As part of the MILAGRO 2006 field campaign, the exchange of atmospheric aerosols with the urban landscape was measured from a tall tower erected in a heavily populated neighborhood of Mexico City. Urban submicron aerosol fluxes were measured using an eddy covariance method with a quadrupole aerosol mass spectrometer during a two week period in March, 2006. Nitrate and ammonium aerosol concentrations were elevated at this location near the city center compared to measurements at other urban sites. Significant downward fluxes of nitrate aerosol, averaging −0.2 μg m−2 s−1, were measured during daytime. The urban surface was not a significant source of sulfate aerosols. The measurements also showed that primary organic aerosol fluxes, approximated by hydrocarbon-like organic aerosols (HOA), displayed diurnal patterns similar to CO2 fluxes and anthropogenic urban activities. Overall, 47% of submicron organic aerosol emissions were HOA, 35% were oxygenated (OOA) and 18% were associated with biomass burning (BBOA). Organic aerosol fluxes were bi-directional, but on average HOA fluxes were 0.1 μg m−2 s−1, OOA fluxes were −0.03 μg m−2 s−1, and BBOA fluxes were −0.03 μg m−2 s−1. After accounting for size differences (PM1 vs PM2.5) and using an estimate of the black carbon component, comparison of the flux measurements with the 2006 gridded emissions inventory of Mexico City, showed that the daily-averaged total PM emission rates were essentially identical for the emission inventory and the flux measurements. However, the emission inventory included dust and metal particulate contributions, which were not included in the flux measurements. As a result, it appears that the inventory underestimates overall PM emissions for this location.

The DEA method partitions the air into two reservoirs based on the magnitude and direction of the vertical wind speed. The VOCs concentrations in both reservoirs are analyzed and used to determine the fluxes with the vertical wind velocities. Although this method is not as precise as other micrometeorological techniques, such as the eddy covariance method, it provides the ability to directly measure the fluxes of an extended number of individual species using off-line sensors without relying on similarity scaling or empirical parameters.

Eddy covariance (EC) flux measurements of the atmosphere/surface exchange of gases over an urban area are a direct way to improve and evaluate emissions inventories, and, in turn, to better understand urban atmospheric chem- istry and the role that cities play in regional and global chem- ical cycles. As part of the MCMA-2003 study, we demon- strated the feasibility of using eddy covariance techniques to measure fluxes of selected volatile organic compounds (VOCs) and CO2 from a residential district of Mexico City (Velasco et al., 2005a, b). During the MILAGRO/MCMA- 2006 field campaign, a second flux measurement study was conducted in a different district of Mexico City to corrob- orate the 2003 flux measurements, to expand the number of species measured, and to obtain additional data for evaluation of the local emissions inventory. Fluxes of CO2 and olefins were measured by the conventional EC technique using an open path CO2 sensor and a Fast Isoprene Sensor calibrated with a propylene standard. In addition, fluxes of toluene, benzene, methanol and C2-benzenes were measured using a virtual disjunct EC method with a Proton Transfer Reaction Mass Spectrometer. The flux measurements were analyzed in terms of diurnal patterns and vehicular activity and were compared with the most recent gridded local emissions in- ventory. In both studies, the results showed that the urban surface of Mexico City is a net source of CO2 and VOCs with significant contributions from vehicular traffic. Evap-

Eddy covariance (EC) flux measurements of the atmosphere/surface exchange of gases over an urban area are a direct way to improve and evaluate emissions inventories, and, in turn, to better understand urban atmospheric chemistry and the role that cities play in regional and global chemical cycles. As part of the MCMA-2003 study, we demonstrated the feasibility of using eddy covariance techniques to measure fluxes of selected volatile organic compounds (VOCs) and CO2 from a residential district of Mexico City (Velasco et al., 2005a, b). During the MILAGRO/MCMA-2006 field campaign, a second flux measurement study was conducted in a different district of Mexico City to corroborate the 2003 flux measurements, to expand the number of species measured, and to obtain additional data for evaluation of the local emissions inventory. Fluxes of CO2 and olefins were measured by the conventional EC technique using an open path CO2 sensor and a Fast Isoprene Sensor calibrated with a propylene standard. In addition, fluxes of toluene, benzene, methanol and C-2-benzenes were measured using a virtual disjunct EC method with a Proton Transfer Reaction Mass Spectrometer. The flux measurements were analyzed in terms of diurnal patterns and vehicular activity and were compared with the most recent gridded local emissions inventory. In both studies, the results showed that the urban surface of Mexico City is a net source of CO2 and VOCs with significant contributions from vehicular traffic. Evaporative emissions from commercial and other anthropogenic activities were significant sources of toluene and methanol. The results show that the emissions inventory is in reasonable agreement with measured olefin and CO2 fluxes, while C-2-benzenes and toluene emissions from evaporative sources are overestimated in the inventory. It appears that methanol emissions from mobile sources occur, but are not reported in the mobile emissions inventory.

The concept of disjunct eddy sampling (DES) for use in measuring ecosystem-level micrometeorological fluxes is re-examined. The governing equations are discussed as well as other practical considerations and guidelines concerning this sampling method as it is applied to either the disjunct eddy covariance (DEC) or disjunct eddy accumulation (DEA) techniques. A disjunct eddy sampling system was constructed that could either be combined with relatively slow sensors (response time of 2 to 40 s) to measure fluxes using DEC, or could also be used to accumulate samples in stable reservoirs for later laboratory analysis (DEA technique). Both the DEC and DEA modes of this sampler were tested against conventional eddy covariance (EC) for fluxes of either CO2 (DEC) or isoprene (DEA). Good agreement in both modes was observed relative to the EC systems. However, the uncertainty in a single DEA flux measurement was considerable (~40%) due to both the reduced statistical sampling and the analytical precision of the concentration difference measurements. We have also re-investigated the effects of nonzero mean vertical wind velocity on accumulation techniques as it relates to our DEA measurements. Despite the higher uncertainty, disjunct eddy sampling can provide an alternative technique to eddy covariance for determining ecosystem-level fluxes for species where fast sensors do not currently exist.

The use of disjunct eddy sampling methods for the determination of ecosystem level fluxes of trace gases. A. A. Turnipseed, S. N. Pressley, T. Karl, B. Lamb, E. Nemitz, E. Allwine, W. A. Cooper, S. Shertz, and A. B. Guenther Atmospheric Chemistry Div., National Center for Atmospheric Research, Boulder, CO, USA Dept. of Civil and Environmental Engineering, Washington State Univ., Pullman, WA, USA Center for Ecology and Hydrology (CEH), Edinburgh, UK Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA The Institute for Integrative and Multidisciplinary Earth Studies (TIIMES), National Center for Atmospheric Research, Boulder, CO, USA Received: 19 May 2008 – Accepted: 18 June 2008 – Published: 15 July 2008 Correspondence to: A. Turnipseed (turnip@ucar.edu) Published by Copernicus Publications on behalf of the European Geosciences Union.

A 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. Four 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 in addition to meteorological processes. 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 alkanes are underestimated in the emissions inventory, while some olefins and aromatics are overestimated.

Our objectives were to: characterize spatial and temporal variation in wind speed, direction and air temperature within a steeply sloping 96 ha forested watershed in the Oregon Cascade Mountains; assess the area contributing to advection in cold air drainage; identify appropriate conditions for sampling advected gases representative of the entire watershed; estimate ecosystem respiration from mass balance estimates. The flow dynamics could be categorized into four periods: daytime flows, evening transition, nighttime conditions (formation of a cold air pool) and morning transition. On summer days, the wind above the canopy flowed upvalley whereas the wind direction below the canopy was often down-valley. During the evening transition, the below-canopy cold air drainage became well mixed after the vertical temperature profile at the tower became isothermal, and sodar and tethersonde data revealed that a second drainage flow developed above the canopy. Between 20:00 and 24:00 h (PST), a cold air pool formed within the valley; cold air drainage continued, but wind speed decreased as the night progressed. After sunrise, CO2 in air passing the tower remained well mixed, but its concentration decreased. After the base of the watershed began to receive direct solar radiation, the incanopy vertical CO, concentration profile became stratified and the cold air drainage ceased or was confined below the canopy. After the cold air pool formed, the entire watershed supplied the respired CO, that advected past the tower. During this period, the potential temperature profile within the watershed indicated a strong inversion within the watershed. Because the air passing the tower was well mixed and the cold air pool encompassed the entire watershed, it is probable that air samples collected during this period could provide the best estimate of watershed-scale respiration and carbon isotope composition of respired CO, (Keeling plot analysis). However, sampling prior to the formation of the cold air pool may confound interpretations as the area contributing to the tower is likely changing. Mass balance calculations using an inert tracer (SF6) released below the canopy demonstrated that data from a single vertical profile could provide plausible estimates of ecosystem respiration at the watershed scale. (c) 2007 Elsevier B.V. All rights reserved.

A proton transfer reaction mass spectrometer (PTR-MS) was redesigned and deployed to monitor selected hydrocarbon emissions from in-use vehicles as part of the Mexico City Metropolitan Area (MCMA) 2003 field campaign. This modified PTR-MS instrument provides the necessary time response (<2s total cycle time) and sensitivity to monitor the rapidly changing hydrocarbon concentrations, within intercepted dilute exhaust emission plumes. Selected hydrocarbons including methanol, acetaldehyde, acetone, methyl tertiary butyl ether (MTBE), benzene and toluene were among the vehicle exhaust emission components monitored. A comparison with samples collected in canisters and analyzed by gas chromatography provides validation to the interpretation of the ion assignments and the concentrations derived using the PTR-MS. The simultaneous detection of multiple hydrocarbons in dilute vehicle exhaust plumes provides a valuable tool to study the impact of driving behavior on the exhaust gas emissions.

Direct measurements of volatile organic compound (VOC) emissions that include all sources in urban areas are a missing requirement to evaluate emission inventories and constrain current photochemical modelling practices. Here we demonstrate the use of micrometeorological techniques coupled with fast‐response sensors to measure urban VOC fluxes from a neighbourhood of Mexico City, where the spatial variability of surface cover and roughness is high. Fluxes of olefins, methanol, acetone, toluene and C2‐benzenes were measured and compared with the local gridded emissions inventory. VOC fluxes exhibited a clear diurnal pattern with a strong relationship to vehicular traffic. Recent photochemical modelling results suggest that VOC emissions are significantly underestimated in Mexico City, but for the olefin class, toluene, C2‐benzenes, and acetone fluxes measured in this work, the results show general agreement with the gridded emissions inventory. While these measurements do not address the full suite of VOC emissions, the comparison with the inventory suggests that other explanations may be needed to explain the photochemical modelling results.

In a densely Populated section of Mexico City, an eddy covariance (EC) flux system was deployed oil a tall urban tower to obtain direct measurements Of CO2 emissions from an urban neighborhood located in a subtropical megacity. The measured fluxes and boundary layer conditions satisfy EC assumptions of stationarity, and cospectral analyses of the turbulence measurements exhibit the required boundary layer patterns for acceptable flux measurements. Results from a field experiment conducted during April 2003 show that the urban surface is a net source of CO2. The CO2 flux measurements showed a clear diurnal pattern, with the highest emissions during the morning (up to 1.60 mg m(-2) s(-1)), and the lowest emissions during nighttime. The measured fluxes were closely correlated to traffic patterns in the area. The mean daily flux was 0.41 mg m(-2) s(-1), which is similar to that observed in European and US cities. (c) 2005 Elsevier Ltd. All rights reserved.

Measurements of natural hydrocarbon emission rates are reported for an old-growth Pacific Northwest coniferous forest. The emission data were collected for the two dominant species Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) during the growing season in 1997 and 1998 using branch enclosure techniques. Samples were collected at different heights from 13 to 51 m within the canopy using the Wind River Canopy Crane facility. The standard emission factor at a temperature of 30°C and the temperature coefficient for Douglas-fir is Es=0.39±0.14 μg C g−1 h−1 and β=0.14±0.05°C−1 and for western hemlock Es=0.95±0.17 μg C g−1 h−1 and β=0.06±0.02°C−1. There was considerable variability among all the emission factors due to seasonal and branch-to-branch variations. Within season emission factors appear to decline from May to September for the Douglas-fir, although there was no corresponding decrease for the western hemlock. There was no significant difference in standard emission factors (Es) or temperature coefficients as a function of sunlit versus shady growth environment (different heights) for Douglas-fir, but western hemlock emission samples collected low in the canopy showed no exponential correlation with temperature. Applying the standard emission factors from this study to a Pacific Northwest domain and comparing the modified emission inventory to the current regulatory-based emission inventory yielded a net decrease of 19% in the domain wide monoterpene emissions. The relatively small difference in biogenic emissions is slightly misleading, as the difference in standard emission rates between this study and current regulatory rates is quite significant, and they offset each other when combined in this domain. When this inventory was input into a regional photochemical air quality simulation using the MM5/CMAQ system, the reduction in biogenic emissions resulted in an insignificant decrease of O3 and a significant decrease in the secondary organic aerosol (domain wide −20%). The emission measurements reported here represent one of the first extensive data sets for an old-growth forest, where sampling conditions are limited to in situ enclosure techniques within the tall, elevated canopy.

Urban areas are acknowledged to be major sources of anthropogenic CO2 (IPCC, 2001), however, there are few direct measurements of CO2 emissions in urban areas. This is particularly true for megacities, such as Mexico City, where there is a rapid growth with a wide range of direct and indirect sources, such as the high levels of traffic and land use changes involved with urbanization. Several authors have attempted to quantify the CO2 levels in urban environments through emission inventories from estimates of fossil fuel consumption, evaluations of the amount of carbon sequestered in urban vegetation, and short term studies of CO2 concentrations. Grimmond et al. (2002) provided a list of studies measuring CO2 concentrations in urban environments. For Mexico City, results of only two studies have been reported; the first was a study during 1981 and 1982, in which CO2 was sampled at different locations in the city and analyzed by gas chromatography (Baez et al., 1988). In the second study CO2 concentrations were measured by a Fourier Transform Infrared (FTIR) spectrometer at a fixed site in the southwest of the city in a residential area during fall 2001 (Grutter, 2003).