Nancy A. Marley

Particulate Air Pollution in Mexico City: A Collaborative Research Project

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 citys 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.

An Empirical Method for the Determination of the Complex Refractive Index of Size-Fractionated Atmospheric Aerosols for Radiative Transfer Calculations

To adequately assess the effects of atmospheric aerosols on climate, their optical constants (scattering and absorption coefficients) must be known. The absorption and scattering coefficients of the aerosols are derived from the real and imaginary parts of the complex refractive index and are dependent on their size and chemical composition. Because aerosol properties vary significantly with location, it is difficult to assign values for the absorption and scattering of solar radiation by aerosols in models of global climate change. This study reports a new method of collecting size-fractionated atmospheric aerosol samples for the purpose of directly measuring their transmission and reflectance spectra followed by the determination of the complex refractive index across the entire atmospherically relevant spectral range. The samples were collected with a modified Sierra high-volume cascade impactor with the usual filter collection surfaces replaced with Teflon sheets machined to hold quartz (ultraviolet [UV]/visible transparent) and/or silver chloride (infrared transparent) sample collection plates. Reflectance and transmission spectra can be obtained on the aerosol samples directly as a function of wavelength, from 280 nm to 2.5 m, with an integrating sphere coupled to an UV/visible or a Fourier transform infrared (FTIR) spectrophotometer. The effective real and imaginary components of the refractive index of the bulk sample material can then be approximated, as a function of wavelength, from the sample spectra. Preliminary results are presented for carbon soot samples generated in the laboratory and for standard diesel soot samples in the UV/visible spectral range. These are compared to results obtained for size-fractionated atmospheric aerosol samples collected near Pasco, WA, West Mesa, AZ, and Argonne, IL.

Peroxyacyl Nitrates (PANs): Their Physical and Chemical Properties

Peroxyacyl nitrates (PANs) are important organic indicators of tropospheric chemical processes. In this chapter, our current understanding of the chemical and physical properties of PANs are reviewed.

Field observations of regional and urban impacts on NO2, ozone, UVB, and nitrate radical production rates in the Phoenix air basin

Abstract In the May and June of 1998, field measurements were taken at a site near the Usery Pass Recreation Area, ∼27 miles from the downtown Phoenix area, overlooking Phoenix and Mesa, Arizona. This site was selected to examine the impacts of the Phoenix urban plume on the Usery Pass Recreation Area and surrounding regions. Data were obtained for ultraviolet-B (UVB) radiation, nitrogen dioxide (NO 2 ), peroxyacetyl nitrate (PAN), ozone (O 3 ), and carbon monoxide (CO). Nocturnal plumes of NO 2 (in tens of ppb), observed near midnight, were correlated with CO and anti-correlated with O 3 . This behavior was consistent with the titration of locally generated NO by boundary layer O 3 to form the nighttime NO 2 plumes that were subsequently transported into the Usery Pass Recreation area. Nitrate radical (NO 3 ) production rates were calculated to be very high on the edges of these nocturnal plumes. Examination of O 3 and PAN data also indicates that Phoenix is being affected by long-range transport of pollutants from the Los Angeles to San Diego areas. A regional smoke episode was observed in May, accompanied by a decrease in UVB of factor of two and a decrease in O 3 and an increase in methyl chloride. Low level back trajectories and chemical evidence confirm that the smoke event originated in northern Mexico and that the reduced O 3 levels observed at Usery Pass could be partially due to reduced photolysis rates caused by carbonaceous soot aerosols transported in the smoke plume. The results are discussed with regard to potential effects of local pollution transport from the Phoenix air basin as well as an assessment of the contributions from long-range transport of pollutants to the background levels in the Phoenix-Usery Pass area.

Heterogeneous NOx chemistry in the polluted PBL

Abstract The significance of heterogeneous mechanisms in controlling gas-phase NO x (NO, NO 2 ) mixing ratios in polluted urban air, especially during nighttime, is not well established. Several recent studies have suggested that carbon soot can provide an effective surface for mediating the inter conversion among several NO y members. However, a number of such reactions reported in the literature have widely varying reaction probabilities and often conflicting pathways. We evaluated several of these reactions and choose the NO 2 conversion to HONO on the surface of soot particles for further analysis with a box photochemical model. These calculations show that the conversion of NO 2 to HONO on particle surfaces produces a large, measurable signal (up to several parts per billion) in nighttime HONO mixing ratios. Inclusion of this reaction was also shown to have significant impacts on ozone, OH and HO 2 in the polluted planetary boundary layer (PBL). The sensitivity of these results to the different reaction rate probabilities ( γ ) and particle surface areas was also examined. Results are then evaluated to find the combination of γ and surface areas that would mostly likely occur in the PBL within the limitations of the model.

Fast gas chromatography with luminol chemiluminescence detection for the simultaneous determination of nitrogen dioxide and peroxyacetyl nitrate in the atmosphere

An instrument has been designed and constructed for the simultaneous determination of nitrogen dioxide (NO2) and peroxyacetyl nitrate (PAN) in atmospheric samples. The instrument’s design is based on separation by fast gas chromatography (GC) with a 30 ft capillary column (DB-1) followed by detection by luminol chemiluminescence. The chemiluminescent reaction between NO2 or PAN and luminol takes place at the gas–liquid interface on the surface of a solid support. The chemiluminescent emission at 425 nm is detected with a photon counting module. The instrument is controlled by a 1.8 GHz Notebook computer with a WINDOWS 2000 operating system and a custom software application programmed in LABVIEW. Detection limits are in the low parts per trillion (ppt) with a time resolution of 30 s to 1 min. The instrument was operated during the Mexico City Metropolitan Area/Mexico City Megacity 2003 collaborative air quality study. Results for NO2 from this fast GC method were compared with results from a co-located dif...

Chemical characterization of size-fractionated humic and fulvic materials in aqueous samples

Abstract Humic and fulvic acids have been shown to be potentially important transport agents for inorganic and organic contaminants, including radionuclides, in surface and groundwaters. In this work, the possibility of characterizing humic and fulvic materials in the colloidal and macromolecular size ranges (i.e., between 3000 molecular weight (1 nm) and 0.45 μm) after size fractionation and collection with hollow-fiber ultrafilters has been investigated. Three surface waters have been examined as test cases. Using ultrafiltration, sized samples of humic and fulvic acids were chemically characterized with 13 C-nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FTIR) using diffuse reflectance, and pyrolysis/gas chromatography (GC). These test cases demonstrate that sufficient material can be obtained by ultrafiltration with minimal or no alteration to the materials. In addition, inorganic analyses performed on the size fractions show that ultrafiltration can also allow the binding capacity of the humic materials in each fraction to be measured. Substantial variability among the different humic and fulvic fractions demonstrates the importance of sizing submicron materials in attempts to understand the mechanisms of pollutant transport by natural humic materials.

Measurements of 7Be and 210Pb in rain, snow, and hail

Abstract Measurements of the levels of 7Be and 210Pb are reported for rain, snow, and hail samples taken at Argonne, Illinois, and Socorro, New Mexico. These natural radioisotopes are indicators of the sources of the aerosols contributing materials to the precipitation samples. The data presented indicate that the more soluble 7Be is enriched in the precipitation samples with respect to 210Pb, as compared to the ratios of these radioisotopes found in aerosol samples. Use of the 210Po/210Pb activity ratios as an internal clock indicated that the aerosols contributing to the precipitation ranged in age from 10 to 47 days. Levels of 7Be ranged from 11 to 55 pCi L−1 for the samples, with the highest levels in a stratus precipitation event and in a thunderstorm with the lowest wet deposition rate. These results are discussed with regard to the potential for use of these radioisotopes in the determination of stratospheric-tropospheric mixing and in their geochemical usage as indicators of sedimentation rates.

Characterization of fine mode atmospheric aerosols by Raman microscopy and diffuse reflectance FTIR.

A combination of Raman microscopy and diffuse reflectance Fourier transform infrared spectroscopy (FTIR) has been used for the characterization of fine mode (<1 μm) tropospheric aerosols. Peak fitting was used to identify five overlapping bands in the Raman spectra. These bands have been identified as due to combustion generated carbon soot as well as large molecular organic carbon species. The fwhm of the D band at 1400 cm(-1) as well as the ratio of intensities of the D3 band at 1550 cm(-1) to the G band at 1580 cm(-1) can serve as a measure of the aerosol organic carbon content. Raman microscopy combined with spectral mapping capabilities was used to investigate the composition of the fine mode aerosols at the particle level, allowing for the direct determination of aerosol mixing state. Results showed that the fine aerosols were predominately internally mixed particles composed of carbon soot coated with molecular organic carbon species. Characterization of the aerosols by diffuse reflectance FTIR showed that the major organic carbon species were polycarboxylates and polysaccharide-like species typical of humic-like substances (HULIS).

In-depth review of atmospheric mercury: sources, transformations, and potential sinks

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