The composition of PM (particulate matter with diameter less than1 µm) in the greater London area was characterized during the Clean Airfor London (ClearfLo) project in winter 2012. Two high-resolution time-of-flight aerosol mass spectrometers (HR-ToF-AMS) were deployed at arural site (Detling, Kent) and an urban site (North Kensington, London). Thesimultaneous and high-temporal resolution measurements at the two sitesprovide a unique opportunity to investigate the spatial distribution ofPM. We find that the organic aerosol (OA) concentration is comparablebetween the rural and urban sites, but the contribution from differentsources is distinctly different between the two sites. The concentration ofsolid 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 concentrationsof oxygenated OA (OOA) are well-correlated between the two sites, the OOAconcentration at the rural site is almost twice that of the urban site. Atthe rural site, more than 70 % of the carbon in OOA is estimated to benon-fossil, which suggests that OOA is likely related to aged biomassburning considering the small amount of biogenic SOA in winter. Thus, it ispossible that the biomass burning OA contributes a larger fraction ofambient OA in wintertime than what previous field studies have suggested.A suite of instruments was deployed downstream of a thermal denuder (TD) toinvestigate the volatility of PM species at the rural Detling site.After heating at 250 °C in the TD, 40 % of the residual mass isOA, indicating the presence of non-volatile organics in the aerosol.Although the OA associated with refractory black carbon (rBC; measured by asoot-particle aerosol mass spectrometer) only accounts for 10 %of the total OA (measured by a HR-ToF-AMS) at 250 °C, the twomeasurements are well-correlated, suggesting that the non-volatile organicshave similar sources or have undergone similar chemical processing as rBC inthe atmosphere. Although the atomic O : C ratio of OOA is substantially largerthan that of solid fuel OA and hydrocarbon-like OA, these three factors havesimilar volatility, which is inferred from the change in mass concentrationafter heating at 120 °C. Finally, we discuss the relationshipbetween the mass fraction remaining (MFR) of OA after heating in the TD andatomic O : C of OA and find that particles with a wide range of O : C could havesimilar MFR after heating. This analysis emphasizes the importance ofunderstanding the distribution of volatility and O : C in bulk OA.
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