Abstract. Recent observations have detected surface active organics in atmospheric aerosols. We have studied the reaction of N 2 O 5 on aqueous natural seawater and NaCl aerosols as a function of sodium dodecyl sulfate (SDS) concentration to test the effect of varying levels of surfactant on gasaerosol reaction rates. SDS was chosen as a proxy for naturally occurring long chain monocarboxylic acid molecules, such as palmitic or stearic acid, because of its solubility in water and well-characterized surface properties. Experiments were performed using a newly constructed aerosol flow tube coupled to a chemical ionization mass spectrometer for monitoring the gas phase, and a differential mobility analyzer/condensation particle counter for determining aerosol surface area. We find that the presence of ∼3.5 wt% SDS in the aerosol, which corresponds to a monolayer surface coverage of ∼2×10 14 molecules cm −2 , suppresses the N 2 O 5 reaction probability, γ N 2 O 5 , by approximately a factor of ten, independent of relative humidity. Consistent with this observation is a similar reduction in the rate of ClNO 2 product generation measured simultaneously. However, the product yield remains nearly constant under all conditions. The degree of suppression is strongly dependent on SDS content in the aerosol, with no discernable effect at 0.1 wt% SDS, but significant suppression at what we predict to be submonolayer coverages with 0.3-0.6 wt% SDS on NaCl and natural seawater aerosols, respectively.

Abstract. Recent observations have detected surface active organics in atmospheric aerosols. We have studied the reaction of N2O5 on aqueous natural seawater and NaCl aerosol as a function of sodium dodecyl sulfate (SDS) concentration to test the effect of varying levels of surfactant on gas-aerosol reaction rates. SDS was chosen as a proxy for naturally occurring long chain monocarboxylic acid molecules, such as palmitic or stearic acid, because of its solubility in water and well-characterized surface properties. Experiments were performed using a newly constructed aerosol flow tube coupled to a chemical ionization mass spectrometer for monitoring the gas phase, and a differential mobility analyzer/condensation particle counter for determining aerosol surface area. We find that the presence of ~3.5 wt% SDS in the aerosol, which corresponds to a monolayer surface coverage of ~2×1014 molecules cm−2, suppresses the N2O5 reaction probability, γN2O5, by approximately a factor of ten, independent of relative humidity. Consistent with this observation is a similar reduction in the rate of ClNO2 product generation measured simultaneously. However, the product yield remains nearly constant under all conditions. The degree of suppression is strongly dependent on SDS content in the aerosol, with no discernable effect at 0.1 wt% SDS, but significant suppression at what we predict to be submonolayer coverages with 0.3–0.6 wt% SDS on NaCl and natural seawater aerosols, respectively.

The blinking and photobleaching dynamics of alizarin (1,2dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) are investigated using single-molecule spectroscopy. The time-dependent emission of alizarin and purpurin on glass under N 2 is analyzed using the change point detection (CPD) method to compile on-and off-event distributions. The number of distinct emissive events per molecule is about four times higher for alizarin relative to purpurin, consistent with an excited-state intramolecular proton transfer (ESIPT) process to populate an emissive tautomer state. To elucidate the mechanism for blinking (i.e., switching between on and off events), maximum likelihood estimation (MLE), goodness-of-fit tests based on the Kolmogorov−Smirnov (KS) statistic, and the loglikelihood ratio (LLR) tests are used to establish the best fits to the on-and offinterval probability distributions. For both alizarin and purpurin the on intervals are log-normally distributed, and off intervals are Weibull distributed, consistent with a dispersive electron-transfer (ET) kinetics model for blinking (i.e., involving Gaussian-like distributions of activation barriers to ET). Further analysis of the blinking dynamics reveals that ET to a long-lived dark state most often precedes molecular photobleaching, where extended residency in the dark state increases the probability of photobleaching. Based on these findings, mechanisms for the blinking and photobleaching of alizarin and purpurin are proposed. The ability of alizarin to undergo ESIPT enables fast excited-state decay and decreases the probability of ET. In contrast, purpurin exhibits faster injection and slower back ET relative to alizarin, leading to increased photobleaching via a dark radical cation state.

The photochemistry of nitrosyl chloride (ClNO) in the solution phase is investigated using Fourier transform infrared (FTIR) and ultrafast time-resolved infrared (TRIR) spectroscopies. The NO-stretch fundamental transition for ClNO dissolved in cyclohexane, carbon tetrachloride, chloroform, dichloromethane, and acetonitrile is measured, with the frequency and line width of this transition demonstrating a strong dependence on solvent polarity. Following the photolysis of ClNO dissolved in acetonitrile at 266 nm, the subsequent optical-density evolution across the entire width of the NO-stretch fundamental is measured. Analysis of the optical-density evolution demonstrates that geminate recombination of the primary photofragments resulting in the reformation of ground state ClNO occurs with a quantum yield of 0.54 +/- 0.06. In addition, an increase in optical density is observed at 1860 cm(-1) that is assigned to the NO-stretch fundamental transition of the photoisomer, ClON, having a formation quantum yield of 0.07 +/- 0.02. This work represents the first definitive observation of ClNO photoisomerization in solution. Finally, essentially no evidence is observed for significant vibrational excitation of the NO fragment following photodissociation, in marked contrast to the behavior observed in the gas phase. An environment-dependent dissociation scheme is proposed in which the interplay between solvent polarity and the location of the ground state potential-energy-surface minimum along the Cl-N coordinate provides for the optical preparation of different excited states thereby affecting the extent of NO vibrational excitation following photolysis.

The interfacial properties of NO 3 − were investigated using reverse micelles (RMs) in solution as proxies for sea spray aerosol (SSA) particles. By tuning the size of bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) RMs doped with NO 3 − we are able to isolate the vibrational signature of interfacial NO 3 − using infrared spectroscopy. The infrared spectrum of interfacial NO 3 − along the asymmetric-stretch coordinate (υ 3 ) is blue-shifted and possesses smaller peak splitting relative to NO 3 − in aqueous solution. These observations are consistent with the reduced hydrogenbonding availability of the interfacial region within the RM aqueous interior. We show that the partitioning of NO 3 − between the interfacial and core regions of the RM interior can be determined using a linear combination of interfacial and aqueous NO 3 − spectra. By fitting the interfacial partitioning curve of NO 3 − we demonstrate a method of determining quantitative interfacial affinity (χ Interface ) for ionic species doped within RMs.