[1] In order to understand and predict the role of organic particles in the atmosphere their deliquescence behavior must be understood. Using an optical microscope coupled to a flow cell, we investigated the deliquescence of malonic, succinic, glutaric, and adipic acid particles with sizes ranging from 2 to 40 mm. Deliquescence relative humidities were determined for temperatures ranging from 293 to 243 K. Over this temperature range both succinic acid and adipic acid deliquesced at approximately 100% relative humidity, whereas malonic acid and glutaric acid deliquesced at significantly lower relative humidities. These results are generally in good agreement with previous studies and are within 3% of calculations based on the UNIQUAC (universal quasi-chemical) Functional Group Activity Coefficients (UNIFAC) model and recently published interaction parameters. Our studies also include measurements at temperatures below the eutectic temperatures. At these temperatures, ice did not nucleate; rather the particles underwent deliquescence to form metastable solution droplets. This indicates that solid dicarboxylic acids are not good ice nuclei above 243 K and hence will probably not play a role in ice cloud formation at these temperatures.

[1] The heterogeneous oxidation of a saturated hydrocarbon monolayer by NO 3 was studied. A flow tube reactor coupled to chemical ionization mass spectrometry was used to determine the reactive uptake coefficient of NO 3 on these surfaces, and X-ray photoelectron spectroscopy (XPS) was used to investigate surface oxidation and to determine if exposure to NO 3 leads to volatilization of the organic substrate. The uptake coefficient of NO 3 by an alkane monolayer is about (8.8 ± 2.5) Â 10 À4 , which may lead to competitive oxidation compared with OH, due to the higher atmospheric abundance of NO 3 under certain conditions. The XPS results are consistent with the formation of 1) C-O groups, 2) ketones or aldehydes, and 3) carboxylic groups. The XPS results also suggest that NO 3 does not rapidly volatilize the organic surface: even under extremely polluted conditions, maximum 10% of the organic layer is volatilized.

[1] In the following, we report the deliquescence relative humidities (DRH) and crystallization relative humidities (CRH) of mixed inorganic-organic particles, specifically ammonium sulfate-glutaric acid and sodium chlorideglutaric acid particles. Knowledge of the DRH and CRH of mixed inorganic-organic particles is crucial for predicting the role of aerosol particles in the atmosphere. Our DRH results are in good agreement with previous measurements, but our CRH results are significantly lower than some of the previous measurements reported in the literature. Our studies show that the DRH and CRH of ammonium sulfate and sodium chloride only decreased slightly when the mole fraction of the acid was less than 0.4. If other organics in the atmosphere behave in a similar manner, then the DRH and CRH of mixed inorganic-organic atmospheric particles will only be slightly less than the DRH and CRH of pure inorganic particles when the organic mole fraction is less than 0.4. Our results also show that if the particles contain a significant amount of organics (mole fraction > 0.5) the crystallization relative humidity decreases significantly and the particles are more likely to remain in the liquid state. Further work is needed to determine if other organics species of atmospheric importance have a similar effect.

[1] The reaction of NO 3 with methane soot, hexane soot, and solid pyrene was investigated using a flow tube reactor. The uptake of NO 3 on fresh soot was fast (uptake coefficient >0.1). Based on this result and an assumed density of reactive sites on soot, the time to process or oxidize 90% of a soot surface in the atmosphere would take only approximately five minutes. This suggests that NO 3 chemistry can rapidly oxidize soot surfaces under atmospheric conditions. After exposing soot films to NO 3 for approximately 180 minutes in the laboratory, the uptake reaches a steady-state value. The steady state uptake coefficients (assuming a geometric surface area) were 0.0054 ± 0.0027 and 0.0025 ± 0.0018 for methane and hexane soot, respectively. These numbers are used to show that heterogeneous reactions between NO 3 and soot are not likely a significant sink of gas-phase NO 3 under most atmospheric conditions. The uptake of NO 3 on fresh pyrene surfaces was also fast (uptake coefficient >0.1), and much faster than previously suggested. We argue that under certain atmospheric conditions reactions between NO 3 and surface-bound polycyclic aromatic hydrocarbons (PAHs) may be an important loss process of PAHs in the atmosphere.

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