NOx regime-dependent effects of biogenic emissions on toluene degradation and product formation.

Interactions between anthropogenic and biogenic emissions play a critical role in shaping atmospheric chemistry, yet their combined effects remain insufficiently understood. To elucidate these interactions, we selected toluene and isoprene as representative anthropogenic and biogenic volatile organic compound (AVOC and BVOC), respectively. A series of photooxidation experiments were conducted under both low and high NOx regimes to investigate how isoprene influences the photooxidation of toluene and the formation of reaction products. In toluene-only experiments, the fractions of toluene consumed were 25 %-28 % under the low NOx regime and 18 %-22 % under the high NOx regime. When isoprene was introduced, toluene consumption decreased to 12 %-14 % under the low NOx regime but increased to 25 %-40 % under the high NOx regime. These contrasting trends are attributed to isoprene's dual role in hydroxyl radicals (OH•) chemistry: under high NOx regimes, isoprene enhanced OH• regeneration via accelerated the hydroperoxy radical-peroxynitric acid cycling, while under low NOx regimes, it competed with toluene for the limited available OH•, thereby suppressing toluene oxidation. Furthermore, the formation of semi-volatile and intermediate-volatile organic compounds was promoted in the presence of isoprene under high NOx regimes, but inhibited under low NOx regimes. These results provide new insights into the nonlinear photochemical behavior of AVOCs influenced by biogenic emissions, and offer valuable implications for air pollution control strategies and atmospheric chemical modeling.