Measurements of volatile organic compounds using proton transfer reaction ? mass spectrometry during the MILAGRO 2006 Campaign

International audienceVolatile organic compounds (VOCs) were measured by proton transfer reaction ? mass spectrometry (PTR-MS) on a rooftop in the urban mixed residential and industrial area North Northeast of downtown Mexico City as part of the Megacity Initiative ? Local and Global Research Observations (MILAGRO) 2006 field campaign. Thirty eight individual masses were monitored during the campaign and many species were quantified including methanol, acetaldehyde, toluene, the sum of C2 benzenes, the sum of C3 benzenes, acetone, isoprene, benzene, and ethyl acetate. The VOC measurements were analyzed to gain a better understanding of the type of VOCs present in this region of the MCMA, their diurnal patterns and their origins. Diurnal profiles of weekday and weekend/holiday aromatic VOC concentrations show the influence of vehicular traffic during the morning rush hours and during the afternoon hours. Plumes including of elevated toluene as high as 216 parts per billion (ppb) and ethyl acetate as high as 183 ppb were frequently observed during the late night and early morning hours, indicating the probability of significant industrial sources of the two compounds in the region. Wind fields during those peak episodes revealed no specific direction for the majority of the toluene plumes but the ethyl acetate plumes arrived at the site when winds were from the Southwest or West. The PTR-MS measurements combined with other VOC measuring techniques at the field site as well as VOC measurements conducted in other areas of the Mexico City Metropolitan Area (MCMA) will help to develop a better understanding of the spatial pattern of VOCs and its variability in the MCMA

Potassium binding adjacent to cationic transition metal fragments: unusual heterobimetallic adducts of a calix[4]arene-based thione ligand

The synthesis of cationic rhodium and iridium complexes of a bis(imidazol-2-thione) functionalised calix[4]arene ligand and their surprising capacity for potassium binding is described. In both cases uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition metal fragment (Rh+∙∙∙K+ = 3.715(1) Å, Ir+∙∙∙K+ = 3.690(1) Å). The formation and constituent bonding of these unusual heterobimetallic adducts has been interrogated through extensive solution and solid-state characterisation, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalised calix[4]arene analogue, and computationally using DFT-based energy decomposition analysis (EDA)

Application of positive matrix factorization to on-road measurements for source apportionment of diesel- and gasoline-powered vehicle emissions in Mexico City

The goal of this research is to quantify diesel- and gasoline-powered motor vehicle emissions within the Mexico City Metropolitan Area (MCMA) using on-road measurements captured by a mobile laboratory combined with positive matrix factorization (PMF) receptor modeling. During the MCMA-2006 ground-based component of the MILAGRO field campaign, the Aerodyne Mobile Laboratory (AML) measured many gaseous and particulate pollutants, including carbon dioxide, carbon monoxide (CO), nitrogen oxides (NOx) [(NO subscript x)], benzene, toluene, alkylated aromatics, formaldehyde, acetaldehyde, acetone, ammonia, particle number, fine particulate mass (PM2.5) [(PM subscript 2.5)], and black carbon (BC). These serve as inputs to the receptor model, which is able to resolve three factors corresponding to gasoline engine exhaust, diesel engine exhaust, and the urban background. Using the source profiles, we calculate fuel-based emission factors for each type of exhaust. The MCMA's gasoline-powered vehicles are considerably dirtier, on average, than those in the US with respect to CO and aldehydes. Its diesel-powered vehicles have similar emission factors of NOx [NO subscript x] and higher emission factors of aldehydes, particle number, and BC. In the fleet sampled during AML driving, gasoline-powered vehicles are found to be responsible for 97% of total vehicular emissions of CO, 22% of NOx [NO subscript x], 95–97% of each aromatic species, 72–85% of each carbonyl species, 74% of ammonia, negligible amounts of particle number, 26% of PM2.5 [PM subscript 2.5], and 2% of BC; diesel-powered vehicles account for the balance. Because the mobile lab spent 17% of its time waiting at stoplights, the results may overemphasize idling conditions, possibly resulting in an underestimate of NOx [NO subscript x] and overestimate of CO emissions. On the other hand, estimates of the inventory that do not correctly account for emissions during idling are likely to produce bias in the opposite direction.The resulting fuel-based estimates of emissions are lower than in the official inventory for CO and NOx [NO subscript x] and higher for VOCs. For NOx [NO subscript x], the fuel-based estimates are lower for gasoline-powered vehicles but higher for diesel-powered ones compared to the official inventory. While conclusions regarding the inventory should be interpreted with care because of the small sample size, 3.5 h of driving, the discrepancies with the official inventory agree with those reported in other studies.National Science Foundation (U.S.) (Grant ATM-0528170)National Science Foundation (U.S.) (Grant ATM-0528227)United States. Dept. of Energy (Grant DE-FG02-05ER63982)United States. National Aeronautics and Space AdministrationMolina Center for Energy and the Environmen

Characterization of submicron particles influenced by mixed biogenic and anthropogenic emissions using high-resolution aerosol mass spectrometry: results from CARES

An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative Effects Study (CARES) that took place in northern California in June 2010. We present results obtained at Cool (denoted as the T1 site of the project) in the foothills of the Sierra Nevada Mountains, where intense biogenic emissions are periodically mixed with urban outflow transported by daytime southwesterly winds from the Sacramento metropolitan area. During this study, the average mass loading of submicrometer particles (PM<sub>1</sub>) was 3.0 μg m<sup>−3</sup>, dominated by organics (80%) and sulfate (9.9%). The organic aerosol (OA) had a nominal formula of C<sub>1</sub>H<sub>1.38</sub>N<sub>0.004</sub>OM<sub>0.44</sub>, thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Two distinct oxygenated OA factors were identified via Positive matrix factorization (PMF) of the high-resolution mass spectra of organics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a surrogate for secondary OA (SOA) influenced by biogenic emissions whereas the less oxidized LO-OOA (O/C = 0.42) was found to represent SOA formed in photochemically processed urban emissions. LO-OOA correlated strongly with ozone and MO-OOA correlated well with two 1st generation isoprene oxidation products (methacrolein and methyl vinyl ketone), indicating that both SOAs were relatively fresh. A hydrocarbon like OA (HOA) factor was also identified, representing primary emissions mainly due to local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for 91% of the total OA mass and 72% of the PM<sub>1</sub> mass observed at Cool. Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool) were identified using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The average PM<sub>1</sub> mass loading was considerably higher in urban plumes than in air masses dominated by biogenic SOA. The change in OA mass relative to CO (ΔOA/ΔCO) varied in the range of 5-196 μg m<sup>−3</sup> ppm<sup>−1</sup>, reflecting large variability in SOA production. The highest ΔOA/ΔCO was reached when air masses were dominated by anthropogenic emissions in the presence of a high concentration of biogenic volatile organic compounds (BVOCs). This ratio, which was 97 μg m<sup>−3</sup> ppm<sup>−1</sup> on average, was much higher than when urban plumes arrived in a low BVOC environment (~36 μg m<sup>−3</sup> ppm<sup>−1</sup>) or during other periods dominated by biogenic SOA (35 μg m<sup>−3</sup> ppm<sup>−1</sup>). These results demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors

Distribution, magnitudes, reactivities, ratios and diurnal patterns of volatile organic compounds in the Valley of Mexico during the MCMA 2002 and 2003 field campaigns

International audienceA wide array of volatile organic compound (VOC) measurements was conducted in the Valley of Mexico during the MCMA-2002 and 2003 field campaigns. Study sites included locations in the urban core, in a heavily industrial area and at boundary sites in rural landscapes. In addition, a novel mobile-laboratory-based conditional sampling method was used to collect samples dominated by fresh on-road vehicle exhaust to identify those VOCs whose ambient concentrations were primarily due to vehicle emissions. Five distinct analytical techniques were used: whole air canister samples with Gas Chromatography/Flame Ionization Detection (GC-FID), on-line chemical ionization using a Proton Transfer Reaction Mass Spectrometer (PTR-MS), continuous real-time detection of olefins using a Fast Olefin Sensor (FOS), and long path measurements using UV Differential Optical Absorption Spectrometers (DOAS). The simultaneous use of these techniques provided a wide range of individual VOC measurements with different spatial and temporal scales. The VOC data were analyzed to understand concentration and spatial distributions, diurnal patterns, origin and reactivity in the atmosphere of Mexico City. The VOC burden (in ppbC) was dominated by alkanes (60%), followed by aromatics (15%) and olefins (5%). The remaining 20% was a mix of alkynes, halogenated hydrocarbons, oxygenated species (esters, ethers, etc.) and other unidentified VOCs. However, in terms of ozone production, olefins were the most relevant hydrocarbons. Elevated levels of toxic hydrocarbons, such as 1,3-butadiene, benzene, toluene and xylenes were also observed. Results from these various analytical techniques showed that vehicle exhaust is the main source of VOCs in Mexico City and that diurnal patterns depend on vehicular traffic. Finally, examination of the VOC data in terms of lumped modeling VOC classes and its comparison to the VOC lumped emissions reported in other photochemical air quality modeling studies suggests that some, but not all, VOC classes are underestimated in the emissions inventory by factors of 1.1 to 3

Measurements of volatile organic compounds at a suburban ground site (T1) in Mexico City during the MILAGRO 2006 campaign: measurement comparison, emission ratios, and source attribution

Volatile organic compound (VOC) mixing ratios were measured with two different instruments at the T1 ground site in Mexico City during the Megacity Initiative: Local and Global Research Observations (MILAGRO) campaign in March of 2006. A gas chromatograph with flame ionization detector (GC-FID) quantified 18 light alkanes, alkenes and acetylene while a proton-transfer-reaction ion-trap mass spectrometer (PIT-MS) quantified 12 VOC species including oxygenated VOCs (OVOCs) and aromatics. A GC separation system was used in conjunction with the PIT-MS (GC-PIT-MS) to evaluate PIT-MS measurements and to aid in the identification of unknown VOCs. The VOC measurements are also compared to simultaneous canister samples and to two independent proton-transfer-reaction mass spectrometers (PTR-MS) deployed on a mobile and an airborne platform during MILAGRO. VOC diurnal cycles demonstrate the large influence of vehicle traffic and liquid propane gas (LPG) emissions during the night and photochemical processing during the afternoon. Emission ratios for VOCs and OVOCs relative to CO are derived from early-morning measurements. Average emission ratios for non-oxygenated species relative to CO are on average a factor of ~2 higher than measured for US cities. Emission ratios for OVOCs are estimated and compared to literature values the northeastern US and to tunnel studies in California. Positive matrix factorization analysis (PMF) is used to provide insight into VOC sources and processing. Three PMF factors were distinguished by the analysis including the emissions from vehicles, the use of liquid propane gas and the production of secondary VOCs + long-lived species. Emission ratios to CO calculated from the results of PMF analysis are compared to emission ratios calculated directly from measurements. The total PIT-MS signal is summed to estimate the fraction of identified versus unidentified VOC species

Investigation of the correlation between odd oxygen and secondary organic aerosol in Mexico City and Houston

Many recent models underpredict secondary organic aerosol (SOA) particulate matter (PM) concentrations in polluted regions, indicating serious deficiencies in the models' chemical mechanisms and/or missing SOA precursors. Since tropospheric photochemical ozone production is much better understood, we investigate the correlation of odd-oxygen ([Ox]≡[O3]+[NO2]) [([O subscript x] ≡ [O subscript 3] + [NO subscript 2])] and the oxygenated component of organic aerosol (OOA), which is interpreted as a surrogate for SOA. OOA and Ox [O subscript x] measured in Mexico City in 2006 and Houston in 2000 were well correlated in air masses where both species were formed on similar timescales (less than 8 h) and not well correlated when their formation timescales or location differed greatly. When correlated, the ratio of these two species ranged from 30 μg [mu g] m−3/ppm [m superscript -3 / ppm] (STP) in Houston during time periods affected by large petrochemical plant emissions to as high as 160 μg [mu g] m−3/ppm [m superscript -3 / ppm] in Mexico City, where typical values were near 120 μg [mu g] m−3/ppm [m superscript -3 / ppm]. On several days in Mexico City, the [OOA]/[Ox] [[OOA] / O subscript x]] ratio decreased by a factor of ~2 between 08:00 and 13:00 local time. This decrease is only partially attributable to evaporation of the least oxidized and most volatile components of OOA; differences in the diurnal emission trends and timescales for photochemical processing of SOA precursors compared to ozone precursors also likely contribute to the observed decrease. The extent of OOA oxidation increased with photochemical aging. Calculations of the ratio of the SOA formation rate to the Ox [O subscript x] production rate using ambient VOC measurements and traditional laboratory SOA yields are lower than the observed [OOA]/[Ox] [[OOA] / O subscript x]] ratios by factors of 5 to 15, consistent with several other models' underestimates of SOA. Calculations of this ratio using emission factors for organic compounds from gasoline and diesel exhaust do not reproduce the observed ratio. Although not successful in reproducing the atmospheric observations presented, modeling P(SOA)/P(Ox) [P (SOA) / P (O subscript x)] can serve as a useful test of photochemical models using improved formulation mechanisms for SOA.National Science Foundation (U.S.) (Grant ATM-528227)National Science Foundation (U.S.) (Grant ATM-0528170)National Science Foundation (U.S.) (Grant ATM-0513116)National Science Foundation (U.S.) (Grant ATM-0449815)United States. Dept. of Energy. Office of Biological and Environmental Research. Atmospheric Science Program (Grant DE-FGO2-05ER63982)United States. Dept. of Energy. Office of Biological and Environmental Research. Atmospheric Science Program (Grant DEFGO2- 05ER63980)United States. Dept. of Energy. Office of Biological and Environmental Research. Atmospheric Science Program (Grant DE-FG02-08ER64627)United States. National Oceanic and Atmospheric Administration (Grant NA08OAR4310656

Vehicle fleet emissions of black carbon, polycyclic aromatic hydrocarbons, and other pollutants measured by a mobile laboratory in Mexico City

International audienceBlack carbon (BC) and polycyclic aromatic hydrocarbons (PAHs) are of concern due to their effects on climate and health. The main goal of this research is to provide the first estimate of emissions of BC and particle-phase PAHs (PPAHs) from motor vehicles in Mexico City. The emissions of other pollutants including carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs), and particulate matter of diameter 2.5 ?m and less (PM2.5) are also estimated. As a part of the Mexico City Metropolitan Area field campaign in April 2003 (MCMA-2003), a mobile laboratory was driven throughout the city. The laboratory was equipped with a comprehensive suite of gas and particle analyzers, including an aethalometer that measured BC and a photoionization aerosol sensor that measured PPAHs. While driving through traffic, the mobile lab continuously sampled exhaust plumes from the vehicles around it. We have developed a method of automatically identifying exhaust plumes, which are then used as the basis for calculation of fleet-average emissions. In the approximately 75 h of on-road sampling during the field campaign, we have identified ~30 000 exhaust measurement points that represent a variety of vehicle types and driving conditions. The large sample provides a basis for estimating fleet-average emission factors and thus the emission inventory. Motor vehicles in the Mexico City area are estimated to emit 1700±200 metric tons BC, 57±6 tons PPAHs, 1 190 000±40 000 tons CO, 120 000±3000 tons NOx, 240 000±50 000 tons VOCs, and 4400±400 tons PM2.5 per year, not including cold start emissions. The estimates for CO, NOx, and PPAHs may be low by up to 10% due to the slower response time of analyzers used to measure these species. Compared to the government's official motor vehicle emission inventory for the year 2002, the estimates for CO, NOx, VOCs, and PM2.5 are 38% lower, 23% lower, 27% higher, and 25% higher, respectively. The distributions of emission factors of BC, PPAHs, and PM2.5 are highly skewed, i.e. asymmetric, while those for benzene, measured as a surrogate for total VOCs, and NOx are less skewed. As a result, the total emissions of BC, PPAHs, and PM2.5 could be reduced by approximately 50% if the highest 20% of data points were removed, but "super polluters" are less influential on overall NOx and VOC emissions