Abstract
Abstract. The composition of PM1 (particulate matter with diameter less than 1 µm) in the greater London area was characterized during the Clean Air for London (ClearfLo) project in winter 2012. Two high-resolution time-of-flight aerosol mass spectrometers (HR-ToF-AMS) were deployed at a rural site (Detling, Kent) and an urban site (North Kensington, London). The simultaneous and high-temporal resolution measurements at the two sites provide a unique opportunity to investigate the spatial distribution of PM1. We find that the organic aerosol (OA) concentration is comparable between the rural and urban sites, but the contribution from different sources is distinctly different between the two sites. The concentration of solid fuel OA at the urban site is about twice as high as at the rural site, due to elevated domestic heating in the urban area. While the concentrations of oxygenated OA (OOA) are well-correlated between the two sites, the OOA concentration at the rural site is almost twice that of the urban site. At the rural site, more than 70 % of the carbon in OOA is estimated to be non-fossil, which suggests that OOA is likely related to aged biomass burning considering the small amount of biogenic SOA in winter. Thus, it is possible that the biomass burning OA contributes a larger fraction of ambient OA in wintertime than what previous field studies have suggested. A suite of instruments was deployed downstream of a thermal denuder (TD) to investigate the volatility of PM1 species at the rural Detling site. After heating at 250 °C in the TD, 40 % of the residual mass is OA, indicating the presence of non-volatile organics in the aerosol. Although the OA associated with refractory black carbon (rBC; measured by a soot-particle aerosol mass spectrometer) only accounts for < 10 % of the total OA (measured by a HR-ToF-AMS) at 250 °C, the two measurements are well-correlated, suggesting that the non-volatile organics have similar sources or have undergone similar chemical processing as rBC in the atmosphere. Although the atomic O : C ratio of OOA is substantially larger than that of solid fuel OA and hydrocarbon-like OA, these three factors have similar volatility, which is inferred from the change in mass concentration after heating at 120 °C. Finally, we discuss the relationship between the mass fraction remaining (MFR) of OA after heating in the TD and atomic O : C of OA and find that particles with a wide range of O : C could have similar MFR after heating. This analysis emphasizes the importance of understanding the distribution of volatility and O : C in bulk OA.
Highlights
Particulate matter (PM) concentration in the greater London area often exceeds European air quality limits, causing adverse effects on the health of habitants in this area (Harrison et al, 2012; Bohnenstengel et al, 2014)
Considering the above issues associated with PMFambient+thermal denuder (TD), we performed positive matrix factorization (PMF) analysis using the ME-2 solver on the TD line measurements by applying the factor profiles determined from PMFambient as a priori information, in order to improve the separation of organic aerosol (OA) factor
The chemical composition of PM1 is dominated by nitrate and organics, which on average accounts for 32 and 31 % of total PM1 mass, respectively
Summary
Particulate matter (PM) concentration in the greater London area often exceeds European air quality limits, causing adverse effects on the health of habitants in this area (Harrison et al, 2012; Bohnenstengel et al, 2014). In a laboratory study on toluene SOA, Hildebrandt Ruiz et al (2015) observed a linear relationship between OS and effective saturation concentration of the aerosol For both ambient measurements and laboratory studies, it is uncertain whether the O : C or OS of bulk OA is a good indicator of volatility. The authors found that between thermally denuded OA and ambient OA, the mass spectrum was similar and the difference in f44 (i.e., fraction of organic signal at m/z 44, which has a linear correlation with O : C) was not statistically significant This indicates that the degree of oxidation does not change after evaporation of relatively volatile species. We performed simultaneous measurements a1t a rural site (Detling, Kent) and an urban site (North Kensington, London) in the greater London area in winter 2012 using two Aerodyne high-resolution time-of-flight mass spectrometers (HR-ToF-AMS) (DeCarlo et al, 2006). We investigated the volatility of different OA sources and systematically evaluated the relationship between O : C and OA volatility
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