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This work examines organic acid and metal concentrations in northeastern Pacific Ocean stratocumulus cloudwater samples collected by the CIRPAS Twin Otter between July and August 2011. Correlations between a suite of various monocarboxylic and dicarboxylic acid concentrations are consistent with documented aqueous-phase mechanistic relationships leading up to oxalate production. Monocarboxylic and dicarboxylic acids exhibited contrasting spatial profiles reflecting their different sources; the former were higher in concentration near the continent due to fresh organic emissions. Concentrations of sea salt crustal tracer species, oxalate, and malonate were positively correlated with low-level wind speed suggesting that an important route for oxalate and malonate entry in cloudwater is via some combination of association with coarse particles and gaseous precursors emitted from the ocean surface. Three case flights show that oxalate (and no other organic acid) concentrations drop by nearly an order of magnitude relative to samples in the same vicinity. A consistent feature in these cases was an inverse relationship between oxalate and several metals (Fe, Mn, K, Na, Mg, Ca), especially Fe. By means of box model studies we show that the loss of oxalate due to the photolysis of iron oxalato complexes is likely a significant oxalate sink in the study region due to the ubiquity of oxalate precursors, clouds, and metal emissions from ships, the ocean, and continental sources.

In June 2010, the NOAA WP-3D aircraft conducted two survey flights around the Deepwater Horizon (DWH) oil spill. The Gulf oil spill resulted in an isolated source of secondary organic aerosol (SOA) precursors in a relatively clean environment. Measurements of aerosol composition and volatile organic species (VOCs) indicated formation of SOA from intermediate-volatility organic compounds (IVOCs) downwind of the oil spill (Science2011, 331, doi 10.1126/science.1200320). In an effort to better understand formation of SOA in this environment, we present mass spectral characteristics of SOA in the Gulf and of SOA formed in the laboratory from evaporated light crude oil. Compared to urban primary organic aerosol, high-mass-resolution analysis of the background-subtracted SOA spectra in the Gulf (for short, "Gulf SOA") showed higher contribution of C(x)H(y)O(+) relative to C(x)H(y)(+) fragments at the same nominal mass. In each transect downwind of the DWH spill site, a gradient in the degree of oxidation of the Gulf SOA was observed: more oxidized SOA (oxygen/carbon = O/C ∼0.4) was observed in the area impacted by fresher oil; less oxidized SOA (O/C ∼0.3), with contribution from fragments with a hydrocarbon backbone, was found in a broader region of more-aged surface oil. Furthermore, in the plumes originating from the more-aged oil, contribution of oxygenated fragments to SOA decreased with downwind distance. Despite differences between experimental conditions in the laboratory and the ambient environment, mass spectra of SOA formed from gas-phase oxidation of crude oil by OH radicals in a smog chamber and a flow tube reactor strongly resembled the mass spectra of Gulf SOA (r(2) > 0.94). Processes that led to the observed Gulf SOA characteristics are also likely to occur in polluted regions where VOCs and IVOCs are coemitted.

The occurrence, source, and sink processes of N-nitrosodimethylamine (NDMA) have been explored by means of combined laboratory, field, and model studies. Observations have shown the occurrence of NDMA in fogs and clouds at substantial concentrations (7.5-397 ng L(-1)). Laboratory studies were conducted to investigate the formation of NDMA from nitrous acid and dimethylamine in the homogeneous aqueous phase. While NDMA was produced in the aqueous phase, the low yields (<1%) observed could not explain observational concentrations. Therefore gaseous formation of NDMA with partitioning to droplets likely dominates aqueous NDMA formation. Box-model calculations confirmed the predominant contributions from gas phase formation followed by partitioning into the cloud droplets. Measurements and model calculations showed that while NDMA is eventually photolyzed, it might persist in the atmosphere for hours after sunrise and before sunset since the photolysis in the aqueous phase might be much less efficient than in the gas phase.

Box model studies have been performed to study the role of aqueous phase chemistry with regard to halogen activation for marine and urban clouds and the marine aerosol as well. Different chemical pathways leading to halogen activation in diluted cloud droplets and highly concentrated sea salt aerosol particles are investigated. The concentration of halides in cloud droplets is significantly smaller than in sea-salt particles, and hence different reaction sequences control the overall chemical conversions. In diluted droplets radical chemistry involving OH, NO(3), Cl/Cl(2)(-)/ClOH(-), and Br/Br(2)(-)/BrOH(-) gains in importance and pH independent pathways lead to the release of halogens from the particle phase whereas the chemistry in aerosol particles with high electrolyte concentrations is controlled by non-radical reactions at high ionic strengths and relatively low pH values. For the simulation of halogen activation in tropospheric clouds and aqueous aerosol particles in different environments a halogen module was developed including both gas and aqueous phase processes of halogen containing species. This module is coupled to a base mechanism consisting of RACM (Regional Atmospheric Chemistry Mechanism) and the Chemical Aqueous Phase Radical Mechanism CAPRAM 2.4 (MODAC-mechanism). Phase exchange is described by the resistance model by Chemistry of Multiphase Atmospheric Systems, NATO ASI Series, 1986. It can be shown that under cloud conditions the bromine atom is mainly produced by OH initiated reactions, i.e. its concentration maximum is reached at noon. In contrast, the concentration level of chlorine atoms is linked to NO(3) radical chemistry leading to a smaller amplitude between day and night time concentrations. The contribution of radical processes to halogen atom formation in the particle phase is evident, e.g. by halogen atoms which undergo direct phase transfer. Furthermore, the application of the multiphase model for initial concentrations for sea-salt aerosols shows that the particle phase can act as a main source of halogen containing molecules (Cl(2), BrCl, Br(2)) which are photolysed in the gas phase to yield halogen atoms (about 70% of all Cl sources and more than 99% for Br).