Mass Absorption Cross-section Values Research Articles

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.

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.

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.

Effect of carbonaceous components of biodiesel combustion particles on optical properties

This paper studies the influence of carbonaceous components on the optical properties of particulate matter (PM) in biodiesel combustion by conducting a bench test on an electronically controlled high-pressure common-rail diesel engine. In addition, the PM produced by the combustion of diesel oil, soybean oil methyl ester (SME), waste edible oil methyl ester (WME), and palm oil methyl ester (PME) was collected. The carbonaceous composition and optical properties of diesel and three biodiesel particulates were then analyzed. The obtained results showed that the ratio of organic carbon (OC) to total carbon (TC) in diesel PM was 0.25 and the ratio of OC/EC was 0.33. The OC to TC ratio of biodiesel PM was significantly greater than that of diesel PM, ranging between 0.59 and 0.65, with OC/EC values in the range of 1.44–1.86. The mass absorption cross-section (MAC) values of three kinds of biodiesel particles were all higher than those of diesel particles. When the incident laser wavelength increased, the difference of MAC values among four kinds of fuel particles gradually decreased. The MAC values of all the three biodiesel particles were higher than those of the diesel particles, and the difference between the MAC values of the four fuel particles gradually decreased with the increase of the incident laser wavelength. Afterwards, the “shell-core” model of particles was developed with 80 nm EC sphere as the core. At the two refractive indices, the scattering cross section, absorption cross section, and extinction cross section of the particles decrease with the increase of the incident light wavelength, and the scattering cross section, absorption cross section, and extinction cross section of the particles increase with the increase of the OC coating thickness.

Optical properties of morphologically complex black carbon aerosols: Effects of coatings

Optical properties are computed for fractal-like aggregate black carbon (BC) aerosols coated with different substances. Two models are used for these aerosols: (i) the coated aggregate model (model I), where coating is added to the voids and surroundings of BC fractal-like aggregates; and (ii) the closed-cell aggregate model (model II), where coating is added concentrically to each monomer of BC fractal-like aggregates. Our results favor choosing the coated aggregate model (model I) to simulate scattering and absorption by coated BC aerosols because this model is morphologically more realistic, and because this model yields mass absorption cross section (MAC) and backscattering linear depolarization ratio (LDR) values that are consistent with field measurements. Moreover the corresponding computed degree of linear polarization (DoLP) and LDR values are very sensitive to changes in the coating volume fraction (fvol) and the coating refractive index (m). With the same absorbing BC core, the MAC value increases steadily with increasing fvol and m values. For example, using a density of 1.8 g/cm3 and BC refractive index 1.95 + i0.79, the calculated MAC values for uncoated BC aggregates range from 6.2 to 6.8 m2/g at 0.55 µm. When coating material is applied to the BC aggregates, the calculated MAC values for model I particles increase to between 9.8 and 13.2 m2/g (depending on m) when fvol = 87.5%. The backscattering LDR values also tend to increase with the increasing m values for the shapes and sizes considered in our study. For model I particles, the backscattering LDR values span a wide range of 4.2–27.8% at a wavelength of 0.35 µm at fvol = 87.5% when m increases from 1.33 to 1.55. Our results are relevant to analyses of polarimetric and lidar observations of smoke particles, especially when these particles undergo hygroscopic growth.

Absorption instruments inter-comparison campaign at the Arctic Pallas station

Abstract. Aerosol light absorption was measured during a 1-month field campaign in June–July 2019 at the Pallas Global Atmospheric Watch (GAW) station in northern Finland. Very low aerosol concentrations prevailed during the campaign, which posed a challenge for the instruments' detection capabilities. The campaign provided a real-world test for different absorption measurement techniques supporting the goals of the European Metrology Programme for Innovation and Research (EMPIR) Black Carbon (BC) project in developing aerosol absorption standard and reference methods. In this study we compare the results from five filter-based absorption techniques – aethalometer models AE31 and AE33, a particle soot absorption photometer (PSAP), a multi-angle absorption photometer (MAAP), and a continuous soot monitoring system (COSMOS) – and from one indirect technique called extinction minus scattering (EMS). The ability of the filter-based techniques was shown to be adequate to measure aerosol light absorption coefficients down to around 0.01 Mm−1 levels when data were averaged to 1–2 h. The hourly averaged atmospheric absorption measured by the reference MAAP was 0.09 Mm−1 (at a wavelength of 637 nm). When data were averaged for &gt;1 h, the filter-based methods agreed to around 40 %. COSMOS systematically measured the lowest absorption coefficient values, which was expected due to the sample pre-treatment in the COSMOS inlet. PSAP showed the best linear correlation with MAAP (slope=0.95, R2=0.78), followed by AE31 (slope=0.93). A scattering correction applied to PSAP data improved the data accuracy despite the added noise. However, at very high scattering values the correction led to an underestimation of the absorption. The AE31 data had the highest noise and the correlation with MAAP was the lowest (R2=0.65). Statistically the best linear correlations with MAAP were obtained for AE33 and COSMOS (R2 close to 1), but the biases at around the zero values led to slopes clearly below 1. The sample pre-treatment in the COSMOS instrument resulted in the lowest fitted slope. In contrast to the filter-based techniques, the indirect EMS method was not adequate to measure the low absorption values found at the Pallas site. The lowest absorption at which the EMS signal could be distinguished from the noise was &gt;0.1 Mm−1 at 1–2 h averaging times. The mass absorption cross section (MAC) value measured at a range 0–0.3 Mm−1 was calculated using the MAAP and a single particle soot photometer (SP2), resulting in a MAC value of 16.0±5.7 m2 g−1. Overall, our results demonstrate the challenges encountered in the aerosol absorption measurements in pristine environments and provide some useful guidelines for instrument selection and measurement practices. We highlight the need for a calibrated transfer standard for better inter-comparability of the absorption results.

Photobleaching reduces the contribution of dissolved organic carbon to glacier melting in the Himalayas and the Tibetan Plateau

Dissolved organic carbon (DOC) makes an important contribution to glacier melting in the Himalayas and the Tibetan Plateau (HTP). Photobleaching can effectively reduce the light absorption ability of DOC, further changing its impact on glacier melting, which is not yet well researched in the HTP. Therefore, snowpit samples from the Bayi, Ganglongjiama (GLJM), Jiemayangzong (JMYZ) and Demula (DML) glaciers were collected to study the influence of photobleaching on the light absorption ability of DOC and its impact on glacier melting. The results showed that the DOC concentration of snowpit samples, which was affected by the melting state and photobleaching, decreased from the northern HTP to the southern HTP. At an early stage of melting, the mass absorption cross-section value at 365 nm (MAC365) values showed a negative correlation with DOC concentrations in the snowpit at the JMYZ and DML glaciers, indicating that colored DOC tended to be concentrated in the snowpit during the melting process. With the aggravation of ablation, some snowpit samples in the GLJM and Bayi glaciers had both low concentrations and MAC365 values of DOC due to the reduced influence of photobleaching on the light absorption ability of DOC. Similarly, two fluorescence components (one protein-like component and one humic-like component) were identified in the extracted DOC at the JMYZ and DML glaciers, while those components were not detected in the GLJM glacier. Based on the sources of fluorescent DOC and five-day backward air mass trajectories, long-distance transport of pollutants from South Asia was an important source of snowpit DOC in the southern HTP. In this study, photobleaching can effectively remove colored and fluorescent DOC from snowpit samples in the HTP, further reducing the radiation forcing and glacier melting caused by DOC.

Implications of Site‐specific Mass Absorption Cross‐section (MAC) to Black Carbon Observations at a High‐altitude Site in the Central Himalaya

Accurate estimation of black carbon (BC) from the widely used optical attenuation technique is important for the reliable assessment of their climatic impact. The optical instruments use Mass Absorption Cross-section (MAC) for converting light attenuation records to BC mass concentrations and Aethalometer is a widely used optical instrument for BC estimation. Several studies have shown large variability in MAC values. It is thus necessary to examine the accuracy and consistency of MAC values obtained using Aethalometer over distinct geographic locations and seasons. In the present study, MAC values are derived using simultaneous observations (2014–2017) from an EC-OC analyzer and an Aethalometer (AE-42) over a high altitude central Himalayan site at Nainital (29.4oN, 79.5oE, 1958 a.m.s.l). The observations reveal that the annual mean value of MAC (5.03 ± 0.03 m2g− 1 at 880nm) is significantly lower than the constant value used by the manufacturer (16.6 m2g− 1 at 880nm). The estimated MAC values also showed significant seasonal variation, spanning over a range from 3.7 to 6.6 m2g− 1. It is found that the seasonal variability of elemental carbon (EC), air mass variation and meteorological parameters play an important role in the changes in MAC values over this region. Multi-wavelength determination of MAC shows the contribution of absorption by species other than EC at shorter wavelengths. MAC does not show a clear diurnal variation, unlike EC and absorption coefficient. The slope of EC vs. corrected equivalent black carbon (eBC) showed a significant improvement during all seasons when compared with uncorrected eBC. This lends credibility to the fact that the use of site-specific MAC leads to more reliable estimates of eBC over the central Himalayan region. It is found that, instead of using the site specific MAC value, had we used the one supplied by the instrument, we would have underestimated the radiative forcing by about 7.8Wm− 2 which amounts to a reduction by 24 %.

Bias in quantification of light absorption enhancement of black carbon aerosol coated with low-volatility brown carbon

Brown carbon (BrC) is an optically defined class of organic carbon (OC) which strongly absorbs light at shorter visible and ultraviolet-A (UVA) wavelengths. Both light absorbing and non-absorbing OC have been found to coat black carbon (BC) and could modulate the optical properties of BC. Thermodenuders are conventionally used in conjunction with instruments measuring particle light absorption, such as photoacoustic spectrometers, to parametrize enhanced BC light absorption (Eabs) due to OC. This method can introduce unquantified measurement artifacts because thermodenuders do not remove low-volatility compounds at the temperatures they are commonly operated at and due to temperature-dependent loss of particles within the instrument. These artifacts could lead to errors in accurately estimating Eabs for coated BC particles. The value of Eabs can also be estimated by comparing the mass absorption cross-section (MAC) of coated BC aerosol to literature-based MAC values of uncoated BC. In this study, we use two integrated photoacoustic-nephelometer spectrometers, operating at wavelengths of 375 nm and 532 nm, respectively, to quantify the differences and errors associated with measuring Eabs using these two methods. Our results indicate differences ranging from 25% to 65% for Eabs measured using both methods at 375 nm. We observed little to no enhancement at either wavelength using the thermodenuder method. In contrast, the Eabs value increased with the OC/BC ratio at 375 nm using the literature-based MAC method. The difference between the two methods was attributed to the presence of low-volatility brown carbon, and these results were corroborated using discrete dipole approximation calculations. Theoretical calculations predicted that these low-volatility OC could have values of the imaginary part of the refractive index up to 0.32 at a wavelength of 375 nm, which is in line with previous studies that found large absorption by low-volatility organics.Copyright © 2021 American Association for Aerosol Research

A new optical-based technique for real-time measurements of mineral dust concentration in PM&amp;lt;sub&amp;gt;10&amp;lt;/sub&amp;gt; using a virtual impactor

Abstract. Atmospheric mineral dust influences Earth's radiative budget, cloud formation, and lifetime; has adverse health effects; and affects air quality through the increase of regulatory PM10 concentrations, making its real-time quantification in the atmosphere of strategic importance. Only few near-real-time techniques can discriminate dust aerosol in PM10 samples and they are based on the dust chemical composition. The online determination of mineral dust using aerosol absorption photometers offers an interesting and competitive alternative but remains a difficult task to achieve. This is particularly challenging when dust is mixed with black carbon, which features a much higher mass absorption cross section. We build on previous work using filter photometers and present here for the first time a highly time-resolved online technique for quantification of mineral dust concentration by coupling a high-flow virtual impactor (VI) sampler that concentrates coarse particles with an aerosol absorption photometer (Aethalometer, model AE33). The absorption of concentrated dust particles is obtained by subtracting the absorption of the submicron (PM1) aerosol fraction from the absorption of the virtual impactor sample (VI-PM1 method). This real-time method for detecting desert dust was tested in the field for a period of 2 months (April and May 2016) at a regional background site of Cyprus, in the Eastern Mediterranean. Several intense desert mineral dust events were observed during the field campaign with dust concentration in PM10 up to 45 µg m−3. Mineral dust was present most of the time during the campaign with an average PM10 of about 8 µg m−3. Mineral dust absorption was most prominent at short wavelengths, yielding an average mass absorption cross section (MAC) of 0.24±0.01 m2 g−1 at 370 nm and an absorption Ångström exponent of 1.41±0.29. This MAC value can be used as a site-specific parameter for online determination of mineral dust concentration. The uncertainty of the proposed method is discussed by comparing and validating it with different methods.

A new method for extraction of methanol-soluble brown carbon: Implications for investigation of its light absorption ability

As an important component of organic carbon (OC), brown carbon (BrC) plays a significant role in radiative forcing in the atmosphere. Water-insoluble OC (WIOC) generally has higher light absorption ability than water-soluble OC (WSOC). The mass absorption cross-section (MAC) of WIOC is normally investigated by dissolving OC in methanol. However, all the current methods have shortcomings due to neglecting the methanol insoluble particulate carbon that is detached from the filter and suspended in methanol extracts, which results in MAC uncertainties of the methanol-soluble BrC and its climate warming estimation. In this study, by investigating typical biomass combustion sourced aerosols from the Tibetan Plateau and ambient aerosols from rural and urban areas in China, we evaluated the light absorption of extractable OC fraction for the existing methods. Moreover, a new method was developed to overcome the methanol insoluble particulate carbon detachment problem to achieve more reliable MAC values. We found that OC can be dissolved in methanol in a short time (e.g., 1 h) and ultrasonic treatment and long-term soaking do not significantly increase the extractable OC fraction. Additionally, we proved that methanol insoluble particulate carbon detachment in methanol does exist in previous methods, causing overestimation of the BrC mass extracted by methanol and thus the underestimation of MAC values. We therefore recommend the newly developed extraction method in this study to be utilized in future related studies to quantitatively obtain the light absorption property of methanol-soluble BrC.

Review of recent literature on the light absorption properties of black carbon: Refractive index, mass absorption cross section, and absorption function

Knowledge of the optical properties of soot black carbon (BC) is required for the prediction of the radiative effects of freshly-emitted and aged BC particles. Here we review BC mass absorption cross section (MAC) and absorption function E(m) measurements, focusing on freshly-emitted BC. First, we review recently reported MACs at 550 nm wavelength as obtained from direct measurements of particulate absorption and mass concentration; we find an average of 8.0 ± 0.7 m2/g from ten measurements, not significantly higher (p > 0.26) than the widely used MAC of 7.5 ± 1.2 m2/g recommended by Bond and Bergstrom [Bond, T. C., and R. W. Bergstrom. 2006. Light absorption by carbonaceous particles: An investigative review. Aerosol Sci. Technol. 40(1):27–67]. Second, we review recently reported E(m), whose retrieval is more complex due to the need to combine measurements with numerical models to estimate the contribution of scattering to extinction. Third, we review recent numerical studies that have aimed to predict the BC MAC using various complex refractive indices (m = n + ik). Most of these studies have used m = 1.95 + 0.79i recommended by Bond and Bergstrom (2006), yet failed to predict a MAC as high as 7.5 or 8.0 m2/g at 550 nm wavelength. Fourth, we summarize a selected range of alternative values of m that has been reported by recent studies and place them in the context of measurements using a contour plot of E(m) on the n–k plane. We show that the widely used m = 1.95 + 0.79i corresponds to an E(m) that is too low to be consistent with the measured MAC values. We conclude that the E(m) of BC in the visible and near infrared should be greater than 0.32, and that the commonly used BC models or the refractive index, or both, are still in need of improvement.

Mass absorption cross-section of flare-generated black carbon: Variability, predictive model, and implications

Global gas flaring is an important source of black carbon (BC) emissions with uncertain climate impacts. The link between atmospheric concentration and direct radiative forcing (DRF) by BC is its mass absorption cross-section (MAC). MAC data for flare-generated BC are lacking in the literature and the only known data conflict with generally-accepted BC MAC values, which are assumed to be source-independent. This paper presents the first measurements of BC MAC for large-scale flares, burning globally-representative, industry-relevant flare gas compositions in a controlled facility. BC MAC was calculated with precisely-quantified uncertainties using photoacoustic and thermal-optical instruments. Flare-generated carbon was found to be primarily elemental in composition (typically >92%), and most probably externally-mixed based on detailed analysis of attenuation vs. evolved carbon data and consideration of flare-specific mechanisms for organic carbon emissions. Flare BC MAC was generally larger than well-cited literature values and had statistically significant variations with fuel and operating conditions. Variability in BC MAC was well-predicted by a novel phenomenological model based on flame radiative characteristics and relative BC production. The derived model consolidates previously-unreconciled disparate data from different sources and suggests that flare BC MAC is likely >1.3–2 times standard values, implying an underestimation of DRF by flare-generated BC.

Effects of mixing state on optical and radiative properties of black carbon in the European Arctic

Abstract. Atmospheric aging promotes internal mixing of black carbon (BC), leading to an enhancement of light absorption and radiative forcing. The relationship between BC mixing state and consequent absorption enhancement was never estimated for BC found in the Arctic region. In the present work, we aim to quantify the absorption enhancement and its impact on radiative forcing as a function of microphysical properties and mixing state of BC observed in situ at the Zeppelin Arctic station (78∘ N) in the spring of 2012 during the CLIMSLIP (Climate impacts of short-lived pollutants in the polar region) project. Single-particle soot photometer (SP2) measurements showed a mean mass concentration of refractory black carbon (rBC) of 39 ng m−3, while the rBC mass size distribution was of lognormal shape, peaking at an rBC mass-equivalent diameter (DrBC) of around 240 nm. On average, the number fraction of particles containing a BC core with DrBC&gt;80 nm was less than 5 % in the size range (overall optical particle diameter) from 150 to 500 nm. The BC cores were internally mixed with other particulate matter. The median coating thickness of BC cores with 220 nm &lt; DrBC&lt; 260 nm was 52 nm, resulting in a core–shell diameter ratio of 1.4, assuming a coated sphere morphology. Combining the aerosol absorption coefficient observed with an Aethalometer and the rBC mass concentration from the SP2, a mass absorption cross section (MAC) of 9.8 m2 g−1 was inferred at a wavelength of 550 nm. Consistent with direct observation, a similar MAC value (8.4 m2 g−1 at 550 nm) was obtained indirectly by using Mie theory and assuming a coated-sphere morphology with the BC mixing state constrained from the SP2 measurements. According to these calculations, the lensing effect is estimated to cause a 54 % enhancement of the MAC compared to that of bare BC particles with equal BC core size distribution. Finally, the ARTDECO radiative transfer model was used to estimate the sensitivity of the radiative balance to changes in light absorption by BC as a result of a varying degree of internal mixing at constant total BC mass. The clear-sky noontime aerosol radiative forcing over a surface with an assumed wavelength-dependent albedo of 0.76–0.89 decreased, when ignoring the absorption enhancement, by −0.12 W m−2 compared to the base case scenario, which was constrained with mean observed aerosol properties for the Zeppelin site in Arctic spring. The exact magnitude of this forcing difference scales with environmental conditions such as the aerosol optical depth, solar zenith angle and surface albedo. Nevertheless, our investigation suggests that the absorption enhancement due to internal mixing of BC, which is a systematic effect, should be considered for quantifying the aerosol radiative forcing in the Arctic region.

Light-absorption of dust and elemental carbon in snow in the Indian Himalayas and the Finnish Arctic

Abstract. Light-absorbing impurities (LAIs) deposited in snow have the potential to substantially affect the snow radiation budget, with subsequent implications for snow melt. To more accurately quantify the snow albedo, the contribution from different LAIs needs to be assessed. Here we estimate the main LAI components, elemental carbon (EC) (as a proxy for black carbon) and mineral dust in snow from the Indian Himalayas and paired the results with snow samples from Arctic Finland. The impurities are collected onto quartz filters and are analyzed thermal–optically for EC, as well as with an additional optical measurement to estimate the light-absorption of dust separately on the filters. Laboratory tests were conducted using substrates containing soot and mineral particles, especially prepared to test the experimental setup. Analyzed ambient snow samples show EC concentrations that are in the same range as presented by previous research, for each respective region. In terms of the mass absorption cross section (MAC) our ambient EC surprisingly had about half of the MAC value compared to our laboratory standard EC (chimney soot), suggesting a less light absorptive EC in the snow, which has consequences for the snow albedo reduction caused by EC. In the Himalayan samples, larger contributions by dust (in the range of 50 % or greater for the light absorption caused by the LAI) highlighted the importance of dust acting as a light absorber in the snow. Moreover, EC concentrations in the Indian samples, acquired from a 120 cm deep snow pit (possibly covering the last five years of snow fall), suggest an increase in both EC and dust deposition. This work emphasizes the complexity in determining the snow albedo, showing that LAI concentrations alone might not be sufficient, but additional transient effects on the light-absorbing properties of the EC need to be considered and studied in the snow. Equally as imperative is the confirmation of the spatial and temporal representativeness of these data by comparing data from several and deeper pits explored at the same time.

Evaluation of ground‐based black carbon measurements by filter‐based photometers at two Arctic sites

AbstractLong‐term measurements of the light absorption coefficient (babs) obtained with a particle soot absorption photometer (PSAP), babs (PSAP), have been previously reported for Barrow, Alaska, and Ny‐Ålesund, Spitsbergen, in the Arctic. However, the effects on babs of other aerosol chemical species coexisting with black carbon (BC) have not been critically evaluated. Furthermore, different mass absorption cross section (MAC) values have been used to convert babs to BC mass concentration (MBC = babs/MAC). We used a continuous soot monitoring system (COSMOS), which uses a heated inlet to remove volatile aerosol compounds, to measure babs (babs (COSMOS)) at these sites during 2012–2015. Field measurements and laboratory experiments have suggested that babs (COSMOS) is affected by about 9% on average by sea‐salt aerosols. MBC values derived by COSMOS (MBC (COSMOS)) using a MAC value obtained by our previous studies agreed to within 9% with elemental carbon concentrations at Barrow measured over 11 months. babs (PSAP) was higher than babs (COSMOS), by 22% at Barrow (PM1) and by 43% at Ny‐Ålesund (PM10), presumably due to the contribution of volatile aerosol species to babs (PSAP). Using babs (COSMOS) as a reference, we derived MBC (PSAP) from babs (PSAP) measured since 1998. We also established the seasonal variations of MBC at these sites. Seasonally averaged MBC (PSAP) decreased at a rate of about 0.55 ± 0.30 ng m−3 yr−1. We also compared MBC (COSMOS) and scaled MBC (PSAP) values with previously reported data and evaluated the degree of inconsistency in the previous data.

A European aerosol phenomenology-5: Climatology of black carbon optical properties at 9 regional background sites across Europe

A reliable assessment of the optical properties of atmospheric black carbon is of crucial importance for an accurate estimation of radiative forcing. In this study we investigated the spatio-temporal variability of the mass absorption cross-section (MAC) of atmospheric black carbon, defined as light absorption coefficient (σap) divided by elemental carbon mass concentration (mEC). σap and mEC have been monitored at supersites of the ACTRIS network for a minimum period of one year. The 9 rural background sites considered in this study cover southern Scandinavia, central Europe and the Mediterranean. σap was determined using filter based absorption photometers and mEC using a thermal-optical technique. Homogeneity of the data-set was ensured by harmonization of all involved methods and instruments during extensive intercomparison exercises at the European Center for Aerosol Calibration (ECAC). Annual mean values of σap at a wavelength of 637 nm vary between 0.66 and 1.3 Mm−1 in southern Scandinavia, 3.7–11 Mm−1 in Central Europe and the British Isles, and 2.3–2.8 Mm−1 in the Mediterranean. Annual mean values of mEC vary between 0.084 and 0.23 μg m−3 in southern Scandinavia, 0.28–1.1 in Central Europe and the British Isles, and 0.22–0.26 in the Mediterranean. Both σap and mEC in southern Scandinavia and Central Europe have a distinct seasonality with maxima during the cold season and minima during summer, whereas at the Mediterranean sites an opposite trend was observed. Annual mean MAC values were quite similar across all sites and the seasonal variability was small at most sites. Consequently, a MAC value of 10.0 m2 g−1 (geometric standard deviation = 1.33) at a wavelength of 637 nm can be considered to be representative of the mixed boundary layer at European background sites, where BC is expected to be internally mixed to a large extent. The observed spatial variability is rather small compared to the variability of values in previous literature, indicating that the harmonization efforts resulted in substantially increased precision of the reported MAC. However, absolute uncertainties of the reported MAC values remain as high as ± 30–70% due to the lack of appropriate reference methods and calibration materials.The mass ratio between elemental carbon and non-light-absorbing matter was used as a proxy for the thickness of coatings around the BC cores, in order to assess the influence of the mixing state on the MAC of BC. Indeed, the MAC was found to increase with increasing values of the coating thickness proxy. This provides evidence that coatings do increase the MAC of atmospheric BC to some extent, which is commonly referred to as lensing effect.

Mass Absorption Cross-Section of Ambient Black Carbon Aerosol in Relation to Chemical Age

Three differing techniques were used to measure ambient black carbon (BC) aerosols in downtown Toronto through 20 December 2006 to 23 January 2007. These techniques were thermal analysis, as performed by a Sunset Labs OCEC Analyzer (OCEC); light attenuation, as performed by an Aethalometer (AE); and photoacoustic analysis, as performed by a Photoacoustic Instrument (PA). These measurements of ambient PM 2.5 were used to investigate the effects of coating thickness on BC Mass Absorption Cross-section (MAC). MAC values were determined by comparing 880 nm and 370 nm AE measurements and PA measurements of b abs (absorption coefficient, Mm–1) to the OCEC measurements. Based on mass size distributions and supporting criteria, the PM 2.5 was classified as fresh, semi-aged, or aged. The average MAC values in these categories, based on the PA measurements, were 9.3 ± 1.8, 9.9 ± 2.0, and 9.3 ± 2.2 m 2 /g (mean ± standard deviation), respectively, suggesting that any difference in coating thickness as a result of ag...

Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area

Abstract. Data taken from the MCMA-2003 and the 2006 MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from a Multi-Filter Rotating Shadowband Radiometer (MFRSR) and an actinic flux spectroradiometer (SR), we derive aerosol single scattering albedo, ϖ0, λ, as a function of wavelength, λ. We find that in the near-UV spectral range (250 to 400 nm) ϖ0, λ is much lower compared to ϖ0, λ at 500 nm indicating enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or gas cannot account for this enhanced absorption leaving the organic carbon component of the aerosol (OA) as the most likely absorber. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred ϖ0, λ to estimate the Mass Absorption Cross section (MAC) for the organic aerosol. We find that the MAC is about 10.5 m2/g at 300 nm and falls close to zero at about 500 nm; values that are roughly consistent with other estimates of organic aerosol MAC. These MAC values can be considered as "radiatively correct", because when used in radiative transfer calculations, the calculated irradiances/actinic fluxes match those measured at the wavelengths considered here. For an illustrative case study described here, we estimate that the light absorption by the "brown" (organic) carbonaceous aerosol can add about 40% to the light absorption of black carbon in Mexico City. This contribution will vary depending on the relative abundance of organic aerosol relative to black carbon. Furthermore, our analysis indicates that organic aerosol would slow down photochemistry by selectively scavenging the light reaching the ground at those wavelengths that drive photochemical reactions. Finally, satellite retrievals of trace gases that are used to infer emissions currently assume that the MAC of organic carbon is zero. For trace gases that are retrieved using wavelengths shorter then 420 nm (i.e. SO2, HCHO, halogenoxides, NO2), the assumption of non-zero MAC values will induce an upward correction to the inferred emissions. This assumption will be particularly relevant in polluted urban atmospheres and areas of biomass burning where organic aerosols are particularly abundant.