Functionality-based formation of secondary organic aerosol from <i>m</i>-xylene photooxidation

Abstract. Photooxidation of volatile organic compounds (VOCs) produces condensable oxidized organics (COOs) to yield secondary aerosol (SOA), but the fundamental chemical mechanism for gas-to-particle conversion remains uncertain. Here we elucidate production COOs and their roles in SOA brown carbon (BrC) formation from m-xylene oxidation by simultaneously monitoring evolution gas-phase products properties an environmental chamber. Four COO types with distinct functionalities dicarbonyls, carboxylic acids, polyhydroxy aromatics/quinones, nitrophenols are identified early-generation oxidation, yields 25 %, 37 5 3 respectively. occurs via several heterogeneous processes, including interfacial interaction, ionic dissociation/acid–base reaction, oligomerization, (20 ± 4) % (32 7) at 10 70 relative humidity (RH), Chemical speciation shows dominant presence oligomers, nitrogen-containing organics, carboxylates high RH low RH. The BrC includes N-heterocycles/N-heterochains nitrophenols, as evident reduced single scattering albedo. measured uptake coefficient (?) is dependent on functionality, ranging 3.7 × 10?4 1.3 10?2. A functionality-based kinetic framework developed predict observed concentrations coefficients COOs, which reproduces measurement oxidation. Our results reveal that photochemical represents a major source under urban environments, because its large abundance, reactivity OH, COOs.