Effects of NO<sub><i>x</i></sub> and SO<sub>2</sub> on the secondary organic aerosol formation from photooxidation of <i>α</i>-pinene and limonene

Abstract

Abstract. Anthropogenic emissions such as NO x and SO 2 influence the biogenic secondary organic aerosol (SOA) formation, but detailed mechanisms and effects are still elusive. We studied the effects of NO x and SO 2 on the SOA formation from the photooxidation of α-pinene and limonene at ambient relevant NO x and SO 2 concentrations (NO x : < 1to 20 ppb, SO 2 : < 0.05 to 15 ppb). In these experiments, monoterpene oxidation was dominated by OH oxidation. We found that SO 2 induced nucleation and enhanced SOA mass formation. NO x strongly suppressed not only new particle formation but also SOA mass yield. However, in the presence of SO 2 which induced a high number concentration of particles after oxidation to H 2 SO 4 , the suppression of the mass yield of SOA by NO x was completely or partly compensated for. This indicates that the suppression of SOA yield by NO x was largely due to the suppressed new particle formation, leading to a lack of particle surface for the organics to condense on and thus a significant influence of vapor wall loss on SOA mass yield. By compensating for the suppressing effect on nucleation of NO x , SO 2 also compensated for the suppressing effect on SOA yield. Aerosol mass spectrometer data show that increasing NO x enhanced nitrate formation. The majority of the nitrate was organic nitrate (57-77 %), even in low-NO x conditions (< ∼ 1 ppb). Organic nitrate contributed 7-26 % of total organics assuming a molecular weight of 200 g mol −1 . SOA from α-pinene photooxidation at high NO x had a generally lower hydrogen to carbon ratio (H / C), compared to low NO x . The NO x dependence of the chemical composition can be attributed to the NO x dependence of the branching ratio of the RO 2 loss reactions, leading to a lower fraction of organic hydroperoxides and higher fractions of organic nitrates at high NO x . While NO x suppressed new particle formation and SOA mass formation, SO 2 can compensate for such effects, and the combining effect of SO 2 and NO x may have an important influence on SOA formation affected by interactions of biogenic volatile organic compounds (VOCs) with anthropogenic emissions.

Aerosol mass yields of selected biogenic volatile organic compounds – a theoretical study with nearly explicit gas-phase chemistry

“…Varying NO x concentrations changes the fate of RO 2 radicals formed during organic oxidations by altering the HO 2 /RO 2 ratio, thereby impacting the distribution of reaction products and aerosol formation (Presto et al, 2005;Zhao et al, 2018;Sarrafzadeh et al, 2016). We modeled the SOA mass yields for the α-pinene-O 3 setup with varying NO x concentrations (NO was varied whereas NO 2 was kept constant for all the runs), for initial α-pinene mixing ratios in the range 0.5-200 ppb (Fig.…”

“…A majority of chamber and flow-tube experiments have focused on HOM formation from the oxidation of various VOCs and their contribution to SOA mass loadings (Ehn et al, 2014;Kristensen et al, 2017). Oxidation of isoprene (Liu et al, 2016), endocyclic monoterpenes containing reactive double bonds such as α-pinene and limonene (Zhao et al, 2015), or exocyclic double-bond-containing compounds such as β-pinene (Jokinen et al, 2015) and sesquiterpenes such as β-caryophyllene (Chen et al, 2012) have been investigated. The SOA forming potential of various BVOCs depends on the isomeric structures (Friedman and Farmer, 2018;Keywood et al, 2004).…”

“…One explanation for this dependence on the isomeric structure is attributed to the formation of HOMs (Ehn et al, 2014). Another important factor influencing HOM formation is the initial oxidant, as pointed out by Zhao et al (2015). They showed that the SOA formation by OH oxidation of α-pinene and limonene was lower when compared to their SOA formed by ozonolysis.…”

Rapid, 3D Chemical Profiling of Individual Atmospheric Aerosols with Stimulated Raman Scattering Microscopy

“…Previous studies have proposed various possible mechanisms and pathways of heterogeneous reactions of NO x and SO 2 on mineral dust particles using aerosol flow tube methods, aerosol chamber methods, SEM/EDS, and Knudsen cell‐mass spectrometer . Recently, MRS showed promise in obtaining microscopic morphology of particles to understand the reaction mechanisms . As we have discussed before, the particle structures are related with different heterogeneous reactions.…”

Aerosol Mass yields of selected Biogenic Volatile Organic Compounds – a theoretical study with near explicit gas-phase chemistry

“…Organic oxidation mechanisms are sensitive to NOx concentrations as they change the fate to RO2 radical formed, thereby impacting the distribution of reaction products and aerosol formation (Presto et al, 2005;Zhao et al, 2018). We modeled the SOA mass yields for α-pinene -O3 setup with varying NOx concentrations (NO was varied whereas NO2 was kept constant for all the runs), for initial α-pinene mixing ratios in the range 0.5 -200 ppb (Fig.…”