scholarly journals Evidence that the spectral dependence of light absorption by aerosols is affected by organic carbon

2004 ◽  
Vol 109 (D21) ◽  
pp. n/a-n/a ◽  
Author(s):  
Thomas W. Kirchstetter ◽  
T. Novakov ◽  
Peter V. Hobbs
Keyword(s):  
Organic Carbon ◽  
Light Absorption ◽  
Spectral Dependence

scholarly journals Photooxidants from Brown Carbon and Other Chromophores in Illuminated Particle Extracts

2018 ◽  
Author(s):  
Richie Kaur ◽  
Jacqueline R. Labins ◽  
Scarlett S. Helbock ◽  
Wenqing Jiang ◽  
Keith Bein ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Dissolved Organic Carbon ◽  
Organic Compounds ◽  
Excited States ◽  
Liquid Water ◽  
Light Absorption ◽  
Singlet Molecular Oxygen ◽  
Brown Carbon ◽  
Aggregate Rate ◽  
Triplet Excited States

Abstract. While photooxidants are important in atmospheric condensed phases, there are very few measurements in particulate matter (PM). Here we measure light absorption and the concentrations of three photooxidants – hydroxyl radical (•OH), singlet molecular oxygen (1O2*) and oxidizing triplet excited states of organic matter (3C*) – in illuminated aqueous extracts of wintertime particles from Davis, California. 1O2* and 3C*, which are formed from photoexcitation of brown carbon (BrC), have not been previously measured in PM. In the extracts, mass absorption coefficients for dissolved organic compounds (MACDOC) at 300 nm range between 13,000–30,000 cm2 g–C–1 and are approximately twice as high as previous values in Davis fogs. The average (± 1σ) •OH steady-state concentration in particle extracts is 4.7 (± 1.9) × 10−16 M, which is very similar to previous values in fog, cloud and rain: although our particle extracts are more concentrated, the resulting enhancement in the rate of •OH photoproduction is essentially cancelled out by a corresponding enhancement in concentrations of natural sinks for •OH. In contrast, concentrations of the two oxidants formed primarily from brown carbon (i.e., 1O2* and 3C*) are both enhanced in the particle extracts compared to Davis fogs, a result of higher concentrations of dissolved organic carbon and faster rates of light absorption in the extracts. The average 1O2* concentration in the PM extracts is 1.6 (± 0.5) × 10−12 M, seven times higher than past fog measurements, while the average concentration of oxidizing triplets is 1.0 (± 0.4) × 10−13 M, nearly double the average Davis fog value. Additionally, the rates of 1O2* and 3C* photoproduction are both well correlated with the rate of sunlight absorption. While concentrations of 1O2* and 3C* are higher in our PM extracts compared to fog, the extracts are approximately 1000 times more dilute than water-containing ambient PM. Since we cannot experimentally measure photooxidants under these ambient conditions, we measured the effect of PM dilution on oxidant concentrations and then extrapolated to ambient particle conditions. As the particle mass concentration in the extracts increases, measured concentrations of •OH remain relatively unchanged, 1O2* increases linearly, and 3C* concentrations increase less than linearly, likely due to quenching by dissolved organics. Based on our measurements, and accounting for additional sources and sinks that should be important under PM conditions, we estimate that [•OH] in particles is essentially the same as in fog waters, [3C*] is higher in PM by nearly a factor of 3, and [1O2*] is enhanced by a factor of roughly 600. Because of these enhancements in 1O2* and 3C* concentrations, the lifetimes of some highly soluble organics appear to be much shorter in particle liquid water than under foggy/cloudy conditions. Based on our extrapolated rates of formation, BrC-derived singlet molecular oxygen and triplet excited states are the dominant sinks for organic compounds in particle liquid water, with an aggregate rate of reaction for each oxidant that is approximately 200–300 times higher than the aggregate rate of reactions for organics with •OH. For individual, highly soluble reactive organic compounds it appears that 1O2* is the major sink in particle water. Triplet excited states are likely also important in the fate of individual particulate organics, but assessing this requires additional measurements of triplet interactions with dissolved organic carbon in natural samples.

scholarly journals Dynamic light absorption of biomass-burning organic carbon photochemically aged under natural sunlight

2014 ◽  
Vol 14 (3) ◽  
pp. 1517-1525 ◽  
Author(s):  
M. Zhong ◽  
M. Jang
Keyword(s):  
Organic Carbon ◽  
Biomass Burning ◽  
Light Absorption ◽  
Dynamic Change ◽  
Wood Smoke ◽  
Integrating Sphere ◽  
Natural Sunlight ◽  
Wood Burning ◽  
Polycyclic Aromatic ◽  
Uv Visible

Abstract. Wood-burning aerosol produced under smoldering conditions was photochemically aged with different relative humidity (RH) and NOx conditions using a 104 m3 dual outdoor chamber under natural sunlight. Light absorption of organic carbon (OC) was measured over the course of photooxidation using a UV–visible spectrometer connected to an integrating sphere. At high RH, the color decayed rapidly. NOx slightly prolonged the color of wood smoke, suggesting that NOx promotes the formation of chromophores via secondary processes. Overall, the mass absorption cross section (integrated between 280 and 600 nm) of OC increased by 11–54% (except high RH) in the morning and then gradually decreased by 19–68% in the afternoon. This dynamic change in light absorption of wood-burning OC can be explained by two mechanisms: chromophore formation and sunlight bleaching. To investigate the effect of chemical transformation on light absorption, wood smoke particles were characterized using various spectrometers. The intensity of fluorescence, which is mainly related to polycyclic aromatic hydrocarbons (PAHs), rapidly decreased with time, indicating the potential bleaching of PAHs. A decline of levoglucosan concentrations evinced the change of primary organic aerosol with time. The aerosol water content measured by Fourier transform infrared spectroscopy showed that wood-burning aerosol became less hygroscopic as photooxidation proceeded. A similar trend in light absorption changes has been observed in ambient smoke aerosol originating from the 2012 County Line wildfire in Florida. We conclude that the biomass-burning OC becomes less light absorbing after 8–9 h sunlight exposure compared to fresh wood-burning OC.

scholarly journals Mass absorption efficiency of elemental carbon and water-soluble organic carbon in Beijing, China

2011 ◽  
Vol 11 (22) ◽  
pp. 11497-11510 ◽  
Author(s):  
Y. Cheng ◽  
K.-B. He ◽  
M. Zheng ◽  
F.-K. Duan ◽  
Z.-Y. Du ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Biomass Burning ◽  
Light Absorption ◽  
Elemental Carbon ◽  
Organic Aerosol ◽  
Absorption Efficiency ◽  
Water Soluble ◽  
Water Soluble Organic Carbon ◽  
Soluble Organic Carbon ◽  
Mass Absorption Efficiency

Abstract. The mass absorption efficiency (MAE) of elemental carbon (EC) in Beijing was quantified using a thermal-optical carbon analyzer. The MAE measured at 632 nm was 8.45±1.71 and 9.41±1.92 m2 g−1 during winter and summer respectively. The daily variation of MAE was found to coincide with the abundance of organic carbon (OC), especially the OC to EC ratio, perhaps due to the enhancement by coating with organic aerosol (especially secondary organic aerosol, SOA) or the artifacts resulting from the redistribution of liquid-like organic particles during the filter-based absorption measurements. Using a converting approach that accounts for the discrepancy caused by measurements methods of both light absorption and EC concentration, previously published MAE values were converted to the equivalent-MAE, which is the estimated value if using the same measurement methods as used in this study. The equivalent-MAE was found to be much lower in the regions heavily impacted by biomass burning (e.g., below 2.7 m2 g−1 for two Indian cities). Results from source samples (including diesel exhaust samples and biomass smoke samples) also demonstrated that emissions from biomass burning would decrease the MAE of EC. Moreover, optical properties of water-soluble organic carbon (WSOC) in Beijing were presented. Light absorption by WSOC exhibited strong wavelength (λ) dependence such that absorption varied approximately as λ−7, which was characteristic of the brown carbon spectra. The MAE of WSOC (measured at 365 nm) was 1.79±0.24 and 0.71±0.20 m2 g−1 during winter and summer respectively. The large discrepancy between the MAE of WSOC during winter and summer was attributed to the difference in the precursors of SOA such that anthropogenic volatile organic compounds (AVOCs) should be more important as the precursors of SOA in winter. The MAE of WSOC in Beijing was much higher than results from the southeastern United States which were obtained using the same method as used in this study, perhaps due to the stronger emissions of biomass burning in China.

scholarly journals Size-resolved measurements of brown carbon and estimates of their contribution to ambient fine particle light absorption based on water and methanol extracts

2013 ◽  
Vol 13 (7) ◽  
pp. 18233-18276 ◽  
Author(s):  
J. Liu ◽  
M. Bergin ◽  
H. Guo ◽  
L. King ◽  
N. Kotra ◽  
...  
Keyword(s):  
Organic Carbon ◽  
Light Absorption ◽  
Fine Particle ◽  
Elemental Carbon ◽  
Fine Particles ◽  
Water Soluble ◽  
Rural Site ◽  
Brown Carbon ◽  
Methanol Extracts ◽  
Carbon Absorption

Abstract. Light absorbing organic carbon, often termed brown carbon, has the potential to significantly contribute to the visible light absorption budget, particularly at shorter wavelengths. Currently, the relative contributions of particulate brown carbon to light absorption, as well as the sources of brown carbon are poorly understood. With this in mind field measurements were made at both urban (Atlanta), and rural (Yorkville) sites in Georgia. Measurements in Atlanta were made at both a central site and a road side site adjacent to a main highway near the city center. Fine particle brown carbon optical absorption is estimated based on Mie calculations using direct size resolved measurements of chromophores in filter extracts. Size-resolved atmospheric aerosol samples were collected using a cascade impactor and analyzed for water-soluble organic carbon (WSOC), organic and elemental carbon (OC and EC), and solution light absorption spectra of water and methanol extracts. Methanol extracts were more light-absorbing than water extracts for all size ranges and wavelengths. Absorption refractive indices of the organic extracts were calculated from solution measurements for a range of wavelengths and used with Mie theory to predict the light absorption by fine particles comprised of these components, under the assumption that brown carbon and other aerosol components were externally mixed. For all three sites, chromophores were predominately in the accumulation mode with an aerodynamic mean diameter of 0.5 μm, an optically effective size range resulting in predicted particle light absorption being a factor of 2 higher than bulk solution absorption. Fine particle absorption was also measured with a Multi-Angle Absorption Photometer (MAAP) and seven-wavelength Aethalometer. Scattering-corrected aethalometer and MAAP absorption were in good agreement at 670 nm and Mie-estimated absorption based on size-resolved EC data were within 30% of these optical instruments. When applied to solution measurements, at all sites, Mie-predicted brown carbon absorption at 350 nm contributed a significant fraction (20 to 40%) relative to total light absorption, with highest contributions at the rural site where organic to elemental carbon ratios were highest. Brown carbon absorption, however, was highest by the roadside site due to vehicle emissions. The multi-wavelength aethalometer did not detect brown carbon, having an absorption Ångstrom exponent near one. Although the results are within the estimated Aethalometer uncertainties, the direct measurement of brown carbon in solution definitively shows that it is present and this Mie analysis suggests it is optically important in the near UV range in both a rural and urban environment during summer when biomass burning emissions are low.

scholarly journals Amplification of black carbon light absorption induced by atmospheric aging: temporal variation at seasonal and diel scales in urban Guangzhou

2019 ◽  
Author(s):  
Jia Yin Sun ◽  
Cheng Wu ◽  
Dui Wu ◽  
Chunlei Cheng ◽  
Mei Li ◽  
...  
Keyword(s):  
Optical Properties ◽  
Organic Carbon ◽  
Black Carbon ◽  
Light Absorption ◽  
Dry Season ◽  
Diurnal Fluctuation ◽  
Absorption Enhancement ◽  
Wet Season ◽  
Diurnal Variability ◽  
The Impact

Abstract. Black carbon (BC) is an important climate forcer in the atmosphere. Amplification of light absorption can occur by coatings on BC aerosols, an effect that remains one of the major sources of uncertainties for accessing the radiative forcing of BC. In this study, the absorption enhancement factor (Eabs) was quantified by the minimum R squared (MRS) method using elemental carbon (EC) as the tracer. Two field campaigns were conducted in urban Guangzhou at the Jinan university super site during both wet season (July 31–September 10, 2017) and dry season (November 15, 2017–January 15, 2018) to explore the temporal dynamics of BC optical properties. The average concentration of EC was 1.94 ± 0.93 and 2.81 ± 2.01 μgC m−3 in the wet and dry seasons, respectively. Mass absorption efficiency at 520 nm by primary aerosols (MAEp520) determined by MRS exhibit a strong seasonality (8.6 m2g−1 in the wet season and 16.8 m2g−1 in the dry season). Eabs520 was higher in the wet season (1.51 ± 0.50) and lower in the dry season (1.29 ± 0.28). Absorption Ångström exponent (AAE470-660) in the dry season (1.46 ± 0.12) were higher than that in the wet season (1.37 ± 0.10). Collective evidence showed that the active biomass burning (BB) in dry season effectively altered optical properties of BC, leading to elevated MAE, MAEp and AAE in dry season comparing to those in wet season. Diurnal Eabs520 was positively correlated with AAE470-660 (R2 = 0.71) and negatively correlated with the AE33 aerosol loading compensation parameter (k) (R2 = 0.74) in the wet season, but these correlations were significantly weaker in the dry season, which may be related to the impact of BB. This result suggests that lensing effect was dominating the AAE diurnal variability during the wet season. The effect of secondary processing on Eabs diurnal dynamic were also investigated. The Eabs520 exhibit a clear dependency on secondary organic carbon to organic carbon ratio (SOC/OC). Eabs520 correlated well with nitrate, implying that gas-particle partitioning of semi-volatile compounds may potentially play an important role in steering the diurnal fluctuation of Eabs520. In dry season, the diurnal variability of Eabs520 was associated with photochemical aging as evidenced by the good correlation (R2 = 0.69) between oxidant concentrations (Ox=O3+NO2) and Eabs520.

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