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Enhanced Reactive Uptake of Nonanal by Acidic Aerosols in the Presence of Particle-Phase Organics

An electrodynamic balance was used to examine the effect of the presence of particle-phase organics on the acid-catalyzed reactive uptake of nonanal (NL) vapor. Uptake experiments were conducted by using sulfuric acid (SA) particles, oleic acid/SA (hydrophobic), and levoglucosan/SA (hydrophilic) mixed particles with 6 ppm (approximately) gas-phase NL at about 3% relative humidity. SA reacted with the mixed organics prior to NL uptake to form organic products, denoted as OleA* and Levo*, and with NL to form hydrophobic NL* (particle-phase organics). Fresh SA particles had small mass increases (5%–13%) at the start of NL exposure (0–40 min) even though they are highly acidic. However, OleA*/SA mixed particles of about 30–70 wt% of OleA* took up NL swiftly during the first 40 min. For example, the mass increase of a 33 wt% OleA* particle jumped to 120%. As the organic product, NL*, accumulated, the uptake rate of SA particle increased and the mass increase surged to 150% at 100 min. Afterwards, the mass increase started to level off which yielded a sigmoid uptake curve. For OleA*/SA particles, the uptake rate gradually slowed down resulting in physical-absorption-like uptake kinetics. The physical uptake of NL by a pure OleA* surrogate was negligible (<1%) showing that the large uptake of OleA*/SA particles were attributed to the enhanced reactive uptake of NL in the presence of hydrophobic OleA*. Conversely, the hydrophilic Levo*/SA particles were incompatible with NL, and they showed insignificant enhanced uptake compared with the SA particles. Overall, the acidic uptake of NL is highly dependent on the chemical nature and weight percentages of particle-phase organics in mixed particles. Presence of hydrophobic organic materials in particles enhanced the reactive uptake of NL.