Organosulfate Formation in Biogenic Secondary Organic Aerosol

Organosulfates of isoprene, α-pinene, and β-pinene have recently been identified in both laboratory-generated and ambient secondary organic aerosol (SOA). In this study, the mechanism and ubiquity of organosulfate formation in biogenic SOA is investigated by a comprehensive series of laboratory photooxidation (i.e., OH-initiated oxidation) and nighttime oxidation (i.e., NO3-initiated oxidation under dark conditions) experiments using nine monoterpenes (α-pinene, β-pinene, d-limonene, l-limonene, α-terpinene, γ-terpinene, terpinolene, Δ3-carene, and β-phellandrene) and three monoterpenes (α-pinene, d-limonene, and l-limonene), respectively. Organosulfates were characterized using liquid chromatographic techniques coupled to electrospray ionization combined with both linear ion trap and high-resolution time-of-flight mass spectrometry. Organosulfates are formed only when monoterpenes are oxidized in the presence of acidified sulfate seed aerosol, a result consistent with prior work. Archived laboratory-generated isoprene SOA and ambient filter samples collected from the southeastern U.S. were reexamined for organosulfates. By comparing the tandem mass spectrometric and accurate mass measurements collected for both the laboratory-generated and ambient aerosol, previously uncharacterized ambient organic aerosol components are found to be organosulfates of isoprene, α-pinene, β-pinene, and limonene-like monoterpenes (e.g., myrcene), demonstrating the ubiquity of organosulfate formation in ambient SOA. Several of the organosulfates of isoprene and of the monoterpenes characterized in this study are ambient tracer compounds for the occurrence of biogenic SOA formation under acidic conditions. Furthermore, the nighttime oxidation experiments conducted under highly acidic conditions reveal a viable mechanism for the formation of previously identified nitrooxy organosulfates found in ambient nighttime aerosol samples. We estimate that the organosulfate contribution to the total organic mass fraction of ambient aerosol collected from K-puszta, Hungary, a field site with a similar organosulfate composition as that found in the present study for the southeastern U.S., can be as high as 30%

Thermodynamic modelling of aqueous aerosols containing electrolytes and dissolved organic compounds. II. An extended Zdanovskii–Stokes–Robinson approach

The Zdanovskii-Stokes-Robinson (ZSR) equation, or linear isopiestic relation, can be used to estimate osmotic and activity coefficients of multicomponent mixtures, based on the properties of pure (single solute) solutions. We have generalised an extension to the ZSR equation (for ternary mixtures) to systems containing an indefinite number of solutes, and derived the corresponding equations for solute activity coefficients. The new model is tested by calculating salt solubilities in Na+/NH4+/Cl-/SO42- aqueous solutions, liquid-liquid phase equilibrium in the acetone/glycerol/water system, and thermodynamic properties of aqueous NaCl/sucrose solutions (all at 298.15 K). The mixture parameters, up to three for each pair of solutes, significantly increase the accuracy of the method. It is least satisfactory for solutions containing both electrolytes and non-electrolytes, and it was found that the ZSR equation predicts activity coefficients of trace amounts of non-electrolytes in salt solutions that do not conform to the Setchenow relationship

Barrierless Reactions with Loose Transition States Govern the Yields and Lifetimes of Organic Nitrates Derived from Isoprene

The chemical reaction mechanism of NO addition to two β and δ isoprene hydroxy–peroxy radical isomers is examined in detail using density functional theory, coupled cluster methods, and the energy resolved master equation formalism to provide estimates of rate constants and organic nitrate yields. At the M06-2x/aug-cc-pVTZ level, the potential energy surfaces of NO reacting with β-(1,2)-HO-IsopOO^• and δ-<i>Z</i>-(1,4)-HO-IsopOO^• possess barrierless reactions that produce alkoxy radicals/NO₂ and organic nitrates. The nudged elastic band method was used to discover a loosely bound van der Waals (vdW) complex between NO₂ and the alkoxy radical that is present in both exit reaction channels. Semiempirical master equation calculations show that the β organic nitrate yield is 8.5 ± 3.7%. Additionally, a relatively low barrier to C–C bond scission was discovered in the β-vdW complex that leads to direct HONO formation in the gas phase with a yield of 3.1 ± 1.3%. The δ isomer produces a looser vdW complex with a smaller dissociation barrier and a larger isomerization barrier, giving a 2.4 ± 0.8% organic nitrate yield that is relatively pressure and temperature insensitive. By considering all of these pathways, the first-generation NO_<i>x</i> recycling efficiency from isoprene organic nitrates is estimated to be 21% and is expected to increase with decreasing NO_<i>x</i> concentration

Effect of Acidity on Secondary Organic Aerosol Formation from Isoprene

The effect of particle-phase acidity on secondary organic aerosol (SOA) formation from isoprene is investigated in a laboratory chamber study, in which the acidity of the inorganic seed aerosol was controlled systematically. The observed enhancement in SOA mass concentration is closely correlated to increasing aerosol acidity (R^2 = 0.979). Direct chemical evidence for acid-catalyzed particle-phase reactions was obtained from the SOA chemical analyses. Aerosol mass concentrations for the 2-methyltetrols, as well as the newly identified sulfate esters, both of which serve as tracers for isoprene SOA in ambient aerosols, increased significantly with enhanced aerosol acidity. Aerosol acidities, as measured in nmol of H^+ m^(-3), employed in the present study are in the same range as those observed in tropospheric aerosol collected from the eastern U.S