Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO 3 radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO 3 reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.atmospheric aerosol | nitrate radical | kinetic growth mechanism | condensation mechanism A irborne particles are well-known to negatively affect human health (1) and to contribute to "haze" associated with urban and regional pollution, leading to a reduction in visibility (2). On a global scale, airborne particles scatter solar radiation and can act as cloud condensation (CCN) and ice nuclei (IN), influencing the radiative balance of the atmosphere (3, 4). Currently these effects represent the largest uncertainty in calculations of climate change (5). A major component of atmospheric particles is secondary organic aerosol (SOA) formed via the oxidation of gaseous anthropogenic and biogenic precursor compounds. The SOA material is formed from low-volatility oxidation products (3, 4). However, the processes and species leading to SOA formation and growth are not fully understood, which precludes reliable quantitative predictions of their impacts on climate, visibility, and human health.Regional and global chemical models have generally underpredicted SOA concentrations compared to those from field measurements (6-9). Inclusion of a number of additional factors such as new SOA precursors, condensed phase chemistry, updated gasphase chemistry and SOA yields, new primary semivolatile and intermediate volatility species, and improved emissions inventories of both gases and p...

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH(4)NO(3), NaNO(3), and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO(3) radical reactions at 22 degrees C and 1 atm in air with alpha- and beta-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the alpha-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z 30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [C(x)H(y)N(z)O(a)](+) were identified using HR-ToF-AMS. The NO(+)/NO(2)(+) ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the alpha- and beta-pinene, 3-carene, and limonene reactions, approximately 5 for the isoprene reaction, 2.4 for NH(4)NO(3) and 80 for NaNO(3). The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO(2)/C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO(+)/NO(2)(+) ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH(4)NO(3) or NaNO(3). However, unique identification of specific organic nitrates is not possible with either method.

Heterogeneous reactions of sea salt aerosol with various oxides of nitrogen lead to replacement of chloride ion by nitrate ion. Studies of the photochemistry of a model system were carried out using deliquesced mixtures of NaCl and NaNO 3 on a Teflon s substrate. Varying molar ratios of NaCl to NaNO 3 (1 : 9 Cl À : NO 3 À , 1 : 1 Cl À : NO 3 À , 3 : 1 Cl À : NO 3 À , 9 : 1 Cl À : NO 3 À) and NaNO 3 at the same total concentration were irradiated in air at 299 AE 3 K and at a relative humidity of 75 AE 8% using broadband UVB light (270-380 nm). Gaseous NO 2 production was measured as a function of time using a chemiluminescence NO y detector. Surprisingly, an enhanced yield of NO 2 was observed as the chloride to nitrate ratio increased. Molecular dynamics (MD) simulations show that as the Cl À : NO 3 À ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl À and subsurface Na + . This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO 2 +O À , increasing the effective quantum yield and hence the production of gaseous NO 2 . The implications of enhanced NO 2 and likely OH production as sea salt aerosols become processed in the atmosphere are discussed.