Plume Mixing Research Articles

Electrochemical sensing of volcanic gases

We report here the development and application of a compact “geochemical nose” incorporating electrochemical sensors for gas measurements in volcanic plumes. A novel element of the instrument design is the arrangement of the sensors in a parallel array that enables near-simultaneous exposure and fast response. Data analysis methods were developed that utilise the multi-sensor output currents to extract gas mixing ratio abundances and eliminate cross-sensitivities. Use of filter methods is demonstrated to remove baseline drift or instrument noise. We introduce a new approach for analysis of measurements from sensors that have a slower response time (e.g. HCl), and apply this model to estimate HCl/SO2 ratios. We deployed the sensor system at Aso volcano, Japan, detecting emissions from its fumarole field hot crater lake, and a mixed plume. We measured SO2, H2S, CO and HCl, ranging in abundance from ~102–104ppbv. Neither NO2 nor Cl2 were detected. For the fumarolic gases, molar ratios were measured as follows: H2S/SO2 is ~0.15, H2/SO2 ~0.25, CO/SO2 ~0.02, HCl/SO2 ~0.1. The crater lake plume's H2S/SO2 is ~0.03. The compositions are discussed in terms of degassing equilibria and plume chemistry. Our instrument design represents a cost-effective, low-power and highly portable system that can be readily adapted for operational surveillance of volcanic gases.

Mapping ocean outfall plumes and their mixing using autonomous underwater vehicles

This paper reports on developing autonomous underwater vehicle (AUV) survey methods for ocean outfall discharge plumes and new insights gained on plume mixing. Unique to the study is mapping the discharge mixing using colored dissolved organic matter (CDOM) calibrated for effluent dilution. AUV mission planning methodologies for discharge plume sampling, plume characterization using onboard temperature, salinity and optical sensors, and comparison of observational data to model results are presented for the Point Loma Ocean Outfall offshore of San Diego, CA. The results are expected to be applicable to the general theme of mixing of submerged buoyant discharges. In the near‐field, the plume is found to mix to a height consistent with the predictions of buoyant jet engineering models. At the far‐field, the fine spatial scales of the plume resolved by the vehicle suggests that shear instabilities caused by internal waves can enhance plume mixing and elevate the discharge plume above the predicted equilibrium rise height. These results suggest that even under variable oceanic conditions, properly planned missions for AUVs equipped with an optical CDOM sensor in addition to traditional physical oceanographic sensors, can accurately map the mixing of ocean outfall plumes at resolutions not possible with traditional boat‐based techniques. Variations of oceanic conditions are found to influence the mixing and fate of the plume at time scales generally not considered in the design of these discharges.

Effects of NOxcontrol and plume mixing on nighttime chemical processing of plumes from coal‐fired power plants

Coal‐fired electric power plants produce a large fraction of total U.S. NOx emissions, but NOx from this sector has been declining in the last decade owing to installation of control technology. Nighttime aircraft intercepts of plumes from two different Texas power plants (Oklaunion near Wichita Falls and W. A. Parish near Houston) with different control technologies demonstrate the effect of these reductions on nighttime NOxoxidation rates. The analysis shows that the spatial extent of nighttime‐emitted plumes to be quite limited and that mixing of highly concentrated plume NOx with ambient ozone is a determining factor for its nighttime oxidation. The plume from the uncontrolled plant had full titration of ozone through 74 km/2.4 h of downwind transport that suppressed nighttime oxidation of NO2 to higher oxides of nitrogen across the majority of the plume. The plume from the controlled plant did not have sufficient NOx to titrate background ozone, which led to rapid nighttime oxidation of NO2 during downwind transport. A plume model that includes horizontal mixing and nighttime chemistry reproduces the observed structures of the nitrogen species in the plumes from the two plants. The model shows that NOx controls not only reduce the emissions directly but also lead to an additional overnight NOx loss of 36–44% on average. The maximum reduction for 12 h of transport in darkness was 73%. The results imply that power plant NOxemissions controls may produce a larger than linear reduction in next‐day, downwind ozone production following nighttime transport.

Detailed analysis of particle launch velocities, size distributions and gas densities during normal explosions at Stromboli

Using high frame rate (33Hz) thermal video data we describe and parameterize the emission and ascent dynamics of a mixed plume of gas and particles emitted during a normal explosion at Stromboli (Aeolian Islands, Italy). Analysis of 34 events showed that 31 of them were characterized by a first phase characterized by an initial diffuse spray of relatively small (lapilli-sized) particles moving at high velocities (up to 213ms−1; average 66–82ms−1). This was followed, typically within 0.1s, by a burst comprising a mixture of ash and lapilli, but dominated by larger bomb-sized particles, moving at lower exit velocities of up to 129ms−1, but typically 46ms−1. We interpret these results as revealing initial emission of a previously unrecorded high velocity gas-jet phase, to which the lapilli are coupled. This is followed by emission of slower moving larger particles that are decoupled from the faster moving gas-phase. Diameters for particles carried by the gas phase are typically around 4cm, but can be up to 9cm, with the diameter of the particles carried by the gas jet (D) decreasing with increased density and velocity of the erupted gas cloud (ρgas and Ugas). Data for 101 particles identified as moving with the gas jet during 32 eruptions allow us to define a new relation, whereby Ugas=Uparticle+a [ρgasD]b. Here, Uparticle is the velocity of bombs whose motion is decoupled from that of the gas cloud, and a and b are two empirically-derived coefficients. This replaces the old relation, whereby Ugas=Uparticle+k D; a relation that requires a constant gas density for each eruption. This is an assumption that we show to be invalid, with gas density potentially varying between 0.04kgm−3 and 9kgm−3 for the 32 cases considered, so that k varies between 54m1/2s−1 and 828m1/2s−1, compared with the traditionally used constant of 150m1/2s−1.

BIOPHYSICS, BIOENERGETICS AND MECHANISTIC APPROACHES TO ECOLOGY

Biophysics – the study of how the structure of individual plants and animals contributes to their function – has rarely been considered on the ecological scale. However, Mark Denny from Stanford University, USA, has a broader vision. Because the mechanisms that underpin individual performance

The influence of streams on nearshore water chemistry, Lake Ontario

Large lakes have characteristics, such as a spring thermal bar; nearshore offshore gradients of nutrients, sediment, and biota; and the development of a nearshore zone often chemically, biologically, and physically different from the offshore region, that are not commonly observed in smaller lakes. The research presented here focuses on the role of rivers affecting the cross margin transport of terrigenous materials delivered during the summer on nearshore waters of Lake Ontario. On the south shore of Lake Ontario, levels of chlorophyll, total phosphorus (TP), and total coliform were higher in the Oak Orchard Creek river plume than out of it. Similarly at the Genesee River polygon, turbidity, specific conductance, temperature, TP, and chlorophyll were substantially higher in the plumes than outside of the plumes. Graphic depictions and the strong correlations (r>=0.70) between specific conductance and TP, soluble reactive phosphorus , temperature, chlorophyll, total Kjeldahl nitrogen, and turbidity indicate the plumes of water are from the Genesee River and Oak Orchard Creek. The load from all creeks on the New York shore of Lake Ontario ranged from ~18 to 25% of the annual phosphorus load of the Niagara River. The cumulative impact of these small streams entering the nearshore and embayments of Lake Ontario is potentially great, as the mixing of plume and lake water appears to be limited to the nearshore areas. Terrigenous inputs likely provide the nutrient, phosphorus, which contributes to the periodic development of benthic algae blooms observed along the south shore of Lake Ontario. The conclusion that river plumes impact and sustain the nearshore of large lakes is suggested by the data.

Probability density function of steady state concentration in two‐dimensional heterogeneous porous media

Spatial variability of hydraulic aquifer parameters causes meandering, squeezing, stretching, and enhanced mixing of steady state plumes in concentrated hot‐spots of mixing. Because the exact spatial distribution of hydraulic parameters is uncertain, the spatial distribution of enhanced mixing rates is also uncertain. We discuss how relevant the resulting uncertainty of mixing rates is for predicting concentrations. We develop analytical solutions for the full statistical distribution of steady state concentration in two‐dimensional, statistically uniform domains with log‐hydraulic conductivity following an isotropic exponential model. In particular, we analyze concentration statistics at the fringes of wide plumes, conceptually represented by a solute introduced over half the width of the domain. Our framework explicitly accounts for uncertainty of streamline meandering and uncertainty of effective transverse mixing (defined at the Darcy scale). We make use of existing low‐order closed‐form expressions that lead to analytical expressions for the statistical distribution of local concentration values. Along the expected position of the plume fringe, the concentration distribution strongly clusters at its extreme values. This behavior extends over travel distances of up to tens of integral scales for the parameters tested in our study. In this regime, the uncertainty of effective transverse mixing is substantial enough to have noticeable effects on the concentration probability density function. At significantly larger travel distances, intermediate concentration values become most likely, and uncertainty of effective transverse mixing becomes negligible. A comparison to numerical Monte Carlo simulations of flow and solute transport show excellent agreement with the theoretically derived expressions.

Limitations in the use of compound-specific stable isotope analysis to understand the behaviour of a complex BTEX groundwater contamination near Brussels (Belgium)

The application of compound-specific stable isotope analysis (CSIA) was evaluated to characterise a complex groundwater contamination. For this purpose, δ13C and δ2H analysis of benzenes and alkylated derivatives were used to interpret both the impact of different sources on a contaminant plume and the presence of degradation processes. The different contaminant sources could be distinguished based on their combined δ13C–δ2H signature of the benzene, toluene, ethylbenzene and xylenes (BTEX) dissolved in the groundwater. Despite this source differentiation, plume characterisation was not possible due to the complex mixing of the respective contaminant plumes. Furthermore, the original isotope signatures of the sources were not preserved across these plumes. To estimate the level of in situ biodegradation independently from concentration data, the Rayleigh equation was used. Although current literature identifies the application of CSIA as very promising in the frame of characterising organic groundwater pollution, this study has indicated that this approach can be limited with respect to successfully distinguish the different plumes and their relation to the known source zones.

Oscillating Dense Plumes

Abstract The flow of dense polar shelf waters down continental slopes is a critical component of the global ocean circulation. Recent observations suggest that such plumes can be heavily impacted by tidal variability, and many of the world’s important dense-water sources are located in tidally active areas. Tides affect the source of dense water (by modulating the location of hydrographic gradients) and control the subsequent plume mixing and flow path. In an effort to separate these effects, dense plumes are modeled here by extending a classical one-dimensional plume model to two unsteady scenarios in which the plume path is fixed. The first case features a pulsed release of dense water into a stagnant ambient, and the model predicts that gravity waves propagate down the plume. Advective waves in plume density travel with the mean velocity of the current and thus have a wavelength of , the product of plume velocity and the oscillation period P. The second case is of a steady-sourced plume flowing through an ambient that has uniformly oscillating flow. This drives fluctuating shear at the plume–ambient interface (and/or seabed) that leads to variable entrainment of ambient fluid into the plume. Perturbed properties are subsequently advected by the plume, leading to standing “entrainment waves” that also have a wavelength of . Pulsed-source effects may be distinguished from variable-entrainment effects by the phase difference between waves in the different state variables of each plume. Both effects are maximized when the ratio , where L is the plume length. This condition is satisfied in the Ross Sea, Antarctica, where observations show dense plumes that are strongly affected by tides. Modeled pulsed-source effects qualitatively agree with the observations, implying that hydrographic variability in Ross Sea plumes is associated with variability in their dense-water source rather than unsteady plume mixing. These results might help inform the gathering and interpretation of oceanographic data in tidally active dense-water source regions.

Modelling a turbidity current in Soyang reservoir (Korea) and its control using a selective withdrawal facility

Persistent turbidity in reservoirs and their downstream after flood events is one of most important environmental issues in Korea. Recently, modification of withdrawal facility and installation of a new selective withdrawal structure (SWS) have been implemented for the mitigation of downstream impact and sediment loading into water treatment facilities. This study was to explore the characteristics of flood density flow induced into Soyang Reservoir and the transport processes of suspended sediments (SS) through application of coupled two-dimensional hydrodynamic and particle dynamic models (TM-1, TM-2 and TM-3). The TM-3 including a turbidity attenuation rate as a lumped parameter showed best performance in reproducing the magnitude and distribution of SS in the reservoir. The validated model was applied to evaluate the effectiveness of SWS, which was designed for the reservoir, with 6 different historical flood events. The magnitude of vertical mixing of the turbidity plume and its persistence within the reservoir were closely correlated to the ratio of the volume of turbidity flow to the total reservoir storage (the theta value). The operation of SWS showed a positive effect as long as theta is between 0.3 and 0.6 but negative when theta = 0.83 for the study reservoir, thus it should be optimized based on the theta value for a better management of the reservoir.

Parameterizing Convective Organization to Escape the Entrainment Dilemma

[1] Lateral mixing parameters in buoyancy-driven deep convection schemes are among the most sensitive and important unknowns in atmosphere models. Unfortunately, there is not a true optimum value for plume mixing rate, but rather a dilemma or tradeoff: Excessive dilution of updrafts leads to unstable stratification bias in the mean state, while inadequate dilution allows deep convection to occur too easily, causing poor space and time distributions and variability. In this too-small parameter space, compromises are made based on competing metrics of model performance. We attempt to escape this “entrainment dilemma” by making bulk plume parameters (chiefly entrainment rate) depend on a new prognostic variable (“organization,” org) meant to reflect the rectified effects of subgrid-scale structure in meteorological fields. We test an org scheme in the Community Atmosphere Model (CAM5) with a new unified shallow-deep convection scheme (UW-ens, a 2-plume version of the University of Washington scheme). Since buoyant ascent involves natural selection, subgrid structure makes convection systematically deeper and stronger than the pure unorganized case: plumes of average (or randomly sampled) air rising in the average environment. To reflect this, org is nonnegative, but we leave it dimensionless. A time scale characterizes its behavior (here ∼3 h for a 2o model). Currently its source is rain evaporation, but other sources can be added easily. We also let org be horizontally transported by advection, as a mass-weighted mean over the convecting layer. Linear coefficients link org to a plume ensemble, which it assists via: 1) plume base warmth above the mean temperature 2) plume radius enhancement (reduced mixing), and 3) increased probability of overlap in a multi-plume scheme, where interactions benefit later generations (this part has only been implemented in an offline toy column model). Since rain evaporation is a source for org, it functions as a time-lagged but positive feedback on deep convection development. This evades the entrainment dilemma, since fully developed org–enhanced convection is not overly dilute, avoiding stability bias, while the pioneering updrafts of new convection are suppressed by entrainment, encouraging more large-scale variability.

Californian forest fire plumes over Southwestern British Columbia: lidar, sunphotometry, and mountaintop chemistry observations

Abstract. Forest fires in Northern California and Oregon were responsible for two significant regional scale aerosol transport events observed in southern British Columbia during summer 2008. A combination of ground based (CORALNet) and satellite (CALIPSO) lidar, sunphotometry and high altitude chemistry observations permitted unprecedented characterization of forest fire plume height and mixing as well as description of optical properties and physicochemistry of the aerosol. In southwestern BC, lidar observations show the smoke to be mixed through a layer extending to 5–6 km a.g.l. where the aerosol was confined by an elevated inversion in both cases. Depolarization ratios for a trans-Pacific dust event (providing a basis for comparison) and the two smoke events were consistent with observations of dust and smoke events elsewhere and permit discrimination of aerosol events in the region. Based on sunphotometry, the Aerosol Optical Thicknesses (AOT) reached maxima of ~0.7 and ~0.4 for the two events respectively. Dubovik-retrieval values of reff, f during both the June/July and August events varied between about 0.13 and 0.15 μm and confirm the dominance of accumulation mode size particles in the forest fire plumes. Both Whistler Peak and Mount Bachelor Observatory data show that smoke events are accompanied by elevated CO and O3 concentrations as well as elevated K+/SO4 ratios. In addition to documenting the meteorology and physic-chemical characteristics of two regional scale biomass burning plumes, this study demonstrates the positive analytical synergies arising from the suite of measurements now in place in the Pacific Northwest, and complemented by satellite borne instruments.

Lidar measurements of Raman scattering at ultraviolet wavelength from mineral dust over East Asia

We developed a novel measurement channel that utilizes Raman scattering from silicon dioxide (SiO2) quartz at an ultraviolet wavelength (361 nm). The excitation of the Raman signals is done at the primary wavelength of 355 nm emitted from a lidar instrument. In combination with Raman signals from scattering from nitrogen molecules, we may infer the mineral-quartz-related backscatter coefficient. This technique thus allows us to identify in a comparably direct way the mineral quartz content in mixed pollution plumes that consist, e.g., of a mix of desert dust and urban pollution. We tested the channel for the complex situation of East Asian pollution. We find good agreement of the inferred mineral-quartz-related backscatter coefficient to results obtained with another mineral quartz channel which was operated at 546 nm (primary emission wavelength at 532 nm), the functionality of which has already been shown for a lidar system in Tsukuba (Japan). The advantage of the novel channel is that it provides a better signal-to-noise ratio because of the shorter measurement wavelength.

Mixing of mineral dust with urban pollution aerosol over Dakar (Senegal): impact on dust physico-chemical and radiative properties

In the framework of the Saharan Mineral Dust Experiment (SAMUM) in 2008, the mixing of the urban pollution plume of Dakar (Senegal) with mineral dust was studied in detail using the German research aircraft Falcon which was equipped with a nadir-looking high spectral resolution lidar (HSRL) and extensive aerosol in situ instrumentation. The mineral dust layer as well as the urban pollution plume were probed remotely by the HSRL and in situ. Back trajectory analyses were used to attribute aerosol samples to source regions.We found that the emission from the region of Dakar increased the aerosol optical depth (532 nm) from approximately 0.30 over sea and over land east of Dakar to 0.35 in the city outflow. In the urban area, local black carbon (BC) emissions, or soot respectively, contributed more than 75% to aerosol absorption at 530 nm. In the dust layer, the single-scattering albedo at 530 nm was 0.96 − 0.99, whereas we found a value of 0.908 ± 0.018 for the aerosol dominated by urban pollution. After 6 h of transport over the North Atlantic, the externally mixed mode of secondary aerosol particles had almost completely vanished, whereas the BC agglomerates (soot) were still externally mixed with mineral dust particles.

Convective venting and surface ozone in Houston during TexAQS 2006

The influence of convective mixing on surface ozone in Houston during TexAQS 2006 is examined. We use airborne lidar measurements of ozone and ship‐based Doppler lidar measurements of winds, together with ship‐ and ground‐based measurements of surface ozone to characterize horizontal and vertical mixing of ozone plumes from the Houston Ship Channel on two high‐ozone days. We show that a stable capping layer trapped the plume in the boundary layer on 31 August, while shallow convection associated with active fair weather cumulus clouds mixed the plume with free tropospheric air on 17 August. Deep convection associated with an isolated air mass thunderstorm further decreased surface ozone near Galveston Bay in the late afternoon. High ozone thus affected a smaller area for a shorter period on 17 August, despite similar background concentrations and local production. We generalize these findings by comparing Houston ozone concentrations to National Weather Service (Lake Charles, LA) radiosondes. We show that for 1 June to 15 September 2006, stable conditions with high background ozone occurred 18% of the days leading to mean daily 8 h concentrations of 73 ± 11 ppbv. Shallow and deep convection associated with moderate to strongly unstable conditions lowered the mean ozone to 50 ± 11 ppbv (∼29% of days), while weaker convection associated with marginally unstable conditions reduced the mean concentrations to 63 ± 13 ppbv (∼11%). We use these observations to derive simple relationships between surface ozone and convective indicators that may prove useful for parameterization of convective venting in air quality models.

An investigation of the sub-grid variability of trace gases and aerosols for global climate modeling

Abstract. One fundamental property and limitation of grid based models is their inability to identify spatial details smaller than the grid cell size. While decades of work have gone into developing sub-grid treatments for clouds and land surface processes in climate models, the quantitative understanding of sub-grid processes and variability for aerosols and their precursors is much poorer. In this study, WRF-Chem is used to simulate the trace gases and aerosols over central Mexico during the 2006 MILAGRO field campaign, with multiple spatial resolutions and emission/terrain scenarios. Our analysis focuses on quantifying the sub-grid variability (SGV) of trace gases and aerosols within a typical global climate model grid cell, i.e. 75×75 km2. Our results suggest that a simulation with 3-km horizontal grid spacing adequately reproduces the overall transport and mixing of trace gases and aerosols downwind of Mexico City, while 75-km horizontal grid spacing is insufficient to represent local emission and terrain-induced flows along the mountain ridge, subsequently affecting the transport and mixing of plumes from nearby sources. Therefore, the coarse model grid cell average may not correctly represent aerosol properties measured over polluted areas. Probability density functions (PDFs) for trace gases and aerosols show that secondary trace gases and aerosols, such as O3, sulfate, ammonium, and nitrate, are more likely to have a relatively uniform probability distribution (i.e. smaller SGV) over a narrow range of concentration values. Mostly inert and long-lived trace gases and aerosols, such as CO and BC, are more likely to have broad and skewed distributions (i.e. larger SGV) over polluted regions. Over remote areas, all trace gases and aerosols are more uniformly distributed compared to polluted areas. Both CO and O3 SGV vertical profiles are nearly constant within the PBL during daytime, indicating that trace gases are very efficiently transported and mixed vertically by turbulence. But, simulated horizontal variability indicates that trace gases and aerosols are not well mixed horizontally in the PBL. During nighttime the SGV for trace gases is maximum at the surface, and quickly decreases with height. Unlike the trace gases, the SGV of BC and secondary aerosols reaches a maximum at the PBL top during the day. The SGV decreases with distance away from the polluted urban area, has a more rapid decrease for long-lived trace gases and aerosols than for secondary ones, and is greater during daytime than nighttime. The SGV of trace gases and aerosols is generally larger than for meteorological quantities. Emissions can account for up to 50% of the SGV over urban areas such as Mexico City during daytime for less-reactive trace gases and aerosols, such as CO and BC. The impact of emission spatial variability on SGV decays with altitude in the PBL and is insignificant in the free troposphere. The emission variability affects SGV more significantly during daytime (rather than nighttime) and over urban (rather than rural or remote) areas. The terrain, through its impact on meteorological fields such as wind and the PBL structure, affects dispersion and transport of trace gases and aerosols and their SGV.

Seasonal variation of primary productivity in the East China Sea: A numerical study based on coupled physical-biogeochemical model

The seasonal variation of phytoplankton growth in the East China Sea (ECS) is simulated with a three-dimensional coupled physical-biogeochemical model, which includes discharges from Changjiang (Yangtze River). The purpose is to determine the main control on the seasonality of primary productivity in the ECS shelf, which nurtures rich biological resources. The model has a horizontal resolution of 1/6° in the domain from 23°N to 41°N and from 116°E to 134°E, excluding the Japan/East Sea, and 33 layers in the vertical. The nitrogen-based biogeochemical model has four compartments: dissolved inorganic nitrogen (DIN), phytoplankton, zooplankton and detritus. The chlorophyll to phytoplankton ratio depends on light and DIN availability. The model is driven by monthly climatological winds with the sea-surface temperature, salinity and DIN relaxed towards the climatological mean values. It successfully reproduces the observed seasonal variation of primary productivity over the ECS shelf with a strong peak in later spring and summer. The modeled annual mean primary production (PP) over the entire ECS shelf is 441 mg C m −2 d −1, which falls within the reported range of 390–529 mg C m −2 d −1. It also reproduces the marked seaward gradient of DIN that decreases away from the Changjiang plume. Strong dependency on the amount of photosynthetically active radiation (PAR) is demonstrated for primary production. For 1% change in PAR 0.7% change in PP is effected. Numerical experiments show that the strong summer peak of DIN load from Changjiang cannot generate the seasonality of PP without the seasonal cycle of PAR. On the other hand, the model can still produces the seasonal pattern with Changjiang nutrient load set to zero, indicating light availability as the major control. Yet the Changjiang DIN load induces a PP increase of 77 mg C m −2 d −1, which represents 13–20% of the observation based estimates of PP. It is noted that the increase in nitrogen uptake associated with PP is 2.7 times the DIN provided by the Changjiang discharge, implying efficient recycling of the riverine nutrients in the shelf water. The model needs improvement on the sluggish dispersion of the Changjiang plume and insufficient vertical mixing. It also needs a more complicated biogeochemical model with more size classes of organisms, multiple-nutrient schemes and additional geochemical processes.

Ozone response to emission changes: a modeling study during the MCMA-2006/MILAGRO Campaign

Abstract. The sensitivity of ozone production to precursor emissions was investigated under five different meteorological conditions in the Mexico City Metropolitan Area (MCMA) during the MCMA-2006/MILAGRO field campaign using the gridded photochemical model CAMx driven by observation-nudged WRF meteorology. Precursor emissions were constrained by the comprehensive data from the field campaign and the routine ambient air quality monitoring network. Simulated plume mixing and transport were examined by comparing with measurements from the G-1 aircraft during the campaign. The observed concentrations of ozone precursors and ozone were reasonably well reproduced by the model. The effects of reducing precursor emissions on urban ozone production were performed for three representative emission control scenarios. A 50% reduction in VOC emissions led to 7 to 22 ppb decrease in daily maximum ozone concentrations, while a 50% reduction in NOx emissions leads to 4 to 21 ppb increase, and 50% reductions in both NOx and VOC emission decrease the daily maximum ozone concentrations up to 10 ppb. These results along with a chemical indicator analysis using the chemical production ratios of H2O2 to HNO3 demonstrate that the MCMA urban core region is VOC-limited for all meteorological episodes, which is consistent with the results from MCMA-2003 field campaign; however the degree of the VOC-sensitivity is higher during MCMA-2006 due to lower VOCs, lower VOC reactivity and moderately higher NOx emissions. Ozone formation in the surrounding mountain/rural area is mostly NOx-limited, but can be VOC-limited, and the range of the NOx-limited or VOC-limited areas depends on meteorology.

Formation of seep bubble plumes in the Coal Oil Point seep field

The fate of marine seep gases (transport to the atmosphere or dissolution, and either bacterial oxidation or diffusion to the atmosphere) is intimately connected with bubble and bubble-plume processes, which are strongly size-dependent. Based on measurements with a video bubble measurement system in the Coal Oil Point seep field in the Santa Barbara Channel, California, which recorded the bubble-emission size distribution (Φ) for a range of seep vents, three distinct plume types were identified, termed minor, major, and mixed. Minor plumes generally emitted bubbles with a lower emission flux, Q, and had narrow, peaked Φ that were well described by a Gaussian function. Major plumes showed broad Φ spanning very small to very large bubbles, and were well described by a power law function. Mixed plumes showed characteristics of both major and minor plume classes, i.e., they were described by a combination of Gaussian and power law functions, albeit poorly. To understand the underlying formation mechanism, laboratory bubble plumes were created from fixed capillary tubes, and by percolating air through sediment beds of four different grain sizes for a range of Q. Capillary tubes produced a Φ that was Gaussian for low Q. The peak radius of the Gaussian function describing Φ increased with capillary diameter. At high Q, they produced a broad distribution, which was primarily described by a power law. Sediment-bed bubble plumes were mixed plumes for low Q, and major plumes for high Q. For low-Q sediment-bed Φ, the peak radius decreased with increasing grain size. For high Q, sediment-bed Φ exhibited a decreased sensitivity to grain size, and Φ tended toward a power law, similar to that for major seep plumes.

An observational and modeling strategy to investigate the impact of remote sources on local air quality: A Houston, Texas, case study from the Second Texas Air Quality Study (TexAQS II)

Quantifying the impacts of remote sources on individual air quality exceedances remains a significant challenge for air quality forecasting. One goal of the 2006 Second Texas Air Quality Study (TexAQS II) was to assess the impact of distant sources on air quality in east Texas. From 23 to 30 August 2006, retrievals of tropospheric carbon monoxide (CO) from NASA's Atmospheric InfraRed Sounder (AIRS) reveal the transport of CO from fires in the United States Pacific Northwest to Houston, Texas. This transport occurred behind a cold front and contributed to the worst ozone exceedance period of the summer in the Houston area. We present supporting satellite observations from the NASA A‐Train constellation of the vertical distribution of smoke aerosols and CO. Ground‐based in situ CO measurements in Oklahoma and Texas track the CO plume as it moves south and indicate mixing of the aloft plume to the surface by turbulence in the nocturnal boundary layer and convection during the day. Ground‐based aerosol speciation and lidar observations do not find appreciable smoke aerosol transport for this case. However, MODIS aerosol optical depths and model simulations indicate some smoke aerosols were transported from the Pacific Northwest through Texas to the Gulf of Mexico. Chemical transport and forward trajectory models confirm the three major observations: (1) the AIRS envisioned CO transport, (2) the satellite determined smoke plume height, and (3) the timing of the observed surface CO increases. Further, the forward trajectory simulations find two of the largest Pacific Northwest fires likely had the most significant impact.