Nonequilibrium atmospheric secondary organic aerosol formation and growth.

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.

Surprising formation of p-cymene in the oxidation of α-pinene in air by the atmospheric oxidants OH, O3, and NO3.

Anthropogenic sources release into the troposphere a wide range of volatile organic compounds (VOCs) including aromatic hydrocarbons, whose major sources are believed to be combustion and the evaporation of fossil fuels. An important question is whether there are other sources of aromatics in air. We report here the formation of p-cymene [1-methyl-4-(1-methylethyl) benzene, C6H4(CH3)(C3H7)] from the oxidation of α-pinene by OH, O3, and NO3 at 1 atm in air and 298 K at low (<5%) and high (70%) relative humidities (RH). Loss of α-pinene and the generation of p-cymene were measured using GC-MS. The fractional yields of p-cymene relative to the loss of α-pinene, Δ [p-cymeme]/Δ [α-pinene], were measured to range from (1.6±0.2)×10(-5) for the O3 reaction to (3.0±0.3)×10(-4) for the NO3 reaction in the absence of added water vapor. The yields for the OH and O3 reactions increased by a factor of 4-8 at 70% RH (uncertainties are ±2s). The highest yields at 70% RH for the OH and O3 reactions, ∼15 times higher than for dry conditions, were observed if the walls of the Teflon reaction chamber had been previously exposed to H2SO4 formed from the OH oxidation of SO2. Possible mechanisms of the conversion of α-pinene to p-cymene and the potential importance in the atmosphere are discussed.

Identification of organic nitrates in the NO3 radical initiated oxidation of alpha-pinene by atmospheric pressure chemical ionization mass spectrometry.

The gas-phase reactions of nitrate radicals (NO3) with biogenic organic compounds are a major sink for these organics during night-time. These reactions form secondary organic aerosols, including organic nitrates that can undergo long-range transport, releasing NOx downwind. We report here studies of the reaction of NO3 with alpha-pinene at 1 atm in dry synthetic air (relative humidity approximately 3%) and at 298 K using atmospheric pressure chemical ionization triple quadrupole mass spectrometry (APCI-MS) to identify gaseous and particulate products. The emphasis is on the identification of individual organic nitrates in the particle phase that were obtained by passing the product mixture through a denuder to remove gas-phase reactants and products prior to entering the source region of the mass spectrometer. Filter extracts were also analyzed by GC-MS and by APCI time-of-flight mass spectrometry (APCI-ToF-MS) with methanol as the proton source. In addition to pinonaldehyde and pinonic acid, five organic nitrates were identified in the particles as well as in the gas phase: 3-oxopinane-2-nitrate, 2-hydroxypinane-3-nitrate, pinonaldehyde-PAN, norpinonaldehyde-PAN, and (3-acetyl-2,2-dimethyl-3-nitrooxycyclobutyl)acetaldehyde. Furthermore, there was an additional first-generation organic nitrate product tentatively identified as a carbonyl hydroxynitrate with a molecular mass of 229. These studies suggest that a variety of organic nitrates would partition between the gas phase and particles in the atmosphere, and serve as a reservoir for NOx.

Comparison of FTIR and particle mass spectrometry for the measurement of particulate organic nitrates.

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.

Enhanced surface photochemistry in chloride-nitrate ion mixtures.

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 NaNO3 on a Teflon substrate. Varying molar ratios of NaCl to NaNO3 (1 : 9 Cl- : NO3-, 1 : 1 Cl- : NO3-, 3 : 1 Cl- : NO3-, 9 : 1 Cl- : NO3-) and NaNO3 at the same total concentration were irradiated in air at 299 +/- 3 K and at a relative humidity of 75 +/- 8% using broadband UVB light (270-380 nm). Gaseous NO2 production was measured as a function of time using a chemiluminescence NO(y) detector. Surprisingly, an enhanced yield of NO2 was observed as the chloride to nitrate ratio increased. Molecular dynamics (MD) simulations show that as the Cl- : NO3- ratio increases, the nitrate ions are drawn closer to the interface due to the existence of a double layer of interfacial Cl- and subsurface Na+. This leads to a decreased solvent cage effect when the nitrate ion photodissociates to NO2+O*-, increasing the effective quantum yield and hence the production of gaseous NO2. The implications of enhanced NO2 and likely OH production as sea salt aerosols become processed in the atmosphere are discussed.

Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of alpha-pinene.

The photooxidation of 0.6-0.9 ppm alpha-pinene in the presence of a deliquesced thin film of NaNO(3), and for comparison increasing concentrations of NO(2), was studied in a 100 L Teflon(R) chamber at relative humidities from 72-88% and temperatures from 296-304 K. The loss of alpha-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO(3) experiments than in NO(2) experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO(3) photolysis. These data indicate enhanced loss of alpha-pinene at the NaNO(3) thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that alpha-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.