Project Description and Overview: Objectives: Determining the overall impact of atmospheric aerosols on radiative balance requires knowledge of the relative amounts of scattering and absorbing aerosols, their distributions, and their chemical and optical properties. This proposal was a continuation of measurements of aerosol scattering and absorption begun in Mexico City in 2003 in collaboration with MCMA 2003 and continuing in the Atmospheric Science Program field study, Megacity Aerosol Experiment-Mexico City, (MAX-Mex) during March of 2006 aimed at determining the variability of aerosol optical properties. A suite of instrumentation was deployed in MAX-Mex at site TO, located in the northern part of the Mexico City Metropolitan Area, (MCMA), for the characterization of the aerosol optical properties in the field. Measurements were made of the following aerosol properties: (1) aerosol absorption as a function of wavelength, measured at two minute intervals with a 7-wavelength Aethalometer (2) aerosol scattering as a function of wavelength, measured at one minute intervals with a 3-wavelength nephelometer; 3) aerosol scattering as a function of relative humidity (RH), measured at one minute intervals with 2 single-wavelength nephelometers operated under dry (10% RH) and wet (80% RH) conditions; and 4) collection of size-fractionated aerosol samples on quartz fiber filters at 12 hour intervals (day/night) for further laboratory characterization. Aerosol filter samples were also collected at site Tl (located north of MCMA) for comparison with those collected in the city center. Preliminary results from in situ measurements have indicated an enhanced UV absorption in the afternoon over that expected from black carbon (BC) aerosols alone. These results are directly applicable to both modeling of aerosol radiative forcing and satellite optical depth retrieval algorithms. Both of these applications assume that the aerosol absorption is due only to BC with a wavelength dependence of A, " whereas results obtained in MAX-Mex show that the aerosol wavelength exponent varies over Mexico City from-0.7 to-1.5. All of the data collected in the field from the measurement sets 1-3 have been made available to the ASP community via the MILAGRO data site housed at NCAR. The laboratory characterization of aerosol samples collected in the ASP MAX-Mex field study compared results from Mexico City to samples collected at other sites, including Chicago, Little Rock, and Mt. Bachelor, OR. The project focused on obtaining complete spectral characterization of aerosols-especially their absorption characteristics as they relate to basic chemical functional groups. Particular attention was given to organics and from biogenic derived organic compounds. This included determinations of the UV-Visible-NIR characteristics of the aerosol absorption as reported as Angstrom Absorption Exponents. Correlation of these results with IR band observations of carboxylic acid, and carboxylate groups were conducted, along with past correlations with carbon...

Abstract. Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Aerosol Mass Spectrometer (AMS) and complementary instrumentation. Positive Matrix Factorization (PMF) of high resolution AMS spectra identified a biomass burning organic aerosol (BBOA) component, which includes several large plumes that appear to be from forest fires within the region. Here, we show that the AMS BBOA concentration at T0 correlates with fire counts in the vicinity of Mexico City and that most of the BBOA variability is captured when the FLEXPART model is used for the dispersion of fire emissions as estimated from satellite fire counts. The resulting FLEXPART fire impact factor (FIF) correlates well with the observed BBOA, acetonitrile (CH3CN), levoglucosan, and potassium, indicating that wildfires in the region surrounding Mexico City are the dominant source of BBOA at T0 during MILAGRO. The impact of distant BB sources such as the Yucatan is small during this period. All fire tracers are correlated, with BBOA and levoglucosan showing little background, acetonitrile having a well-known tropospheric background of ~100–150 pptv, and PM2.5 potassium having a background of ~160 ng m−3 (two-thirds of its average concentration), which does not appear to be related to BB sources. We define two high fire periods based on satellite fire counts and FLEXPART-predicted FIFs. We then compare these periods with a low fire period when the impact of regional fires is about a factor of 5 smaller. Fire tracers are very elevated in the high fire periods whereas tracers of urban pollution do not change between these periods. Dust is also elevated during the high BB period but this appears to be coincidental due to the drier conditions and not driven by direct dust emission from the fires. The AMS oxygenated organic aerosol (OA) factor (OOA, mostly secondary OA or SOA) does not show an increase during the fire periods or a correlation with fire counts, FLEXPART-predicted FIFs or fire tracers, indicating that it is dominated by urban and/or regional sources and not by the fires near the MCMA. A new 14C aerosol dataset is presented. Both this new and a previously published dataset of 14C analysis suggest a similar BBOA contribution as the AMS and chemical mass balance (CMB), resulting in 13% higher non-fossil carbon during the high vs. low regional fire periods. The new dataset has ~15% more fossil carbon on average than the previously published one, and possible reasons for this discrepancy are discussed. During the low regional fire period, 38% of organic carbon (OC) and 28% total carbon (TC) are from non-fossil sources, suggesting the importance of urban and regional non-fossil carbon sources other than the fires, such as food cooking and regional biogenic SOA. The ambient BBOA/ΔCH3CN ratio is much higher in the afternoon when the wildfires are most intense than during the rest of the day. Also, there are large differences in the contributions of the different OA components to the surface concentrations vs. the integrated column amounts. Both facts may explain some apparent disagreements between BB impacts estimated from afternoon aircraft flights vs. those from 24-h ground measurements. We show that by properly accounting for the non-BB sources of K, all of the BB PM estimates from MILAGRO can be reconciled. Overall, the fires from the region near the MCMA are estimated to contribute 15–23% of the OA and 7–9% of the fine PM at T0 during MILAGRO, and 2–3% of the fine PM as an annual average. The 2006 MCMA emissions inventory contains a substantially lower impact of the forest fire emissions, although a fraction of these emissions occur just outside of the MCMA inventory area.

Abstract.A comparison between observed aerosol optical properties from the MILAGRO field campaign, which took place in the Mexico City Metropolitan Area (MCMA) during March 2006, and values simulated by the Weather Research and Forecasting (WRF-Chem) model, reveals large differences. To help identify the source of the discrepancies, data from the MILAGRO campaign are used to evaluate the "aerosol chemical to aerosol optical properties" module implemented in the full chemistry version of the WRF-Chem model. The evaluation uses measurements of aerosol size distributions and chemical properties obtained at the MILAGRO T1 site. These observations are fed to the module, which makes predictions of various aerosol optical properties, including the scattering coefficient, B scat ; the absorption coefficient, B abs ; and the single-scattering albedo, 0 ; all as a function of time. Values simulated by the module are compared with independent measurements obtained from a photoacoustic spectrometer (PAS) at a wavelength of 870 nm. Because of line losses and other factors, only "fine mode" aerosols with aerodynamic diameters less than 2.5 µm are considered here. Over a 10-day period, the simulations of hour-by-hour variations of B scat are not satisfactory, but simulations of B abs and 0 are considerably better. When averaged over the 10-day period, the computed and observed optical properties agree within the uncertainty limits of the measurements and simulations. ues of 0 simulated by the full WRF-Chem model thus cannot be attributed to the "aerosol chemistry to optics" module. The discrepancy is more likely due, in part, to poor characterization of emissions near the T1 site, particularly black carbon emissions.

Abstract. As part of a major atmospheric chemistry and aerosol field program carried out in March of 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 αABS (absorption per unit mass, with unit of m2 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 photometers, 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 specific absorption at T2 was larger than at T1, which is also consistent with the expectation of more aged particles with encapsulated black carbon being found at the more distant location. In situ measurements of single scattering albedo with an aircraft and a ground station showed general agreement with column-averaged values derived from rotating shadowband radiometer data, although some differences were found that may be related to boundary-layer evolution.

Abstract. 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 speCorrespondence to: J. C. Doran (christopher.doran@pnl.gov) cific absorption at T2 was larger than at T1, which is also consistent with the expectation of more aged particles with encapsulated black carbon being found at the more distant location. In situ measurements of single scattering albedo with an aircraft and a ground station showed general agreement with column-averaged values derived from rotating shadowband radiometer data, although some differences were found that may be related to boundary-layer evolution.

Abstract. Thermal/optical methods have been widely used for quantifying total carbon (TC), organic carbon (OC), and elemental carbon (EC) in ambient and source particulate samples. Thermally defined carbon fractions have been used for source identification. Temperature precision in thermal carbon analysis is critical to the allocation of carbon fractions. The sample temperature is determined by a thermocouple, which is usually located in the oven near the sample. Sample and thermocouple temperature may differ owing to different thermal properties between the sample filter punch and the thermocouple, or inhomogeneities in the heating zone. Quick-drying temperature-indicating liquids (Tempil Inc., South Plainfield, NJ) of different liquefying points are used as temperature calibration standards. These consist of chemicals that change their appearance at specific temperatures and can be optically monitored to determine the sample temperature. Temperature measures were evaluated for three different models of carbon analyzers. Sample temperatures were found to differ from sensor temperatures by 10 to 50 • C. Temperature biases of 14 to 22 • C during thermal analysis were found to change carbon fraction measurements. The temperature indicators allow calibration curves to be constructed that relate the sample temperature to the temperature measured by a thermocouple.

Abstract. Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identified three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning. A fourth OA component is a small local nitrogen-containing reduced OA component (LOA) which accounts for 9% of the OA mass but one third of the organic nitrogen, likely as amines. OOA accounts for almost half of the OA on average, consistent with previous observations. OA apportionment results from PMF-AMS are compared to the PM2.5 chemical mass balance of organic molecular markers (CMB-OMM, from GC/MS analysis of filters). Results from both methods are overall consistent. Both assign the major components of OA to primary urban, biomass burning/woodsmoke, and secondary sources at similar magnitudes. The 2006 Mexico City emissions inventory underestimates the urban primary PM2.5 emissions by a factor of ~4, and it is ~16 times lower than afternoon concentrations when secondary species are included. Additionally, the forest fire contribution is underestimated by at least an order-of-magnitude in the inventory.

Abstract.A photoacoustic spectrometer, a nephelometer, an aethalometer, and an aerosol mass spectrometer were used to measure at ground level real-time aerosol light absorption, scattering, and chemistry at an urban site located in North East Mexico City (Instituto Mexicano del Petroleo, Mexican Petroleum Institute, denoted by IMP), as part of the Megacity Impact on Regional and Global Environments field experiment, MILAGRO, in March 2006. Photoacoustic and reciprocal nephelometer measurements at 532 nm accomplished with a single instrument compare favorably with conventional measurements made with an aethalometer and a TSI nephelometer. The diurnally averaged single scattering albedo at 532 nm was found to vary from 0.60 to 0.85 with the peak value at midday and the minimum value at 07:00 a.m. local time, indicating that the Mexico City plume is likely to have a net warming effect on local climate. The peak value is associated with strong photochemical generation of secondary aerosol. It is estimated that the photochemical production of secondary aerosol (inorganic and organic) is approximately 75% of the aerosol mass concentration and light scattering in association with the peak single scattering albedo. A strong correlation of aerosol scattering at 532 nm and total aerosol mass concentration was found, and an average mass scattering efficiency factor of 3.8 m 2 /g was determined. Comparisons of photoacoustic and aethalometer light absorption with oxygenated organic aerosol concentration (OOA) indicate a very small systematic bias of the filter based measurement associated with OOA and the peak aerosol single scattering albedo.