Spectra Aerosol Light Scattering and Absorption for Laboratory and Urban Aerosol (original) (raw)
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Atmospheric Chemistry and Physics, 2009
Hundreds of wildfires in Northern California were sparked by lightning during the summer of 2008, resulting in downwind smoke for the months of June and July. Comparisons are reported for aerosol optics measurements in Reno, Nevada made during the very smoky month of July and the relatively clean month of August. Photoacoustic instruments equipped with integrating nephelometers were used to measure aerosol light scattering and absorption coefficients at wavelengths of 405 nm and 870 nm, revealing a strong variation of aerosol light absorption with wavelength. Insight on fuels burned is gleaned from comparison ofÅngström exponents of absorption (AEA) versus single scattering albedo (SSA) of the ambient measurements with laboratory biomass smoke measurements for many fuels. Measurements during the month of August, which were largely unaffected by fire smoke, exhibit surprisingly low AEA for aerosol light absorption when the SSA is highest, again likely as a consequence of the underappreciated wavelength dependence of aerosol light absorption by particles coated with nonabsorbing organic and inorganic matter. Coated sphere calculations were used to show that AEA as large as 1.6 are possible for wood smoke even with non-absorbing organic coatings on black carbon cores, suggesting care be exercised when diagnosing AEA.
Journal of Geophysical Research: Atmospheres, 2019
Biomass burning (BB) is an important global source of aerosol and trace gases that degrade air quality, decrease visibility, and impact climate and human health. Refractory black carbon (rBC), brown carbon (BrC), and organic aerosol are major components of BB emissions. BB aerosol composition is highly variable at the source and depends on fuel composition and combustion phase. Atmospheric aging alters fresh BB aerosol through processes that are complex and dynamic. To better understand the variability in optical properties, we report fresh aerosol laboratory measurements from burning southwestern U.S. fuels and compare them to aged ambient BB aerosol from wildfires over a range of atmospheric time scales. Our BB aerosol analysis uses the relationship between the absorption Ångström exponent and single-scattering albedo (SSA) to identify rBC, BrC, and organic aerosol-dominated regimes that are defined using Mie theory. This model framework is used to interpret the large variability in optical properties measured in laboratory burns. In contrast, we find the observed absorption Ångström exponent-SSA relationship for ambient BB aerosol to be less variable and more clustered together with increased atmospheric aging. This transition from fresh to aged behavior is attributed to the homogenization of the BB aerosol from mixing and aging over several hours. Finally, BB aerosol in ambient fire plumes that have aged for several hours exhibits larger SSAs than laboratory flaming burns. We conclude that BrC/OC mixtures play a larger role than rBC in the positive climate forcing of BB aerosol than what would be projected from laboratory results.
2006
Thermal analysis of aerosol size distributions provided size resolved volatility up to temperatures of 400 o C during extensive flights over North America (NA) for the INTEX/ICARTT experiment in summer 2004. Biomass burning and pollution plumes identified from trace gas measurements were evaluated for their aerosol physio-chemical and optical signatures. Fast measurements of soluble ionic mass and refractory black carbon (BC) mass, inferred from light-absorption, were combined with volatility to identify residual volatile organic carbon (VolatileOC) and refractory organic carbon, RefractoryOC. This approach characterized distinct constituent mass fractions present in biomass burning and pollution plumes. The "non-plume" regional haze exhibited statistical properties reflecting both plume types but was dominated by pollution characteristics near the surface and biomass burning aloft. VolatileOC included most water-soluble organic carbon. RefractoryOC dominated the enhanced shortwave absorption in plumes from Alaskan and Canadian forest fires. The RefractoryOC mass absorption efficiency was about 0.51 m 2 g-1 at 470 nm and 0.16 m 2 g-1 at 530nm. Biomass burning, pollution and dust aerosol could be stratified by their combined spectral scattering and absorption properties. Concurrent measurements of the humidity dependence of scattering [f(RH)] found the VolatileOC component to be only weakly hygroscopic resulting in a general decrease of overall f(RH) with increasing OC mass fractions. Under ambient humidity conditions, the systematic relations between physio-chemical properties and f(RH) lead to a reduced variability in the single scattering albedo and a simple dependency on the absorption per unit dry mass for these plume types that may be used to challenge modeled optical properties.
Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols
Journal of Geophysical Research, 1999
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. 1998; Remer et al., 1998], combustion of fossil fuels from urban/industrial processes [Rerner and Kaufman, 1998], oceanic wave action producing sea salt aerosol [Hoppel et al., 1990], plants producing biogenic aerosols [Kavouras et al., 1998; Artaxo et al., 1988], volcanic eruptions [Russell et al., 1993] and airborne soil particles [d'Alrneida, 1987]. In addition, once these aerosols have been formed there are often
Journal of Geophysical Research, 2007
1] Thermal analysis of aerosol size distributions provided size resolved volatility up to temperatures of 400°C during extensive flights over North America (NA) for the INTEX/ICARTT experiment in summer 2004. Biomass burning and pollution plumes identified from trace gas measurements were evaluated for their aerosol physiochemical and optical signatures. Measurements of soluble ionic mass and refractory black carbon (BC) mass, inferred from light absorption, were combined with volatility to identify organic carbon at 400°C (VolatileOC) and the residual or refractory organic carbon, RefractoryOC. This approach characterized distinct constituent mass fractions present in biomass burning and pollution plumes every 5-10 min. Biomass burning, pollution and dust aerosol could be stratified by their combined spectral scattering and absorption properties. The ''nonplume'' regional aerosol exhibited properties dominated by pollution characteristics near the surface and biomass burning aloft. VolatileOC included most water-soluble organic carbon. RefractoryOC dominated enhanced shortwave absorption in plumes from Alaskan and Canadian forest fires. The mass absorption efficiency of this RefractoryOC was about 0.63 m 2 g À1 at 470 nm and 0.09 m 2 g À1 at 530 nm. Concurrent measurements of the humidity dependence of scattering, g, revealed the OC component to be only weakly hygroscopic resulting in a general decrease in g with increasing OC mass fractions. Under ambient humidity conditions, the systematic relations between physiochemical properties and g lead to a well-constrained dependency on the absorption per unit dry mass for these plume types that may be used to challenge remotely sensed and modeled optical properties.
Journal of Geophysical Research, 2011
over Bozeman, Montana, during a transitional period in which background polluted rural continental aerosols and well-aged biomass-burning aerosols were the dominant aerosol types of extremely fresh biomass-burning aerosols resulting from forest fires burning in the northwestern United States and Canada. Aerosol optical properties and relative humidity profiles were retrieved using an eye-safe micropulse water vapor differential absorption lidar (DIAL) (MP-DIAL), a single-channel backscatter lidar, a CIMEL solar radiometer as part of the Aerosol Robotic Network (AERONET), a ground-based integrating nephelometer, and aerosol products from Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua. Aerosol optical depths (AODs) measured during the case study ranged between 0.03 and 0.17 (0.015 and 0.075) at 532 nm (830 nm) as episodic combinations of fresh and aged biomass-burning aerosols dominated the optical depth of the pristinely clean background air. Here, a pristinely clean background refers to very low AOD conditions, not that the aerosol scattering and absorption properties are necessarily representative of a clean aerosol type. Diurnal variability in the aerosol extinction to backscatter ratio (S a) of the background atmosphere derived from the two lidars, which ranged between 55 and 95 sr (50 and 90 sr) at 532 nm (830 nm), showed good agreement with retrievals from AERONET sun and sky measurements over the same time period but were consistently higher than some aerosol models had predicted. S a measured during the episodic smoke events ranged on average from 60 to 80 sr (50 to 70 sr) at 532 nm (830 nm) while the very fresh biomass-burning aerosols were shown to exhibit significantly lower S a ranging between 20 and 40 sr. The shortwave direct radiative forcing that was due to the intrusion of biomass-burning aerosols was calculated to be on average −10 W/m 2 and was shown to compare favorably with regional-scale forcing calculations using MODIS-Terra and AERONET data in an effort to assess the accuracy of estimating the regional-scale aerosol direct radiative forcing effect using aerosol optical properties measured from a single rural site such as Bozeman, Montana.
Atmospheric Chemistry and Physics, 2007
In spring 2006, biomass burning aerosols from eastern Europe were transported extensively to Finland, and to other parts of northern Europe. They were observed as far as in the European Arctic. In the first part of this paper, temporal and spatial evolution and transport of these biomass burning aerosols are monitored with MODIS retrieved aerosol optical depth (AOD) imagery at visible wavelengths (0.55 µm). Comparison of MODIS and AERONET AOD is conducted at Tõravere, Estonia. Then trajectory analyses, as well as MODIS Fire Mapper products are used to better understand the type and origin of the air masses. During the studied four-week period AOD values ranged from near zero up to 1.2 at 0.55 µm and the linear correlation between MODIS and AERONET was very high (∼0.97). Temporal variability observed within this fourweek period was also rather well explained by the trajectory analysis in conjunction with the fire detections produced by the MODIS Rapid Response System. In the second part of our study, the surface measurements of global and UV radiation at Jokioinen, Finland are used to study the effect of this haze episode on the levels of surface radiation. We found reductions up to 35% in noon-time surface UV irradiance (at 340 nm) as compared to typical aerosol conditions. For global (total solar) radiation, the reduction was always smaller, in line with the expected wavelength dependence of the aerosol effect.
Chemical, physical, and optical evolution of biomass burning aerosols: a case study
Atmospheric Chemistry and Physics, 2011
In-situ chemical composition measurements of ambient aerosols have been used for characterizing the evolution of submicron aerosols from a large anthropogenic biomass burning (BB) event in Israel. A high resolution Time of Flight Aerosol Mass Spectrometer (Hi-RES-TOF-AMS) was used to follow the chemical evolution of BB aerosols 5 during a night-long, extensive nationwide wood burning event and during the following day. While extensive BB is not common in this region, burning of agricultural waste is a common practice. The aging process of the BB aerosols was followed through their chemical, physical and optical properties. Mass spectrometric analysis of the aerosol organic component showed that aerosol aging is characterized by shifting from 10 less oxidized fresh BB aerosols to more oxidized aerosols. Evidence for aerosol aging during the day following the BB event was indicated by an increase in the organic mass, its oxidation state, the total aerosol concentration, and a shift in the modal particle diameter. The effective broadband refractive index (EBRI) was derived using a white light optical particle counter (WELAS). The average EBRI for a mixed popula-15 tion of aerosols dominated by open fires was m=1.53(±0.03)+0.07i(±0.03), during the smoldering phase of the fires we found the EBRI to be m=1.54(±0.01)+0.04i(±0.01) compared to m=1.49(±0.01)+0.02i(±0.01) of the aged aerosols during the following day. This change indicates a decrease in the overall aerosol absorption and scattering. Elevated levels of particulate Polycyclic Aromatic Hydrocarbons (PAHs) were detected 20 during the entire event, which suggest possible implications for human health during such extensive event.
Atmosphere
Black (EBC) and Brown (BrC) Carbon are ubiquitous constituents of atmospheric particulate matter that affect people’s health, disrupt ecosystems, and modulate local and global climate. Tracking the local deposition and sources of these aerosol particles is essential to better understanding their multidimensional environmental impact. The main goal of the current study is to measure the absorption coefficient (Babs) of particles within the Planetary Boundary Layer (PBL) of the El Paso (US)–Ciudad Juárez (Mexico) airshed and assess the contribution of black and brown carbon particles to the optical absorption. Measurements were taken during a summer, wildfire, and winter season to evaluate the optical properties of BC and non-volatile BrC. The winter season presented a variation from the background Babs in the late evening hours (3:00 PM to midnight) due to an increase in biomass burning driven by lower temperatures. The wildfire season presents the greatest variation in the Babs from...