PERSPECTIVEThe ozonolysis of an approximately one monolayer film of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) on NaCl was followed in real time using diffuse reflection infrared Fourier transform spectrometry (DRIFTS) at 23 1C. Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry and Auger electron spectroscopy were used to confirm the identification of the products. Ozone concentrations ranged from 1.7 Â 10 12 to 7.0 Â 10 13 molecules cm À3 (70 ppb to 2.8 ppm). Upon exposure to O 3 , there was a loss of CQC accompanied by the formation of a strong band at B1110 cm À1 due to the formation of a stable secondary ozonide (1,2,4-trioxolane, SOZ). The yield of the SOZ was smaller when the reaction was carried out in the presence of water vapor at concentrations corresponding to relative humidities between 2 and 25%.

Photolysis of a mixture of NaNO2 with NaCl with an adsorbed coating of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) is shown to lead to oxidation of the OPPC. This oxidation "from the bottom up" is due to the generation of the OH free radical from nitrite ion photochemistry in the presence of water and its addition to the OPPC double bond. Such chemistry may be important in the lower atmosphere.

The UV photolysis at lambda > or = 290 nm in air of a mixture of NaNO(2)/NaCl coated with 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) was followed in real time in the absence and presence of water vapor by using diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) at 23 degrees C. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was used to confirm the identification of the products. Photolysis of NO(2)(-) is known to generate O(-), which in the presence of water forms OH + OH(-). Irradiation of the OPPC/NaNO(2)/NaCl mixture led to a loss of nitrite and the formation of organic nitrates and carbonyl compounds. In the absence of added water vapor, carboxylate ions were also formed. These products are due to oxidation of OPPC by O(-) and OH radicals. The organic products formed per calculated O(-)/OH generated by photolysis increased with relative humidity, consistent with a competition between OPPC and NO(2)(-) for OH. This suggests a new mechanism of oxidation of organics on particles and on surfaces in air that have nitrite ions available for photolysis. Similar chemistry is likely to occur for nitrate ions, which also photolyze to generate O(-).

Experimental and theoretical studies of the interaction of gas phase nitric acid and water with a self-assembled monolayer surface of a germanium infrared-transmitting attenuated total reflectance (ATR) crystal that was coated with a thin layer of silicon oxide (SiO x ). The SAM was exposed at 298 AE 2 K to dry HNO 3 in a flow of N 2 , followed by HNO 3 in humid N 2 at a controlled relative humidity (RH) between 20-90%. For comparison, similar studies were carried out using a similar crystal without the SAM coating. Changes in the surface were followed using Fourier transform infared spectroscopy (FTIR). In the case of the SAMcoated crystal, molecular HNO 3 and smaller amounts of NO 3 À ions were observed on the surface upon exposure to dry HNO 3 . Addition of water vapor led to less molecular HNO 3 and more H 3 O + and NO 3 À complexed to water, but surprisingly, molecular HNO 3 was still evident in the spectra up to 70% RH.This suggests that part of the HNO 3 observed was initially trapped in pockets within the SAM and shielded from water vapor. After increasing the RH to 90% and then exposing the film to a flow of dry N 2 , molecular nitric acid was regenerated, as expected from recombination of protons and nitrate ions as water evaporated. The nitric acid ultimately evaporated from the film. On the other hand, exposure of the SAM to HNO 3 and H 2 O simultaneously gave only hydronium and nitrate ions. Molecular dynamics simulations of defective SAMs in the presence of HNO 3 and water predict that nitric acid intercalates in defects as a complex with a single water molecule that is protected by alkyl chains from interacting with additional water molecules. These studies are consistent with the recently proposed hydrophobic nature of HNO 3 . Under atmospheric conditions, if HNO 3 is formed in organic layers on surfaces in the boundary layer, e.g. through NO 3 or N 2 O 5 reactions, it may exist to a significant extent in its molecular form rather than fully dissociated to nitrate ions. IntroductionNitric acid is considered to be the end-product of oxidation of oxides of nitrogen in the atmosphere. 1 This acid reacts with ammonia and amines to form solid or aqueous phase nitrate particles, and can also be taken up into other types of atmospheric particles. Removal of gaseous HNO 3 and particulate nitrates takes place primarily through wet and dry deposition, in what were once considered to be terminal removal processes. However, there are now a number of laboratory and field studies that suggest that nitric acid can be converted back into gaseous oxides of nitrogen such as NO, NO 2 and HONO. 2-8Nitric acid is a strong acid, but it requires a cluster of at least four water molecules to dissociate in the gas phase. [9][10][11] In bulk aqueous solutions and on ice, nitric acid can form hydrates with 1, 2 or 3 water molecules, depending on the concentration. 12-24 Such hydrates, as well as the HNO 3 dimer, have also been seen on solid surfaces during the hydrolysis of NO 2 at

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