Hydrocarbon-like and oxygenated organic aerosols in Pittsburgh: insights into sources and processes of organic aerosols

Abstract

Abstract.A recently developed algorithm has been applied to deconvolve the mass spectra of organic aerosols acquired with the Aerosol Mass Spectrometer (AMS) in Pittsburgh during September 2002. The results are used here to characterize the mass concentrations, size distributions, and mass spectra of hydrocarbon-like and oxygenated organic aerosol (HOA and OOA, respectively). HOA accounts for 34% of the measured organic aerosol mass and OOA accounts for 66%. The mass concentrations of HOA demonstrate a prominent diurnal profile that peaks in the morning during the rush hour and decreases with the rise of the boundary layer. The diurnal profile of OOA is relatively flat and resembles those of SO 2− 4 and NH + 4 . The size distribution of HOA shows a distinct ultrafine mode that is commonly associated with fresh emissions while OOA is generally concentrated in the accumulation mode and appears to be mostly internally mixed with the inorganic ions, such as SO 2− 4 and NH + 4 . These observations suggest that HOA is likely primary aerosol from local, combustion-related emissions and that OOA is secondary organic aerosol (SOA) influenced by regional contributions. There is strong evidence of the direct correspondence of OOA to SOA during an intense new particle formation and growth event, when condensational growth of OOA was observed. The fact that the OOA mass spectrum from this event is very similar to that from the entire study suggests that the majority of OOA in Pittsburgh is likely SOA. O 3 appears to be a poor indicator for OOA concentration while SO 2− 4 is a relatively good surrogate for this dataset. Since the diurnal averages of HOA track those of CO during day time, oxidation/aging of HOA appears to be very small on the time scaleCorrespondence to: J. L. Jimenez (jose.jimenez@colorado.edu) of several hours. Based on extracted mass spectra and the likely elemental compositions of major m/z's, the organic mass to organic carbon ratios (OM:OC) of HOA and OOA are estimated at 1.2 and 2.2 µg/µgC, respectively, leading to an average OM:OC ratio of 1.8 for submicron OA in Pittsburgh during September. The C:O ratio of OOA is estimated at 1:0.8. The carbon contents in HOA and OOA estimated accordingly correlate well to primary and secondary organic carbon, respectively, estimated by the OC/EC tracer technique (assuming POC-to-EC ratio=1). In addition, the total carbon concentrations estimated from the AMS data agree well with those measured by the Sunset Laboratory Carbon analyzer (r 2 =0.87; slope=1.01±0.11). Our results represent the first direct estimate of the OM:OC ratio from highly timeresolved chemical composition measurements.

Properties and evolution of biomass burning organic aerosol from Canadian boreal forest fires

“…f 60 repre-sents the prevalence of primary combustion products such as levoglucosan and is used as an indicator for fresh BB emissions (Schneider et al, 2006;Alfarra et al, 2007). Conversely, f 44 is associated with the CO + 2 ion derived from more aged OA as hydrocarbon fragments are oxidised to form organic acids (Zhang et al, 2005;Aiken et al, 2008), although m/z 44 is also a constituent of fresh smoke and has been shown to be significantly elevated at source, dependent on combustion conditions (Weimer et al, 2008;Jolleys et al, 2014). While strongly associated with saturated hydrocarbon fragments, m/z 43 can also originate from oxidised compounds such as aldehydes and ketones (Alfarra et al, 2004).…”

Does the POA–SOA split matter for global CCN formation?

“…The relative contributions of POA and SOA to the total OA remains an open issue as different studies report wide ranges of estimates (Kanakidou et al, 2005;Hallquist et al, 2009). Traditionally, models have tended to predict a predominance of POA over SOA (Chung and Seinfeld, 2002;Kanakidou et al, 2005;Pun et al, 2003;Vutukuru et al, 2006), but measurement studies show striking evidence of SOA dominance observed at various locations, even in heavily urbanized locations (Zhang et al, 2005de Gouw et al, 2005;Volkamer et al, 2006). Recent work has suggested that many global models are underestimating OA sources (Heald et al, 2010;Spracklen et al, 2011b) that appear to be SOA, and recent improvements in SOA modeling efforts have addressed this discrepancy (Jathar et al, 2011;Pye and Seinfeld, 2010).…”

“…Recent work has suggested that many global models are underestimating OA sources (Heald et al, 2010;Spracklen et al, 2011b) that appear to be SOA, and recent improvements in SOA modeling efforts have addressed this discrepancy (Jathar et al, 2011;Pye and Seinfeld, 2010). To complicate the matter even further, different measurement techniques result in different SOA/OA fractions, as was the case for the Pittsburgh Air Quality Study, for which SOA/OA values were estimated to be 35-73 % (Subramanian et al, 2007;Cabada et al, 2004;Shrivastava et al, 2007;Zhang et al, 2005). Therefore, there exists great uncertainty regarding the POA-SOA split.…”

Three-dimensional factorization of size-resolved organic aerosol mass spectra from Mexico City

“…The vectormatrix model in which the vector contains a factor size distribution and the matrix shows how the chemical composition of that characteristic size distribution changes with time ( Fig. 1c) would likely identify modes of submicron aerosol and the sources and processes affecting the those aerosol Zhang et al, 2005). In contrast, the vectormatrix model in which the vector contains a factor time series and the matrix shows the size dependence of that factor's chemical composition (Fig.…”