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 (C4H6O4), sulfuric acid (H2SO4) and ammonium sulfate ((NH4)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 C4H6O4 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. However, 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

Surface modification of mineral dust particles by sulphuric acid processing: Implications for ice nucleation abilities

The ability of coated mineral dust particles to act as ice nuclei (IN) was investigated at LACIS (Leipzig Aerosol Cloud Interaction Simulator) during the FROST1- and FROST2-campaigns (Freezing of dust). Sulphuric acid was condensed on the particles which afterwards were optionally humidified, treated with ammonia vapour and/or heat. By means of aerosol mass spectrometry we found evidence that processing of mineral dust particles with sulphuric acid leads to surface modifications of the particles. These surface modifications are most likely responsible for the observed reduction of the IN activation of the particles. The observed particle mass spectra suggest that different treatments lead to different chemical reactions on the particle surface. Possible chemical reaction pathways and products are suggested and the implications on the IN efficiency of the treated dust particles are discussed

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<sub>4</sub>H<sub>6</sub>O<sub>4</sub>), sulfuric acid (H<sub>2</sub>SO<sub>4</sub>) and ammonium sulfate ((NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub>). 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<sub>4</sub>H<sub>6</sub>O<sub>4</sub> or small amounts of H<sub>2</sub>SO<sub>4</sub> were found to be the most efficient ice nuclei (IN). ATD particles coated with (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> 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. However, 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

Formation of 3-methyl-1,2,3-butanetricarboxylic acid via gas phase oxidation of pinonic acid - a mass spectrometric study of SOA aging

This paper presents the results of mass spectrometric investigations of the OH-initiated oxidative aging of alpha-pinene SOA under simulated tropospheric conditions at the large aerosol chamber facility AIDA, Karlsruhe Institute of Technology. In particular, the OH-initiated oxidation of pure pinic and pinonic acid, two well-known oxidation products of alpha-pinene, was investigated. Two complementary analytical techniques were used, on-line atmospheric pressure chemical ionization/mass spectrometry (APCI/MS) and filter sampling followed by liquid chromatography/mass spectrometry (LC/ESI-MS). The results show that 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA), a very low volatile alpha-pinene SOA product and a tracer compound for terpene SOA, is formed from the oxidation of pinonic acid and that this oxidation takes place in the gas phase. This finding is confirmed by temperature-dependent aging experiments on whole SOA formed from alpha-pinene, in which the yield of MBTCA scales with the pinonic acid fraction in the gas phase. Based on the results, several feasible gas-phase radical mechanisms are discussed to explain the formation of MBTCA from OH-initiated pinonic acid oxidation

Aerosol mass spectrometric measurements of stable crystal hydrates of oxalates and inferred relative ionization efficiency of water

We investigated the crystal hydrates of ammonium oxalate, potassium oxalate and oxalic acid as well as sodium oxalate. By taking advantage of the different crystal water contents we determined for the first time the relative ionization efficiency of water (RIEH2O) in an Aerodyne Aerosol Mass Spectrometer (AMS). The RIE is a key parameter for the quantitative measurement of a compounds' aerosol particle concentration with an AMS. Since the liquid water content of an aerosol particle determines its physical and chemical properties to a large extent, the knowledge of the water content is of high scientific interest. The investigation of the three salts led to a RIEH2O of 2. Application of this RIEH2O and further analysis of oxalic acid revealed two major characteristics, which need to be considered in future when handling oxalic acid. First, oxalic acid aerosol particles show a reduced crystal water content compared to the thermodynamically stable and expected value when produced under standard laboratory conditions. Second, solid oxalic acid and its solutions show significant contamination with ammonium when exposed to atmospheric air. These findings were verified by high resolution mass spectrometry and their implications for laboratory studies of hygroscopic growth and CCN properties of oxalic acid are discussed. (C) 2010 Elsevier Ltd. All rights reserved