Organonitrates (ON) are important products of gas-phase oxidation of volatile organic compounds in the troposphere; some models predict, and laboratory studies show, the formation of large, multifunctional ON with vapor pressures low enough to partition to the particle phase. Organosulfates (OS) have also been recently detected in secondary organic aerosol. Despite their potential importance, ON and OS remain a nearly unexplored aspect of atmospheric chemistry because few studies have quantified particulate ON or OS in ambient air. We report the response of a high-resolution time-of-flight aerosol mass spectrometer (AMS) to aerosol ON and OS standards and mixtures. We quantify the potentially substantial underestimation of organic aerosol O/C, commonly used as a metric for aging, and N/C. Most of the ONnitrogen appears as NO þ x ions in the AMS, which are typically dominated by inorganic nitrate. Minor organonitrogen ions are observed although their identity and intensity vary between standards. We evaluate the potential for using NO þ x fragment ratios, organonitrogen ions, HNO þ 3 ions, the ammonium balance of the nominally inorganic ions, and comparison to ion-chromatography instruments to constrain the concentrations of ON for ambient datasets, and apply these techniques to a field study in Riverside, CA. OS manifests as separate organic and sulfate components in the AMS with minimal organosulfur fragments and little difference in fragmentation from inorganic sulfate. The low thermal stability of ON and OS likely causes similar detection difficulties for other aerosol mass spectrometers using vaporization and/or ionization techniques with similar or larger energy, which has likely led to an underappreciation of these species.atmospheric chemistry | organic aerosol | organic nitrate | organic sulfate | SOA O rganonitrates (ON, i.e., RONO 2 ) and organosulfates (OS, i.e., ROSO 3 H) are known to be present in secondary organic aerosol (SOA) (1-4), and are a nearly unexplored but potentially important aspect of atmospheric chemistry. The mechanisms behind ON and OS production and aging are poorly understood and generally ignored in models due in part to a lack of measurement approaches. ON have recently been identified as significant components (15-35%) of NO y in the gas-phase (5, 6) and serve as indicators of ozone production (7). Oceans and certain industrial processes directly emit ON, but these are generally short-chain alkyl nitrates that exist only in the gas-phase and constitute a minor fraction of atmospheric ON (6,8). Most atmospheric ON are produced either by photochemical (OH-initiated) or nocturnal (NO 3 -initiated) oxidation reactions of anthropogenic and biogenic volatile organic compounds (VOCs). These reactions can produce large, multifunctional ON with vapor pressures potentially low enough to condense and form SOA. During photochemical oxidation of VOCs in the presence of nitrogen oxides, ON are minor products of peroxy radical ðRO 2 Þ þ NO reactions. These reactions produce ON with yields of ...

Gas-wall partitioning of organic compounds (OC) that included C 8 -C 16 n-alkanes and 1-alkenes and C 8 -C 13 2-alcohols and 2-ketones was investigated in two Teflon FEP chambers whose walls were either untreated, oxidized in sunlight, or previously exposed to secondary organic aerosol (SOA). Partitioning was nearly independent of chamber treatment, reversible, and obeyed Henry's law. The fraction of an OC that partitioned to the walls at equilibrium ranged from 0 to ∼65%. Values increased with increasing carbon number within an OC class and for OC with similar vapor pressures increased in the order n-alkanes <1-alkenes <2-alcohols <2-ketones. Estimated time constants for achieving partitioning equilibrium ranged from ∼60 min for n-alkanes to ≤8 min for 2-ketones. The observations are consistent with a sorption mechanism in which OC dissolve into the film but are restricted to the near-surface region by a sharp permeability gradient that develops in response to OC-induced stresses in polymer chains.When the results were analyzed using a model analogous to one commonly employed for gas-particle partitioning, it was estimated that the sorption properties of the chamber walls were equivalent to organic aerosol mass concentrations of 2, 4, 10, and 24 mg m -3 with respect to the partitioning of n-alkanes, 1-alkenes, 2-alcohols, and 2-ketones. These values are up to ∼4 orders of magnitude larger than concentrations used in most laboratory studies of SOA, which are typically 1-10 3 µg m -3 , meaning that if full partitioning equilibrium is established in the chamber then semi-volatile OC will reside overwhelmingly in the chamber walls. Model simulations of gas-particle-wall partitioning were also carried out using the experimental data, and demonstrate quantitatively the large potential effects of Teflon walls on measured yields of gas-phase OC products and SOA.

[1] Laboratory experiments simulating chemical aging of carbonaceous aerosol by atmospheric oxidants demonstrate that oxidative processing increases their ability to activate as cloud droplets. A microphysical model shows, however, that the measured increase in hygroscopicity is insufficient to lead to efficient wet scavenging for sub-100 nm particles that are typically emitted from combustion sources. The absence of an efficient atmospheric oxidation pathway for hydrophobic-to-hydrophilic conversion suggests that the fate of carbonaceous aerosol is instead controlled by its interaction with more hydrophilic species such as sulfates, nitrates, and secondary organic aerosol, leading to longer lifetimes, higher burdens, and greater contributions to climate forcing in the free troposphere than are currently estimated. Citation: Petters, M.

Organic aerosols in the atmosphere are composed of a wide variety of species, reflecting the multitude of sources and growth processes of these particles. Especially challenging is predicting how these particles act as cloud condensation nuclei (CCN). Previous studies have characterized the CCN efficiency for organic compounds in terms of a hygroscopicity parameter, κ. Here we extend these studies by systematically testing the influence of the number and location of molecular functional groups on the hygroscopicity of organic aerosols. Organic compounds synthesized via gas-phase and liquid-phase reactions were characterized by high-performance liquid chromatography coupled with scanning flow CCN analysis and thermal desorption particle beam mass spectrometry. These experiments quantified changes in κ with the addition of one or more functional groups to otherwise similar molecules. The increase in κ per group decreased in the following order: hydroxyl ≫ carboxyl > hydroperoxide > nitrate ≫ methylene (where nitrate and methylene produced negative effects, and hydroperoxide and nitrate groups produced the smallest absolute effects). Our results contribute to a mechanistic understanding of chemical aging and will help guide input and parametrization choices in models relying on simplified treatments such as the atomic oxygen:carbon ratio to predict the evolution of organic aerosol hygroscopicity.

Yields of beta-hydroxynitrates and dihydroxynitrates in aerosol formed from OH radical-initiated reactions of linear C(8)-C(17) 1-alkenes and C(14)-C(17) internal alkenes in the presence of NO(x) were measured using a thermal desorption particle beam mass spectrometer coupled to a high-performance liquid chromatograph (HPLC) with UV-vis detector for identification and quantification. For 1-alkenes, total yields of beta-hydroxynitrates normalized for OH radical addition to the CC double bond increased with carbon number, primarily because of enhanced gas-to-particle partitioning, to a plateau of 0.140 +/- 0.009 in the C(14)-C(17) range, with 1-hydroxy/2-hydroxy isomer fractions of 0.7:0.3. When combined with yields measured by O'Brien et al. ( O'Brien , J. M. , Czuba , E. , Hastie , D. R. , Francisco , J. S. , and Shepson , P. S. J. Phys. Chem. A 1998 , 102 , 8903 ) for reactions of smaller alkenes, the results for both 1-alkenes and internal alkenes indicate that the branching ratios for the formation of beta-hydroxynitrates from the reactions of NO with beta-hydroxyperoxy radicals (averaged over both isomers) increase from 0.009 for C(2) up to 0.13-0.15 for C(14) and larger and are approximately half the values determined by Arey et al. ( Arey , J. , Aschmann , S. M. , Kwok , E. S. C. , and Atkinson , R. J. Phys. Chem. A 2001 , 105 , 1020 ) for the corresponding alkyl peroxy radicals. The range of branching ratios may be higher for individual isomers, but this could not be determined. It is estimated that for 1-alkenes, approximately 60-70% of OH radical addition occurred at the terminal carbon atom. Average yields of dihydroxynitrates normalized for OH radical addition were 0.039 +/- 0.006 and 0.006 +/- 0.002 for 1-alkenes and internal alkenes, with differences reflecting enhanced decomposition of beta-hydroxyalkoxy radicals formed from internal alkenes. The addition of NH(3) reduced yields significantly, apparently by altering hydrogen bonding between hydroxy and peroxy groups in hydroxyperoxy radical-NO complexes, whereas adding H(2)O had no obvious effect.