Calibration Of Low-Cost Optical Particle Counters And Application For Atmospheric Particulate Matter Measurement In Urban Nairobi And Rural Nanyuki, Kenya

Abstract

The study of atmospheric particulate matter (APM) is of great significance for both air quality and monitoring studies. Understanding the sources and concentration of these particulates depend on the presence of long-term data, which is largely missing in the Sub-Saharan Africa (SSA). This study aimed at calibrating and using low-cost Optical Particle Counters (OPC-N2) sensors, collocated with an Andersen dichotomous impactor in the provision of the much-needed temporal resolution data for APM monitoring in Nairobi. Three sensors ran by a raspberry pi low-cost microprocessor packaged in weather-proof boxes were deployed in three sites in Kenya; two in Nairobi (urban background and roadside site) and one in Nanyuki, upwind of Nairobi representing the rural background site. Comparison of APM mass concentration obtained by the sensors and gravimetric means aided in the calibration process before deploying them in the three sites for a period of two months (February-March 2017). From the comparisons, differences of concentrations were observed and correction of the OPC data was implemented using factors, 1.63 and 1.70 for PM2.5 and PM10 species respectively, obtained from the collocated calibration sampling. The mean daily concentration observed for PM2.5 using the OPC-N2 at the rural background, urban background and roadside sites were 13 ± 7 μgm-3, 25 ± 14 μgm-3 and 37 ± 26 μgm-3 respectively while those of PM10 were 20 ± 9 μgm-3, 53 ± 34 μgm-3 and 94 ± 45 μgm-3 at the same sites respectively. The concentrations of APM observed at the two urban sites regularly exceeded the WHO limits in both the PM2.5 and PM10 size fractions. A “Lenschow” approach showed a clear urban and roadside increment in the city of Nairobi which was estimated to 7 μgm-3 and 18 μgm-3 for PM2.5 respectively while the same increments for PM10 were 33 μgm-3 and 43 μgm-3 respectively. A clear diurnal pattern in APM mass concentration was noted in the peak traffic periods which was consistent with the roadside increment, spatial variation and source apportionment findings that implicated traffic emissions as a dominant source of air pollution in Nairobi. Analysis of gravimetric samples by Energy Dispersive X-ray Fluorescence Spectroscopy (EDXRF) obtained during the collocation measurements of PM2.5 at the urban background site showed the presence of Fe, Ca, Zn, Mn and Cr. Their mean concentrations were 496±32 ngm-3, 407 ± 106 ngm-3, 42.7 ± 5 ngm-3, 8 ± 1 ngm-3 and 2 ± 1 ngm-3 respectively. The presence of mineral dust elements (Fe and Ca) affirmed the contribution of mineral dust as a significant source of APM in Nairobi while Mn, Cr and Zn indicated the contribution of mixed sources such as biomass burning, industrial and traffic-related emissions. This was consistent with source apportionment findings obtained using the PMF model. Results obtained from this study show that the use of low-cost and calibrated OPC-N2 sensors collocated alongside standard gravimetric reference methods can provide cheap and reliable air quality measurements in Kenya urban centers