Th. F. Mentel

Heterogeneous freezing of droplets with immersed mineral dust particles - measurements and parameterization

During the measurement campaign FROST (FReezing Of duST), LACIS (Leipzig Aerosol Cloud Interaction Simulator) was used to investigate the immersion freezing behavior of size selected, coated and uncoated Arizona Test Dust (ATD) particles with a mobility diameter of 300 nm. Particles were coated with succinic acid (C 4H6O4), sulfuric acid (H2SO4) and ammonium sulfate ((NH 4)2SO4). Ice fractions at mixed-phase cloud temperatures ranging from 233.15 K to 239.15 K ( ±0.60 K) were determined for all types of particles. In this temperature range, pure ATD particles and those coated with C 4H6O4 or small amounts of H2SO4 were found to be the most efficient ice nuclei (IN). ATD particles coated with (NH4)2SO4 were the most inefficient IN. Since the supercooled droplets were highly diluted before freezing occurred, a freezing point suppression due to the soluble material on the particles (and therefore in the droplets) cannot explain this observation. Therefore, it is reasonable to assume that the coatings lead to particle surface alterations which cause the differences in the IN abilities. Two different theoretical approaches based on the stochastic and the singular hypotheses were applied to clarify and parameterize the freezing behavior of the particles investigated. Both approaches describe the experimentally determined results, yielding parameters that can subsequently be used to compare our results to those from other studies. HowCorrespondence to: D. Niedermeier (niederm@tropos.de) ever, we cannot clarify at the current state which of the two approaches correctly describes the investigated immersion freezing process. But both approaches confirm the assumption that the coatings lead to particle surface modifications lowering the nucleation efficiency. The stochastic approach interprets the reduction in nucleation rate from coating as primarily due to an increase in the thermodynamic barrier for ice formation (i.e., changes in interfacial free energies). The singular approach interprets the reduction as resulting from a reduced surface density of active sites.

A study of nighttime nitrogen oxide oxidation in a large reaction chamber—the fate of NO2, N2O5, HNO3, and O3 at different humidities

Inorganic reactions important for the nighttime chemistry of nitrogen oxides in surface air were studied. The experiments were performed in a new, large reaction chamber with a volume of 260 m3 and a surface/volume ratio better than 1 m−1. The inner surface of the chamber is Teflon FEP. The formation of N2O5 and HNO3 in ambient air with an initial content of ≈ 1.3 ppm NO2 and ≈ 1.3 ppm O3 was monitored at 8, 20, and 70% relative humidity for periods of up to five days. The mixing ratios of NO2, N2O5, and HNO3 were measured simultaneously by in-chamber FTIR absorption spectroscopy. O3 and NO were measured by UV absorption and chemiluminescence. Model calculations for the nitrogen oxide/ozone system were performed. By comparison of the model calculations with the experimental data, the rate coefficients of two slow reactions, the unimolecular decomposition of NO3 and the gas-phase formation of HNO3 from N2O5 and water were determined. An upper limit for the rate coefficient for the unimolecular decomposition of NO3 of ⩽ 1.4 × 10−4s−1 was obtained, which corresponds to a lifetime of 120 min. The experiments provide evidence that the conversion of N2O5 with gaseous water to gas-phase HNO3 is a superposition of two slow processes: a second-order reaction, N2O5 + H2O, with a rate coefficient of 2.6( ± 0.1) × 10−22cm3 molecule−1s−1, and a third-order reaction, first order in N2O5 and second order in H2O, with a rate coefficient of 2( ± 0.05) × 10−39 cm6 molecule−2s−1. The third-order process could be due to a reaction of N2O5 with water on the chamber walls or alternatively to a gas-phase reaction, possibly even with water dimers. The implications of both alternatives for the atmospheric lifetime of N2O5 with respect to its gas-phase conversion to HNO3 are discussed.

Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air

This manuscript presents elemental composition data of highly oxidized compounds as clusters of nitrate ion, NO3-, and biogenic volatile organic compounds, especially a-pinene, oxidation products. The authors present a brief description of the APi-ToF instrument and mass calibration procedure for a large mass range, followed by comparison of mass spectra from Jülich chamber and the Hyytiälä field site along with a hypotheses for the formation mechanism of the highly oxidized compounds. Finally, the authors estimate the concentration of neutral molecules from the observed ion clusters. The data presented is very convincing and the manuscript is suitable for publication in ACP after the following comments are addressed.

Size-dependent hygroscopicity parameter (κ) and chemical composition of secondary organic cloud condensation nuclei: SIZE-DEPENDENTκAND COMPOSITION OF SOA

Secondary organic aerosol components (SOA) contribute significantly to the activation of cloud condensation nuclei (CCN) in the atmosphere. The CCN activity of internally mixed submicron SOA particles is often parameterized assuming a size-independent single-hygroscopicity parameter κ. In the experiments done in a large atmospheric reactor (SAPHIR, Simulation of Atmospheric PHotochemistry In a large Reaction chamber, Julich), we consistently observed size-dependent κ and particle composition for SOA from different precursors in the size range of 50 nm–200 nm. Smaller particles had higher κ and a higher degree of oxidation, although all particles were formed from the same reaction mixture. Since decreasing volatility and increasing hygroscopicity often covary with the degree of oxidation, the size dependence of composition and hence of CCN activity can be understood by enrichment of higher oxygenated, low-volatility hygroscopic compounds in smaller particles. Neglecting the size dependence of κ can lead to significant bias in the prediction of the activated fraction of particles during cloud formation.

Physical characterisation of aerosols and heterogeneous reactions in a large atmospheric chamber

Publisher Summary Heterogeneous reactions of trace gases on aerosol surfaces play an important role in the chemistry of the troposphere. Heterogeneous processes on aerosol surfaces provide an important pathway for the removal of nitrogen oxides from the troposphere. Thus, the conversion of N 2 0 5 to HNO 3 on wet aerosol is the largest single removal mechanism for NO x during night and winter time. The heterogeneous conversion of N 2 0 5 to HNO 3 on wet NaNO 3 aerosol is investigated by oxidizing NO 2 by O 3 with and without the presence of aerosol and observing the decay and building of NO 2 , O 3 , N 2 O 5, and HNO 3 . The data are interpreted using model calculations. The chemical box model includes night-time nitrogen oxide chemistry and the heterogeneous conversion of N 2 O 5 to HNO 3 approximated as a gas kinetic collision of N 2 O 5 with NaNO 3 aerosol surface. From comparison of the experimental results with the model, an uptake coefficient of N 2 O 5 on NaNO 3 aerosol is determined.