Secondary organic aerosol (SOA), particulate matter composed of compounds formed from the atmospheric transformation of organic species, accounts for a substantial fraction of tropospheric aerosol. ...The formation of low-volatility (semivolatile and possibly nonvolatile) compounds that make up SOA is governed by a complex series of reactions of a large number of organic species, so the experimental characterization and theoretical description of SOA formation presents a substantial challenge. In this review we outline what is known about the chemistry of formation and continuing transformation of low-volatility species in the atmosphere. The primary focus is chemical processes that can change the volatility of organic compounds: (1) oxidation reactions in the gas phase, (2) reactions in the particle phase, and (3) continuing chemistry (in either phase) over several generations. Gas-phase oxidation reactions can reduce volatility by the addition of polar functional groups or increase it by the cleavage of carbon–carbon bonds; key branch points that control volatility are the initial attack of the oxidant, reactions of alkylperoxy (RO2) radicals, and reactions of alkoxy (RO) radicals. Reactions in the particle phase include oxidation reactions as well as accretion reactions, non-oxidative processes leading to the formation of high-molecular-weight species. Organic carbon in the atmosphere is continually subject to reactions in the gas and particle phases throughout its atmospheric lifetime (until lost by physical deposition or oxidized to CO or CO2), implying continual changes in volatility over the timescales of several days. The volatility changes arising from these chemical reactions must be parameterized and included in models in order to gain a quantitative and predictive understanding of SOA formation.
Low-cost sensors for measuring particulate matter (PM) offer the ability to understand human exposure to air pollution at spatiotemporal scales that have previously been impractical. However, such ...low-cost PM sensors tend to be poorly characterized, and their measurements of mass concentration can be subject to considerable error. Recent studies have investigated how individual factors can contribute to this error, but these studies are largely based on empirical comparisons and generally do not examine the role of multiple factors simultaneously. Here, we present a new physics-based framework and open-source software package (
) for evaluating the ability of low-cost optical particle sensors (optical particle counters and nephelometers) to accurately characterize the size distribution and/or mass loading of aerosol particles. This framework, which uses Mie theory to calculate the response of a given sensor to a given particle population, is used to estimate the fractional error in mass loading for different sensor types given variations in relative humidity, aerosol optical properties, and the underlying particle size distribution. Results indicate that such error, which can be substantial, is dependent on the sensor technology (nephelometer vs. optical particle counter), the specific parameters of the individual sensor, and differences between the aerosol used to calibrate the sensor and the aerosol being measured. We conclude with a summary of likely sources of error for different sensor types, environmental conditions, and particle classes and offer general recommendations for the choice of calibrant under different measurement scenarios.
Relationships between various optical, physical, and chemical properties of
biomass-combustion-derived particles are characterized for particles
produced in the laboratory from a wide range of fuels ...and burn conditions.
The modified combustion efficiency (MCE), commonly used to parameterize
biomass particle emissions and properties, is shown to generally have weak
predictive capabilities, especially for more efficient combustion
conditions. There is, however, a strong relationship between many intensive
optical properties (e.g., single-scatter albedo, Ångström absorption
exponent, mass absorption efficiency) and the organic aerosol-to-black
carbon (OA ∕ BC) mass ratio over a wider range than previously considered
(0.3 to 105). The properties of brown carbon (BrC, i.e., light-absorbing organic carbon) also vary with OA ∕ BC. Coating-induced enhancements
(i.e., “lensing” effects) contribute only a minor amount to BC absorption
for all of the burns despite some burns producing particles having large
ensemble-average coating-to-core mass ratios. The BC–OA mixing state varies
strongly with OA ∕ BC; the fraction of OA that is internally mixed with BC
decreases with OA ∕ BC while the relative amount of OA coated on BC
increases. In contrast, there is little relationship between many OA bulk
chemical properties and OA ∕ BC, with the O : C and H : C atomic ratios and
the relative abundance of a key marker ion (m/z=60, linked to
levoglucosan) all showing no dependence on OA ∕ BC. In contrast, both the
organic nitrate fraction of OA and the OA volatility do depend on the
OA ∕ BC. Neither the total particle nor BC-specific size distributions
exhibit any clear dependence on the burn conditions or OA ∕ BC, although
there is perhaps a dependence on fuel type. Overall, our results expand on
existing knowledge to contribute new understanding of the properties of
particles emitted from biomass combustion.
Atmospheric oxidation reactions are known to affect the chemical composition of organic aerosol (OA) particles over timescales of several days, but the details of such oxidative aging reactions are ...poorly understood. In this study we examine the rates and products of a key class of aging reaction, the heterogeneous oxidation of particle-phase organic species by the gas-phase hydroxyl radical (OH). We compile and reanalyze a number of previous studies from our laboratories involving the oxidation of single-component organic particles. All kinetic and product data are described on a common basis, enabling a straightforward comparison among different chemical systems and experimental conditions. Oxidation chemistry is described in terms of changes to key ensemble properties of the OA, rather than to its detailed molecular composition, focusing on two quantities in particular, the amount and the oxidation state of the particle-phase carbon. Heterogeneous oxidation increases the oxidation state of particulate carbon, with the rate of increase determined by the detailed chemical mechanism. At the same time, the amount of particle-phase carbon decreases with oxidation, due to fragmentation (C–C scission) reactions that form small, volatile products that escape to the gas phase. In contrast to the oxidation state increase, the rate of carbon loss is nearly uniform among most systems studied. Extrapolation of these results to atmospheric conditions indicates that heterogeneous oxidation can have a substantial effect on the amount and composition of atmospheric OA over timescales of several days, a prediction that is broadly in line with available measurements of OA evolution over such long timescales. In particular, 3–13% of particle-phase carbon is lost to the gas phase after one week of heterogeneous oxidation. Our results indicate that oxidative aging represents an important sink for particulate organic carbon, and more generally that fragmentation reactions play a major role in the lifecycle of atmospheric OA.
Biomass burning is an important source of aerosol and
trace gases to the atmosphere, but how these emissions change chemically
during their lifetimes is not fully understood. As part of the Fire
...Influence on Regional and Global Environments Experiment (FIREX 2016), we
investigated the effect of photochemical aging on biomass burning organic
aerosol (BBOA) with a focus on fuels from the western United States.
Emissions were sampled into a small (150 L) environmental chamber and
photochemically aged via the addition of ozone and irradiation by 254 nm
light. While some fraction of species undergoes photolysis, the vast
majority of aging occurs via reaction with OH radicals, with total OH
exposures corresponding to the equivalent of up to 10 d of atmospheric
oxidation. For all fuels burned, large and rapid changes are seen in the
ensemble chemical composition of BBOA, as measured by an aerosol mass
spectrometer (AMS). Secondary organic aerosol (SOA) formation is seen for
all aging experiments and continues to grow with increasing OH exposure, but
the magnitude of the SOA formation is highly variable between experiments.
This variability can be explained well by a combination of differences in OH
exposure and the total concentration of non-methane organic gases (NMOGs) in
the chamber before oxidation, as measured by PTR-ToF-MS (r2 values from
0.64 to 0.83). From this relationship, we calculate the fraction of carbon
from biomass burning NMOGs that is converted to SOA as a function of
equivalent atmospheric aging time, with carbon yields ranging from 24±4 % after 6 h to 56±9 % after 4 d.
Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the ...photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH <20%) environmental chamber experiments. The modeling is supported by (i) new oxidation chamber measurements, (ii) detailed concurrent measurements of SOA precursors in biomass-burning emissions, and (iii) development of SOA parameters for heterocyclic and oxygenated aromatic compounds based on historical chamber experiments. We find that oxygenated aromatic compounds, including phenols and methoxyphenols, account for slightly less than 60% of the SOA formed and help our model explain the variability in the organic aerosol mass (R 2 = 0.68) and O/C (R 2 = 0.69) enhancement ratios observed across 11 chamber experiments. Despite abundant emissions, heterocyclic compounds that included furans contribute to ∼20% of the total SOA. The use of pyrolysis-temperature-based or averaged emission profiles to represent SOA precursors, rather than those specific to each fire, provide similar results to within 20%. Our findings demonstrate the necessity of accounting for oxygenated aromatics from biomass-burning emissions and their SOA formation in chemical mechanisms.
Nitrogen oxides (NO x ) play a key role in regulating the oxidizing capacity of the atmosphere through controlling the abundance of O3, OH, and other important gas and particle species. Some recent ...studies have suggested that particulate nitrate, which is conventionally considered as the ultimate oxidation product of NO x , can undergo “renoxification” via photolysis, recycling NO x and HONO back to the gas phase. However, there are large discrepancies in estimates of the importance of this channel, with reported renoxification rate constants spanning three orders of magnitude. In addition, previous laboratory studies derived the rate constant using bulk particle samples collected on substrates instead of suspended particles. In this work, we study renoxification of suspended submicron particulate sodium and ammonium nitrate through controlled laboratory photolysis experiments using an environmental chamber. We find that, under atmospherically relevant wavelengths and relative humidities, particulate inorganic nitrate releases NO x and HONO less than 10 times as rapidly as gaseous nitric acid, putting our measurements on the low end of recently reported renoxification rate constants. To the extent that our laboratory conditions are representative of the real atmosphere, renoxification from the photolysis of inorganic particulate nitrate appears to play a limited role in contributing to the NO x and OH budgets in remote environments. These results are based on simplified model systems; future studies should investigate renoxification of more complex aerosol mixtures that represent a broader spectrum of aerosol properties to better constrain the photolysis of ambient aerosols.
Secondary organic aerosol (SOA) formation proceeds by bimolecular gas-phase oxidation reactions generating species that are sufficiently low in volatility to partition into the condensed phase. ...Advances in instrumentation have revealed that atmospheric SOA is less volatile and more oxidized than can be explained solely by these well-studied gas-phase oxidation pathways, supporting the role of additional chemical processes. These processesautoxidation, accretion, and organic salt formationcan lead to exceedingly low-volatility species that recently have been identified in laboratory and field studies. Despite these new insights, the identities of the condensing species at the molecular level and the relative importance of the various formation processes remain poorly constrained. The thermodynamics of autoxidation, accretion, and organic salt formation can be described by equilibrium partitioning theory; a framework for which is presented here. This framework will facilitate the inclusion of such processes in model representations of SOA formation.
Photolytic aging has been proposed as a major mass loss mechanism for atmospheric secondary organic aerosol (SOA). However, estimated mass loss rates vary by orders of magnitude, and their impacts on ...modeled SOA loadings and properties are highly uncertain. In this study, photolysis rates and composition changes of α-pinene SOA are analyzed in situ over multiple days in an environmental chamber. After an initial exponential decay (τ ∼ 22 h), the mass loss rate slows dramatically, with more than ∼70–90% of the SOA particulate mass undergoing an essentially negligible photolytic degradation. Scaled to ambient conditions, SOA undergoes rapid photolysis over only its first day in the atmosphere; beyond this, the remaining SOA is photo-recalcitrant, and photolysis ceases to be a major sink compared to wet deposition time scales. Thus, extrapolation of the initial photolysis loss rate to the entire aerosol mass may significantly overestimate the role of photolysis in the removal of atmospheric SOA.
An oxidation flow reactor (OFR) is a vessel inside which the concentration of a chosen oxidant can be increased for the purpose of studying SOA formation and aging by that oxidant. During the ...BEACHON-RoMBAS (Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H2O, Organics & Nitrogen–Rocky Mountain Biogenic Aerosol Study) field campaign, ambient pine forest air was oxidized by OH radicals in an OFR to measure the amount of SOA that could be formed from the real mix of ambient SOA precursor gases, and how that amount changed with time as precursors changed. High OH concentrations and short residence times allowed for semicontinuous 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 timescales 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 3 µg m−3 when LVOC fate corrected) compared to daytime (average 0.9 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 local time. 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 OFR254-70), 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 4.4 times more SOA was formed in the reactor from OH oxidation than could be explained by the VOCs measured in ambient air. To our knowledge this is the first time that this has been shown when comparing VOC concentrations with SOA formation measured at the same time, rather than comparing measurements made at different times. Several recently developed instruments have quantified ambient semivolatile and intermediate-volatility organic compounds (S/IVOCs) that were not detected by a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS). An SOA yield of 18–58 % from those compounds can explain the observed SOA formation. S/IVOCs were the only pool of gas-phase carbon that was large enough to explain the observed SOA formation. This work suggests that these typically unmeasured gases play a substantial role in ambient SOA formation. Our results allow ruling out condensation sticking coefficients much lower than 1. These measurements help clarify the magnitude of potential SOA formation from OH oxidation in forested environments and demonstrate methods for interpretation of ambient OFR measurements.
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