O/C and OM/OC Ratios of Primary, Secondary, and Ambient Organic Aerosols with High-Resolution Time-of-Flight Aerosol Mass Spectrometry

Abstract

A recently developed method to rapidly quantify the elemental composition of bulk organic aerosols (OA) using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is improved and applied to ambient measurements. Atomic oxygen-to-carbon (O/C) ratios characterize the oxidation state of OA, and O/C from ambient urban OA ranges from 0.2 to 0.8 with a diurnal cycle that decreases with primary emissions and increases because of photochemical processing and secondary OA (SOA) production. Regional O/C approaches approximately 0.9. The hydrogen-to-carbon (H/C, 1.4--1.9) urban diurnal profile increases with primary OA (POA) as does the nitrogen-to-carbon (N/C, approximately 0.02). Ambient organic-mass-to-organic-carbon ratios (OM/OC) are directly quantified and correlate well with O/C (R2 = 0.997) for ambient OA because of low N/C. Ambient O/C and OM/OC have values consistent with those recently reported from other techniques. Positive matrix factorization applied to ambient OA identifies factors with distinct O/C and OM/OC trends. The highest O/C and OM/OC (1.0 and 2.5, respectively) are observed for aged ambient oxygenated OA, significantly exceeding values for traditional chamber SOA,while laboratory-produced primary biomass burning OA (BBOA) is similar to ambient BBOA, O/C of 0.3--0.4. Hydrocarbon-like OA (HOA), a surrogate for urban combustion POA, has the lowest O/C (0.06--0.10), similar to vehicle exhaust. An approximation for predicting O/C from unit mass resolution data is also presented.

A simplified description of the evolution of organic aerosol composition in the atmosphere

“…The slope of −1 indicates that for each oxygen atom added upon oxidation, a hydrogen atom is lost. This change is consistent with equal increases in carbonyl and alcohol moieties, such as carboxylic acid addition (Figure 1) as well as with the observed increase in the intensity of the m/z 44 ion (thought to be a marker of acid groups in AMS measurements [Aiken et al, 2008]). …”

“…[6] Figure [Aiken et al, 2008;DeCarlo et al, 2008]. Also shown is the elemental composition of the individual Positive Matrix Factorization factors during MILAGRO.…”

“…Also shown is the elemental composition of the individual Positive Matrix Factorization factors during MILAGRO. These factors have been shown to represent freshly emitted OA (HOA, BBOA) and more oxidized OA (progressing from OOA2 to "aged" OOA1) [Aiken et al, 2008]. Laboratory studies are shown with diamonds.…”

“…Laboratory studies are shown with diamonds. These include a series of vehicle and biomass burning POA (diesel, gasoline, smoke from lodgepole pine and sage/rabbitbrush burning) from Aiken et al [2008], laboratory-generated SOA from a-pinene ozonolysis at decreasing OA mass loadings (from left to right) [Shilling et al, 2009] and the products of heterogeneous oxidation of squalane with increasing OH exposure (from left to right) [Kroll et al, 2009]. Finally, the evolution of the elemental composition of OA for a series of thermo-denuder experiments (with temperature increasing left to right) is shown as open circles [Huffman et al, 2009].…”

Surface tension prevails over solute effect in organic-influenced cloud droplet activation

“…A specific quantification of the elemental composition of the NUM particles separately was not possible to the same level of accuracy, but the comparison of selected mass/charge (m/z) signals between the NUM and accumulation mode populations indicate that the average O:C ratio of the NUM particles may have been significantly lower. According to Aiken et al 81 , the organic aerosol signal at m/z 44 is highly correlated to the average organic O:C ratio (with some uncertainty) and halving the m/z 44 percentage contribution to organic mass would result in a similar reduction in O:C ratio. During the NUM event, the percentage contribution of m/z 44 to total mass for NUM particles is roughly half that of the accumulation mode, 6.2 % and 13.3 % for NUM and ACCU, respectively.…”