The time evolution of aerosol size distribution over the Mexico City plateau (original) (raw)
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The time evolution of aerosol composition over the Mexico City plateau
Atmospheric Chemistry and Physics, 2008
The time evolution of aerosol concentration and chemical composition in a megacity urban plume was determined based on 8 flights of the DOE G-1 aircraft in and downwind of Mexico City during the March 2006 MILA-GRO field campaign. A series of selection criteria are imposed to eliminate data points with non-urban emission influences. Biomass burning has urban and non-urban sources that are distinguished on the basis of CH 3 CN and CO. In order to account for dilution in the urban plume, aerosol concentrations are normalized to CO which is taken as an inert tracer of urban emission, proportional to the emissions of aerosol precursors. Time evolution is determined with respect to photochemical age defined as −Log 10 (NO x /NO y ). The geographic distribution of photochemical age and CO is examined, confirming the picture that Mexico City is a source region and that pollutants become more dilute and aged as they are advected towards T1 and T2, surface sites that are located at the fringe of the City and 35 km to the NE, respectively. Organic aerosol (OA) per ppm CO is found to increase 7 fold over the range of photochemical ages studied, corresponding to a change in NO x /NO y from nearly 100% to 10%. In the older samples the nitrate/CO ratio has leveled off suggesting that evaporation and formation of aerosol nitrate are in balance. In contrast, OA/CO increases with age in older samples, indicating that OA is still being formed. The amount of carbon equivalent to the deduced change in OA/CO with age is 56 ppbC per ppm CO. At an aerosol yield of 5% and 8% for low and high yield aromatic compounds, it is estimated from surface hydrocarbon observations that only ∼9% of the OA formation can be accounted for. A compari-Correspondence to: L. I. Kleinman (kleinman@bnl.gov) son of OA/CO in Mexico City and the eastern U.S. gives no evidence that aerosol yields are higher in a more polluted environment.
On the evolution of aerosol properties at a mountain site above Mexico City
Journal of Geophysical Research, 2000
Size distributions, scattering and absorption coefficients, and the bulk chemical composition of aerosols have been measured at a mountain site 400 m above the southwest sector of the Mexico City basin during a two-week period in November 1997. Variations in these properties are primarily related to local meteorology, i.e., wind direction and relative humidity; however, a link was found between carbon monoxide and ozone and the partitioning of aerosols between Aitken and accumulation mode sizes. Relative humidity was also found to affect this partitioning of aerosol size and volume. In addition, the fraction of sulfate in the aerosols was much higher on a high-humidity day than on a very low humidity day; however, the fraction of the mass contained in organic and elemental carbon was the same regardless of humidity levels. The daily variations of aerosol properties are associated with the arrival of new particles at the research site transported from the city basin and their subsequent mixture with aged aerosols that remain in the residual layer from the night before. 1. Overview Aerosol particles in urban areas are a major issue with respect to their impact on public health, damage to the environment, and changes to regional and global climate. The mutagenic and carcinogenic effects due to human inhalation of urban aerosols, particularly those composed of carbon, have been shown in numerous epidemiological studies [e.g., Docke• et al., 1992, 1993; h•ternational Agency for Research on Cancer (/ARC), !989; Pitts, 1983; Schuetzle, 1983]. Recent studies have also shown that atmospheric aerosols can significantly alter photochemical production rates of ozone; for example, ozone formation can bc enhanced as a result of light scattering by aerosols [Dickerson et al., 1997] or suppressed if actinic fluxes are decreased by aerosol layers that absorb radiation [Raga and Raga, 2000]. When aerosol particles are exported from their urban source regions, they contribute to the regional and global background of natural aerosols, where they may play a role in regional or global climate changes as they alter radiative fluxes or change the physical or optical properties of clouds. Numerous observational and modeling studies of aerosols have been conducted in major cities in industrial countries like the United States and Germany, but only recently has similar attention been focused on major urban areas in other parts of the world, particularly in developing countries [Cahill et al., 1996; Knox, 1996; Mage et al., 1996; World Health Organization, United Nations Environment Programme, (WHO UNEP), 1992]. The air quality situation in megacities like Mexico City underlines the need for comprehensive gas and aerosol studies in the
The T1-T2 study: evolution of aerosol properties downwind of Mexico City
Atmospheric Chemistry and Physics, 2007
As part of a major atmospheric chemistry and aerosol field program carried out in March 2006, a study was conducted in the area to the north and northeast of Mexico City to investigate the evolution of aerosols and their associated optical properties in the first few hours after their emission. The focus of the T1-T2 aerosol study was to investigate changes in the specific absorption α ABS (absorption per unit mass, with unit of m 2 g −1) of black carbon as it aged and became coated with compounds such as sulfate and organic carbon, evolving from an external to an internal mixture. Such evolution has been reported in previous studies. The T1 site was located just to the north of the Mexico City metropolitan area; the T2 site was situated approximately 35 km farther to the northeast. Nephelometers, particle soot absorption photometers, photoacoustic absorption spectrometers, and organic and elemental carbon analyzers were used to measure the optical properties of the aerosols and the carbon concentrations at each of the sites. Radar wind profilers and radiosonde systems helped to characterize the meteorology and to identify periods when transport from Mexico City over T1 and T2 occurred. Organic and elemental carbon concentrations at T1 showed diurnal cycles reflecting the nocturnal and early morning buildup from nearby sources, while concentrations at T2 appeared to be more affected by transport from Mexico City. Specific absorption during transport periods was lower than during other times, consistent with the likelihood of fresher emissions being found when the winds blew from Mexico City over T1 and T2. The spe
Atmospheric Chemistry and Physics, 2008
The concentration, size, and composition of nonrefractory submicron aerosol (NR-PM 1 ) was measured over Mexico City and central Mexico with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) onboard the NSF/NCAR C-130 aircraft as part of the MILA-GRO field campaign. This was the first aircraft deployment of the HR-ToF-AMS. During the campaign the instrument performed very well, and provided 12 s data. The aerosol mass from the AMS correlates strongly with other aerosol measurements on board the aircraft. Organic aerosol (OA) species dominate the NR-PM 1 mass. OA correlates strongly with CO and HCN indicating that pollution (mostly secondary OA, SOA) and biomass burning (BB) are the main OA sources. The OA to CO ratio indicates a typical value for aged air of around 80 µg m −3 (STP) ppm −1 . This is within the range observed in outflow from the Northeastern US, which could be due to a compensating effect between higher BB but lower biogenic VOC emissions during this study. The O/C atomic ratio for OA is calculated from the HR mass spectra and shows a clear increase with photochemical age, as SOA forms rapidly and quickly overwhelms primary urban OA, consistent with and Kleinman et al. (2008). The stability of the OA/CO while O/C increases with photochemical age implies a net loss of carbon from the OA. BB OA is marked by signals at m/z 60 and Correspondence to: J. L. Jimenez (jose.jimenez@colorado.edu) P. F. DeCarlo et al.: Aerosol size and chemistry measurements during MILAGRO Atmos.
Atmospheric Chemistry and Physics, 2008
Measurements of atmospheric gases and fine particle chemistry were made in the Mexico City Metropolitan Area (MCMA) at a site ∼30 km down wind of the city center. Ammonium nitrate (NH 4 NO 3 ) dominated the inorganic aerosol fraction and showed a distinct diurnal signature characterized by rapid morning production and a rapid mid-day concentration decrease. Between the hours of 08:00-12:45, particulate water-soluble organic carbon (WSOC) concentrations increased and decreased in a manner consistent with that of NO − 3 , and the two were highly correlated (R 2 =0.88) during this time. A box model was used to analyze these behaviors and showed that, for both NO − 3 and WSOC, the concentration increase was caused primarily (∼75-85%) by secondary formation, with a smaller contribution (∼15-25%) from the entrainment of air from the free troposphere. For NO − 3 , a majority (∼60%) of the midday concentration decrease was caused by dilution from boundary layer expansion, though a significant fraction (∼40%) of the NO −
Atmospheric Chemistry and Physics Discussions, 2010
Organic aerosol (OA) represents approximately half of the submicron aerosol in Mexico City and the Central Mexican Plateau. This study uses the high time resolution measurements performed onboard the NCAR/NSF C-130 aircraft during the MILAGRO/MIRAGE-Mex field campaign in March 2006 to investigate the sources and chemical processing of the OA in this region. An examination of the OA/ CO ratio evolution as a function of photochemical age shows distinct behavior in the presence or absence of substantial open biomass burning (BB) influence, with the latter being consistent with other studies in polluted areas. In addition, we present results from Positive Matrix Factorization (PMF) analysis of 12-s High-Resolution Timeof-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) OA spectra. Four components were resolved.
Characterization of aerosol particles during a high pollution episode over Mexico City
Scientific Reports
More than 7 thousand wildfires were recorded over Mexico in 2019, affecting almost 640 thousand hectares. Most of these fires occurred during the spring season generating dense smoke plumes, impacting urban areas in the central part of the Mexican plateau. From May 10 to 17, 2019, biomass burning (BB) plumes affected Mexico City (MC) and diffused across the basin, producing PM2.5 levels ~ 2 times higher than the nation's air quality standards. Average PM2.5 concentrations increased sharply from 29.4 ± 7.2 µg m−3 to 65.1 ± 13.6 µg m−3 when the dense smoke plumes were detected. The higher particle concentration impacted the aerosol optical depth (AOD) as values ~ 3 times greater than the annual mean (0.32 ± 0.12) were measured, which resulted in a 17% loss of global horizontal irradiation (GHI). Under these severe pollution conditions, the visibility (Va) was reduced by ~ 80%. The high incidence of strong absorbent particles, such as soot and tarballs was revealed through electron...
Atmospheric Chemistry and Physics, 2009
Due to the presence of diffusive anthropogenic sources in urban areas, the spatiotemporal variability of fine (diameter <1 µm) and ultrafine (<0.1 µm) aerosol particles has been a challenging issue in particle exposure assessment as well as atmospheric research in general. We examined number size distributions of atmospheric aerosol particles (size range 3-800 nm) that were measured simultaneously at a maximum of eight observation sites in and around a city in Central Europe (Leipzig, Germany). Two main experiments were conducted with different time span and number of observation sites (2 years at 3 sites; 1 month at 8 sites). A general observation was that the particle number size distribution varied in time and space in a complex fashion as a result of interaction between local and far-range sources, and the meteorological conditions. To identify statistically independent factors in the urban aerosol, different runs of principal component analysis were conducted encompassing aerosol, gas phase, and meteorological parameters from the multiple sites. Several of the resulting principal components, outstanding with respect to their temporal persistence and spatial coverage, could be associated with aerosol particle modes: a first accumulation mode ("droplet mode", 300-800 nm), considered to be the result of liquid phase processes and far-range transport; a second accumulation mode (centered around diameters 90-250 nm), considered to result from primary emissions as well as aging through condensation and coagulation; an Aitken mode (30-200 nm) linked to urban traffic emissions in addition to an urban and a rural Aitken mode; a nucleation mode (5-20 nm) linked to urban traffic emissions; nucleation modes (3-20 nm) linked to photochemically induced particle formation; an aged nucleation mode (10-50 nm). A number of additional components were identified to represent only local sources at a single site each, or infrequent phenomena. In summary, the analysis of size distributions of high time and size resolution yielded a surprising wealth of statistical aerosol components occurring in the urban atmosphere over one single city. Meanwhile, satisfactory physical explanations could be found for the components with the greatest temporal persistence and spatial 18156
Meteorological dependence of size-fractionated number concentrations of urban aerosol particles
Atmospheric Environment, 2006
We utilized a long-term data set of aerosol particle number size distributions (8-400 nm) in the urban background air of Helsinki during 1998-2000. We analyzed the number concentrations of both ultra-fine particles (UFP diameter o100 nm) and the fraction of fine particles (FP, diameter o2.5 mm) larger than 100 nm (accumulation mode), and we also investigated their dependencies on the relevant meteorological parameters. The meteorological parameters were obtained by a meteorological pre-processing model. Among the meteorological parameters considered in this study (wind speed and direction, temperature, atmospheric pressure, relative humidity, Monin-Obukhov length and mixing height), the ambient temperature and local wind conditions were found to be the most important factors that control the number concentrations of FP. We described the dependencies of FP number concentrations on meteorological variables by using an empirically based mathematical function that contains the ambient temperature, local wind speed and direction as independent variables. According to statistical analyses, the predicted number concentrations of accumulation mode particles follow this relationship more closely than those of UFP's. This is mainly due to the origin and type of aerosol particles in the accumulation mode size range, being mainly regional and long-range transported. The main limitations of the mathematical function presented in this study are during new particle formation events, precipitation and long-range pollution episodes of aerosol particles. This study provides an attempt to predict the particle number concentrations of FP by utilizing a simple model that connects the relationship between the aerosol particle number concentrations and the relevant meteorological parameters. r