Aerosol optical properties measured in Argentina: wavelength dependence and variability based on sun photometer measurements
ABSTRACT This paper deals with the spectral dependence and time variability of Ångström wavelength exponent scaling law (α), which is the spectral varying slope of the logarithmic relationship between aerosol optical depths (τ) and the wavelength (λ). It is commonly used to retrieve intensive air masses optical properties such as aerosol size distribution from extensive quantities (τ) and Ångström turbidity coefficient (β). This spectral variation of α is studied at different wavelengths from measurements taken by ground-based sun photometer covering from near-infrared to ultraviolet range. We analyze the spectral measurement of aerosols optical depths at eight specific selected wavelengths from 340 to 1020 nm using the sun photometer measurements from AErosol RObotic NETwork (AERONET) from NASA. Data from the entire year 2000 were used from instruments deployed at two different sites covering the regions of Argentina as northcentral at Cordoba CETT (31.5S, 64.4W) and “pampa húmeda” at Buenos Aires CEILAP (34.5S, 58.5W). A new approach of Ångström wavelength exponent spectral variation was developed to take into account with a more accurate precision the significant curvature appearing in the logarithmic relation between τ and λ. Using the direct spectral solar radiation set, time series of Ångström coefficient of turbidity and wavelength scaling law was computed with a day to day data base clustering with uncertainty lower than 0.01 in the optical depth reconstruction over the bulk sun photometer measurements. Temporal series of constant and spectral dependence of wavelength exponent scaling law and turbidity coefficient was derived and shown to vary in space and time. Different meteorological forcing for both sites was evidenced using a regression coefficient analysis to well assess the spectral dependence of wavelength exponent coefficient due to the different cumulating mode of particles and air masses origin at different sites. This spectral decomposition is a key issue in aerosols analysis of steady state and regional scale intrusion episodes with strong connection to their potential contribution of pollution episodes in air-quality problems on urban environment.
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ABSTRACT: The direct radiative forcing (DRF) of sulfate and black carbon (BC) aerosols is investigated using a new multispectral radiation code within the R30 Geophysical Fluid Dynamics Laboratory general circulation model (GCM). Two independent sulfate climatologies from chemical transport models are applied to the GCM; each climatology has a different atmospheric burden, vertical profile, and seasonal cycle. The DRF is calculated to be approximately -0.6 and -0.8Wm-2 for the different sulfate climatologies. Additional sensitivity studies show that the vertical profile of the sulfate aerosol is important in determining the DRF; sulfate residing near the surface gives the strongest DRF due to the effects of relative humidity. Calculations of the DRF due to BC reveal that the DRF remains uncertain to approximately a factor of 3 due to uncertainties in the total atmospheric burden, the vertical profile of the BC, and the assumed size distribution. Because of the uncertainties in the total global mass of BC, the normalized DRF (the DRF per unit column mass of aerosol in watts per milligram (Wmg-1)) due to BC is estimated; the range is +1.1 to +1.9Wmg-1 due to uncertainties in the vertical profile. These values correspond to a DRF of approximately +0.4Wm-2 with a factor of 3 uncertainty when the uncertainty in the total global mass of BC is included. In contrast to sulfate aerosol, the contribution to the global DRF from cloudy regions is very significant, being estimated as approximately 60%. The vertical profile of the BC is, once again, important in determining the DRF, but the sensitivity is reversed from that of sulfate; BC near the surface gives the weakest DRF due to the shielding effects of overlying clouds. Although the uncertainty in the estimates of the DRF due to BC remains high, these results indicate that the DRF due to absorption by BC aerosol may contribute a significant positive radiative forcing and may consequently be important in determining climatic changes in the Earth-atmosphere system.Journal of Geophysical Research 01/1998; 103:6043-6058. · 3.17 Impact Factor
- Journal of The Optical Society of America. 01/1957; 47(2).
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ABSTRACT: The Angstrom wavelength exponent ct, which is the slope of the logarithm of aerosol optical depth (xa) versus the logarithm of wavelength () •), is commonly used to characterize the wavelength dependence of xa and to provide some basic information on the aerosol size distribution. This parameter is frequently computed from the spectral measurements of both ground-based sunphotometers and from satellite and aircraft remote sensing retrievals. However, spectral variation of ct is typically not considered in the analysis and comparison of values from different techniques. We analyze the spectral measurements of 'r • from 340 to 1020 nm obtained from ground-based Aerosol Robotic Network radiometers located in various locations where either biomass burning, urban, or desert dust aerosols are prevalent. Aerosol size distribution retrievals obtained from combined solar extinction and sky radiance measurements are also utilized in the analysis. These data show that there is significant curvature in the In 'r • versus In) • relationship for aerosol size distributions dominated by accumulation mode aerosols (biomass burning and urban). Mie theory calculations of ct for biomass burning smoke (for a case of aged smoke at high optical depth) agree well with observations, confirming that large spectral variations in ct are due to the dominance of accumulation mode aerosols. A second order polynomial fit to the In 'r • versus In) • data provides excellent agreement with differences in 'to of the order of the uncertainty in the measurements (-0.01-0.02). The significant curvature in In x,, versus In) • for high optical depth accumulation mode dominated aerosols results in ct values differing by a factor of 3-5 from 340 to 870 nm. We characterize the curvature in In 'r • versus In) • by the second derivative ct' and suggest that this parameter be utilized in conjunction with ct to characterize the spectral dependence of xa. The second derivative of In • o versus In) • gives an indication of the relative influence of accumulation mode versus coarse mode particles on optical properties.Journal of Geophysical Research 01/1999; 104349(27):333-31. · 3.17 Impact Factor