V_d}
$$Integrating both sides,
we get,
$$ \Right arrow \int_{c_1}^{c_2} \frac{dc}{c} = -\int_0^t Rη\frac{dt}{V_d}
$$$$\Right arrow \ln\frac{c_2}{c_1} = -\frac{Rη}{V_d} t
$$$$\Right arrow t = -\frac{V_d}{Rη} \ln\frac{c_2}{c_1}
$$$$\Right arrow t = -\frac{(1000 \ m/ km)^3}{(0.1 \ in./h)(25.4 \ mm/in.)(3600 \ s/h)(0.1)} \ln\frac{10}{100}$$
Here, we converted the rain rate from inches to mm and the volume of air from m³ to L (litres), for easy calculations.$$ \Right arrow t = 2.54 \times 10^6 \ s \approx \boxed{707.22 \ h} $$Hence, the time required for the particle concentration to reduce to 10 μg/m³ is approximately 707.22 hours.
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