Environmental Engineering Reference
In-Depth Information
5.1 Atmospheric Organic Particulate Matter:
Revisiting Its Sources, Properties and Impacts
Spyros N. Pandis
1,2
, Neil M. Donahue
2
, and Allen L. Robinson
2
1
Department of Chemical Engineering, University of Patras, Patra, GR 26500, Greece
2
Center of Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, USA
Abstract
An overview of the development of our understanding of the sources,
formation mechanisms, physical and chemical transformations of atmospheric
organic aerosol (OA) is presented. Until recently, organic particulate material was
simply classified as either primary or secondary with the primary component being
treated in models as nonvolatile and inert. However, this oversimplified view fails
to explain the highly oxygenated nature of ambient OA, the relatively small OA
concentration gradients between urban areas and their surroundings, and the
concentrations of OA during periods of high photochemical activity. A unifying
framework for the description of all components based on their volatility distribution
(the volatility basis set) can be used for the treatment of a wide range of processes
affecting organic aerosol loadings and composition in the atmosphere. These
processes include direct organic particle and vapor emissions, chemical production
of organic PM from volatile precursors, chemical reactions (aging) in all phases,
as well as deposition of both particles and vapors and chemical losses to volatile
products. The combination of this new framework with the recent results of laboratory
studies can resolve some of the discrepancies between OA observations and
laboratory results.
Keywords
Secondary organic aerosol, PMCAMx, Volatility basis set
1. Revisiting the Main Assumptions about Primary Organic
Aerosol
Primary Organic Aerosol (POA) has been traditionally assumed to be non-volatile
and non-reactive in atmospheric aerosol models. Even if these assumptions are
used today by the great majority of Chemical Transport Models (CTMs) and are
integral parts of most mental models of the system, it has been clear for decades
that neither is correct in many cases. Most measurements of ambient organic PM
concentrations were accompanied by serious negative (particle evaporation after
collection on the filter) and/or positive artifacts (vapor adsorption on the filter),
providing strong hints about the semi-volatile nature of organic aerosol.
