A41E-01
Comparison of temporal variations in fine carbonaceous particles at rural and urban sites in Canada and China
Weekly and/or daily PM2.5 samples were collected at a rural site (i.e. Egbert, Ontario, about 80 km Northwest of Toronto, 43°41'N, 79°38'W) since August 2005 and an urban site (i.e. Downsview, Toronto) in Canada since October 2006. Similar collections were simultaneously conducted at a rural site (Miyun, about 100 km Northeast of Beijing, 39°54'N, 116°25'E) and an urban site (i.e. Tsinghua) around Beijing area in China, for two consecutive years (2005 and 2006). In order to characterize the temporal variation (e.g. seasonal changes) of carbonaceous components in ambient PM of big cities, understanding the their impacts on the background levels of PM and identify possible sources and their attributions, individual carbon fractions, i.e. organic carbon (OC) and elemental carbon/black carbon (EC/BC) were measured for these samples from a paired urban-rural sites in both Canada and China. The results show that: A). in Egbert, weekly concentrations varied a lot ranging from 0.3 to 3.1 μg m-3 in OC and from 0.05 to 1.8 μg m-3 in EC with an average of 1.7±0.9 μg m-3 and 0.6±0.3 μg m-3, respectively. OC and EC concentrations were 80% and 50% higher during summertime than during wintertime. Typical episodes related to high variations in OC and EC concentrations during both warm and cold seasons corresponded well to southern-northern wind directions, implying the impact of urban sources from the south. B). In Beijing, an average concentrations at the rural site was 14.4±6.8 μg m-3 in OC and 3.5±2.1 μg m-3 in EC, a factor of 7.5 and 4.8 higher than those at the rural site in Canada, respectively, whereas those at the urban site were even higher by another factor of 0.6 and 1.2 than their corresponding values at the rural site, respectively. In contrast to the seasonal pattern at Egbert, Canada, the average concentrations of both OC and EC were much higher during winter season than those during summer by a factor of 0.6-1.2, respectively. It was likely due to enhanced emissions from coal combustions for space heating coupled with some specific meteorological conditions (e.g. low wind speed, low mixing height and high relative humidity etc.). Stable isotope measurements of individual carbon fractions in both ambient and source PM2.5 are likely to provide additional constraints on source identifications and attributions for ambient carbonaceous PM, including the impacts from local emissions and long-range transport.
A41E-02
CCN and absorption in biomass burning, urban pollution and dust particles observed in-situ over North America
For possible remote sensing of cloud condensation nuclei (CCN) concentration, we stratify their relationship with optical properties using comprehensive aircraft observations over the USA, Mexico and northeastern Pacific Ocean. We use size distribution, volatility, chemical mass, and scattering and absorption spectra measured for biomass burning, urban pollution and dust aerosols from DC8 and C130 aircraft during INTEX-NA, MIRAGE, IMPEX and INTEX-B. Our preliminary results show the scattering wavelength dependence stratifies the link between aerosol extinction and number in a nearly identical manner over the different regions. Also, volatile organic compounds measured with an aerosol mass spectrometer (AMS) appear associated with suppressed particle growth around the CCN critical activation diameter (30 - 140 nm) and steep absorption wavelength dependence. These observations suggest the potential for remotely sensed single scattering albedo spectra to detect the presence of organic compounds and reduced CCN concentration. Coarse dust particles are also associated with high absorption wavelength dependence and low aerosol number, but are distinguished from the dominantly submicron biogenic components by different scattering wavelength dependence. The link between absorption and microphysical properties are addressed for soot and inorganic components as well, with a focus on the variation in volume absorption efficiency with mixing and ageing status. Volatile material condenses upon the soot particles as they age. Heating the aerosol to 400C upstream of a particle soot absorption photometer (PSAP) resulted in 0 - 30% reduction in soot absorption, presumably due to removal of the volatile coatings.
A41E-03
The Impact of Rain Events on Aerosol Optical Properties: Mexico City 2003 and 2006
Atmospheric aerosols are now known to play an important role in global climate because of their ability to both scatter and absorb solar radiation, depending on their chemical composition and size. While fine aerosols (0.1- 1.0 microns in diameter) are most effective in scattering shortwave radiation, the dominant light-absorbing aerosol component is carbon soot or black carbon (BC), produced from combustion. The physical removal of large particles (greater than 1 microns) from the atmosphere occurs by gravitational settling while the removal of ultrafine particles (less than 0.1 microns) occurs by diffusion to other aerosol surfaces. This leads to the accumulation of fine aerosols (0.1-1.0 microns) in the atmosphere where the actions of these physical removal mechanisms are at a minimum. These Fine aerosols can adsorb water and grow to a size where gravitationally settling or impaction by falling hydrometeors can occur. Soluble inorganic aerosols are efficiently wet-deposited in this manner, as they are hydrophilic in nature, add water efficiently, and grow rapidly in volume. However, BC aerosols may be more hydrophobic, resisting wet deposition. Thus, the lifetimes of these fine aerosols will depend upon their surface chemistries and hygroscopicity. Measurements of aerosol BC content, light scattering, and absorption were taken in Mexico City during April of 2003 and March of 2006 as part of the Department of Energy's investigations into megacities as major sources of aerosols that would have impacts on regional and global scales. Here we present the results obtained for fine aerosol absorption and scattering in Mexico City during both field efforts. These data are presented as a function of relative humidity and precipitation intensity to determine the effects of wet deposition on the aerosol optical properties. The results indicate that there are substantial amounts of fresh BC aerosols that are not readily washed out during these rain events leading to a decrease in the aerosol single scattering albedo. 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 research was supported by the Office of Science (BER), U. S. Department of Energy, Grant No. DE-FG02-07ER64329.
A41E-04
Ambient Aerosols in the Mexico Megacity Measured by Single Particle Mass Spectrometry
Continuous ambient measurements with aerosol time-of-flight mass spectrometry (ATOFMS) were carried out in an industrial/residential section in the northern part of Mexico City as part of the Mexico City Metropolitan Area - 2006 campaign (MCMA-2006) between March 7 and 27, 2006. Major particle types were first characterized by size and chemical composition. Then, using temporal profiles and airmass backtrajectories, the most likely source regions for each of the single particle types was obtained. Industrial particle types were found to be constrained to the northern (industrial) portion of Mexico City, while biomass and OC particles exhibited less directional dependence. Biomass and organic carbon (OC) particle types were found to dominate the accumulation mode. The OC and Biomass particle types were the dominant particle types early in the morning. From the late morning until early evening, the biomass type became the largest contributor to accumulation mode mass. The diurnal pattern can be attributed to either coagulational aging or a change in the air mass. A unique nitrogen-containing organic (NOC) particle type was observed, and is hypothesized to be from industrial emissions based on the temporal profile and back trajectory analysis. Particle mixing states during the MCMA-2006 campaign showed that Oxalate was primarily associated with metals and OC. Based on this observation, we hypothesize that the metals are directly bound to Oxalate, which is known to be a strong metal chelator.
A41E-05
A Statistical Summary of Particle Properties in the Cortez Pass in March, 2006
A large suite of instruments for measuring the physical (number and size), optical (scattering and absorption coefficients) and chemical (inorganic and organic composition and hygroscopicity) properties of particles was operated 24 hours a day, seven days a week in the Cortez Pass, 60 km southeast of Mexico City, on the Altzomoni ridge at an altitude of 4010 m ASL. During the night and early morning, the site is in the free troposphere; however, air from the surrounding basins of Mexico City (to the NW), Cuernavaca (SW) and Puebla (SE) rises as the boundary layer in these regions grow as a result of local heating and eventually the measurement site is within the air that comes from one of these regions. This presentation is a statistical summary of particle properties stratified by the air mass origin and, in addition, whether the measurement site was in or out of the free troposphere. The properties with highest correlation are shown along with frequency distributions and relevant time series. The objective of this summary is to offer researchers a quick look at the most significant trends, correlations, means and variances of the particle properties.
A41E-06
Regional Distribution of Metals and C and N Stable Isotopes in the Epiphytic Ball Moss (Tillandsia Recurvata) at the Mezquital Valley, Hidalgo State
As a part of the MILAGRO Field Campaign 2006, the influence of anthropogenic sources to metal air pollution in the Mezquital Valley, Hidalgo State, was explored by biomonitoring techniques. This valley is a major industrial- agriculture area located in central Mexico. An oil refinery, an electrical power plant, several cement plants with open-pit mines, as well as intensive wastewater-based agricultural areas, all within a 50 km radius, are some of the most important local sources of particulate air pollution. The concentrations of 25 metals and elements were determined by ICP-AES (EPA 610C method) for triplicate composite samples of the "ball moss" (T. recurvata ) collected at 50 sites. In addition, the ratios of two stable isotopes ((13C/12C and 15N/14N) were determined by continuous-flow isotope-ratio mass spectrometry in order to assess their potential as tracers for industrial emissions. Preliminary results showed high to very high average contents of several metals in the biomonitor compared to values from similar studies in other world regions, indicating a high degree of local air pollution. In contrast, most samples had Ag, As, Be, Se and Tl contents below detection levels (DL = 0.05 mg/kg of sample dry weight) indicating low levels of pollution by these metals. Metals such as Al, Ba, Ca, Fe, Li, Mo, Ni, Sr, Ti, V and Zn concentrated the most at the South portion of the valley, where the Tepeji-Tula-Apaxco industrial corridor is located. A transect parallel to the along-wind direction (N-S) showed a higher concentration of metals farther away from the sources relative to a cross-wind transect, which is consistent with the eolian transport of metal-enriched particles. Regional distribution maps of metals in the biomonitor showed that Al, Ba, Fe, Mo, Ni, Sr, Ti and V had higher levels at the industrial sampling sites; whereas K, Na and P were more abundant near to agriculture areas. Vanadium, a common element of crude oil, reflected better the influence from the local oil refinery and the oil- fueled power plant. Two distinct Ni:V scatterplot trends suggest that there are two main petrogenic emission sources in the region. Calcium and, to some extent, Mg were higher near the mining areas and a calcium carbonate factory. Lead had a diffuse distribution, probably related to former gasoline vehicle exhaust emissions, rather than to current emissions. Antimony was more abundant at sites far from agriculture and industrial areas, which suggests a natural origin (rocks or soils). The spatial distribution of stable isotopes also showed distinct patterns near the industrial sources with relatively 13C -depleted and 15N -enriched values near the oil refinery and the electrical power plant. Although it is not yet possible to provide quantitative estimates for emission contributions per source type, biomonitoring with T. recurvata provided for the first time a clear picture of the relative deposition patterns for several airborne metals in the Mezquital Valley.
A41E-07
Aerosol Light Absorption and Scattering at Four Sites in and Near Mexico City: Comparison with Las Vegas, Nevada, USA
Four photoacoustic spectrometers (PAS) for aerosol light scattering and absorption measurements were deployed in and near Mexico City in March 2006 as part of the Megacity Impacts on Regional and Global Environments (MIRAGE). The four sites included: an urban site at Instituto Mexicano del Petroleo (Mexican Oil Institute, denoted by IMP); a suburban site at the Technological University of Tecamac; a rural site at "La Biznaga" ranch; and a site at the Paseo de Cortes (altitude 3,810 meters ASL) in the rural area above Amecameca in the State of Mexico, on the saddle between the volcanoes Popocatepetl and Iztaccihuatl. A similar campaign was held in Las Vegas, Nevada, USA in January-February, 2003. The IMP site gave in-situ characterization of the Mexico City plume under favorable wind conditions while the other sites provided characterization of the plume, mixed in with any local sources. The second and third sites are north of Mexico City, and the fourth site is south. The PAS used at IMP operates at 532 nm, and conveniently allowed for characterization of gaseous absorption at this wavelength as well. Instruments at the second and third sites operate at 870 nm, and the one at the fourth site at 780 nm. Light scattering measurements are accomplished within the PAS by the reciprocal nephelometery method. In the urban site the aerosol absorption coefficient typically varies between 20 and 180 Mm-1 during the course of the day and significant diurnal variation of the aerosol single scattering albedo was observed probably as a consequence of secondary aerosol formation. Comparisons with TSI nephelometer scattering at the T0 site will be presented. We will present the diurnal variation of the scattering and absorption as well as the single scattering albedo and fraction of absorption due to gases at the IMP site and compare with Las Vegas diurnal variation. Mexico City 'breaths' more during the course of the day than Las Vegas, Nevada in part because the latitude of Mexico City resulted in more direct solar radiation. Further insight on the meteorological connections will be discussed.
A41E-08
A Tale of two Cities: Photoacoustic and Aethalometer Measurements Comparisons of Light Absorption in Mexico City and Las Vegas, NV, USA
As part of the Megacity Impacts on Regional and Global Environments, MIRAGE-Mex deployment to Mexico City in the period of 30 days, March 2006, a suite of photoacoustic spectrometers (PAS; W. Arnott & G. Paredes), nephelometer scattering, and aetholemeter absorption instruments (N. Marley & J.Gaffney) were installed to measure at ground level the light absorption and scattering by aerosols at the urban site at Instituto Mexicano del Petroleo (Mexican Oil Institute, denoted by IMP). This IMP site gave in-situ characterization of the Mexico City plume under favorable wind conditions. The PAS used at IMP operates at 532 nm, and conveniently allowed for characterization of gaseous absorption at this wavelength as well. Light scattering measurements are accomplished within the PAS by the reciprocal nephelometery method. In the urban site the aerosol absorption coefficient typically varies between 20 and 180 Mm-1 during the course of the day and significant diurnal variation of the aerosol single scattering albedo was observed. The Las Vegas, NV site was located at East Charleston Street on January-February, 2003. In east Las Vegas typical westerly winds carry the city plume across the site. Comparisons of PAS aerosol light absorption and aetholemeter absorption measurements at 521 nm at both Las Vegas NV and Mexico City sites will be presented. We will also present a broad overview of the diurnal variation of the scattering and absorption as well as the single scattering albedo and fraction of absorption due to gases at the sites in relation to secondary aerosol formation.