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

2008 ◽  
Vol 8 (22) ◽  
pp. 6665-6679 ◽  
Author(s):  
J. C. Barnard ◽  
R. Volkamer ◽  
E. I. Kassianov
Keyword(s):  
Organic Carbon ◽  
Mexico City ◽  
Cross Section ◽  
Light Absorption ◽  
Absorption Cross Section ◽  
Trace Gases ◽  
Organic Aerosol ◽  
Absorption Cross ◽  
Enhanced Absorption ◽  
Carbon Component

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.

scholarly journals Estimation of the mass absorption cross section of the organic carbon component of aerosols in the Mexico City Metropolitan Area (MCMA)

2008 ◽  
Vol 8 (3) ◽  
pp. 10189-10225 ◽  
Author(s):  
J. C. Barnard ◽  
R. Volkamer ◽  
E. I. Kassianov
Keyword(s):  
Organic Carbon ◽  
Black Carbon ◽  
Mexico City ◽  
Cross Section ◽  
Light Absorption ◽  
Absorption Cross Section ◽  
Trace Gases ◽  
Absorption Cross ◽  
Enhanced Absorption ◽  
Mass Absorption

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 part of the aerosol as the only possible 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 carbon. 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 carbon 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 carbon 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 will be particularly relevant in polluted urban atmospheres and areas of biomass burning where organic aerosols are particularly abundant.

scholarly journals Black carbon over Mexico: the effect of atmospheric transport on mixing state, mass absorption cross-section, and BC/CO ratios

2010 ◽  
Vol 10 (1) ◽  
pp. 219-237 ◽  
Author(s):  
R. Subramanian ◽  
G. L. Kok ◽  
D. Baumgardner ◽  
A. Clarke ◽  
Y. Shinozuka ◽  
...  
Keyword(s):  
Black Carbon ◽  
Mexico City ◽  
Cross Section ◽  
Absorption Cross Section ◽  
Atmospheric Transport ◽  
Absorption Cross ◽  
Air Masses ◽  
Wet Scavenging ◽  
The City ◽  
Absorption Photometer

Abstract. A single particle soot photometer (SP2) was operated on the NCAR C-130 during the MIRAGE campaign (part of MILAGRO), sampling black carbon (BC) over Mexico. The highest BC concentrations were measured over Mexico City (sometimes as much as 2 μg/m3) and over hill-fires to the south of the city. The age of plumes outside of Mexico City was determined using a combination of HYSPLIT trajectories, WRF-FLEXPART modeling and CMET balloon tracks. As expected, older, diluted air masses had lower BC concentrations. A comparison of carbon monoxide (CO) and BC suggests a CO background of around 65 ppbv, and a background-corrected BC/COnet ratio of 2.89±0.89 (ng/m3-STP)/ppbv (average ± standard deviation). This ratio is similar for fresh emissions over Mexico City, as well as for aged airmasses. Comparison of light absorption measured with a particle soot absorption photometer (PSAP) and the SP2 BC suggests a BC mass-normalized absorption cross-section (MAC) of 10.9±2.1 m2/g at 660 nm (or 13.1 m2/g @ 550 nm, assuming MAC is inversely dependent on wavelength). This appears independent of aging and similar to the expected absorption cross-section for aged BC, but values, particularly in fresh emissions, could be biased high due to instrument artifacts. SP2-derived BC coating indicators show a prominent thinly-coated BC mode over the Mexico City Metropolitan Area (MCMA), while older air masses show both thinly-coated and thickly-coated BC. Some 2-day-old plumes do not show a prominent thickly-coated BC mode, possibly due to preferential wet scavenging of the likely-hydrophilic thickly-coated BC.

scholarly journals Absolute ozone absorption cross section in the Huggins Chappuis minimum (350–470 nm) at 296 K

2011 ◽  
Vol 11 (8) ◽  
pp. 21655-21676 ◽  
Author(s):  
J. L. Axson ◽  
R. A. Washenfelder ◽  
T. F. Kahan ◽  
C. J. Young ◽  
V. Vaida ◽  
...  
Keyword(s):  
Cross Section ◽  
Absorption Cross Section ◽  
Absorption Cross ◽  
Measured Cross Section ◽  
Ozone Absorption ◽  
Enhanced Absorption ◽  
Vapor Partial Pressure ◽  
Order Of Magnitude ◽  
Water Vapor Partial Pressure ◽  
Absorption Spectrometer

Abstract. We report the ozone absolute absorption cross section between 350–470 nm, the minimum between the Huggins and Chappuis bands, where the ozone cross section is less than 10−22 cm2. Ozone spectra were acquired using an incoherent broadband cavity enhanced absorption spectrometer, with three channels centered at 365, 405, and 455 nm. The accuracy of the measured cross section is 2 %. Previous measurements vary by more than an order of magnitude in this spectral region. The measurements reported here provide much greater spectral coverage than the most recent measurements. We report a minimum absorption cross section of 3.4×10−24 cm2 at 381.8 nm, which is 22 % lower than the previously reported value. The effect of O3 concentration and water vapor partial pressure were investigated, however there were no observable changes in the absorption spectrum most likely due to the low optical density of the complex.

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