Air pollution according to the south coast air quality management district, the level of nitrogen dioxide, a brown gas that impairs breathing, present in the atmosphere on a certain may day in downtown los angeles is approximated by
Where a(t) is measured in pollutant standard index (psi) and t is measured in hours, with t = 0 corresponding to 7 a.m. At what time of day is the air pollution increasing, and at what time is it decreasing?