In a square city with length $=15$ km during the evening rush hour traffic there are $200,000$ cars on the road. Each being driven $30$ km between $4$ pm and $6$ pm and each emitting $3$ g$/$km of carbon monoxide $($CO$).$ It is a clear winter evening with radiation inversion $($very stable atmospheric conditions in which dispersion of pollutant is inhibited$)$ which restricts mixing depth to $20$ m$.$ The average wind speed is $1$ m$/$s and the air blowing through the city is clean. What is the ambient CO concentration at $6$ pm in the city?