Absorption Angstrom Exponent Research Articles

Ambient PM2.5, Black Carbon, and Particle Size-Resolved Number Concentrations and the Ångström Exponent Value of Aerosols during the Firework Display at the Lantern Festival in Southern Taiwan

ABSTRACTThe Yanshui Beehive Firework Festival is a traditional folk activity in Southern Taiwan held during the Lantern Festival, and it is the third largest folk celebration in the world. During this festival, more than 200 firecracker towers with hundreds of thousands of firecrackers are ignited, posing a risk to public health because of an abrupt increase in particulate matter concentrations within a short period. In this study, real-time variations of PM2.5 (particles with an aerodynamic diameter less than or equal to 2.5 µm), black carbon (BC), and particle number concentrations were monitored before and during the firework display to understand the effect of the firework display on the short-term air quality. The hourly average concentrations of PM2.5, BC, total particle number, and ultrafine particle number during the firework display (episode period) were 146.9 µg m–3, 2639 ng m–3, 3.37 × 104 # cm–3, and 1.18 × 104 # cm–3, respectively. These values were 6.9, 2.3, 5.9, and 3.7 times greater than those during the same period on reference days (nonepisode period), respectively. The measured ultraviolet BC (UVBC) and BC concentrations indicated that aerosols were bound with ultraviolet-absorbing organic compounds, which were abundant, during the episode period. BC aerosols during the episode originated from vehicular traffic and firecracker burning, and the absorption Angstrom exponent value was 1.4. The particle number size distribution during the episode period showed a major accumulation mode and a minor Aitken mode of 180 and 63 nm, respectively. This particle number size distribution pattern was considerably different from that in the nonepisode period. During the episode period, particle coagulation played a crucial role in removing particles in the nucleation and Aitken modes in the ambient air at high particle number concentrations.

The characteristics of brown carbon aerosol during winter in Beijing

Abstract Brown carbon (i.e., light-absorbing organic carbon, or BrC) exerts important effects on the environment and on climate in particular. Based on spectrophotometric absorption measurements on extracts of bulk aerosol samples, this study investigated the characteristics of BrC during winter in Beijing, China. Organic compounds extractable by methanol contributed approximately 85% to the organic carbon (OC) mass. Light absorption by the methanol extracts exhibited a strong wavelength dependence, with an average absorption Angstrom exponent of 7.10 (fitted between 310 and 450 nm). Normalizing the absorption coefficient ( b abs ) measured at 365 nm to the extractable OC mass yielded an average mass absorption efficiency (MAE) of 1.45 m 2 /g for the methanol extracts. This study suggests that light absorption by BrC could be comparable with black carbon in the spectral range of near-ultraviolet light. Our results also indicate that BrC absorption and thus BrC radiative forcing could be largely underestimated when using water-soluble organic carbon (WSOC) as a surrogate for BrC. Compared to previous work relying only on WSOC, this study provides a more comprehensive understanding of BrC aerosol based on methanol extraction.

Seasonal inhomogeneity of soot particles over the central Indo-Gangetic Plains, India: Influence of meteorology

Black carbon (BC) particles play a unique and important role in earth’s climate system. BC was measured (in-situ) in the central part of the Indo-Gangetic Plains (IGP) at Varanasi, which is a highly populated and polluted region due to its topography and extensive emission sources. The annual mean BC mass concentration was 8.92 ± 7.0 µg m -3, with 34% of samples exceeding the average value. Seasonally, BC was highest during the post-monsoon and winter periods (approximately 18 µg m -3) and lower in the premonsoon/ monsoon seasons (approximately 6 µg m -3). The highest frequency (approximately 46%) observed for BC mass was in the interval from 5 to 10 µg m -3. However, during the post-monsoon season, the most common values (approximately 23%) were between 20 and 25 µg m -3. The nighttime concentrations of BC were approximately twice as much as the daytime values because of lower boundary layer heights at nighttime. The Angstrom exponent was significantly positively correlated (0.55) with ground-level BC concentrations, indicating the impact of BC on the columnar aerosol properties. The estimated mean absorption Angstrom exponent was 1.02 ± 0.08 µg m -3, indicating that the major source of BC was from fossil fuel combustion. Significant negative correlations between BC mass and meteorological parameters indicate a pronounced effect of atmospheric dynamics on the BC mass in this region. The highest mean BC mass concentration (18.1 ± 6.9 µg m -3) as a function of wind speed was under calm wind conditions (38% of the time).

Direct radiative effect by brown carbon over the Indo-Gangetic Plain

Abstract. The importance of light-absorbing organic aerosols, often called brown carbon (BrC), has become evident in recent years. However, there have been relatively few measurement-based estimates for the direct radiative effect of BrC so far. In earlier studies, the AErosol RObotic NETwork (AERONET)-measured aerosol absorption optical depth (AAOD) and absorption Angstrom exponent (AAE) were exploited. However, these two pieces of information are clearly not sufficient to separate properly carbonaceous aerosols from dust, while imaginary indices of refraction would contain more and better justified information for this purpose. This is first time that the direct radiative effect (DRE) of BrC is estimated by exploiting the AERONET-retrieved imaginary indices. We estimated it for four sites in the Indo-Gangetic Plain (IGP), Karachi, Lahore, Kanpur and Gandhi College. We found a distinct seasonality, which was generally similar among all the sites, but with slightly different strengths. The monthly warming effect up to 0.5 W m−2 takes place during the spring season. On the other hand, BrC results in an overall cooling effect in the winter season, which can reach levels close to −1 W m−2. We then estimated similarly also the DRE of black carbon and total aerosol, in order to assess the relative significance of the BrC radiative effect in the radiative effects of other components. Even though BrC impact seems minor in this context, we demonstrated that it is not insignificant. Moreover, we demonstrated that it is crucial to perform spectrally resolved radiative transfer calculations to obtain good estimates for the DRE of BrC.

Significant increase of aerosol number concentrations in air masses crossing a densely trafficked sea area

In this study, we evaluated 10 months data (September 2009 to June 2010) of atmospheric aerosol particle number size distribution at three atmospheric observation stations along the Baltic Sea coast: Vavihill (upwind, Sweden), Uto (upwind, Finland), and Preila (downwind, Lithuania). Differences in aerosol particle number size distributions between the upwind and downwind stations during situations of connected atmospheric flow, when the air passed each station, were used to assess the contribution of ship emissions to the aerosol number concentration (diameter interval 50-400 nm) in the Lithuanian background coastal environment. A clear increase in particle number concentration could be noticed, by a factor of 1.9 from Uto to Preila (the average total number concentration at Uto was 791 cm(-3)), and by a factor of 1.6 from Vavihill to Preila (the average total number concentration at Vavihill was 998 cm(-3)). The simultaneous measurements of absorption Angstrom exponents close to unity at Preila supported our conclusion that ship emissions in the Baltic Sea contributed to the increase in particle number concentration at Preila. (C) 2015 Institute of Oceanology of the Polish Academy of Sciences. (Less)

Application of aerosol optical properties to estimate aerosol type from ground-based remote sensing observation at urban area of northeastern China

Aerosol optical properties were derived from ground-based sunphotometer observations between 2009-2013 at three urban sites of Shenyang, Anshan, Fushun in northeastern China. The annual means for extinction aerosol optical depths (EAOD) at 500nm were 0.57±0.38, 0.52±0.35, and 0.41±0.31 at Shenyang, Anshan, Fushun, respectively. The corresponding annual means for the extinction Angstrom exponents (EAE) computed for the wavelengths of 440 and 870nm were 0.86±0.32, 0.86±0.34 and 0.91±0.35, respectively, indicating that urban area of Northeast China were affected by both coarse and fine particles. Hygroscopic growth in summer and incursions of dust aerosols in spring were evidently revealed from the analysis of the relationship between EAE and δEAE (the EAE difference, δEAE=EAE(440,670)−EAE(670,870)). The annual mean absorption aerosol optical depths (AAOD440nm) values at Shenyang, Anshan, Fushun were 0.15±0.11, 0.10±0.07, 0.08±0.04, respectively. The annual mean absorption Angstrom exponents (AAE440–870nm) values were 0.86±0.24, 1.19±0.39, 1.33±0.36 at Shenyang, Anshan, Fushun, respectively. When the AAEs were close to unity at Anshan, the absorption aerosol particles evidently consisted of black carbon from coal combustion and motor vehicles. Larger AAEs at Fushun were indicative of absorbing aerosols mainly from biomass burning and mineral dust. The AAE at Shenyang was<1 which may be consistent with black carbon particles with absorbing or non-absorbing coatings. Analysis of the relationship between the AAEs and extinction Angstrom exponents showed that the aerosol populations at these three sites could be classified as “mixed-small particles” including anthropogenic particles and secondary organic aerosol with highly variable sphericity fractions.

Comparison of column-integrated aerosol optical and physical properties in an urban and suburban site on the North China Plain

The column-integrated optical properties of aerosol in Beijing and Xianghe, two AErosol RObotic NETwork (AERONET) sites situated on the North China Plain (NCP), are investigated based on Cimel sunphotometer measurements from October 2004 to June 2012. The outstanding feature found is that the seasonal medians of aerosol optical depth (AOD) at the two stations are in good agreement. The correlation coefficients and the absolute differences between AOD at the two stations are larger than 0.84 and less than 0.05, respectively. Good agreement in AOD at these two sites (one urban and the other suburban; 70 km apart) indicates that aerosol pollution in the Greater Beijing area is regional in nature. However, we find significant differences in the absorption Angstrom Exponent (AAE), the real and imaginary part of the refractive index, and thereby the single scattering albedo (SSA), and the difference is seasonally dependent. The feature is found to be more prominent in fall when the fine-mode fraction (FMF) and fine-mode effective radius are significantly different at the two stations, besides the parameters mentioned above. The SSA in Beijing at four wavelengths shows lower values as compared to those in Xianghe, although the difference is not significant in some cases. Significant differences in AAE and fine-mode effective radius indicate that there are differences in aerosol physical and chemical properties in urban and suburban regions on the NCP.

Impacts of biomass-burning on aerosol properties of a severe haze event over Shanghai

Anthropogenic aerosols have significant impacts on the environment and human health in the Yangtze River Delta region, one of the most densely populated regions in the world. A biomass-burning plume swept across this area (Shanghai) in May 2009, leading to changes in the physical and optical properties of aerosols, which were investigated using ground-based remote sensing and in situ measurements via comparisons with dust pollution and background conditions. Experiments show that the biomass-burning plume led to an increase in the average aerosol optical depth (AOD) at 500nm from 0.73 to 1.00 (37% higher), an absorption Angstrom exponent (AAE) of 1.48, and an increase in the Angstrom exponent (α) up to 1.53. Furthermore, local dust aerosols derived from road dust and/or construction dust also led to higher values of AOD (2.68) and AAE (2.16), and a daily average value of α of 1.05. For the biomass-burning plume, the aerosol particles exhibited significant variations in short-wavelength spectra. The single scattering albedo at 670nm decreased remarkably under the influence of the biomass-burning plume, indicating the significant absorptive ability of the biomass-burning pollution and higher ratio of absorption aerosols within the plume. Under the effects of the biomass-burning, the volume concentration of fine-mode aerosols increased significantly and the PM-fine/PM-coarse volume concentration ratio reached 12.33. This relatively large change in fine-mode particles indicates that biomass-burning has a greater impact on fine-mode aerosols than on coarse-mode aerosols.

Spectral Light Absorption of Ambient Aerosols in Urban Beijing during Summer: An Intercomparison of Measurements from a Range of Instruments

ABSTRACTAerosol light absorption is important to radiation balance, but it is difficult to accurately quantify using measurements. An intercomparison experiment for the measurement of the aerosol absorption coefficient (bap) was performed at an urban site in Beijing during the summer of 2012, including the filter-based particle soot absorption photometer (PSAP) and aethalometer (AE-31), and the reference photoacoustic extinctiometer (PAX), CRDS-Neph (cavity-ring down spectroscopy/nephelometer) system, and multi-angle absorption photometer (MAAP). The CRDS-Neph system and PAX performed poorly due to unexpected reasons. The corrected bap of the PSAP agreed well with the reference values determined by the MAAP, implying the applicability of this correction scheme as well as the credibility of the reference bap of the MAAP. A new conversion factor with a value of ~7.1 ± 0.05 m2/g at ~530 nm was established by regressing the reference bap against the AE-31 recorded black carbon (BC) concentrations, which is lower than the previously used value (8.28 m2/g). Accordingly, the absorption Angstrom exponent (αap) was estimated as 0.85 ± 0.21 on average. It was ~1 on clean days but significantly lower during pollution episodes, implying the main contributor to aerosol light absorption is freshly-emitted BC on clean days but aged BC during pollution. BC core sizes and the coating are likely to have a great impact on the αap, which needs further investigation. The mass absorption efficiency of BC was estimated by regressing the bap against the filter-analyzed elemental carbon (EC) concentrations, resulting in a mean of 9.2 ± 0.5 m2/g at 670 nm. It was remarkably higher during pollution episodes than on clean days, implying a high variation of aerosol properties, such as the mixing state, with pollution levels.

Vertical Distribution and Columnar Optical Properties of Springtime Biomass-Burning Aerosols over Northern Indochina during 2014 7-SEAS Campaign

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.

Fog-Induced Changes in Optical and Physical Properties of Transported Aerosols over Sundarban, India

A campaign was conducted at Kalas Island, Sundarban to address fog-induced changes in optical and physical properties of aerosols during the winter period (11–16 January 2014). Being an isolated remote island in the northern coastal region of Bay of Bengal, the measurement site provides a unique opportunity to investigate aerosol properties and foggy conditions during transported aerosol plumes from the Indo-Gangetic Basin (IGB). Two fog events were observed over Sundarban during the campaign increasing Aerosol Optical Depth (AOD) by almost a factor of three (1.6 ± 0.4) compared to “background” AOD of 0.52 ± 0.2 on a normal winter day. Back-trajectory analysis shows that aerosols mostly originate from the IGB contributing to higher fine-mode as well as coarse-mode aerosol concentrations during the foggy days. Black Carbon (BC), known as a tracer for anthropogenic sources, is found to be about 15.2 ± 1.3 µg/m3 at such a remote region and increased by 30% during foggy days indicating strong influence from transported anthropogenic aerosols from nearby urban regions. Similar enhancement is also observed in aerosol absorption coefficient, especially in the UV region. Low ventilation due to calm and cool atmosphere with a shallow boundary layer during foggy days could have ‘trapped’ BC over Sundarban and resulted in such an enhancement. On the other hand, the absorption angstrom exponent reduced indicating dominance of weakly spectral dependent aerosols during foggy period, mostly associated with fossil-fuel combustion. However, this could also be the result of BC coating with water-soluble species under high RH conditions. Enhancement of absorbing aerosols during foggy period reduces the incoming solar radiation, causing large perturbation in the radiation budget over the site.

Determination of wood burning and fossil fuel contribution of black carbon at Delhi, India using aerosol light absorption technique.

A comprehensive measurement program of effective black carbon (eBC), fine particle (PM2.5), and carbon monoxide (CO) was undertaken during 1 December 2011 to 31 March 2012 (winter period) in Delhi, India. The mean mass concentrations of eBC, PM2.5, and CO were recorded as 12.1 ± 8.7 μg/m(3), 182.75 ± 114.5 μg/m(3), and 3.41 ± 1.6 ppm, respectively, during the study period. Also, the absorption Angstrom exponent (AAE) was estimated from eBC and varied from 0.38 to 1.29 with a mean value of 1.09 ± 0.11. The frequency of occurrence of AAE was ~17 % less than unity whereas ~83 % greater than unity was observed during the winter period in Delhi. The mass concentrations of eBC were found to be higher by ~34 % of the average value of eBC (12.1 μg/m(3)) during the study period. Sources of eBC were estimated, and they were ~94 % from fossil fuel (eBCff) combustion whereas only 6 % was from wood burning (eBCwb). The ratio between eBCff and eBCwb was 15, which indicates a higher impact from fossil fuels compared to biomass burning. When comparing eBCff during day and night, a factor of three higher concentrations was observed in nighttime than daytime, and it is due to combustion of fossil fuel (diesel vehicle emission) and shallow boundary layer conditions. The contribution of eBCwb in eBC was higher between 1800 and 2100 hours due to burning of wood/biomass. A significant correlation between eBC and PM2.5 (r = 0.78) and eBC and CO (r = 0.46) indicates the similarity in location sources. The mass concentration of eBC was highest (23.4 μg/m(3)) during the month of December when the mean visibility (VIS) was lowest (1.31 km). Regression analysis among wind speed (WS), VIS, soot particles, and CO was studied, and significant negative relationships were seen between VIS and eBC (-0.65), eBCff (-0.66), eBCwb (-0.34), and CO (-0.65); however, between WS and eBC (-0.68), eBCff (-0.67), eBCwb (-0.28), and CO (-0.53). The regression analysis indicated that emission of soot particles may be localized to fossil fuel combustion, whereas wood/biomass burning emission of black carbon is due to transportation from farther distances. Regression analysis between eBCff and CO (r = 0.44) indicated a similar source as vehicular emissions. The very high loading of PM2.5 along with eBC over Delhi suggests that urgent action is needed to mitigate the emissions of carbonaceous aerosol in the northern part of India.

Water-soluble organic carbon aerosols during a full New Delhi winter: Isotope-based source apportionment and optical properties

Water-soluble organic carbon (WSOC) is a major constituent (~ 20–80%) of the total organic carbon aerosol over the Indian subcontinent during the dry winter season. Due to its multiple primary and secondary formation pathways, the sources of WSOC are poorly characterized. In this study, we present radiocarbon constraints on the biomass versus fossil sources of WSOC in PM2.5 for the 2010/2011 winter period for the megacity Delhi, situated in the northern part of the heavily polluted Indo-Gangetic Plain. The fossil fuel contribution to Delhi WSOC (21 ± 4%) is similar to that recently found at two South Asian background sites. In contrast, the stable carbon isotopic composition of Delhi WSOC is less enriched in 13C relative to that at the two receptor sites. Although potentially influenced also by source variability, this indicates that near-source WSOC is less affected by atmospheric aging. In addition, the light absorptive properties of Delhi WSOC were studied. The mass absorption cross section at 365 nm (MAC365) was 1.1–2.7 m2/g with an Absorption Angstrom Exponent ranging between 3.1 and 9.3. Using a simplistic model the relative absorptive forcing of the WSOC compared to elemental carbon in 2010/2011 wintertime Delhi was estimated to range between 3 and 11%. Taken together, this near-source study shows that WSOC in urban Delhi comes mainly (79%) from biomass burning/biogenic sources. Furthermore, it is less influenced by photochemical aging compared to WSOC at South Asian regional receptor sites and contributes with a relatively small direct absorptive forcing effect.

Study on Black Carbon and Its Characterisation at an Urban Location, Hyderabad during 2010 to 2012

A seven channel Aethalometer (AE31) was used to measure the black carbon (BC) mass concentrations at the premises of National Remote Sensing Centre (NRSC), Hyderabad during the period 2010-2012 to characterize BC temporal concentration and source identification. Diurnal variation of BC shows varied amplitude of BC mass concentration with peak during mornings and nights with minimum during afternoons of all the study periods (2010-2012). Highest absorption coefficient at 520 nm (babs = 38.79 Mm-1) was observed during post monsoon season of 2011 and lowest value observed (babs = 22.06 Mm-1) during winter season of 2010. Absorption Angstrom Exponents (AAE) were analysed in the lower (370 nm-520 nm) and higher wavelength (590 nm-950 nm) region to explain the BC origin. A few number of days with high contrasting AAE in lower and higher wavelengths, responsible for the bio fuel generated BC were found during the study period. Study revealed that majority of the study period, AAE values are around 1, attributed to mainly vehicular emission. Highest AAE of 1.57 (370 nm-950 nm) was observed on 4 January, 2012. Three representative days of pre-monsoon, post-monsoon and winter were considered for analyzing daily variations of AAE in conjunction with back trajectories from NOAA HYSPLIT model to confirm the source of BC as biomass burning origin.

Column-integrated aerosol optical and physical properties at a regional background atmosphere in North China Plain

The AERONET level 2.0 data at Xinglong station from February 2006 to July 2011 were used to characterize the aerosol optical and physical properties, including temporal variability, aerosol absorption, classification and properties under dust and haze conditions. The annual mean aerosol optical depth (AOD) and extinction Angstrom exponent (EAE) are 0.28 ± 0.30 and 1.07 ± 0.38, respectively. The seasonal variations of AOD440nm are higher in spring (0.40 ± 0.3) and summer (0.40 ± 0.42) than in autumn (0.20 ± 0.22) and winter (0.19 ± 0.21). The EAE is low in spring (0.96 ± 0.43) and high in summer (1.22 ± 0.38). The EAE is ∼1.25 with an absorption Angstrom exponent (AAE) of ∼1.0–1.5 in Xinglong, which indicates that the dominant type is mixed aerosol (accounting for 88.2% at AAE > 1.0). Almost all of the dust observations occurred in spring. The volume concentrations of both fine and coarse mode particles increase with increasing AOD. In spring, the increase of coarse particles is greater than that of fine aerosols; however, the reverse phenomenon is observed for other seasons. The high AOD at Xinglong could be associated with the growth of fine mode aerosols and addition of coarse mode particles. This background station is not only impacted by dust aerosols from northwest China and south Mongolia but also influenced by long-range transportation of anthropogenic aerosols from south urban and industrialized regions. The mean AOD was 1.49 on the dust day, while AOD was 1.10 on the haze day. The mean EAEs were 0.09 and 1.43 on dust and haze days, respectively.

Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model

Abstract. According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry and refractive index of particle aggregates. DDSCAT predicts a higher mass absorption coefficient (MAC), lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. The MAC of BC (averaged over the 200–1000 nm range) is amplified when internally mixed with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle size and morphology. This study shows that DDSCAT predicts complex morphology and mixing state dependent aerosol optical properties that have been reported previously and are relevant to radiative transfer, climate modeling, and interpretation of remote sensing measurements.

Identification of key aerosol populations through their size and composition resolved spectral scattering and absorption

Abstract. Characterizing chemical and physical aerosol properties is important to understand their sources, effects, and feedback mechanisms in the atmosphere. This study proposes a scheme to classify aerosol populations based on their spectral optical properties (absorption and scattering). The scheme is obtained thanks to the outstanding set of information on particle size and composition these properties contain. The spectral variability of the aerosol single scattering albedo (dSSA), and the extinction, scattering and absorption Angstrom exponents (EAE, SAE and AAE, respectively) were observed on the basis of two-year measurements of aerosol optical properties (scattering and absorption coefficients at blue, green and red wavelengths) performed in the suburbs of Rome (Italy). Optical measurements of various aerosol types were coupled to measurements of particle number size distributions and relevant optical properties simulations (Mie theory). These latter allowed the investigation of the role of the particle size and composition in the bulk aerosol properties observed. The combination of simulations and measurements suggested a general "paradigm" built on dSSA, SAE and AAE to optically classify aerosols. The paradigm proved suitable to identify the presence of key aerosol populations, including soot, biomass burning, organics, dust and marine particles. The work highlights that (i) aerosol populations show distinctive combinations of SAE and dSSA times AAE, these variables being linked by a linear inverse relation varying with varying SSA; (ii) fine particles show EAE &gt; 1.5, whilst EAE &lt; 2 is found for both coarse particles and ultrafine soot-rich aerosols; (iii) fine and coarse particles both show SSA &gt; 0.8, whilst ultrafine urban Aitken mode and soot particles show SSA &lt; 0.8. The proposed paradigm agrees with aerosol observations performed during past major field campaigns, this indicating that relations concerning the paradigm have a general validity.

Absorption Spectrum of Ambient Aerosol and Its Correlation with Size Distribution in Specific Atmospheric Conditions after a Red Mud Accident

Approximately one month after a red sludge industrial accident, a high concentration of atmospheric aerosol was reported in the affected area. This increased aerosol load originated mainly from the fugitive dust emitted from the red sludge sediment, representing a serious threat to public health. In a study presented here, a recently developed four-wavelength photoacoustic spectrometer (4λ-PAS), which simultaneously measures optical aerosol absorption in the near infrared, visible and ultra-violet spectral ranges with high temporal resolution, was operated in the centre of the sludge hit area. Based on the results of the photoacoustic absorption measurements of temporal changes in light absorption and their correlations with other aerosol variables, including size distribution, were carried out in special atmospheric conditions (including with the presence of “atypical” red mud particles). Correlations between segregated absorption Angstrom exponents (AAEs) and specific size ranges were tabulated, and the resulting data were used to draw conclusions about the reasons for the temporal changes to the measured quantities. The absorption behavior of red mud particles was determined in UV, VIS, and near IR spectral regions under laboratory conditions. The absorption spectrum of red mud aerosol was found to be atypical (i.e., it did not follow a power law), and this was used as a selective parameter for its identification in ambient bulk. Each measurement day, at around noon, dust from the red sludge sediment was identified; while during the rest of the day atmospheric aerosol composition was dominated by carbonaceous aerosols, originating most probably from home heating and/or from traffic. In-situ number concentration and size distribution measurements confirm this interpretation of the measurements obtained with 4λ-PAS.

Light absorption of black carbon aerosol and its enhancement by mixing state in an urban atmosphere in South China

The effects of black carbon (BC) aerosol on climate warming have been the study focus in the recent decade, and the reduction of BC is now expected to have significant near-term climate change mitigation. Large uncertainties of BC optical properties, however, still exist and seriously restrict the ability to quantify BC's climate effects. In this study, advanced instrumentation (a three-wavelength photoacoustic soot spectrometer (PASS-3) and a single particle soot photometer (SP2)) were used to measure black carbon aerosol and analyze its optical properties in a mega-city in South China, Shenzhen, during the summer of 2011. The results indicated that the average BC mass concentration was 4.0 ± 3.1 μg m−3 during the campaign, accounting for ∼11% of the total PM2.5 mass concentration. The PM2.5 light absorption at 405, 532 and 781 nm was 37.1 ± 28.1, 25.4 ± 19.0 and 17.6 ± 12.9 Mm−1, respectively. The average absorption Angstrom exponent of PM2.5 in visual spectrum (AAE405–781 nm) was 1.1 ± 0.1 during the campaign, indicating that the light absorbing carbon mainly came from vehicular emissions, with little contributions from biomass burning emissions. The mass absorption efficiency (MAE) of BC at 532 nm ranged from 5.0 to 8.5 m2 g−1 during the campaign, with an average of 6.5 ± 0.5 m2 g−1, and showed an obvious diurnal pattern with high values in the daytime. The average percentage of internally mixed BC was 24.3 ± 7.9% during the campaign, showing significant positive correlation relationship with the MAE of BC. More quantitative data analysis indicated that the internally mixed BC would amplify MAE by about 7% during the campaign, which stands in accordance with the new finding of a very recent Science magazine paper (Cappa et al., 2012) that the BC absorption enhancement due to internal mixing in the real atmosphere is relatively low, in apparent contrast to theoretical model predictions.

Spectral absorption of biomass burning aerosol determined from retrieved single scattering albedo during ARCTAS

Abstract. Actinic flux, as well as aerosol chemical and optical properties, were measured aboard the NASA DC-8 aircraft during the ARCTAS (Arctic Research of the Composition of the Troposphere from Aircraft and Satellites) mission in Spring and Summer 2008. These measurements were used in a radiative transfer code to retrieve spectral (350–550 nm) aerosol single scattering albedo (SSA) for biomass burning plumes encountered on 17 April and 29 June. Retrieved SSA values were subsequently used to calculate the absorption Angstrom exponent (AAE) over the 350–500 nm range. Both plumes exhibited enhanced spectral absorption with AAE values that exceeded 1 (6.78 &amp;amp;pm; 0.38 for 17 April and 3.34 &amp;amp;pm; 0.11 for 29 June). This enhanced absorption was primarily due to organic aerosol (OA) which contributed significantly to total absorption at all wavelengths for both 17 April (57.7%) and 29 June (56.2%). OA contributions to absorption were greater at UV wavelengths than at visible wavelengths for both cases. Differences in AAE values between the two cases were attributed to differences in plume age and thus to differences in the ratio of OA and black carbon (BC) concentrations. However, notable differences between AAE values calculated for the OA (AAEOA) for 17 April (11.15 &amp;amp;pm; 0.59) and 29 June (4.94 &amp;amp;pm; 0.19) suggested differences in the plume AAE values might also be due to differences in organic aerosol composition. The 17 April OA was much more oxidized than the 29 June OA as denoted by a higher oxidation state value for 17 April (&amp;amp;plus;0.16 vs. −0.32). Differences in the AAEOA, as well as the overall AAE, were thus also possibly due to oxidation of biomass burning primary organic aerosol in the 17 April plume that resulted in the formation of OA with a greater spectral-dependence of absorption.