Uptake of Amines on Acidic Particles: Using Laser Raman Tweezers to Monitor Size and Composition

Atmospheric aerosol plays an important role in the biogeochemical pathways of elements. Rainfall is a significant source of nitrogen being returned to the terrestrial ecosystem. Uptake of nitrogen compounds on aerosol is the main pathway from the atmosphere to the biosphere. Organic nitrogen represents 20-80% of the total nitrogen deposition, typically in the form of amino acids or amines, which are commonly associated with vegetated terrain and which are scavenged from the atmosphere by aqueous droplets. We present here a study using laser tweezers and Raman spectroscopy to examine uptake of diethylamine on droplets of sulphuric acid in water, as a proxy for a typical atmospheric aerosol, via the reaction: HN(CH2CH3)2 (g) + H3O+ (l) H2N(CH2CH3)2 + (aq) + H2O(l). Laser tweezers provide a means to study single, micron-sized droplets in isolation by trapping them in the focus of a laser, and Raman spectroscopy uses the laser as a spectroscopic light source to allow real-time analysis of changes in chemical composition. Optical microscopy was used to determine droplet size. Analysis of the rate of amine uptake reveals the uptake coefficient, and thus the mass accommodation coefficient, for a range of droplet sizes using the analysis techniques of Worsnop et al. [2002]. Initial analysis shows that the mass accommodation coefficient, α = 4 × 10-4. The mass accommodation coefficient then determines whether the reaction occurs at the surface or in the bulk of the droplet. Our studies also show that there is significant droplet size increase with uptake of amine, indicating that there is increased precipitation potential as clouds pass over areas of amine emission.