| dc.description.abstract | Criteria pollutants have health implications that are not only of concern to the urban population but also to governments in formulating their policies. These pollutants are also important to climate change investigations and any sustainable development programs require a properly informed population on their effects, concentration levels and sources. However, without the knowledge of the characteristic aerosol and information on the levels of pollutant concentrations, implementation of controls and enforcement of regulation remains difficult. NO and five species in the group of the criteria pollutants CO, NO2, SO2, PM2.5 and PM10 were measured at an urban background site in Nairobi, Kenya. The site is approximately 20 m above the ground, on top of a building at the main campus, University of Nairobi. Eight hour measurements at intervals of 10 s were obtained using pollutant gas monitors and 24 h samples of PM2.5 and PM10 particles were collected on glass fiber filters by a size segregating impactor. Figure 1 and 2 shows preliminary typical hourly average concentrations trends on a day in a weekend. The results imply a different daytime chemistry of NOX (= NO + NO2) from that reported in the literature. Typical measurements in urban boundary layer shows peaking of NO in early morning hours followed by NO2 and O3 in the afternoon. O3 oxidizes NO in a fast reaction to NO2 and O2. The recorded trend showed high rate of NO concentration replenishment, throughout the measurement period, in competition to its conversion to NO2. Similar trends of NO and NO2 were observed from about 1200 h until the end of the 8 h daytime measurement. This implied a near constant rate of NO oxidation that may have been the indicator of O3 production after 1130 at a concentration which was not capable of decreasing NO substantially. Normally, in urban aerosol, CO concentration is far much higher than the other gaseous pollutants but it was comparable to NO from midday. There was daytime SO2 which persisted at constant concentration of 0.1 ppm, the instrument indicator resolution value. High levels of CO were recorded in the early hours of the night and NO2 that persisted at 0.1 ppm. Considering that NO was high during the mid afternoon the nighttime NO2 concentrations portrayed a nighttime chemistry that converted NO to NO2. This conversion may have been driven by the peroxy radicals that are produced by reactions between NO3 radicals and reactive organic gases in the absence of sunlight. Since NO3 is a product of NO and O3 it followed that concentrations of the latter continued into nighttime | en |
