Incorporating hopane degradation into chemical mass balance model: Improving accuracy of vehicular source contribution estimation

Abstract

Hopanes are useful molecular markers for tracking airborne fine particulate matter (PM2.5) associated with vehicular exhaust in receptor models. However, they undergo atmospheric degradation. This causes deviations from the underlying assumption of mass conservation in receptor models and leads to biased estimation for source contributions. Little work has been conducted to account for this issue in receptor modelling. In this study, we analyzed the PM2.5 chemical speciation data including C27–C31 hopanes from two urban sites in the Pearl River Delta region, China. We developed an approach using ambient-to-source ratio of hopane homologues to show evidence of hopane degradation. Organic carbon (OC) and PM2.5 were then apportioned using the chemical mass balance (CMB) model, with the emphasis on considering hopane degradation. We applied a set of volatility-dependent degradation factors to correct for the loss, and determined the extent of degradation through identifying the statistically optimal CMB solution. The results reveal that neglecting hopane degradation would underestimate the primary vehicular OC and PM2.5 contributions by ∼55% in warm season and by ∼35% in cold season in our subtropical study region. The method developed in this work could be used to improve accuracy of vehicular source contribution estimation in other urban locations.