Updating the Conceptual Model for Fine Particle Mass Emissions from Combustion Systems Allen L. Robinson

Abstract

Atmospheric transformations determine the contribution of emissions from combustion systems to fine particulate matter (PM) mass. For example, combustion systems emit vapors that condense onto existing particles or form new particles as the emissions are cooled and diluted. Upon entering the atmosphere, emissions are exposed to atmospheric oxidants and sunlight, which causes them to evolve chemically and physically, generating secondary PM. This review discusses these transformations, focusing on organic PM. Organic PM emissions are semi-volatile at atmospheric conditions and thus their partitioning varies continuously with changing temperature and concentration. Because organics contribute a large portion of the PM mass emitted by most combustion sources, these emissions cannot be represented using a traditional, static emission factor. Instead, knowledge of the volatility distribution of emissions is required to explicitly account for changes in gas-particle partitioning. This requires updating how PM emissions from combustion systems are measured and simulated from combustion systems. Secondary PM production often greatly exceeds the direct or primary PM emissions; therefore, secondary PM must be included in any assessment of the contribution of combustion systems to ambient PM concentrations. Low-volatility organic vapors emitted by combustion systems appear to be very important secondary PM precursors that are poorly accounted for in inventories and models. The review concludes by discussing the implications that the dynamic nature of these PM emissions have on source testing for emission inventory development and regulatory purposes. This discussion highlights important linkages between primary and secondary PM, which could lead to simplified certification test procedures while capturing the emission components that contribute most to atmospheric PM mass. IMPLICATIONSEmissions from combustion systems such as motor vehicles are a major source of atmospheric fine PM. Quantifying the contribution of these emissions to atmospheric PM requires a bridge in the gap between the tailpipe, where emission measurements are made, and the atmosphere, where the emissions are transformed and ultimately delivered to a receptor, whether measured, deposited, or inhaled. This review describes these transformations and discusses their implications for the measurement, simulation, and regulation of emissions from combustion systems. There are important linkages between primary and secondary PM, which could lead to improved test procedures.

Aging of biogenic secondary organic aerosol via gas-phase OH radical reactions

“…After emissions are diluted to ambient concentrations (evaporating semivolatile compounds), OH exposure results in large increases in aerosol mass and oxygen content (23). However the aging mechanism driving this enhancement is uncertain.…”

Design of a Compact Dilution Sampler for Stationary Combustion Sources

“…where DR t [1] and DR t [2] are determined by eq 1 and eq 3, respectively. The values were less than 7.4%, which indicated that the DR t determined by two ways were comparable.…”

“…18 Robinson et al reviewed recent developments in the understanding of contribution of emissions from combustion process to fine PM mass, focusing on organic aerosols and updated the conceptual model for fine particle mass emissions from combustion systems. 1 A recent review paper by Maricq and Maldonado summaries the spirit of two Coordinating Research Council PM Measurement workshops and one of the consensus is that dilution sampling can provide comparable results in laboratory and real-world applications. 20 The bulkiness of early dilution samplers was a major barrier to their widespread application for field source test.…”

“…Contribution to urban ambient PM from combustion sources can generally be categorized as primary PM and secondary PM. 1 Primary PM is emitted directly from sources and includes the compounds that are solid or liquid at stack conditions, and the compounds that are gas or vapor at stack conditions. The latter compounds become solid or liquid upon exiting the stack where they are rapidly cooled and diluted with ambient air to ambient temperature and pressure conditions.…”

“…The latter compounds become solid or liquid upon exiting the stack where they are rapidly cooled and diluted with ambient air to ambient temperature and pressure conditions. [1][2][3] Characterizing PM emissions from various combustion sources and providing reliable emission data are important for identifying their contributions to urban ambient PM and for designing corresponding control measures.…”