Abstract
Abstract. Ambient particulate matter (PM) samples were collected on quartz filters at a rural site in central Ontario during an intensive study in 2007. The concentrations of organic carbon (OC), pyrolysis organic carbon (POC), and elemental carbon (EC) were determined by thermal analysis. The concentrations are compared to the organic aerosol mass concentration (OM) measured with an Aerodyne C-ToF Aerosol Mass Spectrometer (AMS) and to the particle absorption coefficient (basp) obtained from a Radiance Research Particle Soot Absorption Photometer (PSAP). The total organic mass to organic carbon ratios (OM/OC) and specific attenuation coefficients (SAC=basp/EC) are derived. Proportionality of the POC mass with the oxygen mass in the aerosols estimated from the AMS offers a potential means to estimate OM/OC from thermal measurements only. The mean SAC for the study is 3.8±0.3 m2 g−1. It is found that the SAC is independent of or decrease with increasing particle mass loading, depending on whether or not the data are separated between aerosols dominated by more recent anthropogenic input and aerosols dominated by longer residence time or biogenic components. There is no evidence to support an enhancement of light absorption by the condensation of secondary material to particles, suggesting that present model simulations built on such an assumption may overestimate atmospheric warming by BC.
Highlights
Carbonaceous species, consisting of organic carbon (OC) and elemental carbon (EC), make up a large fraction of the fine atmospheric particulate mass in urban, rural, marine, and forest environments (e.g., Hildemann et al, 1996; Novakov et al, 1997; Middlebrook et al, 1998; Alves et al, 2006; Zhang et al, 2007; Bahadur et al, 2009)
In order to separate different source influences, we focus on measurements taken within two wind sectors: northerly (300◦ to 60◦; relatively clean air with some significant biogenic influence) and southerly (120◦ to 240◦; with a strong anthropogenic influence)
The higher organic aerosol mass concentration (OM)/OCtot from the south reflects contributions from both primary emissions and secondary organic aerosol (SOA) since the oxygenated OC (POC) and OM are highly correlated with EC as well as SO24−
Summary
Carbonaceous species, consisting of organic carbon (OC) and elemental carbon (EC), make up a large fraction of the fine atmospheric particulate mass in urban, rural, marine, and forest environments (e.g., Hildemann et al, 1996; Novakov et al, 1997; Middlebrook et al, 1998; Alves et al, 2006; Zhang et al, 2007; Bahadur et al, 2009). Fine particle OC comes from direct emissions (e.g., fossil fuel combustions, biomass burning) as well as from the formation of secondary organic aerosol (SOA). Aiken et al (2008) used the sum of the CxHyOz and Cx:Hy fragments measured with a High Resolution Time-of-Flight AMS sampling ambient particles in Mexico City and found the average OM/OC was 1.71 and the range over 6 days was 1.41 to 2.15 These previous studies indicate that the OM/OC ratio in ambient aerosols varies from 1.2 to 2.2, due to influences from primary emissions and secondary formations. We examine the relationship between the measured POC mass and the level of oxygenation of the ambient aerosol as well as the variation of the SAC with increased fine particle mass loading
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