Interpretation of Secondary Organic Aerosol Formation from Diesel Exhaust Photooxidation in an Environmental Chamber

Abstract

Secondary organic aerosol (SOA) formation from diesel exhaust was investigated using an environmental chamber. Particle volume measurement based solely on mobility diameter underestimated the SOA formation from diesel exhaust due to the external void space of agglomerate particles. Therefore, particle mass concentration and fractal-like dimension was determined from the particle effective density as a function of particle mass using an aerosol particle mass analyzer and scanning mobility particle sizer (APM-SMPS). Continuous aging of aerosol measured by an increase of atomic ratio (O/C) underscored the importance of multigenerational oxidation of low-volatile organic vapors emitted from diesel engine as a possible significant source of ambient oxygenated SOA. Higher particle effective densities were observed when raw exhaust was injected into a full bag as opposed to filling a bag with diluted exhaust using an ejector diluter. This suggests that the dilution method, in addition to dilution ratio, may impact the evaporation of semivolatile species. This study demonstrates the critical need to evaluate particle mass when evaluating SOA formation onto fractal particles such as diesel exhaust.

Experimentally measured morphology of biomass burning aerosol and its impacts on CCN ability

“…Using the effective density analogue of Eq. (5), the fractal-like dimension was obtained by fitting the power function (Park et al, 2003;Xue et al, 2009;Nakao et al, 2011):…”

“…This is likely due to a coating of SOA forming on the primary emissions (Nakao et al, 2011). The SOA fills in the void space inside the fractal agglomerates.…”

“…While the current literature on the morphology of biomass burning aerosols is sparse, the work done on diesel particles suggests that burn conditions may play a part in particle morphology (Park et al, 2004;Nakao et al, 2011). Along with .…”

“…They obtained fractal-like dimensions of 2.33 < D f < 2.84 depending on engine load. Work done by Nakao et al (2011) used this relationship to show SOA (secondary organic aerosol) formation from diesel exhaust has similar D f . Zhang et al (2008) exposed flamegenerated soot to subsaturated sulfuric acid vapor and obtained a fresh (unexposed) D f of 2.1 and an aged (exposed) D f of 2.8.…”

“…To measure the effect of atmospheric aging on biomass burning aerosol's morphology, the APM-SMPS system was used as described in Nakao et al (2011). The TEM was not used in this section because saturation of the TEM grids required extensive time, longer than that necessary to observe changing morphology that can occur with aging.…”