Exploring Differences and Correlation Between Thermal-optical Transmittance Elemental Carbon (EC) and Reflectometer Black Carbon (BC) from an Urban and a Rural Site in the Philippines

Abstract

The Philippines has among the highest black carbon (BC) and elemental carbon (EC) concentrations in atmospheric particulate matter in Asia. Despite numerous studies, there is no single, generally accepted measure or method of analysis for this group of atmospheric particulates. Given the high concentrations of BC and EC in the country, the Philippines offers an interesting case to study BC and EC. To gain a better understanding of the similarities and differences of these quantities, BC and EC in an urban (Valenzuela City) and a rural site (Angat, Bulacan) were compared from September 2011 to August 2012. BC was measured using reflectometry, while EC was measured using the thermal-optical (TO) transmittance analysis. Mean concentrations of EC and BC were 5.54 ± 2.1 μg/cm3 and 6.54 ± 2.5 μg/cm3 in Valenzuela City and 1.82 ± 0.7 μg/cm3 and 1.28 ± 0.7 μg/cm3 in Angat, Bulacan. Cluster analysis showed that in both urban and rural sites, EC1 had the highest correlation to BC among the three EC fractions. Additionally, EC2 and EC3 were poorly correlated with BC but were highly correlated with each other. Similarly, conditional probability function (CPF) analysis revealed that BC and EC1 originated from nearly the same directions, while EC2 and EC3 do not. These results suggest that BC and EC1 are more related to each other than EC2 and EC3, providing insights into the similarities and differences between BC and EC. To maximize the comparability of BC and EC, optimal values of ε – used in reflectometry – were determined for the urban and rural sites. Valenzuela and Angat had optimal ε values of 6.31 m2g–1 and 1.89x 10–9m2g–1, respectively. The optimal ε value in Valenzuela is close to the generally used ε value, 7.0 m2g–1, while the optimal ε value in Angat is arguably too small and needs further assessment.