Ozone Increment Research Articles

Comprehensive study on air-quality impacts from chemical plant flare emissions under planned or emergency shutdown operations†

Chemical plant shutdown (CPS) operations will employ a flare system to combust waste, unwanted, and off-spec gases dumped from process units to protect the plant safety and the local community. However, the incomplete combustion of flare source under CPS operations including plant planned shutdown (PPS) and plant emergency shutdown (PES) will cause considerable air pollution events. Hitherto, quantitative studies on both primary (CO and NO2) and secondary (ozone) pollutant impacts induced by CPS flaring are still lacking. In this paper, the dynamic effect of flare emissions from two types of CPS (PPS and PES) on ambient air-quality impacts will be systematically and quantitatively studied. The study integrates the dynamic process simulation via Aspen Plus Dynamics with the regional air-quality modelling via Comprehensive Air-quality Model with extensions (CAMx). Case studies indicated that the primary pollutants from flare emissions will be significantly diluted due to the atmospheric transportation, and the maximum concentrations of CO and NO2 can respectively reach 119.0 and 11.0 ppb under PPS, and 47.5 and 4.5 ppb under PES. Meanwhile, the increasing height of the flaring stack will decrease CO and NO2 concentrations but increase the ozone concentration under CPS. Furthermore, CPS in daytime periods will greatly promote ozone generations due to the enhanced photochemical reactions under solar radiation. The maximum 1-hr and 8-hr ozone increment can be 16.6 and 7.1 ppb under PPS while 10.7 and 3.3 ppb under PES, respectively. Besides, the decreasing of flare destruction and removal efficiency will increase ozone and CO under PES.

Effect of springtime thermal forcing over Tibetan Plateau on summertime ozone in Central China during the period 1950–2019

In recent years, ozone pollution has attracted wide attention from the scientific community, policymakers and the public. Previous studies have already confirmed the effects of meteorological conditions on regional ozone pollution events in the last two decades. However, due to the lack of historical ozone observations, the relationship of climate variability and ozone pollution has been less discussed. In our study, we use the NCEP/NCAR Reanalysis data and modeled historical surface ozone from CMIP6 (Coupled Model Intercomparison Project Phase 6) to explore the impact of climate variability related to Tibetan Plateau on summertime (June, July and August) ozone in Central China during 1950–2019. The results show that in interdecadal timescale, the annual summertime ozone over Central China is significantly correlated (R = −0.52; P-value <0.05) with the springtime (March, April and May) thermal forcing, indicated by total atmospheric energy (TPE), over Tibetan Plateau. In high TPE years, the East Asian monsoon (southerly wind flow) and vertical ascending motion have been enhanced in summer. It is conducive to both the diffusion of ozone and its precursors and occurrence of cloudy and high humidity weather. These processes lead to lower ozone concentrations up to 6.97 ppb (compared with values averaged during 1950–2019) over Central China. On the contrary, in low TPE years, summertime surface ozone concentrations have been increased by 2.96 ppb (the mean value is 49.00 ppb and the standard deviation is 11.75 ppb for 1950–2019) on average of Central China, due to the weakened East Asian monsoon and weakened vertical ascent, thus the stable weather with low humidity and small near surface wind. After the year 2000, the TPE has increased with a rapid growth rate of 3 × 104 hPa J kg−1 yr−1. However, similar to the reported surface ozone increments based on the modeling results and observations, the CMIP6 summertime surface ozone has shown an increasing trend in the last two decades (0.45 ppb yr−1 and 0.25 ppb yr−1 over Central China and entire China during 1950–2019, respectively). While a decline trend (0.18 ppb yr−1) of ozone from 1980 to 2019 over Central China is calculated after the removal of emission-attributed trend. These results indicate that the ozone increasing trend dependent on the changes of precursor emissions has offset the ozone mitigation attributed to the climate.

Effect of dynamic low DREs from flare combustion on regional ozone pollution during a chemical plant shutdown

The destruction and removal efficiencies (DREs) for industrial flare combustion could be, in reality, less than the supposed standard values of 98%/99% because of various atmospheric and plant operating conditions. Thus, flaring during chemical plant shutdown (CPS) under low DREs would release larger quantities of VOCs and NOx than expected, which might rapidly worsen the regional ozone pollution under solar radiation. Therefore, it is vital to examine the quantity and sensitivity of ozone impacts owing to low DREs for flare combustion rather than standard values. In this paper, effect of dynamic low DREs on regional ozone impacts during CPS flaring has been systematically conducted by coupling Aspen Plus with CAMx modeling and simulation. Case studies indicated that 8-hr ozone caused by CPS flaring under low DREs could range from 6.08 to 7.28 ppb, which was much greater than that based on the standard DREs ranging from 1.8 to 2.19 ppb. This study also demonstrated that the 8-hr ozone increment could be significantly reduced from the maximum of 6.14 ppb to the minimum of 0.85 ppb by starting the CPS operations at the optimal time. Another important finding was that ozone impacts might slightly increase with the increase of flare stack height due to meteorological conditions including high wind speed and strong solar radiation. This study would provide scientific support for quantitative ozone evaluation caused by CPS flare emissions, which will enrich future solutions for cost-effective regional air-quality management and ozone pollution control.

Effect of industrial flare DREs derived by CFD and WERF on ozone pollution through CAMx simulation

Abstract Flare is the last safety measure for daily operations in oil, gas & chemical process industries (OGCPI). However, an excessive flaring releases large quantity of emissions of VOCs and NOx, which may suddenly enhance local ozone as a secondary pollution. Normally, the flare destruction and removal efficiency (DRE) of 98% or 99% is regulated as the national standard and presumed for industrial practices in the U.S. Unfortunately, real DRE values could be much lower than the standard due to impact factors including various meteorological and operating conditions such as the cross-wind speed, flare jet velocity and heating value of combustion. Thus, it is critically important to explore the sensitivity of the regional ozone impact due to low DREs of OGCPI flare combustions. In this paper, a systematic methodology has been developed to examine ozone impacts due to the low flare DREs, which have never been systematically studied before. The DRE formulas were derived from computational fluid dynamic (CFD) modeling and Water Environment Research Foundation (WERF) results and then employed to recompile the point source emission inventory. After that, comprehensive air quality model with extensions (CAMx) was employede to simulate and quantify local ozone changes impacted by flare emissions of OGCPI. Case studies indicate that the maximum hourly ozone increments due to the low DRE through CFD and WERF modeling is 0.18 ppb and 1.3 ppb, respectively. This study could enrich fundamental understandings of industrial point source emissions and provide the quantitative and valuable support for the ozone pollution caused by OGCPI flare emissions under low DRE instead of standard values.

WITHDRAWN: Effect of Flare destruction and removal efficiencies on regional ozone pollution from oil, gas and chemical process industries in Southeast of Texas through CAMx modeling and simulation

Flare is the last safety measure for daily operations in oil, gas & chemical process industries (OGCPI). However, an excessive flaring releases large quantity of emissions of VOCs and NOₓ, which may suddenly enhance local ozone as a secondary pollution. Normally, the flare destruction and removal efficiency (DRE) of 98% or 99% is regulated as the national standard and presumed for industrial practices in the U.S. Unfortunately, real DRE values could be much lower than the standard due to impact factors including various meteorological and operating conditions such as the cross-wind speed, flare jet velocity and heating value of combustion. Thus, it is critically important to explore the sensitivity of the regional ozone impact due to low DREs of OGCPI flare combustions. In this paper, a systematic methodology has been developed to examine ozone impacts due to the low flare DREs, which have never been systematically studied before. The DRE formulas were derived from computational fluid dynamic (CFD) modeling and Water Environment Research Foundation (WERF) results and then employed to recompile the point source emission inventory. After that, comprehensive air quality model with extensions (CAMx) was employede to simulate and quantify local ozone changes impacted by flare emissions of OGCPI. Case studies indicate that the maximum hourly ozone increments due to the low DRE through CFD and WERF modeling is 0.18 ppb and 1.3 ppb, respectively. This study could enrich fundamental understandings of industrial point source emissions and provide the quantitative and valuable support for the ozone pollution caused by OGCPI flare emissions under low DRE instead of standard values.

Standard and alternative procedures for projecting future ozone in the Houston area using relative reduction factors

Abstract Air quality model results are used in a relative sense as part of the process to demonstrate attainment of the U.S. ozone standard. Ozone model predictions for a future year are divided by corresponding model results for a base year to obtain a Relative Reduction Factor (RRF), and the base-year measured ozone design value (DV) is multiplied by the RRF to estimate the future-year design value (DVF). We developed alternative RRFs and DVFs that focus on the increment of local ozone production above regional background (RB). An advantage of the alternative DVFs is that measurements can be used to determine RB ozone, thereby increasing the use of measurements in the DVF estimates. We simulated ozone formation in the Houston area for May–September 2012 and 2028 using the Comprehensive Air Quality Model with Extensions and compared the standard and alternative RRFs and DVFs. If model results are used for all the RB ozone concentrations in our new equation to calculate DVFs, the alternative and standard DVFs are very similar for Houston-Galveston-Brazoria (HGB). If measurements are used for the RB ozone concentrations we combine with the base-year DVs, the alternative DVFs are noticeably larger than the standard DVFs for about half the sites, with the increase being 7 ppb at the site (Deer Park) with the largest standard and alternative DVFs. The differences arise primarily from including different days in the averaging process for the RRFs versus the base-year DVs. We evaluated model performance for the days included in the RRFs and found that the model over-predicted RB ozone (normalized mean bias (NMB) = 26%) and under-predicted the anthropogenic ozone increment (NMB = -22%) from HGB emissions and therefore can be expected to under-estimate ozone response to HGB emission controls.

Atmospheric burden of ozone depleting substances (ODSs) and forecasting ozone layer recovery

The global ozone layer has been continuously depleted in the last few decades due to the anthropogenic emissions of the ozone depleting substances (ODSs). This has created major concern for the ecosystem as well as mankind. This points to the need to examine the influence of each individual ODS in order to estimate how this burden can change in the future. To that end, we have analysed 52 years (1965–2017) data of the ozone depleting substances using the Box-Jenkins model, and have predicted the future trend of the ODSs until the year 2030. On the basis of the relationship between the trend in variations of the ozone depleting substances and the total ozone column, we have established that how much of the recent ozone increment is attributable to the concentration of the ozone depleting substances. By using the above mentioned model, we have estimated that the present atmospheric abundance of chlorine and bromine is about 3325 and 14.3 parts per trillion (ppt) respectively and have calculated that the atmospheric abundance of these substances will reduce to values of 3217 ppt (∼3.2%) and 13.1 ppt (∼8.3%) respectively. We have also inferred that if the decreasing trend of ODSs continues with the same rate in the coming decades, the ozone column will increase by 9–10% and the ozone level will return to its pre-1980 level by the year 2030.

New insight of ozone pollution impact from flare emissions of chemical plant start-up operations

Flaring is a common and necessary operation for chemical industries, which is designed to manage dangerous process overpressure scenarios or to release and destroy off-spec products during chemical plant upsets or turnarounds. However, excessive flaring can emit large quantities of VOCs and NOx into the atmosphere, which will cause transient and localized ozone pollution events in the presence of sunlight. The objective of this study was to quantify the impact to regional air-quality due to flare emissions from chemical plant start-up operations through the coupling of dynamic process simulations via Aspen Plus and air-quality simulations via CAMx. Simulation results from case studies have indicated that the corresponding ozone increments can vary significantly from 0.2 ppb to 17.8 ppb under different temporal and spatial factors, including the start-up starting hour, starting day, and plant location. Additional ozone sensitivity simulations have also indicated that the corresponding ozone increments are higher when the plant is located in a VOC-limited area than that in a NOx-limited area. The results from this study have delivered a cost-effective air-quality control practice for plant start-ups with a minimum air-quality impact through selecting the optimal starting time within the allowable ranges. The practice has significant potential to benefit all stakeholders, including environmental agencies, chemical industries, and local communities.

Study on regional air quality impact from a chemical plant emergency shutdown

Emergency shutdowns of chemical plants (ESCP) inevitably generate intensive and huge amounts of VOCs and NOx emissions through flaring that can cause highly localized and transient air pollution events with elevated ozone concentrations. However, quantitative studies of regional ozone impact due to ESCP, in terms of how ESCP would affect and to what extent ESCP could impact, are still lacking. This paper reports a systematic study on regional air quality impact from an olefin plant emergency shutdown due to the sudden failure of its cracked gas compressor (CGC). It demonstrates that emergency shutdown may cause significant ozone increment subject to different factors such as the starting time of emergency shutdown, flare destruction and removal efficiency (DRE) and plant location. In our studied case, the 8-hr ozone increment ranges from 0.4 to 3.3 ppb under different starting time, from 3.3 to 24.8 ppb under different DRE, and from 1.6 to 3.3 ppb under different locations. The results enable us to understand how and to what extent emergency operating activities of the chemical process will affect local air quality, which might be beneficial for decision makings on emergency air-quality response and control in the future.

Contributions of foreign, domestic and natural emissions to US ozone estimated using the path-integral method in CAMx nested within GEOS-Chem

Abstract. The Goddard Earth Observing System global chemical transport (GEOS-Chem) model was used at 2° × 2.5° resolution to simulate ozone formation for a base case representing year 2010 and a natural background case without worldwide anthropogenic emissions. These simulations provided boundary concentrations for base and natural background simulations with the Comprehensive Air Quality Model with Extensions (CAMx) on a North American domain (one-way nested) at 12 km × 12 km resolution over March–September 2010. The predicted maximum daily average 8 h (MDA8) background ozone for the US is largest in the mountainous areas of Colorado, New Mexico, Arizona, and California. The background MDA8 ozone in some of these locations exceeds 60 ppb, when averaged over the 10 days with the largest base-case ozone (T10base average). The background ozone generally becomes both a larger fraction of the base-case ozone in the western US and a smaller fraction in the eastern US when proceeding from spring to summer to the T10base average. The ozone difference between the base and background cases represents the increment to ozone from all anthropogenic sources. The path-integral method was applied to allocate this anthropogenic ozone increment to US anthropogenic emissions, Canadian/Mexican anthropogenic emissions, and the anthropogenic components of the lateral and top boundary concentrations (BCs). Using the T10base average MDA8 ozone, the relative importance of the sources is generally US emissions &gt; anthropogenic lateral BCs &gt; Canadian/Mexican emissions ≫ anthropogenic top BCs. Specifically, for 10 US urban areas, the source contributions were 12–53 ppb for US emissions, 3–9 ppb for lateral BCs, 0.2–3 ppb for Canadian/Mexican emissions, and ≤ 0.1 ppb for top BCs. The contributions of the lateral BCs are largest for the higher-elevation US sites in the Intermountain West and along the western boundary of the domain. In Denver, for example, the lateral BCs contribute 44 % to the T10base anthropogenic increment. Averaging over the 10 days in the background case with the largest MDA8 ozone (T10bkgd), the contribution from US emissions is reduced to 3–13 ppb at the 10 urban sites, leading to a reduction in the anthropogenic ozone increment. The contribution of the Canadian/Mexican emissions remains about the same (0.3–2 ppb), and the contribution from the lateral BCs increases (5–10 ppb), especially at the lower-elevation urban sites. The net effect at the urban sites is that the relative importance of the anthropogenic lateral BCs is significantly greater for the T10bkgd average than the T10base average. This is also true for rural sites studied, where the anthropogenic lateral BC contribution becomes as large as 68 % of the anthropogenic increment for the T10bkgd average. In addition to the source apportionment, we also used surface and ozonesonde measurements to evaluate GEOS-Chem and CAMx performance.

Impact of chemical plant start-up emissions on ambient ozone concentration

Flare emissions, especially start-up flare emissions, during chemical plant operations generate large amounts of ozone precursors that may cause highly localized and transient ground-level ozone increment. Such an adverse ozone impact could be aggravated by the synergies of multiple plant start-ups in an industrial zone. In this paper, a systematic study on ozone increment superposition due to chemical plant start-up emissions has been performed. It employs dynamic flaring profiles of two olefin plants’ start-ups to investigate the superposition of the regional 1-hr ozone increment. It also summaries the superposition trend by manipulating the starting time (00:00–10:00) of plant start-up operations and the plant distance (4–32 km). The study indicates that the ozone increment induced by simultaneous start-up emissions from multiple chemical plants generally does not follow the linear superposition of the ozone increment induced by individual plant start-ups. Meanwhile, the trend of such nonlinear superposition related to the temporal (starting time and operating hours of plant start-ups) and spatial (plant distance) factors is also disclosed. This paper couples dynamic simulations of chemical plant start-up operations with air-quality modeling and statistical methods to examine the regional ozone impact. It could be helpful for technical decision support for cost-effective air-quality and industrial flare emission controls.

Impacts of flare emissions from an ethylene plant shutdown to regional air quality

Critical operations of chemical process industry (CPI) plants such as ethylene plant shutdowns could emit a huge amount of VOCs and NOx, which may result in localized and transient ozone pollution events. In this paper, a general methodology for studying dynamic ozone impacts associated with flare emissions from ethylene plant shutdowns has been developed. This multi-scale simulation study integrates process knowledge of plant shutdown emissions in terms of flow rate and speciation together with regional air-quality modeling to quantitatively investigate the sensitivity of ground-level ozone change due to an ethylene plant shutdown. The study shows the maximum hourly ozone increments can vary significantly by different plant locations and temporal factors including background ozone data and solar radiation intensity. It helps provide a cost-effective air-quality control strategy for industries by choosing the optimal starting time of plant shutdown operations in terms of minimizing the induced ozone impact (reduced from 34.1 ppb to 1.2 ppb in the performed case studies). This study provides valuable technical supports for both CPI and environmental policy makers on cost-effective air-quality controls in the future.

The impact of speciated VOCs on regional ozone increment derived from measurements at the UK EMEP supersites between 1999 and 2012

Abstract. The impact of 27 volatile organic compounds (VOCs) on the regional O3 increment was investigated using measurements made at the UK EMEP supersites Harwell (1999–2001 and 2010–2012) and Auchencorth (2012). Ozone at these sites is representative of rural O3 in south-east England and northern UK, respectively. The monthly-diurnal regional O3 increment was defined as the difference between the regional and hemispheric background O3 concentrations, respectively, derived from oxidant vs. NOx correlation plots, and cluster analysis of back trajectories arriving at Mace Head, Ireland. At Harwell, which had substantially greater regional O3 increments than Auchencorth, variation in the regional O3 increment mirrored afternoon depletion of anthropogenic VOCs due to photochemistry (after accounting for diurnal changes in boundary layer mixing depth, and weighting VOC concentrations according to their photochemical ozone creation potential). A positive regional O3 increment occurred consistently during the summer, during which time afternoon photochemical depletion was calculated for the majority of measured VOCs, and to the greatest extent for ethene and m+p-xylene. This indicates that, of the measured VOCs, ethene and m+p-xylene emissions reduction would be most effective in reducing the regional O3 increment but that reductions in a larger number of VOCs would be required for further improvement. The VOC diurnal photochemical depletion was linked to anthropogenic sources of the VOC emissions through the integration of gridded anthropogenic VOC emission estimates over 96 h air-mass back trajectories. This demonstrated that one factor limiting the effectiveness of VOC gridded emissions for use in measurement and modelling studies is the highly aggregated nature of the 11 SNAP (Selected Nomenclature for Air Pollution) source sectors in which they are reported, as monthly variation in speciated VOC trajectory emissions did not reflect monthly changes in individual VOC diurnal photochemical depletion. Additionally, the major VOC emission source sectors during elevated regional O3 increment at Harwell were more narrowly defined through disaggregation of the SNAP emissions to 91 NFR (Nomenclature for Reporting) codes (i.e. sectors 3D2 (domestic solvent use), 3D3 (other product use) and 2D2 (food and drink)). However, spatial variation in the contribution of NFR sectors to parent SNAP emissions could only be accounted for at the country level. Hence, the future reporting of gridded VOC emissions in source sectors more highly disaggregated than currently (e.g. to NFR codes) would facilitate a more precise identification of those VOC sources most important for mitigation of the impact of VOCs on O3 formation. In summary, this work presents a clear methodology for achieving a coherent VOC, regional-O3-impact chemical climate using measurement data and explores the effect of limited emission and measurement species on the understanding of the regional VOC contribution to O3 concentrations.

Path-integral method for the source apportionment of photochemical pollutants

Abstract. A new, path-integral method is presented for apportioning the concentrations of pollutants predicted by a photochemical model to emissions from different sources. A novel feature of the method is that it can apportion the difference in a species concentration between two simulations. For example, the anthropogenic ozone increment, which is the difference between a simulation with all emissions present and another simulation with only the background (e.g., biogenic) emissions included, can be allocated to the anthropogenic emission sources. The method is based on an existing, exact mathematical equation. This equation is applied to relate the concentration difference between simulations to line or path integrals of first-order sensitivity coefficients. The sensitivities describe the effects of changing the emissions and are accurately calculated by the decoupled direct method. The path represents a continuous variation of emissions between the two simulations, and each path can be viewed as a separate emission-control strategy. The method does not require auxiliary assumptions, e.g., whether ozone formation is limited by the availability of volatile organic compounds (VOCs) or nitrogen oxides (NOx), and can be used for all the species predicted by the model. A simplified configuration of the Comprehensive Air Quality Model with Extensions (CAMx) is used to evaluate the accuracy of different numerical integration procedures and the dependence of the source contributions on the path. A Gauss–Legendre formula using three or four points along the path gives good accuracy for apportioning the anthropogenic increments of ozone, nitrogen dioxide, formaldehyde, and nitric acid. Source contributions to these increments were obtained for paths representing proportional control of all anthropogenic emissions together, control of NOx emissions before VOC emissions, and control of VOC emissions before NOx emissions. There are similarities in the source contributions from the three paths but also differences due to the different chemical regimes resulting from the emission-control strategies.

Economic and health benefits of the co-reduction of air pollutants and greenhouse gases

This study examines the monetary value of social costs—private costs and negative externalities—that could be avoided by energy conservation actions. A novel Air Resource Co-Benefits (ARCoB) model has been developed in this study to assess the comprehensive social benefits of greenhouse gas GHG mitigation policy. The rollback model is used to estimate changes in air pollutant concentrations attributed to the emission reductions. Change in ozone concentration is converted from non-methane hydrocarbons based on the maximum ozone increment reactivity. In addition to saved medical expenditure, years of potential life lost (YPLL) is estimated based on the exposure-response coefficients for mortality and is calculated with abridged life table. Two cases of energy efficiency improvement in different scales are analyzed to estimate the annual co-benefits of abatements of air pollutants and greenhouse gas in 2009: 1) the energy intensity reduction in the industrial sector and 2) energy saving at Taipei Taiwan City Hall. Results indicate the saved energy cost accounted for 66 % and 70 % in the first and second case, respectively, and was a major part of the total benefit from energy conservation. The saved air pollution fee was 7.8–8.5 times lower than the averted health cost of medical expenditure, which was US$10.34 million in the first case, in which there were also averted YPLL of 3,478 person-years or averted deaths of 311 persons per year.

On the Merits of Using a 3D-FGAT Assimilation Scheme with an Outer Loop for Atmospheric Situations Governed by Transport

Abstract Three-dimensional variational data assimilation (3D-Var) with the first guess at appropriate time (FGAT) appears to be an attractive compromise between accuracy and overall computing time. It is computationally cheaper than four-dimensional (4D)-Var as the increment is not propagated back and forth in time by a model, yet the comparison between the model and the observations is still computed at the right observation time. An interesting feature of the 4D-Var is the iterative process known as the outer loop. This outer-loop approach can also be used in conjunction with 3D-FGAT. But it requires the application of the 3D-FGAT analysis increment at the beginning of the assimilation window. The pros and cons of using this unusual 3D-FGAT variant are illustrated in this paper on two applications focused on the transport, one of the main phenomena governing the atmospheric evolution. The first one is the one-dimensional advection of a passive tracer. By three representative situations, it shows the benefits of the outer loop, except for practical situations driven by very rapid dynamics such as a zonal wind of 50 m s−1 on the earth’s great circle, when the assimilation window has a size of 3 h. The second application is the 3D-FGAT assimilation of true ozone measurements into a chemical–transport model. It confirms the previous results, showing that the 3D-FGAT analysis with the outer loop produces an overestimation of the ozone increment in regions where the wind speed is high compared to the time length of the assimilation window.

Association of the vertical ozone structure with the lower‐stratospheric circulation

This study aims to examine the role of the atmospheric circulation in the lower stratosphere in relation to the vertical ozone distribution. This analysis is based on the vertical ozone profiles collected by balloon‐borne sondes released at Athens, Greece (38° N, 24° E) during the period 2002–2005 and those derived from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) observations of ENVISAT. The most pronounced special features of the ozone structure have been used in order to create groups of relevant profiles. The occurrence of the laminated ozone structure, the minimum of ozone partial pressure at the height region of 14–17 km and the ozone minima at the height region of 20–25 km as well as the scalable ozone increment from the region of the characteristic ozone minimum up to the ozone maximum concentration region, correlated with the general circulation, leads to several results. They are (1) the laminated ozone profiles are associated with the north‐north‐west circulation in the lower stratosphere; (2) the characteristic ozone minimum is related to the influence of the subtropical jet stream circulation; (3) the observed ozone depletion at the height region of 20–25 km is characterized by the movement of the polar vortex to the mid‐latitudes, resulting in more intense north‐western circulation above our experimental site; and (4) the scalable ozone increment from the characteristic ozone minimum region at 100 hPa up to the region where the ozone partial pressure takes its maximum value was observed on the southern side of the trough during cyclonic flow.

Assimilation of MIPAS limb radiances in the ECMWF system. II: Experiments with a 2‐dimensional observation operator and comparison to retrieval assimilation

AbstractThis study applies and compares three different ways of assimilating MIPAS data in a global numerical weather prediction system. The three methods are: direct assimilation of emitted infrared limb radiances with a 1‐dimensional radiative transfer model that assumes local horizontal homogeneity; direct assimilation of radiances with a 2‐dimensional radiative transfer model that takes into account horizontal gradients in the atmosphere; and assimilation of retrieved profiles of temperature, humidity and ozone, derived under the assumption of horizontal homogeneity. The resulting analyses are compared against each other and against independent retrievals.The use of a 2‐dimensional radiative transfer model in the radiance assimilation leads to relatively small differences in the analyses compared to using a 1‐dimensional observation operator in our experiments. Nevertheless, the results show that the 2‐dimensional operator correctly takes into account the effect of tangent point drift, is capable of extracting a limited amount of horizontal structure from a single MIPAS scan, and leads to smaller first‐guess departures for lower tangent altitudes and more strongly absorbing channels. Consequently, humidity and ozone increments from a 2‐dimensional operator are smaller in the lower stratosphere and upper troposphere in areas where considerable horizontal gradients prevail, and forecasts of humidity and ozone are improved in these regions.In these first trials with assimilation of limb radiances, both the radiance and the retrieval assimilation appear capable of incorporating useful information from MIPAS in the analyses. Both methods introduce broadly similar changes to the mean analyses, with little indication of which method should be favoured. Nevertheless, results from the retrieval assimilation compare better to independent ozone data in the Tropics and over Antarctica. For the radiance assimilation, there are many areas with scope for improvement; these are discussed at the end of the paper. Copyright © 2007 Royal Meteorological Society

Surveillance of acute health effects of air pollution in Mexico City.

A unique, active, timely, low-cost surveillance system for the metropolitan area of Mexico City was established in the mid-1990s. The system obtained upper and lower respiratory tract symptoms and eye symptoms from daily interviews for a systematic sample of the general nonhospitalized population living within a 2-km radius of air pollution monitors during 1996-1997. Ozone increments (10 ppb) were associated with upper respiratory symptoms (odds ratio [OR] = 1.003; 95% confidence interval [CI] = 1.002-1.004) and ocular symptoms indicators (OR = 1.005; CI = 1.004-1.007), and with a higher risk of lower respiratory symptoms indicator among nonsmokers (OR = 1.003; CI = 1.002-1.005). Increases in relative humidity reduced the risk of increments of sulfur dioxide on the 3 acute health indicators. Association of PM10 with health indicators varied among the 5 regions. During emergency episodes, symptoms increased sharply when ozone reached 281 ppb, a finding that resulted in a change in the ozone criteria for emergency declaration from 294 to 281 ppb. This system has been low cost, timely, and useful for local decision making.

The ozone increments in urban plumes

The role of urban plumes in the exposure of suburban/rural areas to pollution by ozone is discussed, and literature on direct measurement of ozone within plumes reviewed. There is a virtual absence of reports of ozone destruction within a NOx-rich urban plume and reasons are given for this gap in the current published data. These negative ozone increments are important because the particular air quality problems encountered downwind of a city will depend on its character as a source or sink of ozone. Simple plume reconstruction methods are used to verify and extend data from measurements in the London plume. There is an optimum time for air-parcel release with respect to increased plume ozone, and a change in the typical diurnal variation of ground-level ozone concentrations when the receptor site is situated within an urban plume. On a surface constructed from air parcel emission times and times travelled downwind of London, a major peak in plume ozone increment is observed 6–8 h downwind of a post rush-hour release in the urban centre. A secondary maximum in the surface is also apparent for air parcels 2–3 h after release in the early afternoon. These are not the emission and travel times usually adopted in modelling studies of the urban plume from London. Sites in the urban plume are also found to have a statistically different diurnal variation compared to rural/suburban sites which are not influenced by the plume. This is due to the titration of ozone mixing into the plume from aloft by plume NO.