Thomas J. Bannan

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

Atmospheric simulation chambers are essential for studying oxidation processes, yet many experiments rely on single-precursor systems at concentrations that limit their representativeness for the chemical complexity of urban air. Here, we present a framework linking emission inventories, chamber experiments, and field observations by constructing emission fingerprints of major urban sources, including vegetation, volatile chemical products, cooking emissions, and gasoline and diesel exhaust. Applied during the CHANEL campaign in the SAPHIR outdoor chamber, these fingerprints advanced toward reproducing gas-phase mixtures representative of Los Angeles (gasoline-dominated traffic), a diesel-dominated city, and a future low-vehicle-emission scenario. The resulting mixtures generated peroxy-to-hydroperoxy radical ratios (RO2/HO2) close to unity, bimolecular RO2 lifetimes comparable with urban atmospheric observations, although lower NO levels favored RO2 + HO2 over RO2 + NO reactions. Comparison with aircraft measurements over Los Angeles using ammonium (NH4+) chemical ionization mass spectrometry showed reasonable agreement, with 465 organic product ions detected in both data sets, representing 78% of the product ions observed in the ambient air. Furthermore, 87% of common product ions agreed within a factor of 3 of the linear correlation, demonstrating that chamber experiments can capture key aspects of both atmospherically relevant radical chemistry and the oxidation product diversity of urban air.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies. We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities. We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.

Nitrogen oxides (NO_x = NO₂ + NO) play a crucial role in secondary organic aerosol (SOA) formation. The effects of NO_x on SOA formation from single precursors have been extensively studied. However, in the real atmosphere, biogenic and anthropogenic precursors often coexist, and it remains unclear whether the effects of NO_x in such mixtures can be directly extrapolated from those observed in single-precursor systems. In this study, we investigated the effects of NO_x on SOA particle mass yields and chemical composition from α-pinene, n-dodecane, and their mixture under high- and moderate-NO_x conditions in the Manchester Aerosol Chamber. The results show that SOA particle mass yields were higher under high-NO_x conditions across all systems. Enhanced oxidant levels and alkoxy radical (RO) isomerisation appear to more than compensate for the negative impacts associated with the formation of more volatile products via the reaction of organic peroxy radicals (RO₂) with NO. However, compared with the single-precursor systems, the increase in SOA particle mass yields in the mixed-precursor system was less pronounced. In the mixed-precursor system, enhancement of RO₂ + NO termination pathways was stronger, and the contribution of α-pinene-derived alkoxy-peroxy pathway may have been comparatively weaker. These changes would be expected to favour the formation of more volatile products and thus suppress SOA formation. Collectively, these observations provide evidence that the effects of NO_x in the mixed-precursor systems cannot be interpreted as a simple combination of behaviours observed in individual precursor systems.