Abstract. Simulated primary organic aerosols (POA), as well as other particulates and trace gases, in the vicinity of Mexico City are evaluated using measurements collected during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaigns. Since the emission inventories, transport, and turbulent mixing will directly affect predictions of total organic matter and consequently total particulate matter, our objective is to assess the uncertainties in predicted POA before testing and evaluating the performance of secondary organic aerosol (SOA) treatments. Carbon monoxide (CO) is well simulated on most days both over the city and downwind, indicating that transport and mixing processes were usually consistent with the meteorological conditions observed during MILAGRO. PreCorrespondence to: J. D. Fast (jerome.fast@pnl.gov) dicted and observed elemental carbon (EC) in the city was similar, but larger errors occurred at remote locations since the overall CO/EC emission ratios in the national emission inventory were lower than in the metropolitan emission inventory. Components of organic aerosols derived from Positive Matrix Factorization of data from several Aerodyne Aerosol Mass Spectrometer instruments deployed both at ground sites and on research aircraft are used to evaluate the model. Modeled POA was consistently lower than the measured organic matter at the ground sites, which is consistent with the expectation that SOA should be a large fraction of the total organic matter mass. A much better agreement was found when modeled POA was compared with the sum of "primary anthropogenic" and "biomass burning" components derived from Positive Matrix Factorization (PMF) on most days, especially at the surface sites, suggesting that the overall magnitude of primary organic particulates released was reasonable. However, simulated POA Published by Copernicus Publications on behalf of the European Geosciences Union. 6192 J. Fast et al.: Evaluating simulated primary anthropogenic and biomass burning organic aerosols from anthropogenic sources was often lower than "primary anthropogenic" components derived from PMF, consistent with two recent reports that these emissions are underestimated. The modeled POA was greater than the total observed organic matter when the aircraft flew directly downwind of large fires, suggesting that biomass burning emission estimates from some large fires may be too high.

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Abstract. The first KErguelen Ocean and Plateau compared Study (KEOPS1), conducted in the naturally iron-fertilised Kerguelen bloom, demonstrated that fecal material was the main pathway for exporting carbon to the deep ocean during summer (January-February 2005), suggesting a limited role of direct export via phytodetrital aggregates. The KEOPS2 project reinvestigated this issue during the spring bloom initiation (October-November 2011), when zooplankton communities may exert limited grazing pressure, and further explored the link between carbon flux, export efficiency and dominant sinking particles depending upon surface plankton community structure. Sinking particles were collected in polyacrylamide gel-filled and standard free-drifting sediment traps (PPS3/3), deployed at six stations between 100 and 400 m, to examine flux composition, particle origin and their size distributions. Results revealed an important contribution of phytodetrital aggregates (49 ± 10 and 45 ± 22 % of the total number and volume of particles respectively, all stations and depths averaged). This high contribution dropped when converted to carbon content (30±16 % of total carbon, all stations and depths averaged), with cylindrical fecal pellets then representing the dominant fraction (56 ± 19 %).At 100 and 200 m depth, iron-and biomass-enriched sites exhibited the highest carbon fluxes (maxima of 180 and 84 ± 27 mg C m −2 d −1 , based on gel and PPS3/3 trap collection respectively), especially where large fecal pellets dominated Published by Copernicus Publications on behalf of the European Geosciences Union. 1008 E. C. Laurenceau-Cornec et al.: The importance of sinking particle types to carbon export over phytodetrital aggregates. Below these depths, carbon fluxes decreased (48±21 % decrease on average between 200 and 400 m), and mixed aggregates composed of phytodetritus and fecal matter dominated, suggesting an important role played by physical aggregation in deep carbon export.Export efficiencies determined from gels, PPS3/3 traps and 234 Th disequilibria (200 m carbon flux/net primary productivity) were negatively correlated to net primary productivity with observed decreases from ∼ 0.2 at low-iron sites to ∼ 0.02 at high-iron sites. Varying phytoplankton communities and grazing pressure appear to explain this negative relationship. Our work emphasises the need to consider detailed plankton communities to accurately identify the controls on carbon export efficiency, which appear to include small spatio-temporal variations in ecosystem structure.

Abstract. Data collected in a measurement campaign in February and March 1997 showed that the Mexico Basin (also called the Valley of Mexico), located atop the Mexican plateau, fails to develop the strong nocturnal inversions usually associated with basins and does not exhibit diurnally reversing valley wind systems. Data analyses, two-and three-dimensional numerical simulations with the Regional Atmospheric Modeling System (RAMS), and a Lagrangian particle dispersion model are used to interpret these observations and to examine the effects of topography and regional diurnal circulations on boundary layer evolution over the Mexico Basin and its surroundings during fair weather periods in the winter dry season. We show that the boundary layer evolution in and above the basin is driven primarily by regional diurnal circulations that develop between the air above the Mexican Plateau and the generally cooler surrounding coastal areas. A convective boundary layer (CBL) grows explosively over the plateau in the late morning to reach elevations of 2250 m agl (4500 rn rnsl) by noon, and a strong baroclinic zone forms on the edges of the plateau separating the warm CBL air from its cooler surroundings. In early afternoon the rates of heating and CBL growth are slowed as cool air leaks onto the plateau and into the basin through passes and over low-lying plateau edges. The flow onto the plateau is retarded, however, by the strongly rising branch of a plain-plateau circulation at the plateau edges, especially where mountains or steep slopes are present. An unusually rapid and deep cooling of the air above the plateau begins in late afternoon and early evening when the surface energy budget reverses, the CBL decays, and air accelerates onto the plateau through the baroclinic zone. Flow convergence near the basin floor and the associated rising motions over the basin and plateau produce cooling in 3 hours that is equivalent to half the daytime heating. While the air that converges onto the plateau comes from elevations at and above the plateau, it is air that was modified earlier in the day by a cool, moist coastal inflow carried up the plateau slopes by the plainplateau circulation.

PM, PM, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the city's particulate matter (PM). Average 24-hr PM concentrations over the period of study at the core sites in the city were 75 H g/m. The 24-hr standard of 150 μ g/m was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 μ g/m. Nearly half of the PM was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM consisted of PM, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.