Abstract. We describe the large-scale meteorological conditions that affected atmospheric chemistry over Mexico during March 2006 when several field campaigns were conducted in the region. In-situ and remote-sensing instrumentation was deployed to obtain measurements of wind, temperature, and humidity profiles in the boundary layer and free atmosphere at four primary sampling sites in central Mexico. Several models were run operationally during the field campaign to provide forecasts of the local, regional, and synoptic meteorology as well as the predicted location of the Mexico City pollutant plume for aircraft flight planning purposes. Field campaign measurements and large-scale analyses are used to define three regimes that characterize the overall meteorological conditions: the first regime prior to 14 March, the second regime between 14 and 23 March, and the third regime after 23 March. Mostly sunny and dry conditions with periods of cirrus and marine stratus along the coast occurred during the first regime. The beginning of the second regime was characterized by a sharp increase in humidity over the central plateau and the development of late afternoon convection associated with the passage of a weak cold surge on 14 March. Over the next several days, the atmosphere over the central plateau became drier so that deep convection gradually diminished. The third regime began with the passage of a strong cold surge that lead to humidity, afternoon convection, and precipitation over the central plateau that was higher than during the second regime. The frequency and intensity of fires, as determined by satelliteCorrespondence to: J. D. Fast (jerome.fast@pnl.gov) measurements, also diminished significantly after the third cold surge. The synoptic-scale flow patterns that govern the transport of pollutants in the region are described and compared to previous March periods to put the transport into a climatological context. The complex terrain surrounding Mexico City produces local and regional circulations that govern short-range transport; however, the mean synoptic conditions modulate the thermally-driven circulations and on several days the near-surface flow is coupled to the ambient winds aloft.

Abstract. MCMA-2003 was a major field campaign investigating the atmospheric chemistry of the Mexico City Metropolitan Area (MCMA) in April of 2003. This paper describes the wind circulation patterns during the campaign both within the Mexico City basin and on the regional scale. ''Time roses'' are introduced to concisely analyze the diurnal wind patterns. Three episode types were identified that explain the conditions encountered: ''O3-South'', ''Cold Surge'' and ''O3-North''. These can be diagnosed from a combination of synoptic and basin observations based on whether the day was predominantly cloudy, or whether the O3 peak was in the north or south of the basin. O3-South days have weak synoptic forcing due to an anti-cyclone over the eastern Pacific. Strong solar heating leads to northerly flows in the basin and an evening shift due to a gap flow from the south-east. Peak ozone concentrations are in the convergence zone in the south of the city. Cold Surge days are associated with ''El Norte'' events, with strong surface northerlies bringing cold moist air and rain. Stable conditions lead to high concentrations of primary pollutants and peak ozone in the city center. O3-North days occur when the sub-tropical jet is closer to Mexico City. With strong westerlies aloft, the circulation pattern is the same as O3-South days except for a wind shift in the mid-afternoon leading to ozone peaks in the north of the city. This classification is proposed as a means of understanding pollutant transport in the Mexico City basin and as a basis for future meteorological and chemical analysis. Furthermore, model evaluation and design of policy recommendations will need to take into account the three episode types.

We describe the large-scale meteorological conditions that affected atmospheric chemistry over Mexico during March 2006 when several field campaigns were conducted in the region. In-situ and remote-sensing instrumentation was deployed to obtain measurements of wind, temperature, and humidity profiles in the boundary layer and free atmosphere at four primary sampling sites in central Mexico. Several models were run operationally during the field campaign to provide forecasts of the local, regional, and synoptic meteorology as well as the predicted location of the Mexico City pollutant plume for aircraft flight planning purposes. Field campaign measurements and large-scale analyses are used to define three regimes that characterize the overall meteorological conditions: the first regime prior to 14 March, the second regime between 14 and 23 March, and the third regime after 23 March. Mostly sunny and dry conditions with periods of cirrus and marine stratus along the coast occurred during the first regime. The beginning of the second regime was characterized by a sharp increase in humidity over the central plateau and the development of late afternoon convection associated with the passage of a weak cold surge on 14 March. Over the next several days, the atmosphere over the central plateau became drier so that deep convection gradually diminished. The third regime began with the passage of a strong cold surge that lead to humidity, afternoon convection, and precipitation over the central plateau that was higher than during the second regime. The frequency and intensity of fires, as determined by satellite Correspondence to: J. D. Fast (jerome.fast@pnl.gov) measurements, also diminished significantly after the third cold surge. The synoptic-scale flow patterns that govern the transport of pollutants in the region are described and compared to previous March periods to put the transport into a climatological context. The complex terrain surrounding Mexico City produces local and regional circulations that govern short-range transport; however, the mean synoptic conditions modulate the thermally-driven circulations and on several days the near-surface flow is coupled to the ambient winds aloft.

The present analysis describes the temporal evolution of various meteorological parameters over the Americas warm pools that are related to the occurrence of the Mid Summer Drought (MSD). Contrary to what has been suggested by some authors, the bimodal structure of precipitation over the Pacific side of Mesoamerica is not a form of intraseasonal variability in convective activity related to the Madden Julian Oscillation, but a characteristic of the annual cycle in precipitation. The MSD – sea surface temperature (SST) ‐ radiation relationship proposed by Magaña et al. [1999] partially hold during the summer of 2001, when the Climate Experiment over the Americas Warm Pools (ECAC) field campaigns were conducted, with the SST exhibiting a bimodal structure in the MSD region. The maximum in tropical convection over the Central America‐Caribbean coast appears to play an important role in modulating convective activity in the surrounding regions, through induced subsidence related to direct circulations. The Caribbean Low Level Jet (CLLJ) is in phase with maximum western Caribbean Sea convective activity, reaching maxima intensities in July.

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Abstract. Sulfur dioxide emissions from the Popocatépetl volcano in central Mexico were measured during the MI-LAGRO field campaign in March 2006. A stationary scanning DOAS (Differential Optical Absorption Spectrometer) was used to monitor the SO 2 emissions from the volcano and the results were compared with traverses done with a COSPEC from the ground and a DOAS instrument on board an ultra-light aircraft. Daytime evolutions as well as day-today variation of the SO 2 emissions are reported. A value of 2.45±1.39 Gg/day of SO 2 is reported from all the daily averages obtained during the month of March 2006, with large variation in maximum and minimum daily averages of 5.97 and 0.56 Gg/day, respectively. The large short-term fluctuations in the SO 2 emissions obtained could be confirmed through 2-D visualizations of the SO 2 plume measured with a scanning imaging infrared spectrometer. This instrument, based on the passive detection of thermal radiation from the volcanic gas and analysis with FTIR spectrometry, is used for the first time for plume visualization of a specific volcanic gas. A 48-h forward trajectory analysis indicates that the volcanic plume was predominantly directed towards the Puebla/Tlaxcala region (63%), followed by the Mexico City and Cuernavaca/Cuautla regions with 19 and 18% occurrences, respectively. 25% of the modeled trajectories going towards the Puebla region reached altitudes lower than 4000 m a.s.l. but all trajectories remained over this altitude for the other two regions.

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