Contrasting impacts of humidity on the ozonolysis of monoterpenes: insights into the multi-generation chemical mechanism

Abstract

Abstract. Secondary organic aerosol (SOA) formed from the ozonolysis of biogenic monoterpenes is a major source of atmospheric organic aerosol. It has been previously found that relative humidity (RH) can influence the SOA formation from some monoterpenes, yet most studies only observed the increase or decrease in SOA yield without further explanations of molecular-level mechanisms. In this study, we chose two structurally different monoterpenes (limonene with an endocyclic double bond and an exocyclic double bond, Δ3-carene with only an endocyclic double bond) to investigate the effect of RH in a set of oxidation flow reactor experiments. We find contrasting impacts of RH on the SOA formation: limonene SOA yield increases by ∼100 % as RH increases, while there is a slight decrease in Δ3-carene SOA yield. Although the complex processes in the particle phase may play a role, we primarily attribute the results to the water-influenced reactions after ozone attack on the exocyclic double bond of limonene, which leads to the increment of lower volatile organic compounds under high-RH conditions. However, as Δ3-carene only has an endocyclic double bond, it cannot undergo such reactions. This hypothesis is further supported by the SOA yield enhancement of β-caryophyllene, a sesquiterpene that also has an exocyclic double bond. These results greatly improve our understanding of how water vapor influences the ozonolysis of biogenic organic compounds and subsequent SOA formation processes.

Evaporation-Induced Transformations in Volatile Chemical Product-Derived Secondary Organic Aerosols: Browning Effects and Alterations in Oxidative Reactivity

“…The dimeric range, particularly, accounted for the majority of signals (65%), suggesting the formation of dimers through alkylperoxy radical (RO 1a). We also observed 62 The drying of the aerosols, as shown in Figure 1b, has only a minor effect on the distribution of the CHO compounds. Figure 1c shows a comparative analysis of the top 15 species in undried VCP-derived SOA and their respective relative signal intensities in the dried samples.…”

“…The top five signal contributors were C 19 H 32 O 8 , C 18 H 30 O 8 , C 19 H 32 O 7 , C 18 H 30 O 7 , and C 18 H 30 O 9 , which were also reported in limonene/O 3 SOA studies − and likely represent ROOR-type peroxides (Figure a). We also observed a range of alcohols, carbonyls, and peroxide within the monomeric spectrum, such as C 10 H 18 O 4 , C 9 H 14 O 4 , C 9 H 14 O 3 , and C 9 H 16 O 4 , potentially resulting from varied radical reactions during limonene ozonolysis …”

“…We also observed a range of alcohols, carbonyls, and peroxide within the monomeric spectrum, such as C 10 H 18 O 4 , C 9 H 14 O 4 , C 9 H 14 O 3 , and C 9 H 16 O 4 , potentially resulting from varied radical reactions during limonene ozonolysis. 62 …”

Effect of relative humidity on the molecular composition of secondary organic aerosols from α-pinene ozonolysis

“…23,40 Yet, in the ozonolysis of other monoterpenes, e.g., limonene compared with D 3 -carene, RHdependent product distribution in the gas-phase has also been reported to play a vital role in the composition and yield of SOA. 62 Therefore, more mechanistic studies are warranted in the future.…”

“…However, the effect of RH on SOA yields remains inconclusive. 10,17,18,62,64,65 The RH-dependent chemical composition can also inuence the CCN activity of SOA. The decrease of the average molecular weight with increasing RH may enhance the hygroscopic parameter k, which is inversely proportional to the molar volume of solute, and thus to the molecular weight (provided that the density of different organics is largely similar).…”