Abstract
Abstract. Atmospheric H2SO4 / H2O nucleation influencing effects have been studied in the flow tube IfT-LFT (Institute for Tropospheric Research – Laminar Flow Tube) at 293 ± 0.5 K and a pressure of 1 bar using synthetic air as the carrier gas. The presence of a possible background amine concentration in the order of 107–108 molecule cm−3 throughout the experiments has to be taken into account. In a first set of investigations, ozonolysis of olefins (tetramethylethylene, 1-methyl-cyclohexene, α-pinene and limonene) for close to atmospheric concentrations, served as the source of OH radicals and possibly other oxidants initiating H2SO4 formation starting from SO2. The oxidant generation is inevitably associated with the formation of organic oxidation products arising from the parent olefins. These products (first generation mainly) showed no clear effect on the number of nucleated particles within a wide range of experimental conditions for H2SO4 concentrations higher than ~107 molecule cm−3. Also the early growth process of the nucleated particles was not significantly influenced by the organic oxidation products in line with the expected growth by organic products using literature data. An additional, H2SO4-independent process of particle (nano-CN) formation was observed in the case of α-pinene and limonene ozonolysis for H2SO4 concentrations smaller than ~107 molecule cm−3. Furthermore, the findings confirm the appearance of an additional oxidant for SO2 beside OH radicals, very likely stabilized Criegee Intermediates (sCI). A second set of experiments has been performed in the presence of added amines in the concentrations range of a few 107–1010 molecule cm−3 applying photolytic OH radical generation for H2SO4 production without addition of other organics. All amines showed significant nucleation enhancement with increasing efficiency in the order pyridine < aniline < dimethylamine < trimethylamine. This result supports the idea of H2SO4 cluster stabilization by amines due to strong H2SO4↔amine interactions. On the other hand, this study indicates that for organic oxidation products (in presence of the possible amine background as stated) a distinct H2SO4 / H2O nucleation enhancement can be due to increased H2SO4 formation caused by additional organic oxidant production (sCI) rather than by stabilization of H2SO4 clusters due to H2SO4↔organics interactions.
Highlights
For more than a decade the formation of new aerosol particles in the atmosphere has been the subject of intense studies in both, field and laboratory
This study indicates that for organic oxidation products a distinct H2SO4 / H2O nucleation enhancement can be due to increased H2SO4 formation caused by additional organic oxidant production rather than by stabilization of H2SO4 clusters due to H2SO4 ↔ organics interactions
This fact can be explained by increasing interactions of the very strong acid (H2SO4) with atmospheric bases for increasing gas-phase basicity according to the proton affinity; ammonia: 854 kJ mol−1 versus e.g. dimethylamine: 923 kJ mol−1 (Jolly, 1991; Kurten et al, 2008; DePalma et al, 2012)
Summary
For more than a decade the formation of new aerosol particles in the atmosphere has been the subject of intense studies in both, field and laboratory. The amines are consistently identified to be more effective in the enhancement of H2SO4 / H2O nucleation compared with ammonia for similar concentration levels. This fact can be explained by increasing interactions of the very strong acid (H2SO4) with atmospheric bases for increasing gas-phase basicity according to the proton affinity (base + H+ → base-H+); ammonia: 854 kJ mol−1 versus e.g. dimethylamine: 923 kJ mol−1 (Jolly, 1991; Kurten et al, 2008; DePalma et al, 2012). Amines are ubiquitous in the atmosphere produced by microbial degradation of organic material or released by a series of human activities (Schade and Crutzen, 1995; Ge et al, 2010). Ammonia can be locally important for the nucleation process compensating its compared to amines lower efficiency by much higher concentration levels
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