B. Zuberi

Massachusetts Institute of Technology, Cambridge, MA, United States

Are you B. Zuberi?

Claim your profile

Publications (14)35.16 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: During the Mexico City Metropolitan Area (MCMA) field campaign of 2003, measurements of the shortwave radiation field allowed the inference of the black carbon (BC) specific absorption, ?<sub>?</sub>, defined as the monochromatic absorption cross section per unit mass (with units of m<sup>2</sup>/g). The averaged values of ?<sub>?</sub> derived from the method here are either 8.9 m<sup>2</sup>/g or 8.2 m<sup>2</sup>/g at 500 nm, depending upon the physical and optical parameters assumed for BC. These results are reasonably consistent with those of Schuster et al. (2005), 9.5 m<sup>2</sup>/g, and Baumgartner et al. (2002), 7.0 m<sup>2</sup>/g, both measured at 550 nm. The ?<sub>?</sub> values reported in this paper should only be considered effective, "radiatively correct" values because when used in radiative transfer calculations the calculated irradiances match the measured irradiances at 500 nm. The specific absorption so defined can assume a wide range of values, depending upon: (1) the assumptions made prior to the retrieval (e.g., shell/core aerosol configuration), and (2) values chosen for BC density and refractive index. The range of possible values is large, corresponding to a "worst case" uncertainty of about ±70%, assuming that all errors are additive and of the same sign so that no error cancellation occurs.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 03/2007; · 5.51 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Aerosols play an important role in the atmosphere but are poorly characterized, particularly in urban areas like the Mexico City Metropolitan Area (MCMA). The chemical composition of urban particles must be known to assess their effects on the environment, and specific particulate emissions sources should be identified to establish effective pollution control standards. For these reasons, samples of particulate matter ≤2.5 μm (PM<sub>2.5</sub>) were collected during the MCMA-2003 Field Campaign for elemental and multivariate analyses. Proton-Induced X-ray Emission (PIXE), Proton-Elastic Scattering Analysis (PESA) and Scanning Transmission Ion Microscopy (STIM) measurements were done to determine concentrations of 19 elements from Na to Pb, hydrogen, and total mass, respectively. The most abundant elements from PIXE analysis were S, Si, K, Fe, Ca, and Al, while the major emissions sources associated with these elements were industry, wind-blown soil, and biomass burning. Wind trajectories suggest that metals associated with industrial emissions came from northern areas of the city whereas soil aerosols came from the southwest and increased in concentration during dry conditions. Elemental markers for fuel oil combustion, V and Ni, correlated with a large SO<sub>2</sub> plume to suggest an anthropogenic, rather than volcanic, emissions source. By subtracting major components of soil and sulfates determined by PIXE analysis from STIM total mass measurements, we estimate that approximately 50% of non-volatile PM<sub>2.5</sub> consisted of carbonaceous material.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2006; · 5.51 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM<sub>1</sub>) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM<sub>2.5</sub> Tapered Element Oscillating Microbalance (TEOM), and a PM<sub>2.5</sub> DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM<sub>2.5</sub> mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM<sub>2.5</sub> at CENICA represents, on average, 54.6% (standard deviation σ=10%) of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM<sub>2.5</sub> (σ=10%); BC mass concentration is about 11% (σ=4%); while soil represents about 6.9% (σ=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO<sub>2</sub> were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.
    Atmospheric Chemistry and Physics. 01/2006;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite during the Mexico City Metropolitan Area field study from 31 March?4 May 2003. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM<sub>1</sub>) with high time and size-resolution. Measurements of Black Carbon (BC) using an aethalometer, and estimated soil concentrations from Proton-Induced X-Ray Emission (PIXE) analysis of impactor substrates are also presented and combined with the AMS in order to include refractory material and estimate the total PM<sub>2.5</sub> mass concentration at CENICA during this campaign. In Mexico City, the organic fraction of the estimated PM<sub>2.5</sub> at CENICA represents 54.6% of the mass, with the rest consisting of inorganic compounds (mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM<sub>2.5</sub>; BC mass concentration is about 11%; while soil represents about 6.9%. The NR species and BC have diurnal cycles that can be qualitatively interpreted as the interplay of direct emissions, photochemical production in the atmosphere followed by condensation and gas-to-particle partitioning, boundary layer dynamics, and/or advection. Bi- and trimodal size distributions are observed for the AMS species, with a small combustion (likely traffic) organic particle mode and an accumulation mode that contains mainly organic and secondary inorganic compounds. The AMS and BC mass concentrations, size distributions, and diurnal cycles are found to be qualitatively similar to those from most previous field measurements in Mexico City.
    Atmospheric Chemistry and Physics 06/2005; · 4.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: During the Mexico City Metropolitan Area (MCMA) field campaign of 2003, measurements of the shortwave radiation field, lidar backscatter, and atmospheric concentrations of black carbon (BC) permitted the inference of the BC carbon specific absorption, ?<sub>?</sub>, defined as the absorption cross section per unit mass (with units of m<sup>2</sup>/g). This diverse set of measurements allowed us to determine ?<sub>?</sub> in two ways. These methods ? labeled I and II ? are distinguished from one another in the manner that the columnar concentration of BC (with units of mg/m<sup>2</sup> is determined. This concentration is found by using either surface measurements of BC concentration and lidar estimates of aerosol mixing heights, or a more rigorous method that relies on the columnar aerosol size distribution. The averaged values of ?<sub>?</sub> derived from these methods agree to about 20%, although we expect that the values obtained from method I are underestimated. These results, along with those of Schuster et al. (2005), suggest that in the MCMA, ?<sub>?</sub> is in a range of 8 to 10 m<sup>2</sup>/g at a wavelength of 550 nm. This range is somewhat lower than the commonly accepted value of 10 m<sup>2</sup>/g for a wavelength of 550 nm, but is consistent with the calculations of Fuller et al. (1999), who suggest that this value is too high.
    Atmospheric Chemistry and Physics 06/2005; · 4.88 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, while another was deployed in the Aerodyne Mobile Laboratory (AML) during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March?4 May 2003 to investigate particle concentrations, sources, and processes. This is the first of a series of papers reporting the AMS results from this campaign. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 µm (NR-PM<sub>1</sub>) with high time and size-resolution. For the first time, we report field results from a beam width probe, which was used to study the shape and mixing state of the particles and to quantify potential losses of irregular particles due to beam broadening inside the AMS. Data from this probe show that no significant amount of irregular particles was lost due to excessive beam broadening. A comparison of the CENICA and AML AMSs measurements is presented, being the first published intercomparison between two quadrupole AMSs. The speciation, and mass concentrations reported by the two AMSs compared relatively well. The differences found are likely due to the different inlets used in both instruments. In order to account for the refractory material in the aerosol, we also present measurements of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a Tapered Element Oscillating Microbalance (TEOM) and a DustTrak Aerosol Monitor) are also presented. The comparisons show that the AMS + BC + soil mass concentration during MCMA-2003 is a good approximation to the total PM<sub>2.5</sub> mass concentration.
    Atmospheric Chemistry and Physics 01/2005; · 4.88 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The global presence of soot has significant effects on regional and global climate, as well as human health. Influence of soot on radiation budget, rain formation and heterogeneous chemistry, and its residence time in the atmosphere are largely dependent on its ability to interact with water. While freshly emitted soot is extremely hydrophobic, oxidation during aging causes soot to become more hydrophilic. Laboratory studies demonstrate that aged soot attracts and retains water, and can be efficiently removed from the troposphere by entrapment in existing liquid cloud droplets or by activation as cloud condensation nuclei.
    Geophysical Research Letters - GEOPHYS RES LETT. 01/2005; 320(1).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Chemical composition, size, and mixing state of atmospheric particles are critical in determining their effects on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and human health, but still relatively little is known of their physical and chemical nature. In addition, the atmospheric residence times and removal mechanisms for soot are neither well understood nor adequately represented in regional and global climate models. To investigate the effect of locality and residence time on properties of soot and mixing state in a polluted urban environment, particles of diameter 0.2–2.0 μm were collected in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 Field Campaign from various sites within the city. Individual particle analysis by different electron microscopy methods coupled with energy dispersed x-ray spectroscopy, and secondary ionization mass spectrometry show that freshly-emitted soot particles become rapidly processed in the MCMA. Whereas fresh particulate emissions from mixed-traffic are almost entirely carbonaceous, consisting of soot aggregates with liquid coatings suggestive of unburned lubricating oil and water, ambient soot particles which have been processed for less than a few hours are heavily internally mixed, primarily with ammonium sulfate. Single particle analysis suggests that this mixing occurs through several mechanisms that require further investigation. In light of previously published results, the internally-mixed nature of processed soot particles is expected to affect heterogeneous chemistry on the soot surface, including interaction with water during wet-removal.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 01/2005; · 5.51 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Chemical composition, particle size, and mixing state are critical in determining the effects of atmospheric aerosol on the environment. There is growing evidence that soot aerosols play a particularly important role in both climate and health, but still relatively little is known of their physical and chemical nature. To investigate the effect of locality and residence time on properties of soot in a polluted urban environment, collections of particles 0.2-2.0 mu m were taken in the Mexico City Metropolitan Area (MCMA) during the MCMA-2003 campaign. Samples were collected using an automated Time-Resolved Aerosol Collector (TRAC) both onboard a mobile laboratory and at stationary sites within the city. Individual particles were analyzed with Computer-Controlled Scanning Electron Microscopy with Energy Dispersed detection of X-rays (CCSEM/EDX), Transmission Electron Microscopy (TEM), and Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS). Our analysis shows that soot is a major component of aerosol in the MCMA, and freshly-emitted soot is rapidly processed, leading to the presence of inorganic species such as ammonium sulfate on the soot surface. While fresh, mixed-traffic tailpipe emissions are almost entirely carbonaceous (consisting of soot aggregate chains and lube-oil components), processed soot particles in the MCMA show evidence of heavy internal mixing. Our analysis also suggests that internal mixing may occur through several possible mechanisms, which require further investigation.
    AGU Fall Meeting Abstracts. 12/2004;
  • [Show abstract] [Hide abstract]
    ABSTRACT: An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed to the CENICA Supersite during the Mexico City Metropolitan Area (MCMA-2003) field study from March 29 - May 4, 2003. Organic aerosols dominate the NR-PM1 aerosol concentrations in Mexico City during MCMA-2003, and are the focus of this presentation (inorganic species: Salcedo et al., this conference). We have applied a procedure based on internal AMS tracers (Zhang et al., this conference) to estimate the concentrations and size distributions of primary and oxygenated organic aerosols (POA and OOA respectively). OOA encompasses secondary organic aerosol (SOA) as well as oxidized POA. This method is based on iterative principal component analysis with a custom method that uses AMS m/z 57 and m/z44 as initial time series for POA and OOA respectively. During MCMA-2003 POA shows good correlation with primary combustion emissions (such as CO, BC, and NOx), and SOA shows correlation with photochemical products (such as O3 and glyoxal). Our results show that organic aerosols during MCMA-2003 were comprised of ˜35% POA, and ˜65% OOA. Intense SOA formation was observed many of the days, which is consistent with the high levels of aerosol precursors (aromatics), UV radiation, and of radicals (OH, HO2) measured by other researchers. OOA in Mexico City is likely dominated by SOA from the oxidation of aromatic species. The aerosol size distribution was often bimodal, with a smaller mode, ˜100 nm in dva,(i.e., aerodynamic diameter in free molecular regime) characteristic of traffic emissions, and a larger accumulation mode ( ˜500 nm dva) dominated by photochemical products. The ultrafine (< 100 nm) composition is analyzed with collocated measurements from a nano-SMPS and the AMS. Traffic-related POA dominates the ultrafine mode every morning during rush hour. Ultrafine particles in the afternoon contain both sulfate and organics (both POA and SOA). A nucleation event was observed inside the city when the aerosol surface area was relatively low.
    AGU Fall Meeting Abstracts. 12/2004;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Particulate matter Na, H, and total aerosol mass, respectively, for each size range. Multivariate analysis including Positive Matrix Factorization (PMF) was applied to group elements by common factors to identify possible aerosol emission sources within each size range. Sudden increases in elements characteristic of industrial emissions and fuel oil suggest manufacturing sources to the north of the city whereas soil aerosols originate from more rural areas to the south. Sulfur contributes to a significant fraction of PM2.5, in agreement with complementary aerosol measurements taken during the campaign. Additional trends and diurnal profiles observed for Mexico City aerosol are presented.
    AGU Fall Meeting Abstracts. 12/2004;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Using optical microscopy, we investigated the heterogeneous nucleation of ice in aqueous (NH4)2SO4-H2O particles containing two types of mineral dusts, kaolinite and montmorillonite. The efficacy of montmorillonite and kaolinite to nucleate ice in (NH4)2SO4-H2O particles is similar. The difference in freezing temperatures, compared to the homogeneous freezing temperatures, is found to vary from 8-20 K and it is larger for particles with concentrations greater than 27 wt %. Our freezing data shows that for temperatures ranging from 239 K to 198 K, ice super-saturations between 1.35 and 1.51 are required for ice to heterogeneously nucleate in NH4SO4-H2O particles containing mineral dust immersions. Based on our results, we conclude mineral dust is an efficient nuclei for ice in NH4SO4-H2O aerosols and as a result, it can initiate the formation of upper tropospheric ice clouds at warmer temperatures and lower super-saturations in comparison to homogeneous freezing.
    Geophysical Research Letters 01/2002; 29(10):142-1. · 3.98 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Heterogeneous freezing of aqueous particles with solid inclusions of crystallized (NH4)2SO4, ice, and letovicite were studied using optical microscopy and differential scanning calorimetry. For (NH4)2SO4−H2O particles, the heterogeneous freezing temperature was found to be dependent on the morphology of the (NH4)2SO4 solid. If the crystallized solid was in the form of microcrystals, the heterogeneous ice-freezing temperature was close to the eutectic temperature and the critical saturation with respect to ice was close to 1. However, if the solid was in the form of one or two large crystals, the heterogeneous freezing temperature was close to the homogeneous freezing temperature. For particles with one or two large (NH4)2SO4 crystals in equilibrium with (NH4)2SO4−H2O solution, we have estimated an upper limit of 1.5 × 10-5 s-1 μm-2 for Jhet (heterogeneous nucleation rate of ice, immersion freezing mode). Our results for NH4HSO4−H2O particles show that when one or two large crystals of either ice or letovicite are present in the solution, the freezing temperature does not deviate significantly from the homogeneous freezing temperature, consistent with the (NH4)2SO4−H2O experiments. Our work shows that the surface area and surface microstructure of crystalline solids present in aqueous aerosols can significantly change the heterogeneous freezing temperature and critical ice saturations and that heterogeneous ice nucleation induced by crystalline salts may be very important in the formation of upper tropospheric clouds.
    Journal of Physical Chemistry A - J PHYS CHEM A. 01/2001; 105(26):6458-6464.