Improvement of the Chemical Mass Balance model for apportioning—sources of non-methane hydrocarbons using composite aged source profiles

Abstract

The Chemical Mass Balance (CMB) receptor model is commonly used to evaluate the relationship between emissions of air pollutants and their concentration in the ambient air. However, it is not clear that it can accurately achieve this goal when evaluating sources of reactive air pollutants such as non-methane hydrocarbons (NMHC). This work examines the ability of CMB Version 8.2 to accurately resolve sources of NMHC simulated for Houston, Texas, and assesses whether model performance can be improved by providing the model with composite aged profiles. The NMHC are simulated using an unsteady multi-stage photochemical transport model which uses reported emission rates from mobile and industrial sources and meteorological measurements as inputs. Composite aged profiles are also developed using the photochemical transport model, and account for the distribution and strength of sources within the common trajectories to the receptor. The ability of the model is examined through 128 cases and 4 case studies, which challenge CMB to resolve the sources of simulated NMHC when reasonable levels of uncertainty are introduced into the fitting species, source profiles, and/or sources provided to the model, or when composite aged source profiles are also made available to the model. CMB performs best when all contributing sources are known and perfect information is available to describe these sources. When reasonable uncertainties are introduced into any of its inputs, performance is poorer. Many cases do not result in a solution. In many of the cases that result in a solution, CMB fails to identify or attribute the mass of a relevant source to within ±30% of the true contribution, or identifies irrelevant sources and attributes to them more than 5% of the total mass. When composite aged profiles are made available to the model, inaccuracy is eliminated but many cases still do not result in a solution.