Mass Absorption Cross Section Research Articles

Light absorption characteristics of primary brown carbon during a complete residential biomass combustion process and their links with aerosol composition

Brown carbon (BrC) has been recognized as an important light-absorbing organic aerosol, and biomass combustion is the main source of BrC. Residential biomass burning is a heterogeneous process, and the dynamics of BrC optical properties during the burning process are still not well understood. Therefore, laboratory test equipped with online instruments (aerosol mass spectrometry and Aethalometer) of residential biomass burning was established to measure the optical properties of BrC. During a complete burn cycle, the light absorption characteristics at the flaming stage are generally similar for wood and crop straws, while it is distinctly different at the burnout stage. BrC contributes 0.141 to 0.609 to light absorption at UV region among different stages. The absorption emission factor and mass absorption cross section of BrC (MACBrC) at the flaming stage was significantly larger than that at the burnout stage. MACBrC was calculated and related to the aerosol composition, aromatic compounds (f77) and PAHs/OA were significantly positively related to MACBrC near the ultraviolet region, suggesting that aromatic compounds and PAHs are the vital contributors to the light absorption of BrC. This time-resolved study provides additional understanding of the dynamics of BrC optical properties during residential biomass burning processes. The results of this study will provide for more accurate assessment of the climate impact of BrC from residential biomass combustion.

Refined mass absorption cross-section of black carbon from typical non-road mobile machinery in China based on real-world measurements

Non-road mobile machinery (NRMM) is becoming a more prominent contribution of black carbon (BC), and mass absorption cross-section (MAC) as an essential parameter to characterize the BC optical property is still not clear. In this study, we explored the impacts of key factors on the MAC of BC based on real-world measurements from 41 typical NRMM. We characterized the organic carbon (OC) and elemental carbon (EC), and found MAC values of BC from NRMM increase as the OC/EC mass ratios increase, since the OC coating can enhance BC light absorption. With more stringent emission standards, the MAC values of all tested NRMM show a significant decreasing trend. Meanwhile, we found the absorption coefficients obtained by filter-based (bfilter) and in-situ-based (bin-situ) methods present good correlation for NRMM in this study, but bfilter are significantly higher than bin-situ when bfilter are above 40,000 Mm−1. Furthermore, we have refined the MAC values under different emission standards, and recommended a more appropriate MAC value (11.5 ± 3.4 m2/g) of NRMM at 550 nm wavelength, which is 1.5 times of the MAC value (7.5 m2/g) commonly used in previous studies. Our results will be indispensable for accurate BC quantification from NRMM and climate radiative effects prediction.

Influence of light absorption rate on the astaxanthin production by the microalga Haematococcus pluvialis during nitrogen starvation

Large amounts of astaxanthin (about 4% DW) can be produced under nitrogen starvation of Haematococcus pluvialis in photobioreactors (PBRs) exposed to high light conditions to induce a light stress. However, in PBR, the large biomass concentration usually achieved leads to strong light attenuation conditions, which makes complex the analysis of this “light stress”. This study aims to elucidate the role of light transfer in astaxanthin cell content and productivity from the microalga Haematococcus pluvialis during nitrogen starvation. Haematococcus pluvialis was cultivated in a flat-panel PBR in a batch mode with sudden nitrogen starvation conditions and an incident photon flux density (PFD) of 250 µmolhν m−2 s−1. Different initial biomass concentrations ({C}_{{x}_{0}}) were evaluated, 0.21, 0.52, 1.39 and 2.21 kg m−3. As a result, spectral mass absorption cross-sections of Haematococcus pluvialis were measured at different times during nitrogen starvation, and were used to relate the mean rate of photon absorption (MRPA) to the astaxanthin productivity. A minimum initial MRPA of 7000 ± 500 µmolhν kgx−1 s−1 was found necessary to trigger large accumulation of astaxanthin in Haematococcus pluvialis cells (up to 3.21% DW) during nitrogen starvation conditions. The results also demonstrated the link between the MRPA and the daily astaxanthin productivity of Haematococcus pluvialis cultures, introducing then the MRPA as a physical quantity of interest for a rational optimization of the light culture conditions in PBRs.Graphical

Optical properties of water soluble and organic soluble carbonaceous aerosols at an urban location in India

We report measurements of the optical properties of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC) in the metropolitan city of Mumbai (19.01° N, 72.92° E), India. The MSOC and WSOC extracts were analysed using UV–visible spectroscopy. The study covered a period of nine months from September 2017 to May 2018. On average, MSOC constituted 30% and WSOC constituted 24% of the PM2.5 mass for the sampling period with peak concentration observed in the winter season. The absorption coefficients of MSOC were on average 1.57 times higher than WSOC for the sampling period. The absorption coefficients of MSOC and WSOC were correlated with the brown carbon absorption coefficients. Mass absorption cross-section (MAC) was calculated by normalizing the absorption coefficients with its concentration, and the absorption angstrom exponent (AAE) was calculated by exponential fitting of the absorption coefficients. The MAC values for WSOC were estimated to be 1.03 ± 0.39 m2 g⁻1, while for MSOC, it was 1.41 ± 0.76 m2 g⁻1. The relative radiative forcing compared to black carbon was estimated at 10.1 ± 5.2% and 6.3 ± 3.8% for MSOC and WSOC, respectively.

Concurrent photochemical whitening and darkening of ambient brown carbon

Abstract. The light-absorbing organic aerosol (OA), known as brown carbon (BrC), has important radiative impacts; however, its sources and evolution after emission remain to be elucidated. In this study, the light absorption at multiple wavelengths, mass spectra of OA and microphysical properties of black carbon (BC) were characterized at a typical suburban environment in Beijing. The absorption of BC is constrained by its size distribution and mixing state, and the BrC absorption is obtained by subtracting the BC absorption from the total aerosol absorption. Aerosol absorption was further apportioned to BC, primary BrC and secondary BrC by applying the least correlation between secondary BrC and BC. The multilinear regression analysis on the factorized OA mass spectra indicated that the OA from traffic and biomass burning emission contributed to primary BrC. Importantly, the moderately oxygenated OA (O / C = 0.62) was revealed to highly correlate with secondary BrC. These OA had higher nitrogen content, in line with the nitrogen-containing functional groups detected by the Fourier transform infrared spectrometer. The photochemical processes were found to reduce the mass absorption cross section (MAC) of primary OA, reducing its contribution to total absorption by 20 %, at the same time increasing MAC for secondary OA, which showed a 30 % enhancement in contribution to total absorbance, implying the concurrent whitening and darkening of BrC. This provides field evidence that the photochemically produced secondary nitrogen-containing OA can considerably compensate for some bleaching effect on the primary BrC, hereby causing radiative impacts.

Dissolved organic carbon fractionation in wet deposition and its potential impact on radiative forcing in the central Tibetan Plateau

As an important component of carbonaceous matters, dissolved organic carbon (DOC) can absorb and scatter the solar radiation at ultraviolet and blue wavelengths. The wet deposition process has great impact on the concentration and light absorption ability of precipitation DOC, affecting the climatic effect caused by DOC in the atmosphere. In this study, light absorption and fluorescence characteristics of precipitation DOC was investigated in the central Tibetan Plateau (TP). The results showed that the mean DOC concentration and mass absorption cross-section measured at 365 nm (MAC365) in Tanggula (TGL) station were 0.59 ± 0.42 mg/L and 0.37 ± 0.19 m2/g, respectively, while both values showed much higher volatilities than those of aerosols. DOC concentrations had significant negative correlation with the precipitation amount, while MAC365 values increase with the precipitation amount in TGL station. Therefore, DOC with high light-absorbing ability was preferred to be retained in the atmosphere during wet deposition. In this study, precipitation DOC contained three fluorescent components (one humic-like component and two tyrosine-like components) mainly from local biomass burning sources. DOC concentration showed a negative relationship with MAC365 value in TGL station. The wet deposition of DOC with low light-absorbing ability can reduce the strong negative radiative forcing caused by secondary organic aerosol due to high proportion of DOC in secondary organic carbon. Similar phenomenon was also found in Nam Co, Lulang and Everest stations of previous study, which may have a potential impact on radiative forcing in the atmosphere of TP.

Combined influences of sources and atmospheric bleaching on light absorption of water-soluble brown carbon aerosols

Light-absorbing Brown Carbon (BrC) aerosols partially offset the overall climate-cooling of aerosols. However, the evolution of BrC light-absorption during atmospheric transport is poorly constrained. Here, we utilize optical properties, ageing-diagnostic δ13C-BrC and transport time to deduce that the mass absorption cross-section (MACWS-BrC) is decreasing by ~50% during long-range oversea transport, resulting in a first-order bleaching rate of 0.24 day‒1 during the 3-day transit from continental East Asia to a south-east Yellow Sea receptor. A modern 14C signal points to a strong inverse correlation between BrC light-absorption and age of the source material. Combining this with results for South Asia reveals a striking agreement between these two major-emission regions of rapid photobleaching of BrC with a higher intrinsic absorptivity for BrC stemming from biomass burning. The consistency of bleaching parameters constrained independently for the outflows of both East and South Asia indicates that the weakening of BrC light absorption, thus primarily related to photochemical processes rather than sources, is likely a ubiquitous phenomenon.

Optical properties and cloud condensation nuclei activity of brown carbon containing α–dicarbonyls and reduced nitrogen compounds

The aqueous–phase reaction of carbonyls with reduced nitrogen compounds (RNCs) is considered an important pathway for the formation of secondary BrC, and these particles could greatly impact the global climate. However, our knowledge of their properties is still limited. In this study, we measured the optical properties, cloud condensation nuclei (CCN) activity (represented as hygroscopicity parameter κCCN), and effective density (ρeff) under varied reaction conditions for BrC mixtures formed by reactions between two α–dicarbonyls (glyoxal GX and methylglyoxal MG) and three RNCs (ammonium sulfate AS, glycine GC, and dimethylamine DA). Our results showed a wide range of particle ρeff from 1.08 to 1.66 g cm−3, depending on the type of RNCs reactant. At 532 nm wavelength, the real part n of complex refractive index ranged from 1.41 to 1.60 and the imaginary part k of BrC ranged from 0.024 to 0.043, and the absorption cross section for MG mixtures were 20.3–51.0% higher than those of GX mixtures. These values did not appear to vary significantly with mixing molar ratios between RNCs and α–dicarbonyls (10:1, 1:1, and 1:3) for mixtures with the same reactants, but the mass absorption cross section for mixtures after a reaction time of 14 days were 1.57–74.2% higher than those of mixtures after reaction times of 0.5 and 7 days. The κCCN values of different BrC were consistent with the order of κCCN for single RNCs, i.e., AS–containing (0.349–0.428) > GC–containing (0.221–0.352) > DA–containing (0.090–0.131), and showed an overall increasing trend (1.47–59.3%) with the RNCs content. Our results highlight the importance of reaction conditions on secondary BrC properties and are helpful for a more accurate assessment of aerosol radiative forcing.

Aggregation-induced enhancements in aerosol absorption and scattering across the black-brown continuum

Global radiative forcing by carbonaceous aerosols is one of the largest sources of uncertainty in current climate models. The radiative impacts of brown carbon (BrC), a type of amorphous organic carbonaceous aerosol formed in biomass burning events, such as wildfires, remains poorly understood. Recently, the aggregation of spheres (monomers) of BrC was observed in wildfire smoke from various parts of the world. Aggregation could alter the optical properties and direct radiative forcing of BrC, yet very little is known about this phenomenon. This study improves upon the spectral optical properties of BrC aggregates observed in past field studies.We are motivated by the framework presented by Saleh et al. (2020) for the optical classification of carbonaceous particles across the black-brown continuum. Here, we simplify Saleh et al.'s BrC categorization into two sub-classes: dark brown carbon (d-BrC) and weakly-absorbing brown carbon (w-BrC). We calculate and compare the mass absorption cross-section (MAC), mass scattering cross-section (MSC), single scattering albedo (SSA), and asymmetry parameter (g) of d-BrC and w-BrC and determine their absorption and scattering enhancements due to aggregation. Polydisperse diffusion-limited cluster-cluster aggregation (p-DLCA) simulations generated 90 aggregates of varying monomer diameters, and discrete dipole approximation (DDA) was used to calculate optical properties of these aggregates at relevant refractive indices, monomer diameters, and incident wavelengths.We find that optical properties of BrC aggregates are more sensitive to the monomer number and mean diameter than to polydispersity. d-BrC has almost twice the MAC of w-BrC at 350 nm, but w-BrC has MSC values almost double that of d-BrC. Both MAC and MSC for both types of aerosol decrease with wavelength. Aggregation enhances optical properties, with smaller size parameters and lower imaginary part of the complex refractive index of aggregates resulting in stronger absorption and scattering.

Assessment of Carbonaceous Aerosol Properties across an Urban Environment during the Cold Season

For air quality management it is becoming increasingly important to be able to assess the quantity and properties of biomass-burning-related aerosol. Due to different chemical compositions and morphologies, black carbon (BC) and brown carbon (BrC) demonstrate diverse optical properties as well as an impact on air quality. In this study, we analyzed the chemical composition and light-absorbing properties of carbonaceous aerosol at an urban background station during the residential heating season. In addition, BC and BrC levels were assessed in six different areas in the city characterized by different predominant domestic heating systems. Positive matrix factorization (PMF) and BrC attribution analysis revealed that BrC mainly consisted of biomass-burning-related organic aerosol (BBOA) (up to 95%). The mass absorption cross-section at 370 nm for BBOA factors varied between 1.41 m2g−1 and 2.63 m2g−1. The study of six different areas in the city showed that the input of BrC to the total light absorption coefficient within the city varied between 33% and 70%. In addition, the worst air quality was present in areas with numerous old wooden houses with outdated heating systems where significantly increased BC and BrC levels were observed.

Microphysical characteristics of black carbon from various emission sources

The complex microphysical characteristics of black carbon aerosol (BC) results in large uncertainties in the evaluation of BC's climatic and environmental effects. In this study, the microphysical properties were investigated in fresh BC emitted from traffic sources (gasoline vehicle and diesel) and aged BC emitted from solid fuel burning sources (coal, cotton stem, rice stem and mulberry). For the BC core size distribution, the number size distribution of traffic sources peaks at a lower value than that of aged solid fuel burning sources. Although traffic sources emit a greater number of small BC cores, traffic sources could also emit BC aerosols with large BC cores (diameter larger than 400 nm) not found in solid fuel sources. The coating thickness of BC from traffic sources is significantly lower than that of aged BC from solid fuel sources. Such differences in size distribution and coating thickness cause large variations in other microphysical parameters, such as light absorption and hygroscopicity, for these two types of BC. The mass absorption cross section (MAC) of aged BC from cotton stems reached 11.69 m2/g, nearly 2 times higher than the MAC (6.08 m2/g) of fresh BC from gasoline vehicles. The hygroscopicity of aged BC from solid fuel burning is also much higher than that of fresh BC from traffic. The critical supersaturation needed to activate BC to become cloud droplets is 0.03% for the former and 0.42% for the latter. This study reports the BC emission characteristics of traffic sources and emphasizes the large property difference between fresh traffic emission BC and aged solid fuel burning BC. The microphysical properties of BC should be carefully considered to better resolve BC concentrations and optical properties in climatic models.

PM2.5 carbonaceous components and mineral dust at a COALESCE network site - Bhopal, India: Assessing the impacts of COVID-19 lockdowns on site-specific optical properties

Thermal elemental carbon (EC), optical black carbon (BC), organic carbon (OC), mineral dust (MD), and 7-wavelength optical attenuation of 24-hour ambient PM2.5 samples were measured/estimated at a regionally representative site (Bhopal, central India) during a business-as-usual year (2019) and the COVID-19 lockdowns year (2020). This dataset was used to estimate the influence of emissions source reductions on the optical properties of light-absorbing aerosols. During the lockdown period, the concentration of EC, OC, BC880 nm, and PM2.5 increased by 70 % ± 25 %, 74 % ± 20 %, 91 % ± 6 %, and 34 % ± 24 %, respectively, while MD concentration decreased by 32 % ± 30 %, compared to the same time period in 2019. Also, during the lockdown period, the estimated absorption coefficient (babs) and mass absorption cross-section (MAC) values of Brown Carbon (BrC) at 405 nm were higher (42 % ± 20 % and 16 % ± 7 %, respectively), while these quantities for MD, i.e., babs-MD and MACMD values were lower (19 % ± 9 % and 16 % ± 10 %), compared to the corresponding period during 2019. Also, babs-BC-808 (115 % ± 6 %) and MACBC-808 (69 % ± 45 %) values increased during the lockdown period compared with the corresponding period during 2019. It is hypothesized that although anthropogenic emissions (chiefly industrial and vehicular) reduced drastically during the lockdown period compared to the business-as-usual period, an increase in the values of optical properties (babs and MAC) and concentrations of BC and BrC, were likely due to the increased local and regional biomass burning emissions during this period. This hypothesis is supported by the CBPF (Conditional Bivariate Probability Function) and PSCF (Potential Source Contribution Function) analyses for BC and BrC.

Constraining the complex refractive index of black carbon particles using the complex forward-scattering amplitude

Black carbon is the largest contributor to global aerosol’s shortwave absorption in the current atmosphere and is an important positive climate forcer. The complex refractive index, m = mr + imi, the primary determinant of the absorbed and scattered energies of incident radiation per unit volume of particulate material, has not been accurately known for atmospheric black carbon material. An accurate value at visible wavelengths has been difficult to obtain due to the black carbon’s wavelength-scale irregularity and variability of aggregate shape, distribution in particle size, and mixing with other aerosol compounds. Here, we present a method to constrain a plausible (mr, mi) domain for black carbon from the observed distribution of the complex forward-scattering amplitude S(0°). This approach suppresses the biases due to the above-mentioned complexities. The S(0°) distribution of black carbon is acquired by performing single particle S(0°) measurements in a water medium after collecting atmospheric aerosols into water. We demonstrate the method operating at λ = 0.633 μm for constraining the refractive index of black carbon aerosols in the north-western Pacific boundary layer. From the plausible (mr, mi) domain consistent with the observed S(0°) distributions and the reported range of mass absorption cross-section, we conservatively select 1.95 + 0.96i as a recommendable value of the refractive index for uncoated black carbon at visible wavelengths. The recommendable value is 0.17 larger in mi than the widely used value 1.95 + 0.79i in current aerosol-climate models, implying a ∼16% underestimate of shortwave absorption by black carbon aerosols in current climate simulations.

Light absorption by Volvocaceae colonies consisting of equidistant optically soft photosynthetic cells in a transparent spherical extracellular matrix

This study aims to develop a methodology to predict the absorption cross-section of colonial microalgae such as those of the Volvocaceae family including Eudorina, Pleodorina, and Volvox consisting of an ordered assembly of large and optically soft absorbing cells embedded within a non-absorbing spherical extracellular matrix (ECM). The absorption cross-section of spherical colonies, such as Eudorina, containing 16, 32, and 64 equidistant photosynthetic cells distributed on the surface of a sphere within a concentric spherical ECM was predicted by the superposition T-matrix method for ECM size parameters as large as 500 and by the Monte Carlo ray-tracing (MCRT) method for ECM size parameters as large as 900. The predicted absorption cross-sections given by the two methods were in excellent agreement despite the fact that the conditions for geometric optics were not rigorously satisfied. The absorption cross-section of the microalgae colonies considered was found to increase with increasing cell radius, absorption index or cell pigment concentration, and/or number of cells. Shading among cells decreased the mass absorption cross-section and was increasingly important for colonies with strongly absorbing cells, large cell radius, and/or large number of cells. These results demonstrate that accounting for shading effects is necessary to accurately predict the absorption cross-section of microalgae colonies. Furthermore, the study demonstrated that the MCRT method is an accurate and efficient method for modeling light absorption by an ensemble of many large, ordered, and optically soft particles. Finally, the impact of colony formation as well as pigment and biomass concentrations on the local (LRPA) and mean (MRPA) rates of photon absorption within a microalgae culture was also assessed. At low biomass concentrations, the LRPA and MRPA decreased in the presence of colonies. This effect was more pronounced at higher pigment concentrations.

PM2.5 carbonaceous components and mineral dust at a COALESCE network site - Bhopal, India: Estimating site-specific optical characteristics

Atmospheric PM2.5 thermal elemental carbon (EC), optical black carbon (BC), brown carbon (BrC), and mineral dust (MD) were characterized during a field campaign at a regionally representative site (Bhopal, central India) all year-long during 2019. In this study, the optical characteristics of PM2.5 during ‘EC-rich’, ‘OC-rich’, and ‘MD-rich’ days were used in a three-component model to estimate site-specific Absorption Ångström exponent (AAE) and absorption coefficient (babs) of light-absorbing PM2.5 constituents. The AAE for ‘EC-rich’, ‘OC-rich’, and ‘MD-rich’ days were 1.1 ± 0.2, 2.7 ± 0.3, and 3.0 ± 0.9, respectively. The percentage contribution of calculated babs of EC, BrC, and MD to the total babs at 405 nm was dominated by EC during the entire study period (EC; 64 % ± 36 %, BrC: 30 % ± 5 %, MD: 10 % ± 1 %). Further, site-specific mass absorption cross-section (MAC) values were calculated to assess the impact of their use over the use of manufacturer-specified MAC values in estimating BC concentrations. The r2 between thermal EC and optical BC was higher (r2 = 0.67, slope = 1.1) when daily site-specific MAC values were used rather than using the default MAC value (16.6 m2 g−1; r2 = 0.54 and slope = 0.6). Overall, had the default MAC880 been used instead of the site-specific values, we would have underestimated the BC concentration by 39 % ± 18 % during the study period.

Heterogeneous characteristics and absorption enhancement of refractory black carbon in an urban city of China

In this study, field measurement was conducted using an integrated online monitoring system to characterize heterogeneous properties and light absorption of refractory black carbon (rBC). rBC particles are mainly from the incomplete combustion of carbonaceous fuels. With the data collected from a single particle soot photometer, thickly coated (BCkc) and thinly coated (BCnc) particles are characterized with their lag times. With different responses to the precipitation, a dramatical decline of 83 % in the number concentration of BCkc is shown after rainfall, while that of BCnc decreases by 39 %. There is a contrast in core size distribution that BCkc is always with larger particle sizes but has smaller core mass median diameters (MMD) than BCnc. The mean rBC-containing particle mass absorption cross-section (MAC) is 6.70 ± 1.52 m2 g−1, while the corresponding rBC core is 4.90 ± 1.02 m2 g−1. Interestingly, there are wide variations in the core MAC values which range by 57 % from 3.79 to 5.95 m2 g−1, which are also closely related to those of the whole rBC-containing particles with a Pearson correlation of 0.58 (p < 0.01). Errors would be made if we eliminate the discrepancies and set the core MAC as a constant when calculating absorption enhancement (Eabs). In this study, the mean Eabs is 1.37 ± 0.11 while the source apportionment shows that there are five contributors of Eabs including secondary aging (37 %), coal combustion (26 %), fugitive dust (15 %), biomass burning (13 %) and traffic-related emissions (9 %). Secondary aging is found to be the highest contributor due to the liquid phase reactions in formations of secondary inorganic aerosol. Our study characterizes property diversities and provides insights into the sources impacting the light absorption of rBC and will be helpful for controlling it in the future.

Black carbon aerosols over Indian Ocean have unique source fingerprint and optical characteristics during monsoon season

Effects of aerosols such as black carbon (BC) on climate and buildup of the monsoon over the Indian Ocean are insufficiently quantified. Uncertain contributions from various natural and anthropogenic sources impede our understanding. Here, we use observations over 5 y of BC and its isotopes at a remote island observatory in northern Indian Ocean to constrain loadings and sources during little-studied monsoon season. Carbon-14 data show a highly variable yet largely fossil (65 ± 15%) source mixture. Combining carbon-14 with carbon-13 reveals the impact of African savanna burning, which occasionally approach 50% (48 ± 9%) of the total BC loadings. The BC mass-absorption cross-section for this regime is 7.6 ± 2.6 m2/g, with higher values during savanna fire input. Taken together, the combustion sources, longevity, and optical properties of BC aerosols over summertime Indian Ocean are different than the more-studied winter aerosol, with implications for chemical transport and climate model simulations of the Indian monsoon.

Size and light absorption of miniature-inverted-soot-generator particles during operation with various fuel mixtures

The Argonaut miniature inverted soot generator (MISG) serves as a convenient and stable source of soot black-carbon aerosol particles. The MISG performance has been previously characterized for ethylene and propane, in a simple two-flow setup of fuel and air. These fuels produce soot size distributions with geometric median diameters (GMDs) of 150–200 nm. These sizes are substantially larger than the soot particles emitted from typical internal-combustion and aviation gas turbine engines. Here, we sought to produce substantially smaller soot particles, representative of aviation gas turbine engine emissions, by using five different fuels (acetylene, methane, ethylene, propane, and dimethyl ether) and by enriching the combustion air with oxygen. We developed a rapid screening protocol for each fuel, to allow the full range of stable flame conditions to be explored. We monitored the size distribution, mass concentration, and mass absorption cross section (MAC) of denuded MISG soot in real time. We considered only operating conditions where stable soot mass concentrations above 3 μg/m3 were emitted, corresponding to a non-flickering and open-tipped flame. For virtually all operating conditions, the measured MAC was consistent with literature values for mature soot. The only exception was a propane-acetylene mixture, for which the measured MAC was much lower than literature values. The observed GMDs fell into two groups, and we did not identify any trends leading to the mixtures falling into either group. The two groups were at approximately 100 nm and 160 nm. Therefore, while this study identified conditions that reduced the soot particles to 100 nm, it did not identify conditions where even smaller soot particles could be produced. This study has demonstrated that the soot properties produced by the MISG are highly repeatable and relatively insensitive to fuel composition.

Impacts of biomass burning and photochemical processing on the light absorption of brown carbon in the southeastern Tibetan Plateau

Abstract. Brown carbon (BrC) in the atmosphere can greatly influence aerosol's radiative forcing over the Tibetan Plateau (TP) because it has the non-negligible capacity of light absorption compared to black carbon (BC); however, our understanding of optical properties, sources, and atmospheric processes of BrC in this region remains limited. In this study, a multiple-wavelength Aethalometer coupled with a quadrupole aerosol chemical speciation monitor was deployed to investigate the highly time resolved BrC in the submicron aerosol at the southeastern edge of the TP during the pre-monsoon season. The results showed that BrC made substantial contributions (20.0 %–40.2 %) to the light absorption of submicron aerosol from 370 to 660 nm. Organic aerosol (OA), an alternative to BrC, was split into a biomass burning OA (BBOA) with aging processes and a photochemical-oxidation-processed oxygenated OA (po-OOA) by a hybrid environmental receptor model analysis. Combined with the light absorption coefficient of BrC (babs-BrC), the source-specific mass absorption cross sections of BBOA (0.61–2.78 m2 g−1) and po-OOA (0.30–1.43 m2 g−1) at 370–660 nm were retrieved. On average, babs-BrC from po-OOA (1.3–6.0 Mm−1) was comparable to that from BBOA (1.3–6.0 Mm−1) at all wavelengths. The concentration-weighted trajectory analysis showed that the most important potential source regions for babs-BrC values from BBOA and po-OOA were located in northern Myanmar and along the China–Myanmar border, indicating the cross-border transport of BrC from Southeast Asia. A “simple forcing efficiency” evaluation further illustrated the importance of the BrC radiative effect with the high fractional radiative forcing by two OAs relative to BC (48.8 ± 15.5 %). This study highlights a significant influence of BrC of biomass burning origin and secondary formation on climate change over the TP region during the pre-monsoon season.

Wintertime aerosol properties of urban desert region of western India: Implications in regional climate assessment

This study assessed the inter-relation between physiochemical and optical characteristics of aerosols measured at a desert-urban region affected by anthropogenic sources and desert dust during October 2020 to January 2021. Based on horizontal visibility and measured PM2.5 concentration, clear (37 %), light (33 %) and high (31 %) pollution periods were identified. Elemental and organic carbon (50 ± 15 μgm−3; 31 %) and secondary inorganics (53 ± 21 μgm−3; 33 %) dominated the PM2.5 mass (160 ± 4 μgm−3) during high pollution period with low dust (14 ± 7 μgm−3; 8 %) content. Interestingly, the clear pollution period was also influenced by carbonaceous fraction (19 ± 8 μgm−3; 32 %) and secondary inorganics (19 ± 5 μgm−3; 32 %), but the PM2.5 concentrations (59 ± 9 μgm−3) were ∼ one-third as compared to high pollution period. High scattering coefficients were observed which were comparable to highly polluted Indian city like Delhi. An exponential increase in non-absorbing material was observed and showed clear influence on light absorption capacity of EC and dust due to coating/mixing. High absorption Ångström exponent (AAE) >0.6 was observed for the ratio of non-absorbing to light absorbing components (LAC) in the range of 1–2.5 and EC/PM2.5 fraction of 7–14 %. While further increase in non-absorbing to absorbing components ratio > 4 and low amount of EC (<4 %) tend to decrease AAE below 0.4. Higher mass absorption cross-section (>30 m2g−1 of EC) was observed when 4–10 % EC fraction of PM2.5 associated with 1.5–3.5 times non-absorbing components to total absorbing components. Likewise, absorption enhanced by three to five folds compared to uncoated EC for low EC fraction (3–6 %) in PM2.5, but high non-absorbing to absorbing component ratio (>2.5). Interestingly, absorption was minimally amplified for nominal coating fraction associated with significant core materials or vice-versa. These findings have implications not only in regional climate assessment but also for other regions with comparable geography and source-mixes.