Abstract
Abstract. A method using thermal desorption sampling and analysis by proton transfer reaction mass spectrometry (PTR-MS) to measure long chain alkanes (C12–C18) and other larger organics associated with diesel engine exhaust emissions is described. Long chain alkanes undergo dissociative proton transfer reactions forming a series of fragment ions with formula CnH2n+1. The PTR-MS is insensitive to n-alkanes less than C8 but displays an increasing sensitivity for larger alkanes. Fragment ion distribution and sensitivity is a function of drift conditions. At 80 Td the most abundant ion fragments from C10 to C16 n-alkanes were m/z 57, 71 and 85. The mass spectrum of gasoline and diesel fuel at 80 Td displayed ion group patterns that can be related to known fuel constituents, such as alkanes, alkylbenzenes and cycloalkanes, and other compound groups that are inferred from molecular weight distributions such as dihydronapthalenes and naphthenic monoaromatics. It is shown that thermal desorption sampling of gasoline and diesel engine exhausts at 80 Td allows for discrimination against volatile organic compounds, allowing for quantification of long chain alkanes from the abundance of CnH2n+1 fragment ions. The total abundance of long chain alkanes in diesel engine exhaust was measured to be similar to the total abundance of C1–C4 alkylbenzene compounds. The abundance patterns of compounds determined by thermal desorption sampling may allow for emission profiles to be developed to better quantify the relative contributions of diesel and gasoline exhaust emissions on organic compounds concentrations in urban air.
Highlights
Vehicle emissions are a major source of primary air pollutants that impact human health (HEI, 2010)
This discrepancy may arise from underestimation of secondary organic aerosol (SOA) formation rates from larger organic compounds emitted from diesel engine vehicles because exhaust emission inventories for organic compounds are biased towards the condensed phase by virtue of how emissions sampling is done (Robinson et al, 2007; Shirvastava et al, 2008)
This paper reports the technical details of the technique, the sensitivity of the proton transfer reaction mass spectrometer (PTR-MS) to n-alkanes, and measurements of diesel and gasoline engine exhausts by thermal desorption sampling for intermediate volatile organic compounds (IVOCs)
Summary
Vehicle emissions are a major source of primary air pollutants that impact human health (HEI, 2010). While the photochemistry of ozone formation is reasonably well understood, the formation of secondary organic aerosol (SOA) in urban areas is a less understood, more complex process, and it has been noted that some models significantly underestimate organic aerosol concentrations in urban areas compared to observations (de Gouw et al, 2005; Volkamer et al, 2006; Kleinman et al, 2008) This discrepancy may arise from underestimation of SOA formation rates from larger organic compounds emitted from diesel engine vehicles because exhaust emission inventories for organic compounds are biased towards the condensed phase by virtue of how emissions sampling is done (Robinson et al, 2007; Shirvastava et al, 2008). This paper reports the technical details of the technique, the sensitivity of the PTR-MS to n-alkanes, and measurements of diesel and gasoline engine exhausts by thermal desorption sampling for IVOCs
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