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.

Emissions from traditional cooking practices in low- and middle-income countries have detrimental health and climate effects; cleaner-burning cookstoves may provide “co-benefits”. Here we assess this potential via in-home measurements of fuel-use and emissions and real-time optical properties of pollutants from traditional and alternative cookstoves in rural Malawi. Alternative cookstove models were distributed by existing initiatives and include a low-cost ceramic model, two forced-draft cookstoves (FDCS; Philips HD4012LS and ACE-1), and three institutional cookstoves. Among household cookstoves, emission factors (EF; g (kg wood)⁻¹) were lowest for the Philips, with statistically significant reductions relative to baseline of 45% and 47% for fine particulate matter (PM_2.5) and carbon monoxide (CO), respectively. The Philips was the only cookstove tested that showed significant reductions in elemental carbon (EC) emission rate. Estimated health and climate cobenefits of alternative cookstoves were smaller than predicted from laboratory tests due to the effects of real-world conditions including fuel variability and nonideal operation. For example, estimated daily PM intake and field-measurement-based global warming commitment (GWC) for the Philips FDCS were a factor of 8.6 and 2.8 times higher, respectively, than those based on lab measurements. In-field measurements provide an assessment of alternative cookstoves under real-world conditions and as such likely provide more realistic estimates of their potential health and climate benefits than laboratory tests.