Secondary production of brown carbon particles with an agglomerate structure

Brown carbon: An underlying driving force for rapid atmospheric sulfate formation and haze event

Brown carbon (BrC) aerosols impact climate and air quality through light absorption, but their size-resolved characteristics remain unclear. This study employs a novel positive matrix factorization (PMF) approach constrained by light absorption and marker fragment to derive the size distribution of the BrC number concentration and light absorption at high time and size resolutions. Our results show distinct bimodal patterns in the BrC number concentration for the sub-500 nm particles at Mace Head, the west coast of Ireland, with peaks at ~20 nm and 107 nm, attributable to new particle formation (nucleation mode) and subsequent growth processes to the accumulation mode, respectively. Light absorption also exhibited a bimodal distribution, with peaks at 137 nm and increasing values to 484 nm. This difference highlights that the larger particles (e.g. around 484 nm), though fewer in number (~3.6% of the total BrC particles), contribute significantly (~70%) to light absorption due to high mass concentration. The direct solar absorption of BrC relative to black carbon ranges from 1.7% to 4.8%, with a slight increase for particles larger than 100 nm, emphasizing the importance of larger particles in BrC radiative effects. These results offer insights into the size-resolved properties of sub-500 nm BrC, enhancing our understanding of BrC properties and potentially reducing uncertainties in aerosol-radiation interactions.

Abstract. A source-oriented version of the Weather Research and Forecasting model with chemistry (SOWC, hereinafter) was developed. SOWC separately tracks primary particles with different hygroscopic properties rather than instantaneously combining them into an internal mixture. This approach avoids artificially mixing light absorbing black + brown carbon particles with materials such as sulfate that would encourage the formation of additional coatings. Source-oriented particles undergo coagulation and gas-particle conversion, but these processes are considered in a dynamic framework that realistically "ages" primary particles over hours and days in the atmosphere. SOWC more realistically predicts radiative feedbacks from anthropogenic aerosols compared to models that make internal mixing or other artificial mixing assumptions. A three-week stagnation episode (15 December 2000 to 6 January 2001) in the San Joaquin Valley (SJV) during the California Regional PM10 / PM2.5 Air Quality Study (CRPAQS) was chosen for the initial application of the new modeling system. Primary particles emitted from diesel engines, wood smoke, high-sulfur fuel combustion, food cooking, and other anthropogenic sources were tracked separately throughout the simulation as they aged in the atmosphere. Differences were identified between predictions from the source oriented vs. the internally mixed representation of particles with meteorological feedbacks in WRF/Chem for a number of meteorological parameters: aerosol extinction coefficients, downward shortwave flux, planetary boundary layer depth, and primary and secondary particulate matter concentrations. Comparisons with observations show that SOWC predicts particle scattering coefficients more accurately than the internally mixed model. Downward shortwave radiation predicted by SOWC is enhanced by ~1% at ground level chiefly because diesel engine particles in the source-oriented mixture are not artificially coated with material that increases their absorption efficiency. The extinction coefficient predicted by SOWC is reduced by an average of 0.012 km−1 (4.8%) in the SJV with a maximum reduction of ~0.2 km−1. Planetary boundary layer (PBL) height is increased by an average of 5.2 m (1.5%) with a~maximum of ~100 m in the SJV. Particulate matter concentrations predicted by SOWC are 2.23 μg m−3 (3.8%) lower than the average by the internally mixed version of the same model in the SJV because increased solar radiation at the ground increases atmospheric mixing. The changes in predicted meteorological parameters and particle concentrations identified in the current study stem from the mixing state of black carbon. The source-oriented model representation with realistic aging processes predicts that hydrophobic diesel engine particles remain largely uncoated over the +7 day simulation period, while the internal mixture model representation predicts significant accumulation of secondary nitrate and water on diesel engine particles. Similar results will likely be found in any air pollution stagnation episode that is characterized by significant particulate nitrate production. Future work should consider episodes where coatings are predominantly sulfate and/or secondary organic aerosol.

ABSTRACTThis paper is focused on the retrieval of industrial aerosol optical thickness (AOT) and microphysical properties by means of airborne imaging spectroscopy. Industrial emissions generally lead to optically thin plumes requiring an adapted detection method taking into account the weak proportion of particles sought in the atmosphere. To this end, a semi-analytical model combined with the Cluster-Tuned Matched Filter (CTMF) algorithm is presented to characterize those plumes, requiring the knowledge of the soil under the plume. The model allows the direct computation of the at-sensor radiance when a plume is included in the radiative transfer. When applied to industrial aerosol classes as defined in this paper, simulated spectral radiances can be compared to ‘real’ MODTRAN (Moderate Resolution Atmospheric Transmission) radiances using the Spectral Angle Mapper (SAM). On the range from 0.4 to 0.7 µm, for three grounds (water, vegetation, and bright one), SAM scores are lower than 0.043 in the worst case (a both absorbing and scattering particle over a bright ground), and usually lower than 0.025. The darker the ground reflectance is, the more accurate the results are (typically for reflectance lower than 0.3). Concerning AOT retrieval capabilities, with a pre-calculated model for a reference optical thickness of 0.25, we are able to retrieve plume AOT at 550 nm in the range 0.0 to 0.4 with an error usually ranging between 9% and 13%. The first test case is a CASI (Compact Airborne Spectrographic Imager) image acquired over the metallurgical industry of Fos-sur-Mer (France). First results of the use of the model coupled with CTMF algorithm reveal a scattering aerosol plume with particle sizes increasing with the distance from the stack (from detection score of 54% near the stack for particles with a diameter of 0.1 µm, to 69% away from it for 1.0 µm particles). A refinement is made then to estimate more precisely aerosol plume properties, using a multimodal distribution based on the previous results. It leads to find a mixture of sulfate and brown carbon particles with a plume AOT ranging between 0.2 and 0.5. The second test case is an AHS (Airborne Hyperspectral Scanner) image acquired over the petrochemical site of Antwerp (Belgium). The first CTMF application results in detecting a brown carbon aerosol of 0.1 µm mode (detection score is 51%). Refined results show the evolution of the AOT decreasing from 0.15 to 0.05 along the plume for a mixture of brown carbon fine mode and 0.3 µm radius of sulfate aerosol.

A great deal of attention has been paid to brown carbon aerosol in the troposphere because it can both scatter and absorb solar radiation, thus affecting the Earth's climate. However, knowledge of the optical and chemical properties of brown carbon aerosol is still limited. In this study, we have investigated different aspects of the optical properties of brown carbon aerosol that have not been previously explored. These properties include extinction spectroscopy in the mid-infrared region and light scattering at two different visible wavelengths, 532 and 402 nm. A proxy for atmospheric brown carbon aerosol was formed from the aqueous reaction of ammonium sulfate with methylglyoxal. The different optical properties were measured as a function of reaction time for a period of up to 19 days. UV/vis absorption experiments of bulk solutions showed that the optical absorption of aqueous brown carbon solution significantly increases as a function of reaction time in the spectral range from 200 to 700 nm. The analysis of the light scattering data, however, showed no significant differences between ammonium sulfate and brown carbon aerosol particles in the measured scattering phase functions, linear polarization profiles, or the derived real parts of the refractive indices at either 532 or 402 nm, even for the longest reaction times with greatest visible extinction. The light scattering experiments are relatively insensitive to the imaginary part of the refractive index, and it was only possible to place an upper limit of k ≤ 0.01 on the imaginary index values. These results suggest that after the reaction with methylglyoxal the single scattering albedo of ammonium sulfate aerosol is significantly reduced but that the light scattering properties including the scattering asymmetry parameter, which is a measure of the relative amount of forward-to-backward scattering, remain essentially unchanged from that of unprocessed ammonium sulfate. The optical extinction properties in the mid-IR range (800 to 7000 cm(-1)) also showed no significant changes in either the real or the imaginary parts of the refractive indices for brown carbon aerosol particles when compared to ammonium sulfate. Therefore, changes in the optical properties of ammonium sulfate in the mid-IR spectral range due to reaction with methylglyoxal appear to be insignificant. In addition to these measurements, we have characterized additional physicochemical properties of the brown carbon aerosol particles including hygroscopic growth using a tandem-differential mobility analyzer. Compared to ammonium sulfate, brown carbon aerosol particles are found to have lower deliquescence relative humidity (DRH), efflorescence relative humidity (ERH), and hygroscopic growth at the same relative humidities. Overall, our study provides new details of the optical and physicochemical properties of a class of secondary organic aerosol which may have important implications for atmospheric chemistry and climate.

In the Large Aerosol Chamber of the IOA (LAC) in 2020, complex measurements were made of the spectral coefficients of aerosol attenuation at wavelengths 0.45-3.9 microns, angular scattering (nephelometer) and absorption (multi - wave aetalometry) - 0.46, 0.53, 0.59, 0.63 microns in pine combustion fumes. Investigated the variability of the optical and microphysical characteristics of aerosols at the stage of smoke formation, depending on the value of the mass mixing parameter for the modes of flame combustion (generation of black carbon microparticles) and smoldering combustion (brown carbon particles), the total mass of the material. The dynamics of the characteristics of fumes under 2-day aging under dark conditions and under the influence of ultraviolet radiation in the wavelength range of 300-400 nm is analyzed. Aerosol measurements were accompanied by sampling of particles on aerosol filters to determine the concentrations of elemental EC and organic carbon OS in the smoke aerosol by gas chromatography. For the studied pyrolysis and mixed combustion fumes, the spectral dependences of the aerosol attenuation and absorption coefficients, the mutual relationship of the aerosol attenuation, scattering and absorption coefficients are analyzed. Estimates of the variability of the adsorption parameters of Angstrom and the specific mass concentration of brown carbon in the combustion fumes of LGM are obtained, which are important for comparison with the data of satellite sounding of the fumes of Siberian forest fires.

Black carbon (BC) and light-absorbing organic carbon (brown carbon, BrC) play key roles in warming the atmosphere, but the magnitude of their effects remains highly uncertain. Theoretical modelling and laboratory experiments demonstrate that coatings on BC can enhance BC's light absorption, therefore many climate models simply assume enhanced BC absorption by a factor of ∼1.5. However, recent field observations show negligible absorption enhancement, implying models may overestimate BC's warming. Here we report direct evidence of substantial field-measured BC absorption enhancement, with the magnitude strongly depending on BC coating amount. Increases in BC coating result from a combination of changing sources and photochemical aging processes. When the influence of BrC is accounted for, observationally constrained model calculations of the BC absorption enhancement can be reconciled with the observations. We conclude that the influence of coatings on BC absorption should be treated as a source and regionally specific parameter in climate models.

Black (EBC) and Brown (BrC) Carbon are ubiquitous constituents of atmospheric particulate matter that affect people’s health, disrupt ecosystems, and modulate local and global climate. Tracking the local deposition and sources of these aerosol particles is essential to better understanding their multidimensional environmental impact. The main goal of the current study is to measure the absorption coefficient (Babs) of particles within the Planetary Boundary Layer (PBL) of the El Paso (US)–Ciudad Juárez (Mexico) airshed and assess the contribution of black and brown carbon particles to the optical absorption. Measurements were taken during a summer, wildfire, and winter season to evaluate the optical properties of BC and non-volatile BrC. The winter season presented a variation from the background Babs in the late evening hours (3:00 PM to midnight) due to an increase in biomass burning driven by lower temperatures. The wildfire season presents the greatest variation in the Babs from the background absorption due to EBC- and BrC-rich smoke plumes arriving at this region from the US West seasonal wildfires. It was found that the international bridges’ vehicular traffic, waiting time to cross back and forth between both cities, added to other local anthropogenic activities, such as brick kiln emissions in Ciudad Juarez, have created a background of air pollution in this region. These pollutants include carbon monoxide, sulfur dioxide, nitrogen and nitric oxides, coarse and fine particulate matter dominated by BC and BrC. The absorption coefficients due to EBC and BrC of this background constitute what we have called a baseline EBC and BrC. Aided by two photoacoustic Extinctiometers (PAX), operating at 405 nm and 870 nm wavelengths, connected to a 340 °C thermal denuder to remove volatile organics, the optical properties were documented and evaluated to identify the impact of long-range transported emissions from western wildfires. The Single Scattering Albedo and the Absorption Ångstrom exponent were calculated for the winter and summer season. The Angstrom exponent showed a decrease during the wildfire events due to the aging process. The High-Resolution Rapid Refresh Smoke model, HRRR, and the Hybrid Single-Particle Lagrangian Integrated Trajectory model, HYSPLIT, were used to estimate the sources of the particles. In addition, a Vaisala Ceilometer was employed to study the vertical profile of particulate matter within the planetary boundary layer.

Observations from a wintertime and summertime field campaign are used to assess the relationship between black and brown carbon (BC and BrC, respectively) optical properties and particle composition and coating state. The wintertime campaign, in Fresno, CA, was impacted by primary emissions from residential wood burning, secondary organic and inorganic particle formation, and BC from motor vehicles. Two major types of BrC were observed in wintertime. One occurred primarily at night—the result of primary biomass burning emissions. The second was enhanced in daytime and strongly associated with particulate nitrate and the occurrence of fog. The biomass‐burning‐derived BrC absorbed more strongly than the nitrate‐associated BrC but had a weaker wavelength dependence. The wintertime BC‐specific mass absorption coefficient (MACBC) exhibited limited dependence on the ensemble‐average coating‐to‐BC mass ratio (Rcoat‐rBC) at all wavelengths, even up to Rcoat‐rBC of ~5. For the summertime campaign, in Fontana, CA, BC dominated the light absorption, with negligible BrC contribution even after substantial photochemical processing. The summertime MACBC exhibited limited dependence on Rcoat‐rBC, even up to ratios of >10. Based on the four classes of BC‐containing particles identified by Lee et al. (2017, https://doi.org/10.5194/acp‐17‐15055‐2017) for the summertime measurements, the general lack of an absorption enhancement can be partly—although not entirely—attributed to an unequal distribution of coating materials between the BC‐containing particle types. These observations demonstrate that in relatively near‐source environments, even those impacted by strong secondary aerosol production, the ensemble‐average, mixing‐induced absorption enhancement for BC due to coatings can be quite small.

Diurnal and seasonal variation of the PM2.5 apparent particle density in Beijing, China

The apparent particle density of particulate matter with aerodynamic diameter < 2.5 microm (rho2.5) was determined at an urban site in Augsburg, Germany and its correlation with chemical composition and meteorological conditions was investigated. rho2.5 showed strong day-to-day variation from 1.05 to 2.36 g cm(-3) (5 to 95% percentile), and nearly 64% of the daily variability could be explained by a multiple variable regression model. A minimum in the morning and afternoon (about 1.5 g cm(-3)), and a maximum (near 1.8 g cm(-3)) during midday was observed. The minima represent fresh primary aerosol emissions, which were related to traffic soot particles with low density due to their agglomerate structure, especially observed in the early morning hours of weekdays. The maximum is likely due to increased secondary particle production and the presence of more aged particles with the built-up of the convectively mixed boundary layer. rho2.5 has the potential to serve as a crude tracer for chemical composition and atmospheric processing and might play an important role when considering the associations between health effects and ambient particles.

As a typical metal free inorganic semiconductor, graphitic C3N4 (g-C3N4) has attracted intensive attention for H2 generation, pollutant degradation and CO2 reduction. It is well-known that the band gap of g-C3N4 is about 2.7 eV, which can absorb visible light up to 460 nm. Furthermore, the conduction band minimum of g-C3N4 is extremely negative, so photogenerated electrons should have high reduction ability. However, the photocatalytic efficiency of the pure g-C3N4 is limited by the high recombination rate of its photo-generated electron–hole pairs. One of the techniques for increasing the separation efficiency of photo-generated electron–hole pairs is to load a nonmetal element into a photocatalyst. For the increase of the photocatalytic activity, an extensively studied carbonaceous candidate is carbon particles with tunable size and band gap, abundant hydrophilic surface groups, and long term stability. Particularly, carbon particles can act as spectral converters to overcome the contradiction between optical absorption and chemical reaction dynamics of various semiconductors with UV and visible light activity because of its large cross-section of upconversion absorption and a wide range of emission spectra that boost the energy band-gap matching. In this study, novel carbon particles and g-C3N4 composite photocatalysts were prepared through a facile two-step calcination using carbon particles and melamine as a starting material. The synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), nitrogen-sorption, UV-Vis diffuse reflectance spectra (DRS), photoluminescence spectra (PL), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Photocatalytic activity of carbon particles modified g-C3N4 (C/g-CN) was evaluated by photocatalytic hydrogen production under visible irradiation (λ ≧ 420 nm). Firstly, 3.6 g of glucose was dissolved in distilled water (40 mL) to form a clear solution, then 40 mL of concentrated HCl solution was dropped into the solution of glucose. Afterward, the mixture solution was transferred into a 100 mL Teflon liner followed by hydrothermal treatment at 180 ℃ for 6 h. After hydrothermal reaction, the black precipitates were centrifuged and get brown carbon particles solution. Carbon particles and g-CN composite photocatalysts were prepared by two step calcination of the mixture of carbon particles and melamine. Before calcination, 5 mL of carbon particles suspension in ethanol was added into 3g of melamine, then ethanol was evaporated. This obtained ingredient was applied two times calcination, first heating at 550 ℃ for 3h, second heating at 500 ℃ for 10h. Physicochemical measurement of the samples were characterized by XRD, FT-IR, XPS, nitrogen-sorption, DRS, PL, SEM, and TEM. The pyrex column vessel reactor (inner volume: 123 mL) was used for the photocatalytic H2 production from aqueous solution (40 mL) containing 10 vol% triethanolamine (TEA) as a sacrificial donor. 2 wt% Pt loaded on the surface of the photocatalyst by the in situ photodeposition method using H2PtCl6. Before irradiation, N2 gas was bubbled into the reaction solution for 30 min to remove a dissolved O2. Typically, 40 mg of the photocatalyst were added into the reaction solution. A 300 W Xe lamp with a UV cut filter (λ<420 nm) was applied as a light source. The concentration of H2 production from the aqueous TEA solution was analyzed by GC with TCD. The rate of hydrogen production for pure g-CN was approximately 7.9 μmol/g・h. After modified carbon particles, photocatalytic performance was higher. The highest photocatalytic hydrogen evolution rate for 0.5 wt% C/g-CN was approximately 340 μmol/g・h, which was about 43 times faster than that of pure g-CN. DRS spectra showed that the absorbing wavelength region expanded after loading carbon particles. Especially, 5.0 wt% C/g-CN could be absorbed over NIR. However, the photocatalysts which is the highest photocatalytic performance is 0.5 wt% C/g-CN. This result was attributed to the light shading effect. PL spectra represented the peak intensity of C/g-CN were weaker than that of pure g-CN. In this case, this result was implied to preventing recombination of electron-hole pairs which derived from carbon particles loaded. In conclusion, the highest photocatalytic hydrogen evolution rate for 0.5 wt% C/g-CN was approximately 340 μmol /g・h, which was about 43 times higher than that of pure g-CN. The improved photocatalytic activity was attributed to the fact that carbon particles acted electron reservoirs to trap the electrons for promoting charge separation and light harvesters to enhance the visible light absorption.

Understanding formation mechanism of heterogeneous porous structure of catalyst layer in polymer electrolyte fuel cell

ABSTRACTIn this study, the aerosol optical properties and vertical distributions in major biomass-burning emission area of northern Indochina were investigated using ground-based remote sensing (i.e., four Sun-sky radiometers and one lidar) during the Seven South East Asian Studies/Biomass-burning Aerosols & Stratocumulus Environment: Lifecycles & Interactions Experiment conducted during spring 2014. Despite the high spatial variability of the aerosol optical depth (AOD; which at 500 nm ranged from 0.75 to 1.37 depending on the site), the temporal variation of the daily AOD demonstrated a consistent pattern among the observed sites, suggesting the presence of widespread smoke haze over the region. Smoke particles were characterized as small (Angstrom exponent at 440–870 nm of 1.72 and fine mode fraction of 0.96), strongly absorbing (single-scattering albedo at 440 nm of 0.88), mixture of black and brown carbon particles (absorption Angstrom exponent at 440–870 nm of 1.5) suspended within the planetary boundary layer (PBL). Smoke plumes driven by the PBL dynamics in the mountainous region reached as high as 5 km above sea level; these plumes subsequently spread out by westerly winds over northern Vietnam, southern China, and the neighboring South China Sea. Moreover, the analysis of diurnal variability of aerosol loading and optical properties as well as vertical profile in relation to PBL development, fire intensity, and aerosol mixing showed that various sites exhibited different variability based on meteorological conditions, fuel type, site elevation, and proximity to biomass-burning sources. These local factors influence the aerosol characteristics in the region and distinguish northern Indochina smoke from other biomass-burning regions in the world.

A series of simultaneous measurements of the spectral aerosol extinction coefficients at wavelengths of 0.45-3.9 μm, angular scattering and absorption in the wavelength range of 0.46-0.63 μm in pine combustion smokes was carried out in the Big Aerosol Chamber of IAO (BAC) in 2019-2021. Relationships between the aerosol optical characteristics at the initial stage of smoke formation are considered depending on: a) the value of the mass mixing parameter for open flame combustion (generation of black carbon microparticles) and smoldering combustion (brown carbon particles); b) the total mass of the combusted material. The spectral dependences of the aerosol scattering and absorption coefficients of pyrolysis (12 realizations) and mixed (28 realizations) smokes in the wavelength range from 0.45 to 3.9 μm were estimated.