A23D-01 13:40h
VARIABILITY OF AMBIENT AEROSOL IN THE MEXICO CITY METROPOLIAN AREA
The spatial and temporal variations of the ambient aerosol in the Mexico City Metropolitan area was characterized during the springs of 2002 and 2003 using a mobile laboratory equipped with gas and particulate measurement instrumentation. The laboratory was operated at various fixed sites locations in and at the edge of the metropolitan area (Xalostoc, Merced, Cenica, Pedregal, and Santa Ana). Size-resolved aerosol mass and chemical composition was measured with an aerosol mass spectrometer and selected trace gas species (low mass organic compounds, NO, NO2, NOy, O3, SO2, CH2O, NH3, CO2) were measured using a proton transfer reaction mass spectrometer and various optical systems. The aerosol was predominantly organic in composition with lesser amounts of ammonium nitrate, sulfate, and chloride. The organic component was composed of mixed primary and secondary organic compounds. The mass loading and chemical composition of the aerosol was influenced by local and regional air pollution sources and the meteorology in Mexico City. Most urban sites were influenced by a strong diurnal particulate mass trend indicative of primary organic emissions from traffic during early morning and subsequently oxidized/processed organics and ammonium nitrate particles starting in the mid-morning (~9 AM) and continuing throughout the day. Morning traffic-related primary organic emissions were strongest at La Merced (center of Mexico City, located near a busy food market), more subdued at other fixed sites further from the city center, and varied depending upon the day of week and holiday schedules. Particle-bound polycyclic aromatic hydrocarbons were observed within Mexico City fixed sites and were correlated with traffic organic PM emissions. Oxidized organic and ammonium nitrate events occurred during mid-morning at all city sites and were well correlated with gas phase photochemical activity. The daily ammonium nitrate aerosol event occurred later at sites near the city limits, likely due to transported emissions from the city center. The sulfate particulate mass measured throughout most of the Mexico City area did not show a consistent diurnal pattern, characteristic of aged regional aerosol. Local refuse burns were observed to be a source of inorganic particulate chloride.
A23D-02 13:55h
Aerosol number size distribution measurements in Beijing, China
Continuous measurements of aerosol number size distributions in the size range from 3~nm to 10~$\mu$m have been first performed within the city area of Beijing since the beginning of March 2004. The number size distributions of the first 45 measurement days (March 05 to April 18, 2004) were investigated in terms of their high variability. Two dust storm events were observed indicated by high number concentrations greater than 1~$\mu$m and total mass concentrations around 1~mg~m$^{-3}$. Continental highly polluted air was observed during 12~days indicated by a number concentration peak in the accumulation mode range, and submicrometer particle volume concentrations of more than 150~$\mu$m$^3$~cm$^{-3}$ were observed. Newly formed particles with more than 100,000~cm$^{-3}$ were observed on 25 days of measurement period. The measurements show generally no significant subsequent particle growth of the newly formed particles.
A23D-03 14:10h
Diurnal and Seasonal Variations of Elemental Carbon in Tokyo, Japan
Concentrations and mixing state of elemental carbon (EC) were measured, together with carbon monoxide (CO) at RCAST in Tokyo in different seasons in 2003-2004. Measurements of EC were made by using semi-continuous thermal-optical, optical-absorption, and laser induced incandescence (LII) techniques. Measured EC concentrations by these techniques showed very tight correlations, confirming reliability of the EC measurements. The average EC and CO concentrations were about 2.5 ug/m3 and 400 ppbv, respectively. They showed tight correlations throughout the measurement periods. The slope of the EC-CO correlation was highest in the early morning, when the ratio contribution of vehicles with diesel engines to the whole vehicular traffic in Tokyo was highest, suggesting that exhaust from diesel engines is an important source of EC in Tokyo. The EC-CO slope also showed seasonal variation, lowest in winter when the mean temperature was about 0-10C. Changes in mixing state of EC derived from LII are also presented. Despite regulation of emissions of EC from diesel engines started in October 2003, diurnally averaged EC-CO slope did not show significant decease. Possible causes will be discussed.
A23D-04 14:25h
Heterogeneous Chemistry of Individual Mineral Dust Particles with Nitric Acid. Laboratory and Field Observations.
This study presents laboratory and field evidence of complete, irreversible processing of solid calcium carbonate (calcite)-containing particles and quantitative formation of liquid calcium nitrate particles apparently as result of heterogeneous reaction of calcite with nitric acid. The heterogeneous chemistry of individual dust particles from four authentic dust samples with gas-phase nitric acid is presented. Morphology and compositional changes of individual particles as they react with nitric acid were observed using conventional scanning electron microscopy with energy dispersive analysis of X-rays (SEM/EDX) and computer controlled SEM/EDX. Environmental Scanning Electron Microscopy (ESEM) was utilized to investigate the hygroscopic behavior of mineral dust particles reacted with nitric acid. Differences in the reactivity of mineral dust particles from these four different dust source regions with nitric acid were observed. Mineral dust from source regions containing high levels of calcium, namely China loess dust and Saudi coastal dust, were found to react to the greatest extent. Formation of nitrates from individual calcite particles was also followed as a function of time in field aerosol samples collected at Shoresh, Israel. We show that calcium carbonate rich dust aerosols may exhibit continuous, extensive reactivity with nitric acid resulting in formation of highly hygroscopic calcium nitrate particles. Transformation of hygroscopically inert dry mineral dust particles into hygroscopic wet aerosol may have substantial impacts on the light scattering properties, the ability to modify clouds and heterogeneous chemistry.
A23D-05 14:40h
Concentrations, Sources, and Transformation of Particulate Polycyclic Aromatic Hydrocarbons in Mexico City
Understanding ambient concentrations of polycyclic aromatic hydrocarbons (PAHs) and their transformation in the atmosphere is important because of their potent mutagenicity and carcinogenicity and potential role as indicators for aerosol aging. The measurement of particulate polycyclic aromatic hydrocarbons by three different methods during the Mexico City Metropolitan Area air quality study in 2003 (MCMA-2003) presents a unique opportunity for characterization of these compounds. The three methods are (1) time-integrated filter samples for detailed speciation of PAHs; (2) aerosol photoemission for quantification of surface-bound PAHs at high time resolution; and (3) aerosol mass spectrometry for measurement of total and size-resolved PAHs. While the results from the three techniques generally agree, certain disparities emphasize methodological limitations or interesting PAH behavior. Total particle-bound PAH concentrations measured by photoemission and by traditional filter methods are correlated, except during the morning, when filter reaction artifacts may lead to underestimation of actual concentrations. Speciation profiles and diurnal patterns suggest that motor vehicles are the predominant daytime source of PAH emissions in the MCMA. The presence of triphenylbenzene and retene indicate that garbage and wood burning are also important sources of PAHs. For the first time, ambient PAHs were detected by an aerosol mass spectrometer (AMS). The AMS is able to quantify bulk phase PAHs that aerosol photoemission, a surface technique, cannot. Ambient particle-bound PAH concentrations exhibit a strong diurnal pattern. They typically peak at ~100 ng m$^{-3}$ during the morning rush hour and then rapidly decrease to a steady daytime level of ~10 ng m$^{-3}$, with the photoemission signal dropping much faster than black carbon concentrations. We explore two hypotheses to explain this behavior: coating of soot particles by secondary aerosol compounds causing the suppression of the photoemission signal, and reaction of particle-bound PAHs with OH. The first explanation is important because coating of soot particles greatly affects their light absorption properties, with implications for climate and visibility. The second explanation could have important implications for PAH-related health effects, due to the formation through heterogeneous reactions of compounds that are either more or less toxic than the original PAHs. Additionally, condensation and/or heterogeneous reactions may transform soot particles into more effective condensation nuclei.
A23D-06 14:52h
Deconvolution and Quantification of Primary and Oxygenated Organic Aerosols: Technique Development and Applications to the Pittsburgh AMS Datasets
A new technique has been developed to deconvolve and quantify the mass concentrations of primary and oxygenated organic aerosol (POA and OOA) using highly time-resolved organic mass spectral data obtained with an Aerodyne Aerosol Mass Spectrometer (AMS). OOA may comprise secondary organic aerosol (SOA) as well as oxidized POA. This technique involves a series of multivariate linear regressions that use mass-to-charge ratios (m/z's) 57 (mostly C4H9+) and 44 (mostly CO2+), the identified AMS mass spectral tracers for POA and OOA, respectively, as the initial principal components followed by an iterative algorithm to evaluate and "purify" POA and OOA mass spectral tracers. We have applied this technique to the AMS organic aerosol data acquired at the EPA Pittsburgh Supersite during September 2002 and have observed excellent agreement between the reconstructed organic concentrations (= POA + OOA) and the measured values (r2 = 0.997, slope = 0.998). The reconstructed organic data matrix (size = 3199 time steps x 300 m/z's) explains 99% of the variance in the measured time series. The extracted mass spectrum of POA shows high similarity to those of diesel exhaust sampled during a chase study, lab-measured lubricating oil, and freshly emitted traffic aerosols measured in urban environments. The spectrum of OOA closely resembles those of aged organic aerosols sampled in remote areas and also shows similarity with the spectrum of fulvic acid-a humic-like substance that is ubiquitous in the environment and has previously been used as an analogue to represent polyacid compounds found in highly processed and oxidized atmospheric organic aerosols. Organic aerosols in Pittsburgh during Sept. 2002 are mainly oxygenated, on average consisting of ~ 70% OOA (likely mainly secondary in nature). Pronounced diurnal variations in the POA/OOA contributions to organic mass have been observed, with the contribution of POA peaking in the morning rush hours while that of OOA is largest in the afternoon between 3-4 pm. We compared these results to those from the EC/OC tracer method for POA/SOA estimation, and observed qualitative agreement in estimated concentrations between the two methods. The diurnal variations in the mass concentrations and size distributions of POA and OOA will be reported. We have also performed case studies on how the chemistry of POA and OOA changes during nucleation events and acidic particle periods. From these results, implications related to roles of organics in new particle growth and acid catalyzed secondary organic aerosol formation will be discussed. Applications of this algorithm to worldwide AMS datasets will be presented as in an invited talk by Jimenez et al. (this conference).
A23D-07 15:04h
Characterization and Atmospheric Processing of Soot Aerosols in the Mexico City Metropolitan Area
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.
A23D-08 15:16h
Characterization of Ambient Aerosols in Mexico City during the MCMA-2003 Campaign using an Aerosol Mass Spectrometer: Overall Trends and the Inorganic Component
An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite during the Mexico City Metropolitan Area (MCMA-2003) field study from March 29 to May 4, 2003. The AMS provides real time information on mass concentration, and composition of the particulate matter (PM) with high time and size resolution. A beam width probe was used to evaluate whether particles were lost due to excessive broadening of the particle beam. Results show that the shape-related particle collection efficiency was approximately 1 during the campaign. The organic fraction of the PM represents two thirds of the mass, while the rest consists of inorganic compounds. The aerosol was neutralized most of the time; however during the periods with the highest sulfate concentration, the aerosol was acidic. All of the species measured with the AMS have diurnal cycles which can be qualitatively interpreted as the interplay of direct emissions; photochemical production in the atmosphere followed by condensation; gas-to-particle partitioning; boundary layer dynamics; and horizontal advection. The species size distributions were found to be mainly bimodal. The smaller mode is dominated by traffic emission particles, and the accumulation mode contains mainly secondary inorganic and organic compounds. We present the size distribution evolution during one day of the campaign as a case study for the formation and transformation of the PM in Mexico City. We compare the AMS results with measurements done using an Optical Particle Counter (LASAIR), an aerosol photometer, and an R&P PM2.5 TEOM that were also deployed during the MCMA-2003 campaign. Finally we compare our data with those from previous campaigns in Mexico City.
A23D-09 15:28h
Black Carbon Absorption Over Kanpur (an Industrial City in the Indo-Gangetic Basin) Retrieved From AERONET Data
Black carbon (BC) is the main anthropogenic absorbing particles in the atmosphere causing large positive radiative forcing. In India, BC concentration is being monitored by aethalometer at few locations, mostly in campaign modes. However, the Indo-Gangetic basin in northern India, one of the most populated basins in the world is devoid of any such direct measurements. An automatic sun/sky radiometer has been deployed in Kanpur, an industrial city in the heart of the Indo-Gangetic basin, in collaboration with NASA under Aerosol Robotic Network (AERONET) to monitor aerosol parameters since January 2001. In this paper, we have retrieved column-integrated BC concentration and specific absorption cross section ($\alpha$_{a}$) due to BC using Maxwell Garnett dielectric functions assuming internal mixing of BC with other aerosols. Values of refractive indices (real and imaginary parts) retrieved by AERONET have been used in BC volume fraction calculations. We have considered spherical retrieval for winter (December-February) and post-monsoon (September-November) seasons, spheroid retrieval for pre-monsoon (April-May) and monsoon (June-August) seasons and combined spherical and spheroid retrievals for March and September during the transition months based on the aerosol optical properties. BC concentration over Kanpur is found to be higher during the winter and post-monsoon seasons compared to that during the pre-monsoon and monsoon seasons. Yearly averaged column-integrated BC concentrations over Kanpur are found to be 8.185, 5.564 and 7.723 mg/m2, whereas the corresponding values of $\alpha$_{a}$ are found to be 9.562, 11.976 and 10.07 m2/g at 0.44 $\mu$m wavelength for 2001, 2002 and 2003 respectively. $\alpha$_{a}$ is found to be higher during the pre-monsoon season due to the preponderance of coarser dust particles. During the post-monsoon to winter seasons, anthropogenic pollutants are transported over the region by northwesterly winds. Spectral signature of single scattering albedo also supports the idea that the post-monsoon and winter seasons are characterized by relatively higher concentrations of absorbing particles. In this paper, the inter-annual, seasonal and diurnal variations of BC concentrations over Kanpur will be discussed in view of their effect on the regional climatology.