In a particular region of the earth's atmosphere, the electric field above the earth's surface has been measured to be 155 N/C downward at an altitude of 300 m and 172 N/C downward at an altitude of 450 m. Calculate the volume charge density of the atmosphere, assuming it to be uniform between 300 and 450 m. (Hint: You may neglect the curvature of the earth. Why?)