RESUMO

A successive moving average subtraction method is developed and applied to black carbon measured over 5-min intervals at a downtown location near many small emitters and at a suburban residential site within the urban plume but distant from specific emitters. Short-duration pulses assumed to originate from nearby sources are subtracted from the concentrations at each site and are summed to estimate middle-scale (approximately 0.1-1 km) contributions. The difference of the remaining baselines at the urban and suburban monitors is interpreted as the contribution to the downtown monitor from source emissions mixed over a neighborhood scale (1-5 km). The baseline at the suburban site is interpreted as the contribution of the mixture of black carbon sources for the entire city. When applied to a 24-day period from February and March 1997 in Mexico City, the analysis showed that 65% of the 24-hr black carbon was part of the urban mixture, 23% originated in the neighborhood surrounding the monitor, and only 12% was contributed from nearby sources. These analyses indicate that a fixed-site monitor can reasonably represent exposures in its surrounding neighborhood even when many local sources, such as exhaust from diesel buses and trucks, affect the monitor.

Assuntos

RESUMO

Chemical profiles for particle emissions are needed for source apportionment studies using the chemical mass balance (CMB) receptor model. Source measurements of geological sources, motor vehicle exhaust, vegetative burning (e.g. asparagus, field burning, charbroil cooking), and industrial sources (e.g. oil-fueled glass plant, manure-fueled power plants) were acquired as part of the Imperial/Mexicali Valley Cross Border PM10 Transport Study in 1992. Six different source sampling techniques (i.e. hot- and diluted-exhaust sampling, ground-based source sampling, particle sweeping/grab sampling, vacuum sampling, and laboratory resuspension sampling) were applied to acquire filter samples of PM 2.5 and PM10 (particulate matter with aerodynamic diameters < 2.5 and 10 microm, respectively). Filter samples were analyzed for mass by gravimetry, elements (Na to U) by X-ray fluorescence, anions (Cl(-), NO3(-), SO4(=)) by ion chromatography, ammonium (NH4(+)) by automated colorimetry, soluble sodium (Na+) and potassium (K+) by atomic absorption spectrophotometry, and organic and elemental carbon (OC, EC) by thermal/optical reflectance. Concentration data were acquired for a total of approximately 50 chemical species. Elevated abundances of crustal components (Al, Si, K, Ca, Fe) from geological material, carbon (OC, EC) and trace elements (Br, Pb) from vehicle exhausts, carbon (OC, EC) and ions (K(+), Cl(-)) from vegetative burning, ions (SO4(=), NH4(+), Na(+), K(+), Cl(-)) and elements (Cl, Se) from a manure-fueled power plants, and sulfur and trace elements (Na(+), Pb, Se, Ni, V) from an oil-fueled glass plant were found in the resulting source profiles. Abundances of crustal species (e.g. Al, Si, Ca) in the Imperial/Mexicali Valley geological profiles are more than twice those found in central and southern California. Abundances of lead in motor vehicle exhausts indicate different vehicle fleets in border cities. Emission profiles from field burning and charbroil cooking specific to the border area show that a majority (>60%) of emissions are comprised of carbon, with high organic to total carbon ratios (0.93 to 0.97). Abundances of sulfate and ammonium account for nearly 60% of the manure-fueled power plant's emissions. Elevated levels of metals (Na(+), Pb, Cd, Se) and byproducts of petroleum combustion (S, Ni, V) were found in the oil-fueled glass plant's emissions.

Assuntos

RESUMO

The 'Imperial/Mexicali Valley Cross-Border PM10 Transport Study' acquired a database of meteorological and air quality measurements to determine source contributions to elevated PM10 concentrations and to estimate transport of PM10 between the US and Mexico. The study was conducted from 13 March 1992 to 29 August 1993, in a 80-km long by 20-km wide area spanning the US/Mexico border approximately 200 km inland from the coast of the Pacific Ocean, with monitoring sites located in the Imperial Valley on the US side and in the Mexicali Valley on the Mexico side. Measurements of PM 10 (particles with aerodynamic diameters less than 10 microm) mass, elements, water-soluble cations (i.e. sodium, potassium, ammonium) and anions (i.e. chloride, nitrate, sulfate), organic and elemental carbon and particle light absorption were acquired at two base sites on an every-sixth-day schedule supplemented by daily monitoring during winter and 4 times per day monitoring during intensive periods. Measurements were also taken at as many as 30 neighborhood (satellite) sites during week-long intensive monitoring periods in spring, summer and winter. This paper examines the zones of representation of long-term PM10 monitors by comparing their measurements with those from a spatially dense network of satellite sites. PM10 concentrations at the Mexicali site were consistently 30 to 50% higher than those observed at the Calexico site, even though the two sites were only 12 km apart. Distinct diurnal variations were found, with 6-h average PM10 concentrations often varying by a factor of 2 throughout the day - lowest during afternoon (12.00-18.00 h PST) and highest during night time (18.00-24.00 h PST). On average, crustal material accounted for 32-35% of annual-average PM10, carbonaceous aerosol for 20-30%, and ionic species for 8-10%. Levels of trace elements and sea salt were in the range of 1-4% of PM10. Significant concentration variations were found within the study area. PM10 concentrations in Mexico were double those in the US, decreasing with increasing northerly distance.

Assuntos

RESUMO

PM10, PM25, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the city's particulate matter (PM). Average 24-hr PM10 concentrations over the period of study at the core sites in the city were 75 H g/m3. The 24-hr standard of 150 µ g/m3 was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 µ g/m3. Nearly half of the PM10 was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM10 consisted of PM2.5, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM2.5. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.