Where given:

molecular mass of NO$2=46$ g$/$mol

molar volume of air at $1$ atm $=0\mathrm{.}02445$ m$3/$mol

NO$2$ concentration $=57\mathrm{.}7\backslash$mu g$/$m$3=57\mathrm{.}7$x$10-6$ g$/$m$3$

Calculations:

$\%$ Decrease $=[($ Previous value $-$ New Value$)$: Previous Value$]$ x $100\%$

Decrease$=?$