A city is spread over an area of 20 km × 40 km. Wind, at an average speed of 4 m/s, enters perpendicular to the 20 km long side. On a winter day the inversion layer exists over the city at a height of 100 m. PM₁ is emitted from the city at a rate of 1 kg/s. The steady state PM₁ concentration in the city air, in µg/m³, assuming Box model, is ______. (round off to one decimal place)