Analysis of rainwater samples: Comparison of single particle residues with ambient particle chemistry from the northeast Pacific and Indian oceans

Abstract

Individual particles produced from atomized rainwater samples collected in California and the Indian Ocean were analyzed with an aerosol time‐of‐flight mass spectrometer (ATOFMS) to investigate the chemical composition of the individual rain residue particles. Insoluble residue particle types were determined on the basis of a comparison of the rainwater particle mass spectra with ambient particle spectra. Major particle types found in rainwater include dust, organic carbon with sodium, aromatic organic carbon, vegetative detritus, and an internally mixed sea salt and elemental carbon class. A unique internally mixed sea salt–elemental carbon particle type was detected in both the ambient and rainwater samples, suggesting this particle type was most likely formed by cloud processing occurring during long‐range transport. The presence of this particle type in remote marine locations has important climate ramifications as it is anticipated it will be strongly absorbing on the basis of the combination of an absorbing particle (elemental carbon) mixed with a high refractive index material (sea salt). Most of the particle types detected in rainwater were detected in the ambient particles with the exception of a unique aromatic particle type detected in rainwater samples from both locations. The presence of the aromatic type coupled with the absence of biomass particles in the rainwater samples leads to the hypothesis the aromatic components were originally associated with atmospheric biomass burning particles. The ubiquitous presence of this aromatic type in rainwater samples highlights the potential importance of biomass burning and/or humic‐like substances (HULIS) compounds in cloud formation and rain processes.