Ο-xylene is an important aromatic volatile organic compound (VOC) in the atmosphere over urban areas. In this work, the effect of nitrogen dioxide (NO2) concentration and relative humidity (RH) on ...the mass concentration of secondary organic aerosols (SOA) formed from ο-xylene OH oxidization was investigated in a photooxidation chamber. The ο-xylene SOA mass concentration increased from 54.2 μg m−3 to 127.2 μg m−3 during dry conditions, but decreased from 177.7 μg m−3 to 146.5 μg m−3 during high RH conditions when the initial NO2 concentration increased form 0 ppbv to about 900 ppbv. An increase in the ratio of NO3−/Org and a decrease in the oxidation state of carbon (OSC) of SOA suggested that acid-catalyzed heterogeneous reaction was responsible for enhancing SOA formation with increasing NO2 concentrations in dry conditions. In contrast, in humid conditions, the high molecular diffusion capacity of SOA could promote the reactivity of OH towards the interior of SOA, and the enhancement of nitrous acid (HONO) formation under high NO2 conditions could promote the SOA aging processes and be responsible for the decreasing trend of SOA formation with NO2. Light absorption by SOA was also measured, and both NO2 and RH enhanced the mass absorption coefficient (MACλ = 365 nm) value for the optical properties of ο-xylene SOA. The highest MACλ = 365 nm value of ο-xylene SOA was 0.89 m2 g−1, observed during humid conditions with an initial NO2 concentration of 862 ppbv, which was 3.9 times higher than in the experiment conducted in the absence of NO2 under dry conditions. The formation of nitrogen-containing organic compounds (NOCs) and humic-like substances (HULIS) were responsible for the increased MACλ = 365 nm values of ο-xylene derived SOA. This study provides new insight into the effect of NO2 on SOA formation through the change in ο-xylene photooxidation under different RH conditions, and the complex effect of multiple environmental factors on SOA formation was also important and should not be ignored.
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•A chamber study of the compound effect of NO2 and RH on the ο-xylene SOA formation through OH-oxidation.•The evolution characteristics of the ο-xylene SOA chemical compositions were studied by the HR-ToF-AMS.•The ο-xylene SOA mass concentration increased with initial NO2 concentration at dry condition, but decreased at high RH condition.•Both the increasing NO2 concentration and RH condition is beneficial to increase the MAC value of ο-xylene-derived SOA.
To quantify optical and chemical properties of PM2.5 brown carbon (BrC) in Xi'an, 58 high-volume ambient PM2.5 samples were collected during 2 November 2009 to 13 October 2010. Mass concentrations of ...chemical components were determined, including water-soluble ions, water-soluble organic carbon, levoglucosan, organic carbon (OC), and element carbon (EC). BrC, as an unidentified and wavelength-dependent organic compound, was also measured from water-soluble carbon (WSOC) at 340 nm using UV–vis spectrometer. The wavelength-dependent absorption coefficient (babs) and mass absorption coefficient (MAC) were much abundant at 340 nm, and the high Absorption Ångström coefficient (AAC) values were observed around 5.4, corresponding to the existence of BrC in ambient PM2.5, especially in winter. Good correlations (R > 0.60) between babs and biomass burning markers, such as levoglucosan and K+, in winter indicated significant amounts of primary BrC from biomass burning emissions. Secondary organic carbon BrC (SOCBrC) was more abundant in winter than in summer. SOCBrC in winter was mainly fresh SOC formed from aqueous phase reactions while in summer, aged SOC from photo-chemical formation. Source profiles of BrC optical parameters were detected, which verified sources of BrC from biomass burning and coal burning emissions in areas surrounding Xi'an. The rapidly decreasing babs-340nm values from biomass burning smoldering to straw pellet burning suggested that burning straw pellet instead of burning straw directly is an effective measure for reducing BrC emissions.
•Light absorption of brown carbon (BrC) in PM2.5 was investigated.•Biomass burning is an important source for winter BrC.•Secondary BrC was mainly fresh SOC in winter and aged one in summer.
In this study, both PM2.5 and size-resolved source samples were collected from a “heated kang” and an advanced stove to investigate the optical properties of brown carbon (BrC). The light-absorption ...coefficient (babs), the absorption Ångström exponent (AAE), and the mass absorption cross-section (MAC) of both water and methanol-extracted BrC were investigated. The methanol-extracted BrC (BrCmethanol) had higher light absorption than water-extracted BrC (BrCwater). The value of PM2.5 babs of BrCmethanol at 365nm (babs365,methanol) dramatically decreased from 64,669.8Mm−1 for straw burning in the “heated kang” to 1169.2Mm−1 for maize straw briquettes burning in the advanced stove at the same burning rate. The value of PM2.5 MAC for BrCmethanol at 365nm (MAC365,methanol) decreased from 1.8m2g−1 in the “heated kang” to 1.3m2g−1 in the advanced stove. For smoldering burning in the “heated kang”, babs365,methanol, MAC365,methanol, and K+ showed a unimodal distribution that peaked at sizes <0.4μm. However, the babs365,methanol and MAC365,methanol size distributions of the briquette burning in the advanced stove showed a bimodal pattern, with a large peak at sizes <0.4μm and a minor peak in the size range of 4.7–5.8μm. The babs365,methanol value for sizes <0.4μm (277.4Mm−1) was only 12.3% compared to those obtained from the “heated kang”. The burning rate did not influence the size distribution pattern of either the “heated kang” or the advanced stove. Results from a radiative model show that biomass burning is an important factor for light absorptivity, and the use of an advanced stove can reduce the simple forcing efficiency value by nearly 20% in UV bands compared to the “heated kang”. Our results indicate that changing the combustion style from maize straw smoldering to briquette burning in an advanced stove can effectively reduce BrC emissions during heating seasons in rural areas of Guanzhong Plain.
Simple forcing efficiency of MSOC at UV region (300–400nm) from PM2.5 biomass burning under different combustion conditions. Display omitted
•Optical profiles of BrC in size-resolved PM from biomass burning were investigated.•Different size distribution pattern of BrC optical properties were found between “heated kang” and the advance stove.•The advanced stove was effective on BrC emission reduction compared to “heated kang”.
Filter samples of secondary organic aerosols (SOA) generated from the ozone (O3)- and hydroxyl radical (OH)-initiated oxidation of various biogenic (isoprene, α-pinene, limonene, α-cedrene, ...α-humulene, farnesene, pine leaf essential oils, cedar leaf essential oils) and anthropogenic (tetradecane, 1,3,5-trimethylbenzene, naphthalene) precursors were exposed to humid air containing approximately 100 ppb of gaseous ammonia (NH3). Reactions of SOA compounds with NH3 resulted in production of light-absorbing “brown carbon” compounds, with the extent of browning ranging from no observable change (isoprene SOA) to visible change in color (limonene SOA). The aqueous phase reactions with dissolved ammonium (NH4+) salts, such as ammonium sulfate, were equally efficient in producing brown carbon. Wavelength-dependent mass absorption coefficients (MAC) of the aged SOA were quantified by extracting known amounts of SOA material in methanol and recording its UV/Vis absorption spectra. For a given precursor, the OH-generated SOA had systematically lower MAC compared to the O3-generated SOA. The highest MAC values, for brown carbon from SOA resulting from O3 oxidation of limonene and sesquiterpenes, were comparable to MAC values for biomass burning particles but considerably smaller than MAC values for black carbon aerosols. The NH3/NH4+ + SOA brown carbon aerosol may contribute to aerosol optical density in regions with elevated concentrations of NH3 or ammonium sulfate and high photochemical activity.
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► Secondary organic aerosols change color from white to brown in presence of ammonia. ► Browning reaction occurs for a wide range of biogenic and anthropogenic aerosols. ► Aqueous reaction with ammonium ion is equally efficient in producing brown carbon. ► The mass absorption coefficient is comparable to that of biomass burning aerosols. ► Secondary brown carbon may contribute to absorption of solar radiation by aerosols.
Optical properties of incipient soot Minutolo, Patrizia; Commodo, Mario; D'Anna, Andrea
Proceedings of the Combustion Institute,
2023, 2023-00-00, Letnik:
39, Številka:
1
Journal Article
Recenzirano
The exact knowledge of the optical properties of soot nanoparticles is fundamental for several aspects including the correct determination of the soot concentration in combustion environments using ...optical diagnostics and the correct estimation of the environmental impact of the emitted particles. Although extensive researches over the years have led to a substantial agreement on the optical properties of mature soot particles, the optical properties of the incipient soot nanoparticles are still uncertain. From the particle inception point to the formation of large and more mature soot particles, the evolution of the optical properties must account for variations due to the size and the physicochemical transformation of the investigated particles. This work aims to determine the refractive index and optical properties of inception particles formed in lightly sooting flames. A previous determination based on in-situ light absorption and scattering measurements is revisited taking advantage of particle size measurements by differential mobility analysis. The spectral dependencies of the optical properties are derived by the Kramers-Krӧnig analysis of the ex-situ VUV-NIR light absorption measurements. Results confirm the strong decrease in the absorptivity in the vis-NIR region of inception particles with unimodal size distribution and d63∼3 nm, and confirm a strong size dependency of soot optical properties. The thermal-optical analysis of the sampled particles shows that the particle mass absorption coefficient also correlates with organic carbon content.
Brown carbon (BrC) plays an important role in global radiative budget but there have been few studies on BrC in Arctic despite rapid warming and increasing wildfires in this region. Here we ...investigate the optical properties of BrC from boreal fires in Alaska summer, with two sets of measurements from PILS-LWCC-TOC (Particle-Into-Liquid-Sampler – Liquid-Waveguide Capillary flow-through optical Cell - Total-Organic-Carbon analyzer) and filter measurements. We show that during intense wildfires, the mass absorption coefficient at 365 nm (MAC365) from water soluble organic carbon (WSOC) remained stable at ∼1 m2 g−1. With all plumes sampled and derived transport time, we show a decrease of MAC365 with plume age, with a shorter photobleaching lifetime (∼11 h) at 365 nm compared to 405 nm (∼20 h). The total absorption by organic aerosols measured from filters at 365 nm is higher than the absorption by WSOC by a factor 2–3, suggesting a dominant role of insoluble organic carbon. Overall BrC dominates absorption in the near-ultraviolet and visible radiation during wildfire season in Alaska summer.
Household cookstove emissions are an important source of carbonaceous aerosols globally. The light-absorbing organic carbon (OC), also termed brown carbon (BrC), from cookstove emissions can impact ...the Earth's radiative balance, but is rarely investigated. In this work, PM2.5 filter samples were collected during combustion experiments with red oak wood, charcoal, and kerosene in a variety of cookstoves mainly at two water boiling test phases (cold start CS, hot start HS). Samples were extracted in methanol and extracts were examined using spectrophotometry. The mass absorption coefficients (MACλ, m2 g−1) at five wavelengths (365, 400, 450, 500, and 550 nm) were mostly inter-correlated and were used as a measurement proxy for BrC. The MAC365 for red oak combustion during the CS phase correlated strongly to the elemental carbon (EC)/OC mass ratio, indicating a dependency of BrC absorption on burn conditions. The emissions from cookstoves burning red oak have an average MACλ 2–6 times greater than those burning charcoal and kerosene, and around 3–4 times greater than that from biomass burning measured in previous studies. These results suggest that residential cookstove emissions could contribute largely to ambient BrC, and the simulation of BrC radiative forcing in climate models for biofuel combustion in cookstoves should be treated specifically and separated from open biomass burning.
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•BrC absorption from wood-cookstove emissions is uniquely measured in this work.•BrC absorption depends on burn conditions only for cold start wood combustion.•BrC absorption from cookstove emissions are much stronger than open biomass burning.
The radiative forcing of light-absorbing OC from cookstove emissions need to be considered in climate models, and should be treated separately from open biomass burning.
•Size and carbon content of various kind of nascent soot particles are analyzed.•Thermo-optical-transmission (TOT) and Raman spectroscopy (RS) are used.•Organic (OC), pyrolytic (PC), and elemental ...(EC) carbon are compared to RS results.•Mass absorption coefficient (MAC) is derived from TOT and is correlated with OC/EC.
Elemental and organic carbon content of flame-generated soot nanoparticles produced under different operating conditions of equivalence ratio and particle residence time is determined by thermo-optical-transmission measurements, while particle carbon structure is characterized by Raman spectroscopy analysis. The sampled soot nanoparticles are produced in slightly-rich, almost bluish, flames and comprise small 2–4 nm nascent soot nanoparticles, often neglected in the primary carbonaceous emissions from combustion systems, and larger 4–15 nm soot particles.
The application of the NIOSH-like protocol for the thermo-optical-transmission analysis allows the separation of the total carbon (TC) into fractions of organic carbon (OC), pyrolytic carbon (PC), and elemental carbon (EC). The lowest amount of elemental carbon is measured in young soot nanoparticles and it is progressively larger in mature soot particles. The organic carbon has the opposite trend and shows that it is the “pyrolyzable” carbon fraction of the young soot nanoparticles that transforms into elemental carbon in the primary soot.
Raman spectroscopy showed consistent results as compared to the thermo-optical-transmission analysis indicating a analogous trend in the graphitization order of the different soot particles. Most importantly, the logarithm of the intensity of the measured photoluminescence background shows a linear dependence with the OC/EC ratio.
Mass absorption coefficients (MAC) were retrieved by the thermo-optical-transmission analysis for the four soot samples; the estimated MAC coefficient ranged from 2 to 6 m2/g, dependently from the relative percentage of EC component. The data are in excellent agreement with currently available literature data and extend the determination of MACs to carbonaceous particles with a larger OC/TC content.
Mass absorption coefficient (MAC) values were measured for secondary organic aerosol (SOA) samples produced by flow tube ozonolysis and smog chamber photooxidation of a wide range of volatile organic ...compounds (VOC), specifically: α-pinene, β-pinene, β-myrcene, d-limonene, farnesene, guaiacol, imidazole, isoprene, linalool, ocimene, p-xylene, 1-methylpyrrole, and 2-methylpyrrole. Both low-NOx and high-NOx conditions were employed during the chamber photooxidation experiments. MAC values were converted into effective molecular absorption cross sections assuming an average molecular weight of 300 g/mol for SOA compounds. The upper limits for the effective photolysis rates of SOA compounds were calculated by assuming unity photolysis quantum yields and convoluting the absorption cross sections with a time-dependent solar spectral flux. A more realistic estimate for the photolysis rates relying on the quantum yield of acetone was also obtained. The results show that condensed-phase photolysis of SOA compounds can potentially occur with effective lifetimes ranging from minutes to days, suggesting that photolysis is an efficient and largely overlooked mechanism of SOA aging.
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•We measured mass absorption coefficients for a number of different types of SOA.•We estimated the rates of condensed-phase photochemical processes in SOA.•These processes can potentially be as fast as oxidation of SOA by gas-phase radicals.
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