ALBERT

Description: The MILAGRO field campaign was a multi-agency international collaborative project to evaluate the regional impacts of the Mexico City air pollution plume as a means of understanding urban impacts on the global climate. Mexico City lies on an elevated plateau with mountains on three sides and has complex mountain and surface-driven wind flows. This paper asks what the wind transport was in the basin during the field campaign and how representative it was of the climatology. Surface meteorology and air quality data, radiosondes and radar wind profiler data were collected at sites in the basin and its vicinity. Cluster analysis was used to identify the dominant wind patterns both during the campaign and within the past 10 years of operational data from the warm dry season. Our analysis shows that March 2006 was representative of typical flow patterns experienced in the basin. Six episode types were identified for the basin-scale circulation providing a way of interpreting atmospheric chemistry and particulate data collected during the campaign. Decoupling between surface winds and those aloft had a strong influence in leading to convection and poor air quality episodes. Hourly characterisation of wind circulation during the MILAGRO, MCMA-2003 and IMADA field campaigns enables the comparisons of similar air pollution episodes and the evaluation of the impact of wind transport on measurements of the atmospheric chemistry taking place in the basin.

Description: This study examines dynamical and microphysical features of convective clouds that affect mercury (Hg) wet scavenging and concentrations in rainfall. Using idealized numerical model simulations in the Regional Atmospheric Modeling System (RAMS), we diagnose vertical transport and scavenging of soluble Hg species – gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), collectively Hg(II) – in thunderstorms under typical environmental conditions found in the Northeast and Southeast United States (US). Mercury scavenging efficiencies from various initial altitudes are diagnosed for a case study of a typical strong convective storm in the Southeast US. Assuming that soluble mercury concentrations are initially vertically uniform, the model results suggest that 60% of mercury deposited to the surface in rainwater originates from above the boundary layer (〉 2 km). The free troposphere could supply a larger fraction of mercury wet deposition if GOM and HgP concentrations increase with altitude. We use radiosonde observations in the Northeast and Southeast to characterize three important environmental characteristics that influence thunderstorm morphology: convective available potential energy (CAPE), vertical shear (0–6 km) of horizontal wind (SHEAR) and precipitable water (PW). The Southeast US generally has lower SHEAR and higher CAPE and PW. We then use RAMS to test how PW and SHEAR impact mercury scavenging and deposition, while keeping the initial Hg(II) concentrations fixed in all experiments. We found that the mercury concentration in rainfall is sensitive to SHEAR with the nature of sensitivity differing depending upon the PW. Since CAPE and PW cannot be perturbed independently, we test their combined influence using an ensemble of thunderstorm simulations initialized with environmental conditions for the Northeast and Southeast US. These simulations, which begin with identical Hg(II) concentrations, predict higher mercury concentrations in rainfall from thunderstorms forming in the environmental conditions over the Southeast US compared to the Northeast US. A final simulation of a stratiform rain event produces lower mercury concentrations than in thunderstorms forming in environments typical of the Southeast US. The stratiform cloud scavenges mercury from the lowest ~ 4 km of the atmosphere, while thunderstorms scavenge up to ~ 10 km.

Description: This study examines dynamical and microphysical features of convective clouds that affect mercury (Hg) wet scavenging and concentrations in rainfall. Using idealized numerical model simulations in the Regional Atmospheric Modeling System (RAMS), we diagnose vertical transport and scavenging of soluble Hg species in thunderstorms under typical environmental conditions found in the Northeast and Southeast United States (US). Three important environmental characteristics that impact thunderstorm morphology were studied: convective available potential energy (CAPE), vertical shear (0–6 km) of horizontal wind (SHEAR) and precipitable water (PW). We find that in a strong convective storm in the Southeast US that about 40% of mercury in the boundary layer (0–2 km) can be scavenged and deposited to the surface. Removal efficiencies are 35% or less in the free troposphere and decline with altitude. Nevertheless, if we assume that soluble Hg species are initially uniformly mixed vertically, then about 60% deposited mercury deposited by the thunderstorm originates in the free troposphere. For a given CAPE, storm morphology and Hg deposition respond to SHEAR and PW. Experiments show that the response of mercury concentration in rainfall to SHEAR depends on the amount of PW. For low PW, increasing SHEAR decreases mercury concentrations in high-rain amounts (〉13 mm). However, at higher PW values, increasing SHEAR decreases mercury concentrations for all rainfall amounts. These experiments suggest that variations in environmental characteristics relevant to thunderstorm formation and evolution can also contribute to geographical difference in wet deposition of mercury. An ensemble of thunderstorm simulations was also conducted for different combinations of CAPE, SHEAR and PW values derived from radiosonde observations at five sites in the Northeast United States (US) and at three sites in the Southeast US. Using identical initial concentrations of gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), from the GEOS-Chem model, the simulations predict higher mercury concentrations in rainfall from thunderstorms forming in the environmental conditions over the Southeast US compared to the Northeast US. Mercury concentrations in rainfall are also simulated for a typical stratiform rain event and found to be less than in thunderstorms forming in environments typical of the Southeast US. The stratiform cloud scavenges mercury from the lower ~4 km of the atmosphere, while thunderstorms scavenge up to ~10 km.

Description: A series of experiments (the Southern Oxidant and Aerosol Study-SOAS) took place in central Alabama in June–July 2013 as part of the broader Southern Atmosphere Study (SAS). These projects were aimed at studying oxidant photochemistry and formation and impacts of aerosols at a detailed process level in a location where high biogenic organic vapor emissions interact with anthropogenic emissions, and the atmospheric chemistry occurs in a subtropical climate in North America. The majority of the ground-based experiments were located at the Southeastern Aerosol Research and Characterization (SEARCH) Centreville (CTR) site near Brent, Alabama, where extensive, unique aerometric measurements of meteorology, trace gases and particles have been made from the early 1990s through 2013. The SEARCH network data permits a characterization of temporal and spatial context of the SOAS findings. The long-term measurements show that the SOAS experiments took place during the second wettest and coolest year in the 2000–2013 period, with lower than average solar radiation. The pollution levels at CTR and other SEARCH sites were the lowest since full measurements began in 1999. This dataset provides a perspective for the SOAS program in terms of long-term average chemistry (chemical climatology) and short-term comparisons of summer average spatial variability across the Southeast at high temporal (hourly) resolution. Changes in anthropogenic gas and particle emissions between 1999 and 2013, account for the decline in pollutant concentrations at the monitoring sites in the region. The long-term and short-term data provide an opportunity to contrast SOAS results with temporally and spatially variable conditions in support for the development of tests for the robustness of SOAS findings.

Description: During the MILAGRO campaign centered in the Mexico City area, Pacific Northwest National Laboratory (PNNL) and Argonne National Laboratory (ANL) operated atmospheric profiling systems at Veracruz and at two locations on the Central Mexican Plateau in the region around Mexico City. These systems included radiosondes, wind profilers, a sodar, and an aerosol backscatter lidar. An additional wind profiler was operated by the University of Alabama in Huntsville (UAH) at the Mexican Petroleum Institue (IMP) near the center of Mexico City. Because of the opportunity afforded by collocation of profilers, radiosondes, and a lidar, and because of the importance of boundary layer depth for aerosol properties, we have carried out a comparison of mixing layer depth as determined independently from these three types of measurement systems during the campaign. We have then used results of this comparison and additional measurements to develop a detailed description of the daily structure and evolution of the boundary layer on the Central Mexican Plateau during MILAGRO. Our analysis indicates that the profilers were more consistently successful in establishing the mixing layer depth during the daytime. The boundary layer growth was similar at the three locations, although the mixing layer tended to be slightly deeper in the afternoon in central Mexico City. The sodar showed that convection began about an hour after sunrise. Maximum daily mixing layer depths always reached 2000 m a.g.l. and frequently extended to 4000 m. The rate and variability of mixing layer growth was essentially the same as that observed during the IMADA-AVER campaign in the same season in 1997. This growth did not seem to be related to whether deep convection was reported on a given day. Wind speeds within the boundary layer exhibited a daily low-altitude maximum in the late afternoon with lighter winds aloft, consistent with previous reports of diurnal regional circulations. Norte events, which produced high winds at Veracruz, did not appreciably modulate the winds on the plateau. Finally, despite the typically dry conditions at the surface, radiosonde profiles showed that relative humidity often exceeded 50% in the early morning and in the upper part of the boundary layer. This suggests that aerosol particles would have experienced hygroscopic growth within the boundary layer on many days.

Description: The MILAGRO field campaign was a multi-agency international collaborative project to evaluate the regional impacts of the Mexico City air pollution plume as a means of understanding urban impacts on the global climate. Mexico City lies on an elevated plateau with mountains on three sides and has complex mountain and surface-driven wind flows. This paper asks what the wind transport was in the basin during the field campaign and how representative it was of the climatology. Surface meteorology and air quality data, radiosoundings and radar wind profiler data were collected at sites in the basin and its vicinity. Cluster analysis is used to identify the dominant wind patterns both during the campaign and within the past 10 years of operational data from the warm dry season. Our analysis shows that March 2006 was representative of typical flow patterns experienced in the basin. Six episode types were identified for the basin scale circulation providing a way of interpreting atmospheric chemistry and particulate data collected during the campaign. Decoupling between surface winds and those aloft had a strong influence in leading to convection and poor air quality episodes. Hourly characterisation of wind circulation during the MILAGRO, MCMA-2003 and IMADA field campaigns will enable the comparisons of similar air pollution episodes and the evaluation of the impact of wind transport on measurements of the atmospheric chemistry taking place in the basin.

Description: A series of experiments (the Southern Oxidant and Aerosol Study – SOAS) took place in central Alabama in June–July, 2013 as part of the broader Southern Atmosphere Study (SAS). These projects were aimed at studying oxidant photochemistry and formation and impacts of aerosols at a detailed process level in a location where high biogenic organic vapor emissions interact with anthropogenic emissions, and the atmospheric chemistry occurs in a subtropical climate in North America. The majority of the ground-based experiments were located at the Southeastern Aerosol Research and Characterization (SEARCH) Centreville (CTR) site near Brent, Alabama, where extensive, unique aerometric measurements of trace gases and particles and meteorology were made beginning in the early 1990s through 2013. The SEARCH network data permits a characterization of the temporal and spatial context of the SOAS findings. Our earlier analyses of emissions and air quality trends are extended through 2013 to provide a perspective for continued decline in ambient concentrations, and the implications of these changes to regional sulfur oxide, nitrogen–ozone, and carbon chemistry. The narrative supports the SAS program in terms of long-term average chemistry (chemical climatology) and short-term comparisons of early summer average spatial variability across the southeastern US at high temporal (hourly) resolution. The long-term measurements show that the SOAS experiments took place during the second wettest and coolest year in the 2000–2013 period, with lower than average solar radiation. The pollution levels at CTR and other SEARCH sites were the lowest since full measurements began in 1999. Changes in anthropogenic gas and particle emissions between 1999 and 2013 account for the decline in pollutant concentrations at the monitoring sites in the region. The data provide an opportunity to contrast SOAS results with temporally and spatially variable conditions in support of the development of tests for the robustness of SOAS findings.