A32A-01 INVITED
Aerosol Absorption Measurements in MILAGRO.
During the month of March 2006, a number of instruments were used to determine the absorption characteristics of aerosols found in the Mexico City Megacity and nearby Valley of Mexico. These measurements were taken as part of the Department of Energy's Megacity Aerosol Experiment – Mexico City (MAX-Mex) that was carried out in collaboration with the Megacity Interactions: Local and Global Research Observations (MILAGRO) campaign. MILAGRO was a joint effort between the DOE, NSF, NASA, and Mexican agencies aimed at understanding the impacts of a megacity on the urban and regional scale. A super-site was operated at the Instituto Mexicano de Petroleo in Mexico City (designated T-0) and at the Universidad Technologica de Tecamac (designated T-1) that was located about 35 km to the north east of the T-0 site in the State of Mexico. A third site was located at a private rancho in the State of Hidalgo approximately another 35 km to the northeast (designated T-2). Aerosol absorption measurements were taken in real time using a number of instruments at the T-0 and T-1 sites. These included a seven wavelength aethalometer, a multi-angle absorption photometer (MAAP), and a photo-acoustic spectrometer. Aerosol absorption was also derived from spectral radiometers including a multi-filter rotating band spectral radiometer (MFRSR). The results clearly indicate that there is significant aerosol absorption by the aerosols in the Mexico City megacity region. The absorption can lead to single scattering albedo reduction leading to values below 0.5 under some circumstances. The absorption is also found to deviate from that expected for a "well-behaved" soot anticipated from diesel engine emissions, i.e. from a simple 1/lambda wavelength dependence for absorption. Indeed, enhanced absorption is seen in the region of 300-450 nm in many cases, particularly in the afternoon periods indicating that secondary organic aerosols are contributing to the aerosol absorption. This is likely due to carbonyl- and nitro- functional groups on conjugated and aromatic organic structures (e.g. PAH, and terpene derived products). Using 12-hour fine (0.1-1.0 micron) aerosol samples collected in the field on quartz filters, uv/vis and infrared spectra were obtained in the laboratory using integrating spheres and diffuse reflectance spectroscopy, respectively. An inter-comparison of the "real-time" measurements made by the photo-acoustic, aethalometer and MAAP techniques have been described. In addition, the in situ aethalometer (seven-channel) results are compared with continuous integrating sphere uv-visible spectra to examine the angstrom absorption coefficient variance. These results will be briefly overviewed and the specific posters detailing these results will be highlighted highlighted. This work was performed as part of the Department of Energy's Megacity Aerosol Experiment - Mexico City under the support of the Atmospheric Science Program. "This researchwas supported by the Office of Science (BER), U. S. Department of Energy, Grant No. DE-FG02-07ER64329.
A32A-02
Atmospheric heating of the Houston urban boundary layer by black carbon aerosol absorption of solar radiation
The direct radiative forcing associated with absorption by black carbon (BC) aerosol has important regional and global effects. We have calculated the solar absorption of BC aerosol using simultaneous in situ measurements of actinic flux, BC mass, and BC's association with coating materials. The measurements were made during the TexAQS/GoMACCS campaign in the Houston, TX, metropolitan area in September/October 2006 from the NOAA WP-3D research aircraft operating primarily in the boundary layer. The BC measurements were made with a Single-Particle Soot Photometer, which provides detailed physical and optical information about BC on a particle-by-particle basis, while actinic flux was measured over the wavelength range of 280-690 nm with a CCD-based spectroradiometer collecting light from 4π steradians. The absorption was calculated along the flight tracks and converted to atmospheric heating rates in the urban area, as well as in biomass burning plumes. These results will improve our understanding of the relationship between BC emissions and radiative forcing in polluted urban areas, and help evaluate the merit of short-term climate-change mitigation strategies involving reduction of BC emissions.
A32A-03
Estimation of the Mass Absorption Cross Section of the Organic Carbon Component of Aerosols in the Mexico City Metropolitan Area (MCMA)
Data taken from the MCMA-2003 and the MILAGRO field campaigns are used to examine the absorption of solar radiation by the organic component of aerosols. Using irradiance data from an MFRSR and an actinic flux spectroradiometer, we find aerosol single scattering albedo (SSA) as a function of wavelength. We find that in near-UV spectral range (defined here as 250 nm to 400 nm) the SSA is much lower than the SSA at 500 nm suggesting enhanced absorption in the near-UV range. Absorption by elemental carbon, dust, or trace gases (i.e., NO2, SO2, and others) cannot account for this enhanced absorption leaving only the organic part of the aerosol to account for it. We use data from a surface deployed Aerodyne Aerosol Mass Spectrometer (AMS) along with the inferred SSA and aerosol optical thickness to estimate the Mass Absorption Cross-section (MAC) for the organic carbon. We find that the MAC is about 10 m2/g at 300 nm and falls close to zero at about 500 nm. These MAC values can be considered as "radiatively correct" because when used in radiative transfer calculations the calculated solar fluxes match the measured fluxes at the wavelengths considered here.
A32A-04
Diurnal Cycles of Aerosol Optical Properties at Pico Tres Padres, Mexico City: Evidences for Changes in Particle Morphology and Secondary Aerosol Formation
Aerosol optical properties affect planetary radiative balance and depend on chemical composition, size distribution, and morphology. During the MILAGRO field campaign, we measured aerosol absorption and scattering in Mexico City using the Los Alamos aerosol photoacoustic (LAPA) instrument operating at 781 nm. The LAPA was mounted on-board the Aerodyne Research Inc. mobile laboratory, which hosted a variety of gaseous and aerosol instruments. During the campaign, the laboratory was moved to different sites, capturing spatial and temporal variability. Additionally, we collected ambient aerosols on Nuclepore filters for scanning electron microscopy (SEM) analysis. SEM images of selected filters were taken to study particle morphology. Between March 7th and 19th air was sampled at the top of Pico Tres Padres, a mountain on the north side of Mexico City. Aerosol absorption and scattering followed diurnal patterns related to boundary layer height and solar insulation. We report an analysis of aerosol absorption, scattering, and morphology for three days (9th, 11th and 12th of March 2006). The single scattering albedo (SSA, ratio of scattering to total extinction) showed a drop in the tens-of-minutes-to-hour time frame after the boundary layer grew above the sampling site. Later in the day the SSA rose steadily reaching a maximum in the afternoon. The SEM images showed a variety of aerosol shapes including fractal-like aggregates, spherical particles, and other shapes. The absorption correlated with the CO2 signal and qualitatively with the fraction of fractal-like particles to the total particle count. In the afternoon the SSA qualitatively correlated with a relative increase in spherical particles and total particle count. These observed changes in optical properties and morphology can be explained by the dominant contribution of freshly emitted particles in the morning and by secondary particle formation in the afternoon. SSA hourly averaged values ranged from ~0.63 in the morning to ~0.83 in the afternoon/night, indicating light absorbing aerosols, which have a large impact on radiative forcing.
A32A-05
Light Absorbing Carbon Emissions from Commercial Shipping: Impacts for Local Air quality and the Arctic
Emissions of light absorbing carbon (LAC) are a major uncertainty when considering the climatic effect of this aerosol. LAC contributes to direct radiative warming, can affect cloud formation and dynamics and can alter the albedo of snow and ice. Commercial shipping vessels contribute significant amounts of LAC to the atmosphere by burning residual fuels with little emissions regulation. Knowledge of the actual emissions of LAC from shipping is highly uncertain yet critical to help constrain budgets of LAC. This is of particular concern in populated port areas where continuous loading operations can contribute to adverse air quality and in regions like the Arctic that will become exposed to larger volumes of shipping traffic as sea ice retreats. A single directly-measured emission factor from shipping has previously been reported. Here we present an extensive set of measurements of the emission characteristics of LAC from commercial shipping. We show that the highest emitters (mass of LAC per unit fuel burnt) are tug boats, thus making significant contributions to local air quality in major ports, and that emission of LAC is unrelated to primary emissions of NOX and SO2. Using 2001 shipping fuel consumption data we calculate a global LAC contribution from shipping of 133 Gg yr-1, or ~ 1.7 % of global LAC. The optical characteristics of these emissions are also discussed. We also present results from global transport modeling of the estimated impact of increases in commercial shipping through Arctic sea lanes.
A32A-06
Ultraviolet Characteristics of PBL Aerosol in Mexico City
The optical properties of aerosols are relatively unknown at ultraviolet wavelengths (UV, 280-400 nm in the troposphere), and may be quite different than those at the better-studied visible wavelengths. The UV band has a direct effect on atmospheric photochemistry and surface exposures of biota, so that perturbation to the natural UV radiation field may be quite important, especially in regions experiencing heavy aerosol loading. Here, we use data from the MILAGRO campaign (Mexico City, March 2006) to examine spectrally-resolved properties of an aerosol layer. Specifically, we compare spectral (280-420 nm, 1 nm FWHM resolution) actinic fluxes observed at the surface (the MILAGRO T1 supersite) with down-welling and up-welling spectral actinic fluxes measured from the C-130 aircraft during T1 overpasses. Ancillary observations includes ground based radiation (MFRSR, UV- MFRSR, AERONET); ground- and aircraft-based lidars, gaseous absorbers (O3, NO2, SO2), and in-situ aerosol properties; a few vertical profiles of O3 from tethered balloons and ozone sondes; and satellite-based measurements of O3, NO2, and aerosols. These observations, combined with the Tropospheric Ultraviolet- Visible (TUV) model, allow estimation of the transmission and reflection of the aerosol layer below the aircraft, as well as the ground albedo, all spectrally resolved over the UV wavelengths. A major question to be examined is whether the UV properties (e.g. single scattering albedo) can be estimated by Mie-guided extrapolation from visible wavelengths, or whether UV-specific absorbers are present (e.g., from chromophores associated with secondary organic aerosols).
A32A-07
Surface Reflectance of Mexico City
During the MILAGRO field campaign that took part in March of 2006 the Sky Research J31 aircraft carried the fourteen channel NASA Ames Research Center Advanced Airborne Tracking Sunphotometers (AATS-14) and the Spectral Scanning Flux Radiometer (SSFR) together with the SpecTIR Corp. Research Scanning Polarimeter (RSP). These instruments were used to obtain low altitude measurements of the upwelling spectral flux and multi-spectral multi-angle polarized radiances together with the downwelling spectral flux and the multi-spectral measurements of the downwelling direct solar beam over Mexico City. The measurements are used to characterize the surface bidirectional reflectance distribution function (BRDF) and the polarized reflectance of the surface. The BRDF and polarized reflectance estimates are used to improve the capability to remotely characterize the aerosols above the surface of the city and estimate the burden, type, size and single scattering albedo (ssa) of the aerosols . The BRDF estimates together with the estimated aerosol burden and ssa allow the variation in absorbed surface solar radiation to be estimated over the course of the day. Estimates of the effect of the aerosols on the surface radiation budget are given and an evaluation of what types of model are sufficient to represent the surface polarized BRDF of an urban environment.
A32A-08
Evaluating Treatments of Aerosol Optical Properties and their Effect on Radiative Forcing using MILAGRO Measurements
Global climate model predictions still contain relatively large uncertainties associated with aerosol radiative forcing. Part of this problem may result from the coarse spatial grid spacing employed by global climate models that does not permit large gradients in particulates to be resolved and does not account for sub-grid scale variability in non-linear aerosol chemistry. The treatment of aerosol optical properties likely contributes to the uncertainties in radiative forcing as well. For example, a modal approach is typically used to represent the aerosol size distribution in global climate models because of its simplicity and computationally efficiency. More complex treatments for the aerosol size distribution, such as the sectional approach, may become feasible for the next generation of climate models as computational power continues to increase. In this study, we employ a fully-coupled meteorology-chemistry-aerosol model, WRF-chem, to evaluate the predicted optical properties and downwind of Mexico City using both modal and sectional approaches. We also examine the sensitivity of the model predictions to "volume-averaging", "shell-core" and other approaches that employ Mie theory. The impact of the various approaches on predicted aerosol radiative forcing is then quantified. Model performance is assessed by using the extensive measurements from surface-based, aircraft, and satellite platforms made during the MILAGRO field campaign in March 2006. We employ aerosol optical depth measurements at several surface sites as well as those obtained from twice-daily satellite instrumentation, single scattering measurements from surface sites and the G-1 aircraft measurements, and vertical profiles of extinction and backscatter obtained from the NASA Langley Research Center airborne High Spectral Resolution Lidar (HSRL). The lidar measurements provided evidence of multiple layers of particulates and large spatial variability of particulate composition (as determined by depolarization) over central Mexico. To account for particulate variability in the region, the model employs an outer domain with a grid spacing of 12 km that encompasses Mexico and an inner domain with a grid spacing of 3 km that encompasses central Mexico and most of the in-situ measurements.