$2\mathrm{.}1.$ A modest$-$sized urban area experiences an average emission rate of $1,000k\frac{g}{?}$ hour of $P{M}_{2\mathrm{.}5}.$ Its footprint is $3$ km from north to south and $1$ km from east to west. The prevailing wind direction is from the west with a typical wind speed of $4\frac{m}{sec}.$ However, the area sometimes has wind that comes from the south and with a wind speed of $6\frac{m}{sec}.$ If the mixing height is the same in each case $(800$ m $),$ will either condition violate the $24-$hour standard for $P{M}_{2\mathrm{.}5}$ if the upwind concentration is $5\frac{g}{m^3}.$

$[$Note: the $24-$hour standard for $P{M}_{2\mathrm{.}5}$ is $35\frac{g}{m}3]$

$[20]$