In a particular city, the source rate of oxides of nitrogen (NOx) is 175 kg/hr. (kilograms per hour) The residence time of NOx is 2 days (or 48 hours). The volume of the city is such that the inversion height (which is the height of the box in which pollutants mix) is 1 km, which means the volume of the air in which pollution mixes is 100 km3, if the area of the city is 100 km2. In cubic meters, this volume is 100 × 109 m3, since there are a billion cubic meters in one cubic kilometer. What is the steady-state concentration of NOx in the city under these conditions? _____ μg/m3