Chlorine as a primary radical: evaluation of methods to understand its role in initiation of oxidative cycles

摘要: Abstract. The role of chlorine atoms (Cl) in atmospheric oxidation has been traditionally thought to be limited the marine boundary layer, where they are produced through heterogeneous reactions involving sea salt. However, recent observation photolytic Cl precursors (ClNO2 and Cl2) formed from anthropogenic pollution expanded potential importance include coastal continental urban areas. Measurements ClNO2 Los Angeles during CalNex (California Nexus – Research at Air Quality Climate Change) showed it an important primary (first generation) radical source. Evolution ratios volatile organic compounds (VOCs) proposed as a method quantify oxidation, but we find no evidence this approach for significant Angeles. We use box model with Master Chemical Mechanism (MCM v3.2) chemistry scheme, constrained by observations Angeles, examine sensitivity commonly used VOC function NOx secondary production. Model results indicate tracer could not detect influence unless ratio [OH] [Cl] was less than 200 least day. also show that (second OH production resulting VOCs is strongly influenced NOx, effect obscures oxidant. Calculated concentrations maximum mid-morning due source loss primarily VOCs. below approximately 3 h morning, evident measured Instead, simulations causes evolution follow expected despite input photolysis morning. Even though far dominant oxidant do play photochemistry there, constituting 9% Furthermore, Cl–VOC reactivity differs OH, being more order magnitude larger dominated VOCs, such alkanes, reactive toward OH. Primary slightly effective its greater propensity initiate propagation chains via relative chain termination reaction nitrogen oxides.