Shining New Light on the Kinetics of Water Uptake by Organic Aerosol Particles

The uptake of water vapor by various organic aerosols is important in a number of applications ranging from medical delivery of pharmaceutical aerosols to cloud formation in the atmosphere. The coefficient that describes the probability that the impinging gas-phase molecule sticks to the surface of interest is called the mass accommodation coefficient, α_M. Despite the importance of this coefficient for the description of water uptake kinetics, accurate values are still lacking for many systems. In this Feature Article, we present various experimental techniques that have been evoked in the literature to study the interfacial transport of water and discuss the corresponding strengths and limitations. This includes our recently developed technique called photothermal single-particle spectroscopy (PSPS). The PSPS technique allows for a retrieval of α_M values from three independent, yet simultaneous measurements operating close to equilibrium, providing a robust assessment of interfacial mass transport. We review the currently available data for α_M for water on various organics and discuss the few studies that address the temperature and relative humidity dependence of α_M for water on organics. The knowledge of the latter, for example, is crucial to assess the water uptake kinetics of organic aerosols in the Earth's atmosphere. Finally, we argue that PSPS might also be a viable method to better restrict the α_M value for water on liquid water.

Large contribution to secondary organic aerosol from isoprene cloud chemistry

Aerosols still present the largest uncertainty in estimating anthropogenic radiative forcing. Cloud processing is potentially important for secondary organic aerosol (SOA) formation, a major aerosol component: however, laboratory experiments fail to mimic this process under atmospherically relevant conditions. We developed a wetted-wall flow reactor to simulate aqueous-phase processing of isoprene oxidation products (iOP) in cloud droplets. We find that 50 to 70% (in moles) of iOP partition into the aqueous cloud phase, where they rapidly react with OH radicals, producing SOA with a molar yield of 0.45 after cloud droplet evaporation. Integrating our experimental results into a global model, we show that clouds effectively boost the amount of SOA. We conclude that, on a global scale, cloud processing of iOP produces 6.9 Tg of SOA per year or approximately 20% of the total biogenic SOA burden and is the main source of SOA in the mid-troposphere (4 to 6 km).

The Uptake of Methyl Vinyl Ketone, Methacrolein, and 2-Methyl-3-butene-2-ol onto Sulfuric Acid Solutions

To investigate the link between molecular structure, reactivity, and partitioning of oxygenated organic compounds in acidic aerosols, the uptake of three compounds found in the atmosphere, methyl vinyl ketone (MVK), methacrolein (MACR), and 2-methyl-3-butene-2-ol (MBO), by sulfuric acid solutions has been measured using a rotated wetted-wall reactor (RWW) coupled to a chemical ionization mass spectrometer (CIMS). MVK was found to partition reversibly into 20-75 wt % H(2)SO(4) solutions, and we report Henry's law coefficients between 20 and 7000 M atm(-1) over this range. A chemical reaction for MVK was likely responsible for the uptake observed for 80-96 wt % H(2)SO(4) solutions. We derive an upper limit to the aldol self-reaction rate coefficient for MVK in 80 wt % solution of approximately 3 M(-1) s(-1). MACR partitioned reversibly over most of the acidity range, and in contrast to that for MVK, the Henry's law coefficient was relatively independent of H(2)SO(4) content. These differences indicate that the increase of the coefficient with acidity is likely due to the ability of the carbonyl molecule to form an enol. These results indicate that aldol condensation can be facile in concentrated sulfuric acid solutions, but it should be negligibly slow in dilute acid solutions such as tropospheric aerosols. MBO uptake could be explained by a Henry's law coefficient that decreases slightly as acid content varies from 20 to 55 wt % H(2)SO(4); we also measured the value in water, 70 M atm(-1) at 298 K. A steady-state uptake of MBO was observed onto 40-80 wt % H(2)SO(4) solutions, a reaction product was observed, and the reaction was tentatively identified as Pinacol rearrangement. Similar rearrangements could be at the origin of some substituted oxygenated species found in atmospheric aerosols.

Uptake and Reaction Kinetics of Acetone, 2-Butanone, 2,4-Pentanedione, and Acetaldehyde in Sulfuric Acid Solutions

This work presents a study of the uptake of acetone, 2-butanone (methyl ethyl ketone), 2,4-pentanedione, and acetaldehyde by sulfuric acid solutions with an aim at understanding the reactivity of carbonyl compounds present in the atmosphere toward acidic aerosols. Experiments were performed in a rotating wetted-wall reactor coupled to a mass spectrometer at room temperature (298 +/- 3 K) with 0-96 wt % H(2)SO(4) solutions. For all compounds, a reactive uptake was observed at high acidity (>or=64 wt % H(2)SO(4)). The corresponding reactions were found to follow a second-order kinetics, and their rate constants, k (M(-1) s(-1)) were found to increase exponentially with acidity. These rate constants and their variations with acid concentration were in good agreement with the kinetic behavior of acid-catalyzed aldol condensation reported in the organic chemical literature, except for 2,4-pentanedione. The results of this work suggest that aldol condensation should be too slow to account for the enhanced organic aerosol mass observed in smog chamber studies and should have an even smaller contribution under atmospheric conditions. The rate constants of other compounds, such as large aldehydes, remain however to be measured. However, in order to contribute significantly to organic aerosol formation, a liquid phase reaction would have to result in an uptake coefficient of the order of 10(-2).

Mass Accommodation of H2SO4 and CH3SO3H on Water−Sulfuric Acid Solutions from 6% to 97% RH

The uptake of H2SO4 and CH3SO3H onto particles composed of water and sulfuric acid was studied in a laminar flow reactor at atmospheric pressure. Their first-order gas-phase loss rate coefficients were determined using a chemical ionization mass spectrometer. Relative humidity was varied from 6% to 97% at 295-297.5 K. The mass accommodation coefficient, alpha, was found to be close to unity for both species. These findings show that alpha does not limit particle growth rates resulting from H2SO4 and CH3SO3H uptake. Diffusion coefficients in N2 for these two species are also reported and a significant dependence upon relative humidity was seen for H2SO4 but not for CH3SO3H. Last, production of small particles was observed due to the presence of SO2 in particle chargers. Formation of these particles can be significantly reduced by adding an OH scavenger such as propane.

Photoenhanced uptake of gaseous NO2 on solid organic compounds: a photochemical source of HONO?

In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present laboratory study, the effect of light (in the range 300-500 nm) on the uptake kinetics of NO2 on various surfaces taken as proxies for organic surfaces encountered in the troposphere (as organic aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing aromatic compounds was significantly enhanced when irradiated with light of 300-420 nm, and HONO was formed with high yield when the gas was humidified. Especially organic substrates containing a combination of electron donors, such as phenols, and of compounds yielding excited triplet states, such as aromatic ketones, showed a high reactivity towards NO2. Based on the results reported a mechanism is suggested, in which photosensitised electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atmosphere at much longer wavelengths compared to the recently proposed nitrate photolysis.