NO at low concentration can enhance the formation of highly oxygenated biogenic molecules in the atmosphere

Abstract

The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO₂) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 - 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO₂ loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO₂-NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer.

Fig. 1. Comparison of measured and modelled mass yields of highly oxygenated organic molecules (HOM).

In (a) a pure NO₂ experiment run with 1200 pptv monoterpene, (b) a constant NO/NO₂ experiment with 1200 pptv monoterpene, (c) a constant NO/NO₂ experiment with 300 pptv monoterpene and (d) a varying NO/NO₂ experiment with around 1 ppbv NO₂ and 1200 pptv monoterpene. The brown solid circle in (d) denotes the HOM mass yield obtained from the constant NO/NO₂ experiment with around 1 ppbv NO₂ and 7 pptv NO (the brown dash line in (b)). Model simulated HOM represents molecules formed without NO participation and are denoted in green; HOM_NO represents molecules formed with NO’s involvement and are denoted in blue. The propagated error of the HOM yield varies within 6–8% among different experiments with the calculation method provided in the Methods.

Fig. 2. The mechanism through which NO can enhance the formation of highly oxygenated organic molecules (HOM) from monoterpene oxidation at 278 K.

(a) Loss rates of p-HOM-RO₂ (RO₂ that can undergo autoxidation with n_O < 7) as a function of NO concentration in the varying NO/NO₂ experiment. Reactions of RO₂ + HO₂, RO₂ + NO, and RO₂ + RO₂ are marked by pink, blue, and red, respectively. (b) The fraction of p-HOM-RO₂ that can undergo autoxidation to HOM-RO₂ with (the sum the blue and green fillings) and without (green fillings) NO-induced RO autoxidation in the varying NO/NO₂ experiment. (c) Comparison between HOM yields with (solid blue line) and without NO (dashed green line) RO autoxidation. In the simulation of Fig. 2c, the concentrations of monoterpene, O₃, and NO₂ are constrained at 1200 pptv, 40 ppbv and 1 ppbv.

Fig. 3. Model simulated non-linear dependence of the yields of highly oxygenated organic molecules (HOM) on NO₂ and NO at 278 K.

(a) total HOM, (b) CHO-HOM, and (c) CHON-HOM with 1200 pptv monoterpene. O₃ concentrations are constrained at 40 ppbv at each experiment. The contour lines indicate the corresponding HOM yields.

Fig. 4. Contour plot of the yields of highly oxygenated organic molecules (HOM) from monoterpene oxidation vs monoterpene and NO_x concentration.

The observation data of monoterpene and NO_x from the SMEAR II boreal forest station are used to predict the HOM yield (black points). A series of simulations were conducted with monoterpene concentration varying from 100 to 1800 pptv, and NO_x concentration from 0.01 to 100 ppbv. With the assumption NO is solely formed from NO₂ photolysis, NO/NO₂ varied to follow the observed diurnal cycle at SMEAR II station from almost zero at night to 14.3% at 10 am. We take the daytime average HOM yield as the HOM yield at fixed NO_x and monoterpene concentration.