In this study we compile and present results from the factor analysis of 43 Aerosol Mass Spectrometer (AMS) datasets (27 of the datasets are reanalyzed in this work). The components from all sites, when taken together, provide a holistic overview of Northern Hemisphere organic aerosol (OA) and its evolution in the atmosphere. At most sites, the OA can be separated into oxygenated OA (OOA), hydrocarbon-like OA (HOA), and sometimes other components such as biomass burning OA (BBOA). We focus on the OOA components in this work. In many analyses, the OOA can be further deconvolved into low-volatility OOA (LV-OOA) and semi-volatile OOA (SV-OOA). Differences in the mass spectra of these components are characterized in terms of the two main ions <i>m/z</i> 44 (CO₂⁺) and <i>m/z</i> 43 (mostly C₂H₃O⁺), which are used to develop a new mass spectral diagnostic for following the aging of OA components in the atmosphere. The LV-OOA component spectra have higher <i>f</i>₄₄ (ratio of <i>m/z</i> 44 to total signal in the component mass spectrum) and lower <i>f</i>₄₃ (ratio of <i>m/z</i> 43 to total signal in the component mass spectrum) than SV-OOA. A wide range of <i>f</i>₄₄ and O:C ratios are observed for both LV-OOA (0.17&plusmn;0.04, 0.73&plusmn;0.14) and SV-OOA (0.07&plusmn;0.04, 0.35&plusmn;0.14) components, reflecting the fact that there is a continuum of OOA properties in ambient aerosol. The OOA components (OOA, LV-OOA, and SV-OOA) from all sites cluster within a well-defined triangular region in the <i>f</i>₄₄ vs. <i>f</i>₄₃ space, which can be used as a standardized means for comparing and characterizing any OOA components (laboratory or ambient) observed with the AMS. Examination of the OOA components in this triangular space indicates that OOA component spectra become increasingly similar to each other and to fulvic acid and HULIS sample spectra as <i>f</i>₄₄ (a surrogate for O:C and an indicator of photochemical aging) increases. This indicates that ambient OA converges towards highly aged LV-OOA with atmospheric oxidation. The common features of the transformation between SV-OOA and LV-OOA at multiple sites potentially enable a simplified description of the oxidation of OA in the atmosphere. Comparison of laboratory SOA data with ambient OOA indicates that laboratory SOA are more similar to SV-OOA and rarely become as oxidized as ambient LV-OOA, likely due to the higher loadings employed in the experiments and/or limited oxidant exposure in most chamber experiments

Recent work in our laboratory has shown that the photooxidation of isoprene (2-methyl-1,3-butadiene, C 5 H 8 ) leads to the formation of secondary organic aerosol (SOA). In the current study, the chemical composition of SOA from the photooxidation of isoprene over the full range of NO x conditions is investigated through a series of controlled laboratory chamber experiments. SOA composition is studied using a wide range of experimental techniques: electrospray ionization-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry, high-resolution mass spectrometry, online aerosol mass spectrometry, gas chromatography/mass spectrometry, and an iodometric-spectroscopic method. Oligomerization was observed to be an important SOA formation pathway in all cases; however, the nature of the oligomers depends strongly on the NO x level, with acidic products formed under high-NO x conditions only. We present, to our knowledge, the first evidence of particle-phase esterification reactions in SOA, where the further oxidation of the isoprene oxidation product methacrolein under high-NO x conditions produces polyesters involving 2-methylglyceric acid as a key monomeric unit. These oligomers comprise ∼22-34% of the high-NO x SOA mass. Under low-NO x conditions, organic peroxides contribute significantly to the low-NO x SOA mass (∼61% when SOA forms by nucleation and ∼25-30% in the presence of seed particles). The contribution of organic peroxides in the SOA decreases with time, indicating photochemical aging. Hemiacetal dimers are found to form from C 5 alkene triols and 2-methyltetrols under low-NO x conditions; these compounds are also found in aerosol collected from the Amazonian rainforest, demonstrating the atmospheric relevance of these low-NO x chamber experiments.

Blank runs. Irradiation of the chambers in the absence of isoprene is always found to lead to negligible aerosol growth. Prior to the start of the experiment (introduction of reagents), the chambers are relatively particle-free, with number densities of <100 particles/cm 3 , corresponding to volume densities below 0.2 µ m 3 /cm 3 . Irradiation of ~3 ppm H 2 O 2 leads to no increase in particle number or volume, as measured by the DMA.In the presence of ammonium sulfate seed, which provides surface area for the condensation of semivolatile compounds, very small growth cannot be measured by the DMA, due to loss of particles to the chamber walls and signal-to-noise considerations.However, in this case the AMS detects only a very minimal increase in organic mass (<0.1 µ g/m 3 ), also suggesting negligible aerosol growth in the absence of isoprene.Similar results are obtained when NO x is added to the system. Irradiation of a mixture of ~3 ppm H 2 O 2 and 300 ppb NO leads to no measurable increase in either volume or organic mass, both in the absence and the presence of inorganic seed. Hence we are confident that aerosol growth we measure, using both the DMA and the AMS, is a result of gas-particle partitioning of isoprene oxidation products.

[1] Recent experimental evidence indicates that heterogeneous chemical reactions play an important role in the gas-particle partitioning of organic compounds, contributing to the formation and growth of secondary organic aerosol in the atmosphere. Here we present laboratory chamber studies of the reactive uptake of simple carbonyl species (formaldehyde, octanal, trans,trans-2,4-hexadienal, glyoxal, methylglyoxal, 2,3-butanedione, 2,4-pentanedione, glutaraldehyde, and hydroxyacetone) onto inorganic aerosol. Gas-phase organic compounds and aqueous seed particles (ammonium sulfate or mixed ammonium sulfate/sulfuric acid) are introduced into the chamber, and particle growth and composition are monitored using a differential mobility analyzer and an Aerodyne Aerosol Mass Spectrometer. No growth is observed for most carbonyls studied, even at high concentrations (500 ppb to 5 ppm), in contrast with the results from previous studies. The single exception is glyoxal (CHOCHO), which partitions into the aqueous aerosol much more efficiently than its Henry's law constant would predict. No major enhancement in particle growth is observed for the acidic seed, suggesting that the large glyoxal uptake is not a result of particle acidity but rather of ionic strength of the seed. This increased partitioning into the particle phase still cannot explain the high levels of glyoxal measured in ambient aerosol, indicating that additional (possibly irreversible) pathways of glyoxal uptake may be important in the atmosphere.