If the volume mixing ratio of gaseous ammonia (nh3) is measured to be 500 parts per trillion (pptv) at sea level pressure and t 15 °c: a) what is the mass mixing ratio of ammonia relative to dry air? b) what is the mass concentration of ammonia in ugm3? c) what is the molecular concentration of ammonia, and what would it be if the same air were lifted upward to a location where the pressure was 400 mb and the temperature 230 k? what would its mass concentration be at this pressure and temperature? d) considering your answer to part (c), why is it convenient to have mixing ratio as a way to express the amount of a pollutant? e) if mixing ratios are convenient, why is it necessary to use concentrations at all?