Abstract
Abstract. Organic compounds represent a significant fraction of submicrometer atmospheric aerosol mass. Even if most of these compounds are semi-volatile in atmospheric concentrations, the ambient organic aerosol volatility is quite uncertain. The most common volatility measurement method relies on the use of a thermodenuder (TD). The aerosol passes through a heated tube where its more volatile components evaporate, leaving the less volatile components behind in the particulate phase. The typical result of a thermodenuder measurement is the mass fraction remaining (MFR), which depends, among other factors, on the organic aerosol (OA) vaporization enthalpy and the accommodation coefficient. We use a new method combining forward modeling, introduction of "experimental" error, and inverse modeling with error minimization for the interpretation of TD measurements. The OA volatility distribution, its effective vaporization enthalpy, the mass accommodation coefficient and the corresponding uncertainty ranges are calculated. Our results indicate that existing TD-based approaches quite often cannot estimate reliably the OA volatility distribution, leading to large uncertainties, since there are many different combinations of the three properties that can lead to similar thermograms. We propose an improved experimental approach combining TD and isothermal dilution measurements. We evaluate this experimental approach using the same model, and show that it is suitable for studies of OA volatility in the lab and the field.
Highlights
Atmospheric aerosol particles play an important role in the Earth’s energy balance by absorbing and scattering solar radiation and influencing the properties and lifetime of clouds (IPCC, 2007)
Since the oxidation pathways for volatile organic compounds (VOCs) are complex and the reactions lead to hundreds or thousands of oxygenated products, our understanding of organic aerosol formation mechanisms and the Organic aerosol (OA) chemical and physical properties is still incomplete
We explore methods for estimating the OA volatility distribution, together with the effective vaporization enthalpy and mass accommodation coefficient
Summary
Atmospheric aerosol particles play an important role in the Earth’s energy balance by absorbing and scattering solar radiation (direct effect) and influencing the properties and lifetime of clouds (indirect effects) (IPCC, 2007). Cappa and Wilson (2011) focused on the evolution of organic aerosol mass spectra from lubricating oil and secondary aerosol from a-pinene oxidation upon heating, using the Cappa (2010) model They adopted volatility distributions from previous studies (Pathak et al, 2007; Grieshop et al, 2009), and a vaporization enthalpy based on the Epstein et al (2010) relationship. One of their major conclusions was that there were high mass transfer resistances (estimated accommodation coefficients on the order of 10−4) during the evaporation of the a-pinene SOA. We examine the utility of using two residence times, using isothermal dilution instead of thermodenuder measurements (Grieshop et al, 2009), and combining TD and isothermal dilution measurements
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