Horizontal Transport Processes Research Articles

High O3 pollution initiated by cold front passage over Pearl River Estuary

The influence of cold fronts on the formation of ozone (O3) pollution, particularly the contributions of physical and chemical processes to O3 abundance during the passage of cold fronts in winter over the Pearl River Estuary (PRE) region was still unclear. This study provided an in-depth analysis on the occurrence of high O3 pollution based on data from field measurements and simulation of a chemical transport model (i.e., WRF-Chem) over the PRE region during winter 2021 when cold fronts were frequently observed. Totally 7 high O3 episode days were observed during pre- and post-frontal stages at DWS (i.e., Da Wan Shan island), a downwind island site of the inland Pearl River Delta (PRD) region, which were accompanied with the prevailing northerly wind patterns with insignificant variations of wind speeds in a stable boundary layer over the whole PRE region. The quantitative analysis based on simulation results indicated that horizontal transport and chemical processes accounted for the buildup of O3 abundance at DWS, with the highest mean rates of 7.3 and 5.6 ppbv/h at pre- and post-frontal stages, respectively. The high contributions from horizontal transport to O3 levels at DWS were associated with the persistent northerly winds with small variations, while the contributions of chemical processes were varied with the abundance of precursors and meteorological conditions. However, vertical transport made negative contributions to O3 abundance, with the most significant mean diffusion rate of −2.6 ppbv/h at the frontal stage because of the control of high-pressure system, while the less influence of vertical transport at pre- and post-frontal stages was associated with the weak pressure gradient forces and wind patterns. The sensitivity analysis further demonstrated that the emissions of precursors from the inland PRD region contributed 50–76% to O3 abundance over PRE at different stages. Overall, the above results quantitatively confirmed the dominant role of transport from inland PRD to the O3 pollution over the downwind PRE region and were helpful for better understanding of O3 pollution in winter over PRE.

Characteristics and mechanism of a persistent ozone pollution event in Pearl River Delta induced by typhoon and subtropical high

Ozone (O3) is a significant air pollutant in the Pearl River Delta (PRD) region, with severe pollution events primarily associated with the Western Pacific Subtropical High (WPSH) and typhoon periphery synoptic weather patterns. To investigate the continuity and differences in O3 pollution processes influenced by these patterns, we analyze a severe O3 pollution event in Zhuhai from September 10 to 13, 2022. The study collects vertical profiles of O3 from the differential absorption O3 lidar and wind fields observed by the Doppler wind profile lidar. We utilize ground-level monitoring data from national ambient air quality monitoring network and meteorological stations to analyze diurnal variations of air pollutant concentrations and ground-level meteorological conditions. The study reveals that both the WPSH and the typhoon periphery create favorable meteorological conditions for severe O3 pollution events. The highest temperatures during the WPSH and typhoon periphery stages are 32.1 °C and 37.5 °C, respectively. During the WPSH control stage, the combined effects of the inversion layer and light static wind hinder O3 dispersion. The ground-level O3 concentration reaches a maximum of 106 ppb. Then the typhoon periphery control stage, the ground-level O3 peak increases to 170 ppb, showing a 60% increase compared to the WPSH stage. O3 horizontal and vertical transport processes play a crucial role in the O3 episode during this stage. The northerly winds predominantly transport pollutants from upstream urban clusters to the local area, and intense downdrafts facilitate the downward transport of O3 from the residual layer. The study investigates the mechanisms of three-dimensional O3 transport within the boundary layer influenced by the WPSH and typhoon periphery in the PRD region. The results contribute valuable insights to pollution events in the PRD region under typical synoptic weather patterns.

Modelling and simulation of subsurface horizontal flow constructed wetlands

As a consequence of the poor implantation of sanitation systems, more than 80% of the wastewater from human activity is dumped, which poses a threat to public health and generates water stress. In contrast, its depuration and reuse for irrigation improves the efficiency of the water management cycle and reduces the use of fresh water. In the context of grey water purification in rural areas of Europe, constructed wetlands are the preferred option. Furthermore, in developing countries, they are particularly attractive as an alternative to conventional purification systems. The design of such facilities tends to avoid the complexity of the interactions between solids, pollutants, microorganisms, and plants. In this work, a more rigorous mathematical model is proposed, which considers the mass balances of the substrate and the microorganisms, in addition to the degradation kinetics typical of biotechnological processes, the life cycles of microorganisms, and their horizontal and vertical transport processes. This model was tuned based on the responses of already installed systems located in Zapote (Costa Rica) and San Salvador (El Salvador). The model foresees start-up times close to three months, in addition to depuration efficiencies reaching above 90%, which corresponds with previously reported values. The developed simulation tool is also employed to study the response of the model to various disturbances, such as punctual stops or seasonal variations in the incoming flow.

Risk assessment of horizontal transport system in a copper mine

The risk assessment determines threats that could appear during the execution of production system goals. Thus, manufacturing companies need to evaluate and react to the risk as well as it is possible. Due to the complexity and variable character of the mining system, as well as different types of parameters obtained at subsequent stages of the horizontal transport process, the risk was assessed according to various methods. Three stages were distinguished in the horizontal transport system of copper ore studied in the paper: tyre haulage of the shot copper ore to the transfer point, the so-called grate, transferring the output to belt conveyors and crushing solid rock into smaller pieces; transport on belt conveyors. Risk factors have been characterized for each stage. To assess the risk of the loading and haulage processes, an ANN model was built to predict the amount of ore extracted. A general linear neural network model was also built to confirm the results of the correlation analysis. One of the methods recommended for risk assessment in manufacturing companies is the Failure Mode and Effects Analysis (FMEA), which allows for calculating the risk and prioritizing it. The FMEA method allows identifying only these elements of the production system that are the most sensitive to the impact of risk factors. For the next stages of the horizontal transport process, the risk was assessed according to the FMEA method. The risk levels at each stage were different. Production systems are composed of many elements that create the reliability structure of this system. Unfortunately, the FMEA method does not allow for analyzing the reliability structure of the production system. Therefore, the new idea presented in this paper is a method of total risk assessment of the horizontal transport system of copper ore, which does take its reliability structure into account. The currently used methods of risk analysis and assessment do not take into consideration the reliability structure of the production system. The proposed method can be applied for risk assessment in other production systems characterized by a diversified structure.

Arctic Lower-Tropospheric Warm and Cold Extremes: Horizontal and Vertical Transport, Diabatic Processes, and Linkage to Synoptic Circulation Features

AbstractThe thermodynamic processes and synoptic circulation features driving lower-tropospheric temperature extremes in the high Arctic (>80°N) are investigated. Based on 10-day kinematic backward trajectories from the 5% most intense potential temperature anomalies, the contributions of horizontal and vertical transport, subsidence-induced warming, and diabatic processes to the generation of the Arctic temperature anomaly are quantified. Cold extremes are mainly the result of sustained radiative cooling due to a sheltering of the Arctic from meridional airmass exchanges. This is linked to a strengthening of the tropospheric polar vortex, a reduced frequency of high-latitude blocking, and in winter also a southward shift of the North Atlantic storm track. The temperature anomaly of 60% of wintertime extremely warm air masses (90% in summer) is due to transport from a potentially warmer region. Subsidence from the Arctic midtroposphere in blocking anticyclones is the most important warming process with the largest contribution in summer (70% of extremely warm air masses). In both seasons, poleward transport of already warm air masses contributes around 20% and is favored by a poleward shift of the North Atlantic storm track. Finally, about 40% of the air masses in winter are of an Arctic origin and experience diabatic heating by surface heat fluxes in marine cold air outbreaks. Our study emphasizes the importance of processes in the Arctic and the relevance of anomalous blocking—in winter in the Barents, Kara, and Laptev Seas and in summer in the high Arctic—for the formation of warm extremes.

Connectivity Dynamics in Dryland Litter Cycles: Moving Decomposition beyond Spatial Stasis

AbstractDrylands (arid and semiarid ecosystems) cover nearly half of Earth's terrestrial surface, but biogeochemical pools and processes in these systems remain poorly understood. Litter can account for a substantial portion of carbon and nutrient pools in these systems, with litter decomposition exerting important controls over biogeochemical cycling. Dryland decomposition is typically treated as a spatially static process in which litter is retained and decomposed where it is initially deposited. Although this assumption is reasonable for mesic systems with continuous plant canopy cover and a stable subcanopy litter layer, dryland pools generally reflect discontinuous inputs from heterogeneous canopy cover followed by substantial litter transport. In the present article, we review horizontal and vertical transport processes that move litter from the initial deposition point and retention elements that influence litter accumulation patterns. Appreciation of the spatially dynamic litter cycle, including quantitative assessment of transport patterns, will improve estimates of the fate and distribution of organic matter in current and future drylands.

Representation of Horizontal Transport Processes in Snowmelt Modeling by Applying a Footprint Approach

The energy balance of an alpine snow cover significantly changes once the snow cover gets patchy. The local advection of warm air causes above-average snow ablation rates at the upwind edge of the snow patch. As lateral transport processes are typically not considered in models describing surface exchange, e.g. for hydrological or meteorological applications, small-scale variations in snow ablation rates are not resolved. The overall model error in the hydrological model Alpine3D is split into a contribution from the pure “leading edge effect” and a contribution from an increase in the mean air temperature due to a positive snow-albedo feedback mechanism. We found an overall model error for the entire ablation period of 4 % for the almost flat alpine test site Gletschboden and 14 % for the Wannengrat area, which is located in highly complex terrain including slopes of different aspects. Terrestrial laser scanning measurements at the Gletschboden test site were used to estimate the pure “leading edge effect” and reveal an increase in snow ablation rates of 25-30 % at the upwind edge of a snow patch and a total of 4-6 % on a catchment scale for two different ablation days with a snow cover fraction lower than 50 %. The estimated increase of local snow ablation rates is then around 1-3 % for an entire ablation period for the Gletschboden test site and approximately 4 % for the Wannengrat test site. Our results show that the contribution of lateral heat advection is smaller than typical uncertainties in snow melt modelling due to uncertainties in boundary layer parameters but increases in regions with smaller snow patch sizes and long-lasting patchy snow covers in the ablation period. We introduce a new temperature footprint approach, which reproduces a 15 % increase of snow ablation rates at the upwind edge of the snow patch, whereas observations indicate that this value is as large as 25 %. This conceptual model approach could be used in hydrological models. In addition to improved snow ablation rates, the footprint model better represents snow mask maps and turbulent sensible heat fluxes from eddy-covariance measurements.

Accumulation rates, focusing factors, and chronologies from depth profiles of 210Pb and 137Cs in sediments of the Laurentian Great Lakes

Abstract Sediment cores from 41 sites were collected from the Laurentian Great Lakes during 2010–2014, sectioned into 0.5–2.0 cm intervals, and the activities of 210Pb, 137Cs, and 226Ra were measured in the upper 25 to 40 cm of the sediment column by gamma spectrometry. Sediment mass accumulation rates (dry mass) calculated from 210Pb profiles range from 0.006 ± 0.001 to 0.59 ± 0.06 g cm−2 yr −1 and are similar to those reported in previous Great Lakes sediment studies. Sediment mass accumulation rates decreased with increasing water depth. 210Pb-based models in cores exhibiting favorable characteristics (i.e., those having the highest unsupported-210Pb activity at the sediment-water interface, exponential decrease of unsupported-210Pb with increasing depth in sediment cores, and a clear peak in 137Cs activity at some depth below the sediment-water interface) give calendar date profiles that are largely concordant with the maximum 137Cs peak activity at 1963. Sediment focusing factors derived from unsupported-210Pb inventories range from 0.09 to >5.34, and are well correlated with those derived from 137Cs inventories that range from 0.07 to 4.04, demonstrating the ubiquitous occurrence of horizontal sediment transport processes within the lakes. This more recent survey provides a Great Lakes-wide chronological framework for comparing the depositional histories and inventories of a wide variety of persistent, bioaccumulative and toxic pollutants that have been measured in the same sediment cores. This information will be useful for resolving scientific and practical issues pertaining to the environmental quality and management of contaminated sediments in the Laurentian Great Lakes ecosystem.

Vertical fluxes of coccolithophores and foraminifera and their contributions to CaCO3 flux off the coast of Ensenada, Mexico

Coccoliths and foraminifera fluxes and their contributions to CaCO3 were estimated by analyzing the material collected by a sediment trap installed in the area adjacent to Ensenada, Baja California (Mexico). Coccoliths were identified and counted from images obtained with a Zeiss-Supra 55VP scanning electron microscope. Foraminifera were identified and analyzed with a Leica EZ4 stereoscope. Total CaCO3 was determined by coulometry. A total of 33 species of coccoliths were recorded; the most abundant species were Emiliania huxleyi type A, Gephyrocapsa oceanica (Equatorial), E. huxleyi type B, and G. oceanica (Larger), contributing 74% of total coccolith flux. The maximum flux of coccoliths was 1,539 × 106 m–2·d–1 in sample A-8, the minimum zero in sample A-10, and the average 1,028 × 106 m–2·d–1. Coccolith contribution to CaCO3 content ranged from 0.14 mg·m–2·d–1 in sample A-12 to 25.00 mg·m–2·d–1 in sample A-1, with an average of 5.60 mg·m–2·d–1. Helicosphaera carteri made the highest contribution with 10.00 mg·m–2·d–1 in sample A-1, and the rest of the species contributed <7.00 mg·m–2·d–1. Six genera of planktonic foraminifera were identified: Globigerina, Neogloboquadrina, Globorotalia, Pulleniatina, Globigerinoides, and Orbulina. The maximum flux of foraminifera was recorded in summer (513 m–2·d–1) and the minimum in spring (9 m–2·d–1). Benthic foraminifera were found to make an important contribution, even greater than the contribution from planktonic foraminifera, reflecting horizontal resuspension and/or transport processes. CaCO3 contributions by coccoliths and foraminifera were <1% of total CaCO3, and the rest was contributed by fragments of the other calcareous groups.

Effects of the North Pacific Current on the productivity of 163 Pacific salmon stocks

AbstractHorizontal ocean transport can influence the dynamics of higher‐trophic‐level species in coastal ecosystems by altering either physical oceanographic conditions or the advection of food resources into coastal areas. In this study, we investigated whether variability in two North Pacific Current (NPC) indices was associated with changes in productivity of North American Pacific salmon stocks. Specifically, we used Bayesian hierarchical models to estimate the effects of the north‐south location of the NPC bifurcation (BI) and the NPC strength, indexed by the North Pacific Gyre Oscillation (NPGO), on the productivity of 163 pink, chum, and sockeye salmon stocks. We found that for salmon stocks located in Washington (WA) and British Columbia (BC), both the BI and NPGO had significant positive effects on productivity, indicating that a northward‐shifted bifurcation and a stronger NPC are associated with increased salmon productivity. For the WA and BC regions, the estimated NPGO effect was over two times larger than the BI effect for pink and chum salmon, whereas for sockeye salmon the BI effect was 2.4 times higher than the NPGO. In contrast to WA and BC stocks, we found weak effects of both horizontal ocean transport processes on the productivity of salmon stocks in Alaska. Our results indicated that horizontal transport pathways might strongly influence population dynamics of Pacific salmon in the southern part of their North American ranges, but not the northern part, suggesting that different environmental pathways may underlie changes in salmon productivity in northern and southern areas for the species under consideration.

The conservative characteristic FD methods for atmospheric aerosol transport problems

In the paper, we develop the new conservative characteristic finite difference methods (C-CFD) for the atmospheric aerosol transport problems. We propose the time second-order and spatial high-order conservative characteristic finite difference methods for the aerosol vertical advection-diffusion process and the two-dimensional conservative characteristic finite difference methods for aerosol horizontal transport process in the second-order splitting algorithm. Based on the characteristic form of advection-diffusion equations tracking back along the characteristic curve, we treat the integrals over the tracking cells at the previous time level by the conservative interpolations and propose to treat the diffusion terms by the average along the characteristics, where the high-order discrete fluxes are obtained by approximating the cumulative mass function and are continuous at the tracking points. The important feature is that the proposed C-CFD schemes preserve mass and have second-order accuracy in time and high-order accuracy in space. Numerical tests are taken to show the accuracy in time and space and mass conservation of our C-CFD schemes, compared with the standard CFD method. A real case of air quality modelling during the 2008 Beijing Olympics and a severe haze in North China are further simulated and analyzed by using our C-CFD algorithm. Simulated results are in good agreement with observations. The developed C-CFD algorithm can be used for efficiently solving large scale atmospheric aerosol transport problems.

Cyanobacteria in the complex river-connected Poyang Lake: horizontal distribution and transport

Bloom-forming cyanobacteria have been observed with increasing frequency in Poyang Lake, China since 2000. In the present study, we examined the mechanisms responsible for the horizontal distribution of these blooms in the main body of Poyang Lake. Specifically, we addressed the hypothesis that colonies of cyanobacteria are first generated in the Eastern Bay (EB, lentic region) and then advected offshore by large-scale horizontal transport processes, with the colonies subsequently found in Northern Poyang Lake (NPL, lotic region). The cyanobacteria biomass in the EB was significantly greater than that in NPL (P < 0.0001), and negative correlations were observed between cyanobacteria biomass and nutrient concentrations in the EB. However, significant correlations were not observed between cyanobacteria biomass and nutrient concentrations in NPL. The peak abundance of cyanobacteria was first observed in the EB, with the cyanobacterial abundance peak in NPL lagging the EB peak by approximately 1 month. These results are all consistent with the cyanobacteria distribution hypothesis and show the potential for the accumulation of cyanobacteria in NPL that are normally considered unsuitable for in situ growth in EB. Variations of cyanobacteria biomass in eutrophic lakes, including Yangtze-connected (Poyang Lake and Dongting Lake) and Yangtze-isolated (Taihu) lakes, were monitored during summer (July–August), and eutrophic or even polytrophic conditions were not observed to support the development of cyanobacteria. Instead, a high rate of water flow with short retention times was the key factor preventing the accumulation of cyanobacteria in these eutrophic lakes. Therefore, the mean cyanobacteria biomass was significantly lower in the Yangtze-connected lakes (Poyang Lake, 1.01 mg l−1; Dongting Lake, 1.71 mg l−1) than in Taihu Lake at Meiliang Bay (13.54 mg l−1) or the mouth (3.45 mg l−1) (P < 0.0001); however, the biomass more closely resembled Taihu Lake at the center (0.88 mg l−1). As expected, the cyanobacteria biomass was lower in the lakes with hydraulic connections to the Yangtze River compared with those isolated from the Yangtze River. This study revealed that hydrological parameters dominated the accumulation of cyanobacterial blooms in the Yangtze-connected eutrophic lakes in eastern China.

Influence of along-valley terrain heterogeneity on exchange processes over idealized valleys

Abstract. Idealized numerical simulations of thermally driven flows over various valley–plain topographies are performed under daytime conditions. Valley floor inclination and narrowing valley cross sections are systematically varied to study the influence of along-valley terrain heterogeneity on the developing boundary layer structure, as well as horizontal and vertical transport processes. Valley topographies with inclined valley floors of 0.86° increase upvalley winds by a factor of about 1.9 due to smaller valley volumes (volume effect) and by a factor of about 1.6 due to additional upslope buoyancy forces. Narrowing the valley cross section by 20 km per 100 km along-valley distance increases upvalley winds by a factor of about 2.6. Vertical mass fluxes out of the valley are strongly increased by a factor between 1.8 and 2.8 by narrowing the valley cross sections and by a factor of 1.2 by inclining the valley floor. Trajectory analysis shows intensified horizontal transport of parcels from the foreland into the valley within the boundary layer in cases with inclined floors and narrowing cross sections due to increased upvalley winds.

Modeling Fecal Indicator Bacteria Concentrations in Natural Surface Waters: A Review

Microbiological quality of waters must be assessed to ensure that no health risk due to pathogenic microorganisms is associated with its use. As it is impossible to measure the abundance of all possible pathogens, it is general practice to quantify the abundance only of one or a few fecal indicator bacteria (FIB), organisms which are selected to be indicative of fecal pollution and can therefore serve as general indicators of microbiological water quality. Yet, even by focusing on this limited number of indicator organisms, it is still unfeasible to experimentally monitor their levels at the high spatiotemporal resolution often needed in real applications. Therefore, direct FIB measurements are increasingly combined with the use of models. The aim of this review is to present and evaluate the wide variety of models used so far in the scientific literature to simulate and predict FIB concentrations in natural surface waters. First, the distinction is made between regression-based and mechanistic models. While the first are particularly useful in operational contexts and indeed can produce reliable short-term predictions, they do not allow an in-depth understanding of the processes. Because the sources and processes are not modeled explicitly, they cannot be used to test the effect of changes in these internal and external forcings, e.g., to evaluate the impact of different management options. These questions can only be addressed by the use of mechanistic models, which are often based on rather complex computational methods. These models explicitly consider the effect on FIB concentrations due to horizontal transport, external sources, decay, and/or sediment-related processes. It is not possible to make statements about “best” practices, given the broad range of study domains and questions. Instead, we attempted to compile the modeling approaches published so far in a comprehensive and transparent way, hoping that the resulting overview would help to better understand current models and more efficiently set up future ones.

Process analysis of a regional air pollution episode over Pearl River Delta Region, China, using the MM5-CMAQ model

This study focuses on the influences of a warm high-pressure meteorological system on aerosol pollutants, employing the simulations by the Models-3/CMAQ system and the observations collected during October 10–12, 2004, over the Pearl River Delta (PRD) region. The results show that the spatial distributions of air pollutants are generally circular near Guangzhou and Foshan, which are cities with high emissions rates. The primary pollutant is particulate matter (PM) over the PRD. MM5 shows reasonable performance for major meteorological variables (i.e., temperature, relative humidity, wind direction) with normalized mean biases (NMB) of 4.5–38.8% and for their time series. CMAQ can capture one peak of all air pollutant concentrations on October 11, but misses other peaks. The CMAQ model systematically underpredicts the mass concentrations of all air pollutants. Compared with chemical observations, SO2 and O3 are predicted well with a correlation coefficient of 0.70 and 0.65. PM2.5 and NO are significantly underpredicted with an NMB of 43% and 90%, respectively. The process analysis results show that the emission, dry deposition, horizontal transport, and vertical transport are four main processes affecting air pollutants. The contributions of each physical process are different for the various pollutants. The most important process for PM10 is dry deposition, and for NOx it is transport. The contributions of horizontal and vertical transport processes vary during the period, but these two processes mostly contribute to the removal of air pollutants at Guangzhou city, whose emissions are high. For this high-pressure case, the contributions of the various processes show high correlations in cities with the similar geographical attributes. According to the statistical results, cities in the PRD region are divided into four groups with different features. The contributions from local and nonlocal emission sources are discussed in different groups.Implications: The characteristics of aerosol pollution episodes are intensively studied in this work using the high-resolution modeling system MM5/SMOKE/CMAQ, with special efforts on examining the contributions of different physical and chemical processes to air concentrations for each city over the PRD region by a process analysis method, so as to provide a scientific basis for understanding the formation mechanism of regional aerosol pollution under the high-pressure system over PRD.

Impacts of the January 2005 solar particle event on noctilucent clouds and water at the polar summer mesopause

Abstract. The response of noctilucent clouds to the solar particle event in January 2005 is investigated by means of icy particle and ion chemistry simulations. It is shown that the decreasing occurrence rate of noctilucent clouds derived from measurements of the SCIAMACHY/Envisat instrument can be reproduced by one-dimensional model simulations if temperature data from the MLS/Aura instrument are used. The model calculations indicate that the sublimation of noctilucent clouds leads to significant changes of the water distribution in the mesopause region. These model results are compared with H2O measurements from the MLS and the MIPAS/Envisat satellite instruments. The pronounced modelled water enhancement below the icy particle layer and its decrease during the SPE are not observed by the satellite instruments. At altitudes &gt;85 km the satellite measurements show an increase of H2O during the SPE in qualitative agreement with the model predictions. The discrepancies between model H2O and observations at lower altitudes might be attributed to the one-dimensional model approach which in particular neglects inhomogeneities and horizontal transport processes. Additionally, it is revealed that the water depletion due to reactions of proton hydrates during the considered solar particle event has only a minor impact on the icy particles.

Diagnostic evaluation of ozone production and horizontal transport in a regional photochemical air quality modeling system

Abstract A diagnostic model evaluation effort has been performed to focus on photochemical ozone formation and the horizontal transport process since they strongly impact the temporal evolution and spatial distribution of ozone (O3) within the lower troposphere. Results from the Community Multiscale Air Quality (CMAQ) modeling system are evaluated against surface and upper air measurements from field studies during summer 2002 when several high O3 episodes occurred in the eastern United States. Modeled O3 and winds are compared to research aircraft measurements and wind profiler data, respectively, to investigate whether model underestimates of daily maximum 8-h ozone concentrations during high O3 episodes might be attributable to discrepancies in either or both of these modeled processes. Comparisons of 10 AM surface O3 concentrations, which are representative of O3 levels in the residual layer aloft, revealed that model underestimation was greater at higher observed ozone levels. Mid-morning vertical ozone profiles corroborated this surface-level finding, as modeled concentrations tended to be lower than observed O3 aloft. Net ozone production efficiency (OPE) results suggested photochemical ozone formation was comparable between the model and observations with composite OPE values of 6.7 and 7.6, respectively, within the afternoon planetary boundary layer. Evaluation of wind profiles revealed modeled wind speeds with the base four-dimensional data assimilation (FDDA) approach underestimated observed speeds by more than 2 m s−1 and direction was biased by about 20° in the nocturnal residual layer aloft as coarse resolution analysis fields involved in FDDA were found to inhibit modeled winds. These differences could produce large spatial displacements in modeled and observed ozone patterns within the region. Although sensitivity simulation results with the WRF meteorological model with FDDA using all available upper air profile observations displayed improvements in capturing wind fields aloft, CMAQ maximum 8-h O3 results using the improved wind fields also underestimated observations.

Influence of meteorological conditions on summer ozone levels in the central Kanto area of Japan

We investigated the influence of meteorological factors that affect ozone in summer using both measurement analysis and numerical simulation. The results show that there is a close relationship between changes in meteorological conditions and variations in ozone concentrations over the central Kanto area. In summer, up to 84% of long-term variations in peak ozone concentrations may be accounted for by changes in the seasonally averaged daily maximum temperatures and seasonally averaged wind speeds. The ozone episodes in the Kanto region are dominated by three major patterns, of which Patterns I and II are regular summertime pressure patterns with a 26% and 16% frequency of occurrence, respectively. A detailed process analysis of ozone formation under urban heat island (UHI) at two areas in the Kanto region – urban and rural area – indicates that ozone formation is mainly controlled by chemistry, dry deposition, vertical transport, and horizontal transport processes. The groundlevel ozone concentrations are enhanced mainly by the vertical mixing of ozone-rich air from aloft, whereas dry deposition process mainly depletes ozone. Horizontal transport and chemistry processes play opposite roles in the net change of ozone concentration between the two areas. The results of numerical simulations also indicate that the sea breeze has significant effects on the ozone accumulation and distribution in the Kanto area. The high ozone was first observed in urban area and then was transported to the rural area by sea breeze. At rural area, the highest ozone concentrations were found in late afternoon, about two hours later in comparison with the urban area.

Two-dimensional time-dependent model of the transport of minor species in the Venus night side upper atmosphere

We present a numerical tool developed to quantify the role of processes controlling the spatio-temporal distribution of the NO ultraviolet and O 2 ( Δ g 1 ) infrared nightglows in the Venus night side upper atmosphere, observed with the VIRTIS and SPICAV instruments on board Venus Express. This numerical tool consists in a two-dimensional chemical-transport time-dependent model which computes in a hypothetical rectangular solving domain the spatio-temporal distributions of the number densities of the four minor species at play in these two nightglow emissions. The coupled nonlinear system of the four partial differential equations, describing the spatio-temporal variations of the minor species, has been solved using a finite volume method with a forward Euler method for the time integration scheme. As an application, we have first simulated a time-constant supply of atoms through the upper boundary of the solving domain. The fluxes are inhomogeneous relative to its horizontal direction, in order to simulate regions of enhanced downward flow of oxygen and nitrogen giving rise to NO and O 2 brightening. Given that these two emissions show large time variations, we have also simulated a time-dependent downward flux of O and N atoms. It results from these simulations that the lack of correlation between the NO and O 2 ( Δ g 1 ) nightglows largely result from to the coupling between horizontal and vertical transport processes and the very different chemical lifetimes of the two species. In particular, we have quantified the role of each process generating spatio-temporal de-correlations between the NO and O 2 ( Δ g 1 ) nightglows.

Understanding of regional air pollution over China using CMAQ, part II. Process analysis and sensitivity of ozone and particulate matter to precursor emissions

Following model evaluation in part I, this part II paper focuses on the process analysis and chemical regime analysis for the formation of ozone (O 3) and particulate matter with aerodynamic diameter less than or equal to 10 μm (PM 10) in China. The process analysis results show that horizontal transport is the main contributor to the accumulation of O 3 in Jan., Apr., and Oct., and gas-phase chemistry and vertical transport contribute to the production and accumulation of O 3 in Jul. Removal pathways of O 3 include vertical and horizontal transport, gas-phase chemistry, and cloud processes, depending on locations and seasons. PM 10 is mainly produced by primary emissions and aerosol processes and removed by horizontal transport. Cloud processes could either decrease or increase PM 10 concentrations, depending on locations and seasons. Among all indicators examined, the ratio of P HNO 3 / P H 2 O 2 provides the most robust indicator for O 3 chemistry, indicating a VOC-limited O 3 chemistry over most of the eastern China in Jan., NO x-limited in Jul., and either VOC- or NO x-limited in Apr. and Oct. O 3 chemistry is NO x-limited in most central and western China and VOC-limited in major cities throughout the year. The adjusted gas ratio, AdjGR, indicates that PM formation in the eastern China is most sensitive to the emissions of SO 2 and may be more sensitive to emission reductions in NO x than in NH 3. These results are fairly consistent with the responses of O 3 and PM 2.5 to the reductions of their precursor emissions predicted from sensitivity simulations. A 50% reduction of NO x or AVOC emissions leads to a reduction of O 3 over the eastern China. Unlike the reduction of emissions of SO 2, NO x, and NH 3 that leads to a decrease in PM 10, a 50% reduction of AVOC emissions increases PM 10 levels. Such results indicate the complexity of O 3 and PM chemistry and a need for an integrated, region-specific emission control strategy with seasonal variations to effectively control both O 3 and PM 2.5 pollution in China.