Analysis of Black Carbon Emission Factors Based on Carbon Dioxide Concentrations in an Urban Street Canyon

Abstract

Air quality monitoring has always been an important topic of concern in urban environments. In cities traffic was always considered to be one of the main sources of emissions. In addition to gaseous emissions, traffic emits substantial amount of particulate matter which has diverse effects on human health and climate. Out of all traffic-related particulate emissions black carbon presents the most interest and concern. Many recent studies have stated that black carbon poses more danger to climate and human health than commonly used PM2.5.

In this work black carbon was continuously measured by Helsinki Regional Environmental Services (HSY) for one year in the Mäkelänkatu Supersite in Makelankatu street canyon in Helsinki. In addition to black carbon, the concentrations of carbon dioxide as well as other traffic-related pollutants were measured at this station. Background levels of carbon dioxide were recorded by Finnish Meteorological Institute (FMI) at Kumpula measurement station (SMEAR-III) for the same time period. The main aim of this work was to study the emission factors of black carbon and their temporal variations throughout the observation period. Another aim was to understand the effects of background concentrations of trace gas on the emission factors and determine the approach to minimise the effects. To achieve these aims, the data about concentrations of black carbon and carbon dioxide was analysed during different time constraints and separating work days from weekends, and used during calculations of emission factors. To mitigate the effects of the background on emission factor calculations, a special algorithm was implemented and used in the calculations of the final results.

The data analysis has revealed a monthly variation in emission factors to be from 0.08 to 0.10 gBC kgfuel-1 with highest levels being during summer period. The main assumption is that such a variation is related to traffic behaviour, however slight drop in carbon dioxide concentrations during summer time hint on the effects of vegetation. Moreover, the overall levels of emission factors were found to be the highest during the working days. The diurnal analysis of emission factors has shown one consistent peak in values during morning rush hours with individual peaks during other times. The reason for this was not studied in this work however assumed to be related to traffic distribution and rates.