scholarly journals Secondary organic aerosol formation from in situ OH, O<sub>3</sub>, and NO<sub>3</sub> oxidation of ambient forest air in an oxidation flow reactor

2017 ◽  
Vol 17 (8) ◽  
pp. 5331-5354 ◽  
Author(s):  
Brett B. Palm ◽  
Pedro Campuzano-Jost ◽  
Douglas A. Day ◽  
Amber M. Ortega ◽  
Juliane L. Fry ◽  
...  
Keyword(s):  
Mass Loss ◽  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Ambient Air ◽  
Organic Aerosol ◽  
Oxidation Products ◽  
Organic Nitrates ◽  
Aerosol Formation ◽  
Wide Range ◽  
Synoptic Conditions

Abstract. Ambient pine forest air was oxidized by OH, O3, or NO3 radicals using an oxidation flow reactor (OFR) during the BEACHON-RoMBAS (Bio–hydro–atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics and Nitrogen – Rocky Mountain Biogenic Aerosol Study) campaign to study biogenic secondary organic aerosol (SOA) formation and organic aerosol (OA) aging. A wide range of equivalent atmospheric photochemical ages was sampled, from hours up to days (for O3 and NO3) or weeks (for OH). Ambient air processed by the OFR was typically sampled every 20–30 min, in order to determine how the availability of SOA precursor gases in ambient air changed with diurnal and synoptic conditions, for each of the three oxidants. More SOA was formed during nighttime than daytime for all three oxidants, indicating that SOA precursor concentrations were higher at night. At all times of day, OH oxidation led to approximately 4 times more SOA formation than either O3 or NO3 oxidation. This is likely because O3 and NO3 will only react with gases containing C  =  C bonds (e.g., terpenes) to form SOA but will not react appreciably with many of their oxidation products or any species in the gas phase that lacks a C  =  C bond (e.g., pinonic acid, alkanes). In contrast, OH can continue to react with compounds that lack C  =  C bonds to produce SOA. Closure was achieved between the amount of SOA formed from O3 and NO3 oxidation in the OFR and the SOA predicted to form from measured concentrations of ambient monoterpenes and sesquiterpenes using published chamber yields. This is in contrast to previous work at this site (Palm et al., 2016), which has shown that a source of SOA from semi- and intermediate-volatility organic compounds (S/IVOCs) 3.4 times larger than the source from measured VOCs is needed to explain the measured SOA formation from OH oxidation. This work suggests that those S/IVOCs typically do not contain C  =  C bonds. O3 and NO3 oxidation produced SOA with elemental O : C and H : C similar to the least-oxidized OA observed in local ambient air, and neither oxidant led to net mass loss at the highest exposures, in contrast to OH oxidation. An OH exposure in the OFR equivalent to several hours of atmospheric aging also produced SOA with O : C and H : C values similar to ambient OA, while higher aging (days–weeks) led to formation of SOA with progressively higher O : C and lower H : C (and net mass loss at the highest exposures). NO3 oxidation led to the production of particulate organic nitrates (pRONO2), while OH and O3 oxidation (under low NO) did not, as expected. These measurements of SOA formation provide the first direct comparison of SOA formation potential and chemical evolution from OH, O3, and NO3 oxidation in the real atmosphere and help to clarify the oxidation processes that lead to SOA formation from biogenic hydrocarbons.

scholarly journals Secondary organic aerosol formation from in situ OH, O<sub>3</sub>, and NO<sub>3</sub> oxidation of ambient forest air in an oxidation flow reactor

2017 ◽  
Author(s):  
Brett B. Palm ◽  
Pedro Campuzano-Jost ◽  
Douglas A. Day ◽  
Amber M. Ortega ◽  
Juliane L. Fry ◽  
...  
Keyword(s):  
Mass Loss ◽  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Ambient Air ◽  
Organic Aerosol ◽  
Oxidation Products ◽  
Organic Nitrates ◽  
Aerosol Formation ◽  
Wide Range ◽  
Synoptic Conditions

Abstract. Ambient pine forest air was oxidized by OH, O3, or NO3 radicals using an oxidation flow reactor (OFR) during the BEACHON-RoMBAS (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics &amp; Nitrogen–Rocky Mountain Biogenic Aerosol Study) campaign to study biogenic secondary organic aerosol (SOA) formation and organic aerosol (OA) aging. A wide range of equivalent atmospheric photochemical ages was sampled, from hours up to days (for O3 and NO3) or weeks (for OH). Ambient air processed by the OFR was typically sampled every 20–30 min, in order to determine how the availability of SOA precursor gases in ambient air changed with diurnal and synoptic conditions, for each of the three oxidants. More SOA was formed during nighttime than daytime for all three oxidants, indicating that SOA precursor concentrations were higher at night. At all times of day, OH oxidation led to approximately 4 times more SOA formation than either O3 or NO3 oxidation. This is likely because O3 and NO3 will only react with gases containing C=C bonds (e.g., terpenes) to form SOA, but won’t react appreciably with many of their oxidation products or any species in the gas phase that lacks a C=C bond (e.g., pinonic acid, alkanes). In contrast, OH can continue to react with compounds that lack C=C bonds to produce SOA. Closure was achieved between the amount of SOA formed from O3 and NO3 oxidation in the OFR and the SOA predicted to form from measured concentrations of ambient monoterpenes and sesquiterpenes using published chamber yields. This is in contrast to previous work at this site (Palm et al., 2016), which has shown that a source of SOA from semi- and intermediate-volatility organic compounds (S/IVOCs) 3.4 times larger than the source from measured VOCs is needed to explain the measured SOA formation from OH oxidation. This work suggests that those S/IVOCs typically do not contain C=C bonds. O3 and NO3 oxidation produced SOA with elemental O:C and H:C similar to the least oxidized OA observed in local ambient air, and neither oxidant led to net mass loss at the highest exposures, in contrast with OH oxidation. An OH exposure in the OFR equivalent to several hours of atmospheric aging also produced SOA with O:C and H:C values similar to ambient OA, while higher aging (days–weeks) led to formation of SOA with progressively higher O:C and lower H:C (and net mass loss at the highest exposures). NO3 oxidation led to the production of particulate organic nitrates (pRONO2), while OH and O3 oxidation (under low NO) did not, as expected. These measurements of SOA formation provide the first direct comparison of SOA formation potential and chemical evolution from OH, O3 and NO3 oxidation in the real atmosphere, and help to clarify the oxidation processes that lead to SOA formation from biogenic hydrocarbons.

scholarly journals In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor

2015 ◽  
Vol 15 (21) ◽  
pp. 30409-30471 ◽  
Author(s):  
B. B. Palm ◽  
P. Campuzano-Jost ◽  
A. M. Ortega ◽  
D. A. Day ◽  
L. Kaser ◽  
...  
Keyword(s):  
Organic Compounds ◽  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Ambient Air ◽  
Organic Aerosol ◽  
Pine Forest ◽  
Oxidation Products ◽  
Oh Radicals ◽  
Aerosol Formation ◽  
Photochemical Aging

Abstract. Ambient air was oxidized by OH radicals in an oxidation flow reactor (OFR) located in a montane pine forest during the BEACHON-RoMBAS campaign to study biogenic secondary organic aerosol (SOA) formation and aging. High OH concentrations and short residence times allowed for semi-continuous cycling through a large range of OH exposures ranging from hours to weeks of equivalent (eq.) atmospheric aging. A simple model is derived and used to account for the relative time scales of condensation of low volatility organic compounds (LVOCs) onto particles, condensational loss to the walls, and further reaction to produce volatile, non-condensing fragmentation products. More SOA production was observed in the OFR at nighttime (average 4 μg m-3 when LVOC fate corrected) compared to daytime (average 1 μg m-3 when LVOC fate corrected), with maximum formation observed at 0.4–1.5 eq. days of photochemical aging. SOA formation followed a similar diurnal pattern to monoterpenes, sesquiterpenes, and toluene + p-cymene concentrations, including a substantial increase just after sunrise at 07:00 LT. Higher photochemical aging (> 10 eq. days) led to a decrease in new SOA formation and a loss of preexisting OA due to heterogeneous oxidation followed by fragmentation and volatilization. When comparing two different commonly used methods of OH production in OFRs (OFR185 and OFR254), similar amounts of SOA formation were observed. We recommend the OFR185 mode for future forest studies. Concurrent gas-phase measurements of air after OH oxidation illustrate the decay of primary VOCs, production of small oxidized organic compounds, and net production at lower ages followed by net consumption of terpenoid oxidation products as photochemical age increased. New particle formation was observed in the reactor after oxidation, especially during times when precursor gas concentrations and SOA formation were largest. Approximately 6 times more SOA was formed in the reactor from OH oxidation than could be explained by the VOCs measured in ambient air. Several recently-developed instruments quantified ambient semi- and intermediate-volatility organic compounds (S/IVOCs) that were not detected by a PTR-TOF-MS. An SOA yield of 24–80 % from those compounds can explain the observed SOA, suggesting that these typically unmeasured S/IVOCs play a substantial role in ambient SOA formation. Our results allow ruling out condensation sticking coefficients much lower than 1. Our measurements help clarify the magnitude of SOA formation in forested environments, and demonstrate methods for interpretation of ambient OFR measurements.

scholarly journals The effect of particle acidity on secondary organic aerosol formation from α-pinene photooxidation under atmospherically relevant conditions

2016 ◽  
Author(s):  
Yuemei Han ◽  
Craig A. Stroud ◽  
John Liggo ◽  
Shao-Meng Li
Keyword(s):  
Secondary Organic Aerosol ◽  
Chemical Properties ◽  
Organic Aerosol ◽  
Reaction Chamber ◽  
Oxidation Products ◽  
Strong Increase ◽  
Nitrate Content ◽  
Organic Nitrate ◽  
Organic Nitrates ◽  
Aerosol Formation

Abstract. Secondary organic aerosol (SOA) formation from OH-initiated photooxidation of α-pinene has been investigated in a photochemical reaction chamber under varied particle acidity levels. The effect of particle acidity on SOA yield and chemical composition was examined under high- and low-NOx conditions. The SOA yield (4.0 %–7.3 %) increased nearly linearly with the increase in particle acidity under high-NOx conditions. In contrast, the SOA yield (27.9 %–35.6 %) was substantially higher under low-NOx conditions, but its dependency on particle acidity was insignificant. A relatively strong increase in SOA yield (up to 220 %) was observed in the first hour of α-pinene photooxidation under high-NOx conditions, suggesting that SOA formation was more effective for early α-pinene oxidation products in the presence of fresh acidic particles. The SOA yield decreased gradually with the increase in organic mass under high-NOx conditions, which is likely due to the inaccessibility of the acidity over time with the coating of α-pinene SOA. The formation of later-generation SOA was enhanced by particle acidity even under low-NOx conditions when introducing acidic seed particles after α-pinene photooxidation. The fraction of oxygen-containing organic fragments (CxHyO1+ 33–35 % and CxHyO2+ 16–17 %) in the total organics and the O/C ratio (0.49–0.54) of α-pinene SOA were lower under high-NOx conditions than those under low-NOx conditions (39–40 %, 17–19 %, and 0.60–0.62), suggesting that α-pinene SOA was less oxygenated in the studied high-NOx conditions. The fraction of nitrogen-containing organic fragments (CxHyNz+ and CxHyOzNp+) in the total organics was enhanced with the increases in particle acidity under high-NOx conditions, indicating that organic nitrates may be formed heterogeneously through a mechanism catalyzed by particle acidity. The results of this study suggest that inorganic acidity have a significant role to play in determining various organic aerosol chemical properties such as oxidation state, mass yields and organic nitrate content. It is also an important parameter in the modeling of SOA, which is further dependent on the time scale of SOA formation. Additional research is required to understand the complex physical and chemical interactions facilitated by aerosol acidity.

scholarly journals Rate constant and secondary organic aerosol formation from the gas-phase reaction of eugenol with hydroxyl radicals

2019 ◽  
Vol 19 (3) ◽  
pp. 2001-2013 ◽  
Author(s):  
Changgeng Liu ◽  
Yongchun Liu ◽  
Tianzeng Chen ◽  
Jun Liu ◽  
Hong He
Keyword(s):  
Rate Constant ◽  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Relative Rate ◽  
Organic Aerosol ◽  
Organic Nitrates ◽  
Phase Reaction ◽  
Aerosol Formation ◽  
Product Model ◽  
Formation Potential

Abstract. Methoxyphenols are an important organic component of wood-burning emissions and considered to be potential precursors of secondary organic aerosol (SOA). In this work, the rate constant and SOA formation potential for the OH-initiated reaction of 4-allyl-2-methoxyphenol (eugenol) were investigated for the first time in an oxidation flow reactor (OFR). The rate constant was 8.01±0.40×10-11 cm3 molecule−1 s−1, determined by the relative rate method. The SOA yield first increased and then decreased as a function of OH exposure and was also dependent on eugenol concentration. The maximum SOA yields (0.11–0.31) obtained at different eugenol concentrations could be expressed well by a one-product model. The carbon oxidation state (OSC) increased linearly and significantly as OH exposure rose, indicating that a high oxidation degree was achieved for SOA. In addition, the presence of SO2 (0–198 ppbv) and NO2 (0–109 ppbv) was conducive to increasing SOA yield, for which the maximum enhancement values were 38.6 % and 19.2 %, respectively. The N∕C ratio (0.032–0.043) indicated that NO2 participated in the OH-initiated reaction, subsequently forming organic nitrates. The results could be helpful for further understanding the SOA formation potential from the atmospheric oxidation of methoxyphenols and the atmospheric aging process of smoke plumes from biomass burning emissions.

scholarly journals The effect of particle acidity on secondary organic aerosol formation from <i>α</i>-pinene photooxidation under atmospherically relevant conditions

2016 ◽  
Vol 16 (21) ◽  
pp. 13929-13944 ◽  
Author(s):  
Yuemei Han ◽  
Craig A. Stroud ◽  
John Liggio ◽  
Shao-Meng Li
Keyword(s):  
Gas Phase ◽  
Secondary Organic Aerosol ◽  
Organic Aerosol ◽  
Oxidation Products ◽  
Strong Increase ◽  
Nitrate Content ◽  
Organic Nitrate ◽  
Organic Nitrates ◽  
Particle Phase ◽  
Aerosol Formation

Abstract. Secondary organic aerosol (SOA) formation from photooxidation of α-pinene has been investigated in a photochemical reaction chamber under varied inorganic seed particle acidity levels at moderate relative humidity. The effect of particle acidity on SOA yield and chemical composition was examined under high- and low-NOx conditions. The SOA yield (4.2–7.6 %) increased nearly linearly with the increase in particle acidity under high-NOx conditions. In contrast, the SOA yield (28.6–36.3 %) was substantially higher under low-NOx conditions, but its dependency on particle acidity was insignificant. A relatively strong increase in SOA yield (up to 220 %) was observed in the first hour of α-pinene photooxidation under high-NOx conditions, suggesting that SOA formation was more effective for early α-pinene oxidation products in the presence of fresh acidic particles. The SOA yield decreased gradually with the increase in organic mass in the initial stage (approximately 0–1 h) under high-NOx conditions, which is likely due to the inaccessibility to the acidity over time with the coating of α-pinene SOA, assuming a slow particle-phase diffusion of organic molecules into the inorganic seeds. The formation of later-generation SOA was enhanced by particle acidity even under low-NOx conditions when introducing acidic seed particles after α-pinene photooxidation, suggesting a different acidity effect exists for α-pinene SOA derived from later oxidation stages. This effect could be important in the atmosphere under conditions where α-pinene oxidation products in the gas-phase originating in forested areas (with low NOx and SOx) are transported to regions abundant in acidic aerosols such as power plant plumes or urban regions. The fraction of oxygen-containing organic fragments (CxHyO1+ 33–35 % and CxHyO2+ 16–17 %) in the total organics and the O ∕ C ratio (0.52–0.56) of α-pinene SOA were lower under high-NOx conditions than those under low-NOx conditions (39–40, 17–19, and 0.61–0.64 %), suggesting that α-pinene SOA was less oxygenated in the studied high-NOx conditions. The fraction of nitrogen-containing organic fragments (CxHyNz+ and CxHyOzNp+) in the total organics was enhanced with the increases in particle acidity under high-NOx conditions, indicating that organic nitrates may be formed heterogeneously through a mechanism catalyzed by particle acidity or that acidic conditions facilitate the partitioning of gas-phase organic nitrates into particle phase. The results of this study suggest that inorganic acidity has a significant role to play in determining various organic aerosol chemical properties such as mass yields, oxidation state, and organic nitrate content. The acidity effect being further dependent on the timescale of SOA formation is also an important parameter in the modeling of SOA.

scholarly journals Real-time measurements of secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor in the Los Angeles area

2015 ◽  
Vol 15 (15) ◽  
pp. 21907-21958 ◽  
Author(s):  
A. M. Ortega ◽  
P. L. Hayes ◽  
Z. Peng ◽  
B. B. Palm ◽  
W. Hu ◽  
...  
Keyword(s):  
Los Angeles ◽  
Real Time ◽  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Vertical Mixing ◽  
Ambient Air ◽  
Organic Aerosol ◽  
Aerosol Formation ◽  
Scanning Mobility Particle Sizer ◽  
Heterogeneous Oxidation

Abstract. Field studies in polluted areas over the last decade have observed large formation of secondary organic aerosol (SOA) that is often poorly captured by models. The study of SOA formation using ambient data is often confounded by the effects of advection, vertical mixing, emissions, and variable degrees of photochemical aging. An Oxidation Flow Reactor (OFR) was deployed to study SOA formation in real-time during the CalNex campaign in Pasadena, CA, in 2010. A high-resolution aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) alternated sampling ambient and reactor-aged air. The reactor produced OH concentrations up to 4 orders of magnitude higher than in ambient air, achieving equivalent atmospheric aging from hours up to several weeks in 3 min of processing. OH radical concentration was continuously stepped, obtaining measurements of real-time SOA formation and oxidation at multiple equivalent ages from 0.8 days–6.4 weeks. Enhancement of OA from aging showed a maximum net SOA production between 0.8–6 days of aging with net OA mass loss beyond 2 weeks. Reactor SOA mass peaked at night, in the absence of ambient photochemistry, and correlated with trimethylbenzene concentrations. Reactor SOA formation was inversely correlated with ambient SOA and Ox, which along with the short-lived VOC correlation, indicates the importance of relatively reactive (τOH ∼ 0.3 day) SOA precursors in the LA-Basin. Evolution of the elemental composition in the reactor was similar to trends observed in the atmosphere (O : C vs. H : C slope ∼ -0.65). Oxidation state of carbon (OSC) in reactor SOA increased steeply with age and remained elevated (OSC ∼ 2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (ΔCO, ambient CO above regional background) vs. photochemical age is similar to previous studies at low to moderate ages and also extends to higher ages where OA loss dominates. The mass added at low-to-intermediate ages is due primarily to condensation of oxidized species, not heterogeneous oxidation. The OA decrease at high photochemical ages is dominated by heterogeneous oxidation followed by fragmentation/evaporation. A comparison of urban SOA formation in this study with a similar study of vehicle SOA in a tunnel supports the dominance of vehicle emissions in urban SOA. Pre-2007 SOA models underpredict SOA formation by an order of magnitude, while a more recent model performs better but overpredicts at higher ages. These results demonstrate the value of the reactor as a tool for in situ evaluation of the SOA formation potential and OA evolution from ambient air.

scholarly journals Secondary organic aerosol formation from the β-pinene+NO<sub>3</sub> system: effect of humidity and peroxy radical fate

2015 ◽  
Vol 15 (13) ◽  
pp. 7497-7522 ◽  
Author(s):  
C. M. Boyd ◽  
J. Sanchez ◽  
L. Xu ◽  
A. J. Eugene ◽  
T. Nah ◽  
...  
Keyword(s):  
Secondary Organic Aerosol ◽  
Peroxy Radical ◽  
Organic Aerosol ◽  
Formation Mechanisms ◽  
Organic Nitrate ◽  
Organic Nitrates ◽  
Aerosol Formation ◽  
System Effect ◽  
Aerosol Mass ◽  
Wide Range

Abstract. The formation of secondary organic aerosol (SOA) from the oxidation of β-pinene via nitrate radicals is investigated in the Georgia Tech Environmental Chamber (GTEC) facility. Aerosol yields are determined for experiments performed under both dry (relative humidity (RH) < 2 %) and humid (RH = 50 % and RH = 70 %) conditions. To probe the effects of peroxy radical (RO2) fate on aerosol formation, "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are performed. Gas-phase organic nitrate species (with molecular weights of 215, 229, 231, and 245 amu, which likely correspond to molecular formulas of C10H17NO4, C10H15NO5, C10H17NO5, and C10H15NO6, respectively) are detected by chemical ionization mass spectrometry (CIMS) and their formation mechanisms are proposed. The NO+ (at m/z 30) and NO2+ (at m/z 46) ions contribute about 11 % to the combined organics and nitrate signals in the typical aerosol mass spectrum, with the NO+ : NO2+ ratio ranging from 4.8 to 10.2 in all experiments conducted. The SOA yields in the "RO2 + NO3 dominant" and "RO2 + HO2 dominant" experiments are comparable. For a wide range of organic mass loadings (5.1–216.1 μg m−3), the aerosol mass yield is calculated to be 27.0–104.1 %. Although humidity does not appear to affect SOA yields, there is evidence of particle-phase hydrolysis of organic nitrates, which are estimated to compose 45–74 % of the organic aerosol. The extent of organic nitrate hydrolysis is significantly lower than that observed in previous studies on photooxidation of volatile organic compounds in the presence of NOx. It is estimated that about 90 and 10 % of the organic nitrates formed from the β-pinene+NO3 reaction are primary organic nitrates and tertiary organic nitrates, respectively. While the primary organic nitrates do not appear to hydrolyze, the tertiary organic nitrates undergo hydrolysis with a lifetime of 3–4.5 h. Results from this laboratory chamber study provide the fundamental data to evaluate the contributions of monoterpene + NO3 reaction to ambient organic aerosol measured in the southeastern United States, including the Southern Oxidant and Aerosol Study (SOAS) and the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study.

scholarly journals Secondary organic aerosol formation from ambient air in an oxidation flow reactor in central Amazonia

2018 ◽  
Vol 18 (1) ◽  
pp. 467-493 ◽  
Author(s):  
Brett B. Palm ◽  
Suzane S. de Sá ◽  
Douglas A. Day ◽  
Pedro Campuzano-Jost ◽  
Weiwei Hu ◽  
...  
Keyword(s):  
Flow Reactor ◽  
Secondary Organic Aerosol ◽  
Standard Addition ◽  
Ambient Air ◽  
Organic Aerosol ◽  
Aerosol Formation ◽  
Multilinear Regression ◽  
Atmospheric Aging ◽  
Aerosol Mass ◽  
Multilinear Regression Analysis

Abstract. Secondary organic aerosol (SOA) formation from ambient air was studied using an oxidation flow reactor (OFR) coupled to an aerosol mass spectrometer (AMS) during both the wet and dry seasons at the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) field campaign. Measurements were made at two sites downwind of the city of Manaus, Brazil. Ambient air was oxidized in the OFR using variable concentrations of either OH or O3, over ranges from hours to days (O3) or weeks (OH) of equivalent atmospheric aging. The amount of SOA formed in the OFR ranged from 0 to as much as 10 µg m−3, depending on the amount of SOA precursor gases in ambient air. Typically, more SOA was formed during nighttime than daytime, and more from OH than from O3 oxidation. SOA yields of individual organic precursors under OFR conditions were measured by standard addition into ambient air and were confirmed to be consistent with published environmental chamber-derived SOA yields. Positive matrix factorization of organic aerosol (OA) after OH oxidation showed formation of typical oxidized OA factors and a loss of primary OA factors as OH aging increased. After OH oxidation in the OFR, the hygroscopicity of the OA increased with increasing elemental O:C up to O:C∼1.0, and then decreased as O:C increased further. Possible reasons for this decrease are discussed. The measured SOA formation was compared to the amount predicted from the concentrations of measured ambient SOA precursors and their SOA yields. While measured ambient precursors were sufficient to explain the amount of SOA formed from O3, they could only explain 10–50 % of the SOA formed from OH. This is consistent with previous OFR studies, which showed that typically unmeasured semivolatile and intermediate volatility gases (that tend to lack C=C bonds) are present in ambient air and can explain such additional SOA formation. To investigate the sources of the unmeasured SOA-forming gases during this campaign, multilinear regression analysis was performed between measured SOA formation and the concentration of gas-phase tracers representing different precursor sources. The majority of SOA-forming gases present during both seasons were of biogenic origin. Urban sources also contributed substantially in both seasons, while biomass burning sources were more important during the dry season. This study enables a better understanding of SOA formation in environments with diverse emission sources.

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