Exploiting particle-to-particle heat transfer for optical detection of hetero-aggregate aerosols

We present a novel laser-optical in situ method for detecting gas-borne hetero-aggregates and distinguishing them from their individual components. The detection concept utilizes a pump–probe laser scheme, where infrared (IR) absorbing carbon-black (CB) particles are heated upon pump(IR) excitation, and white thermographic phosphor (ZnO:Zn) particles serve as optical markers, providing temperature-sensitive luminescence upon probe ultraviolet (UV) excitation. A temperature-induced spectral red-shift of the centroid of ZnO:Zn luminescence is observed only in hetero-aggregates, where thermal contact enables heat transfer from CB to ZnO:Zn. The method was validated using samples of pristine ZnO:Zn, CB, and pre-prepared hetero-aggregates aerosolized via a dry-powder particle seeder. The temperature sensitivity of the ZnO:Zn luminescence red-shift was determined to be 0.05 nm/K, consistent across pristine and aggregated samples, demonstrating no loss in sensitivity upon hetero-aggregation. The technique successfully discriminates between pump(IR)-heated and cold hetero-aggregates, even at low CB content (2.5 and 4.9 wt%). Varying the delay between pump(IR) and probe(UV) laser pulses revealed a ∼ 35 K maximum temperature increase in ZnO:Zn within the first 140 ns within hetero-aggregates containing 2.5 wt% CB. Doubling the CB content to 4.9 wt% resulted in an increased temperature change at the same laser delay. We observed that probe(UV)-induced heating can influence the temperature measurements in ZnO:Zn/CB aggregates, particularly at UV laser fluences above 0.3 J/cm². However, at lower probe(UV) laser fluences, the UV-induced heating can inherently be minimized.