Geranyl ester components of volatile chemical products readily induce black carbon restructuring

Volatile chemical products (VCPs) are an emerging contaminant in the urban environment. Black carbon (BC) aggregates, initially highly branched, are also a prominent pollutant in the urban environment, emitted by diesel engines and other combustion sources outside and indoors, and they have effects on climate and public health that are influenced by their physical properties, e.g., size and morphology. Here, we investigate the effects of geranyl esters, abundant components of fragrances in VCPs, on the morphology of laboratory generated BC aggregates. BC with an initial mobility diameter of 200 nm, generated by an inverted diffusion burner, was selected using a differential mobility analyzer (DMA) for coating experiments. We find that geranyl formate, acetate, propionate, and butyrate are extremely efficient at inducing BC restructuring. Complete compaction to a diameter growth factor of less than 0.75 occurs already at a volume growth factor as little as 1.01, i.e., when the coating volume is 1% of the initial BC volume, in contrast to dioctyl sebacate, a common coating taken as a reference material. Compaction was also observed by scanning electron microscopy. Using a publicly available model of condensation to BC, we rationalize that this efficient restructuring occurs because the geranyl esters partition exclusively to point contacts between primary particles of BC through capillary condensation. Our results highlight the potential interactions of BC and VCPs, particularly in the indoor environment, where, e.g., scented candles co-emit BC and fragrances.