Outdoor air pollution: keys to understand, inform and prevent

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Outdoor air quality is now a major challenge for society. This article aims to clarify the phenomena involved in air pollution episodes and describe their health effects and economic impacts. This awareness has led national authorities and the European Union to offer air quality monitoring and forecasting services, such as the Prev’air and Copernicus Atmosphere Regional Systems.

According to the latest figures from the WHO and Santé Publique France, the combined effects of outdoor and indoor air pollution would be responsible, in 2012, for the premature death of 6.5 million people worldwide and 600,000 in Europe [1]. While ambient air pollution is a very old phenomenon, a consequence of human concentrations in urban areas (read Air pollution), today it is now a major challenge for public health as well as the environment (see Figure 1).

vallee de la garonne - pic pollution - pollution vallee de la garonne - encyclopedie environnement - air pollution - pollution measurement traffic roads
Figure 1. Reduced visibility in the Garonne valley on a day of particulate pollution, and measurements taken on car traffic that same day (sign on the A64 south of Toulouse). 25 February 2018. [Source: © M. Pithon and S. Guidotti]
The Air and Rational Use of Energy Act of 30 December 1996 defines air pollution as “the introduction by man, directly or indirectly, into the atmosphere and enclosed spaces, of substances with harmful consequences likely to endanger human health, harm biological resources and ecosystems, influence climate change, damage material property and cause excessive odor nuisance“.

Pollutant emitting sources can be of anthropogenic or natural origin. Air quality then results from complex phenomena to which these pollutants will be subjected in the atmosphere under the action of weather conditions. Meteorological situations favorable to the concentration of pollutants or, on the contrary, to their dilution or dispersion in the atmosphere are presented and illustrated by examples. The harmful effects on the body of exposure to suspended particles or reactive gases have been the subject of numerous studies. The results of these studies require the implementation of regulations and monitoring of exceedances of the authorized thresholds of the main pollutants, for the information of the public and decision-makers. In order to better understand and predict the factors that generate this pollution, numerical modelling is one of the essential tools of operational warning and decision support systems.

1. Weather conditions: an important factor in the concentration of pollutants in the atmosphere

The “atmosphere” refers to all the gases surrounding the Earth under the influence of gravity. The composition of the atmosphere, the physical and chemical characteristics of its different layers are described in The atmosphere and the gaseous envelope of the earth. Pollution phenomena concern the first two layers of the atmosphere, the troposphere (altitudes below about 12 km depending on latitude) and the stratosphere (up to about 60 km). Pollution affecting the stratosphere is mainly manifested by an additional greenhouse effect (see Air Pollution) and by the destruction of the protective ozone layer resulting from the combination of special weather conditions and the contribution of halogenated compounds (chlorofluorocarbons or CFCs) from human activities.

1.1. The atmospheric boundary layer

The greatest variations in pollutant concentrations according to weather conditions are observed in the troposphere and, in particular, in the atmospheric boundary layer. This boundary layer is the portion of the atmosphere subject to the influence of the Earth’s surface; its thickness varies from a few hundred metres to a few kilometres depending on larger-scale atmospheric conditions and surface characteristics [2]. Friction of the ground and diurnal variations in its temperature have an impact on air flow and temperature.

It is also in the atmospheric boundary layer that the main pollutants linked to human activity (nitrogen oxides, sulphur oxides, hydrocarbons and fine particles) or of natural origin (desert dust, sea spray, volatile organic compounds emitted by vegetation,…) are released. After their emission into the atmosphere, the pollutants, whose origins and formation are described in the article Air pollution, will then disperse or accumulate depending on weather conditions (see Figure 2). They are more or less quickly transported, diluted, dissolved and leached under:

  • the effect of the wind, responsible for horizontal transport over distances of up to several hundred kilometres. The higher the wind speed, the less pollutants accumulate in the vicinity of the emission source.
  • the effect of vertical turbulence. The latter, within the boundary layer, has two origins, a thermal cause due to underfloor heating and a mechanical cause due to the effect of surface obstacles that cause horizontal and vertical wind shear. This turbulence also favours the dilution and dry deposition of pollutants in contact with the ground, vegetation and various obstacles encountered.
  • the effect of temperature, which directly affects the rate of chemical reactions between pollutants and thus their transformation into secondary pollutants.
  • the effect of precipitation (rain, snow, fog). Some gaseous pollutants are soluble in water and will dissolve in fog droplets or clouds. This is referred to as wet deposition or, in the case of particles, aerosol capture in the cloud phase. As for rainfall, it causes particles and gases to leach out under the clouds.

transport of pollutants
Figure 2. Simplified diagram of the transport of pollutants and their transformations. [Rights reserved]
It is therefore the meteorological conditions that influence the stability or instability of this atmospheric boundary layer.

Thus, in clear skies during the day, with ground temperatures higher than those of the air at the surface, an unstable layer develops in thickness as the ground warms. This instability allows the pollutants to be diluted in the surrounding air. At night, however, the soil cools by radiation. We then observe a temperature inversion over a few hundred meters. In this layer, the stability of the air then affects the dispersion of pollutants.

1.2. Winter pollution

This situation is particularly noticeable in winter, during anticyclonic conditions with light winds. Temperature inversions in this season persist for several hours due to the lower solar radiation during the day and its shorter duration.

Emissions of pollutants (NO2, SO2, fine particles) due to human activities (transport, district heating, industrial installations, etc.) stagnate in the lower layers and accumulate there. In addition, the increased use of heating related to these cold episodes increases particulate emissions. Only a change in weather conditions, the arrival of a disturbance, reinforcement or change in wind direction, can put an end to these episodes by leaching the pollutants or mixing them vertically.

Finally, particular situations (valley effect, relief, sea breeze or urban heat island) are also likely to lead to pollution phenomena by accumulation of harmful chemical elements. The recurrent episodes of particulate pollution in the Arve Valley in winter are an illustration of this. The main causes are the massive use of wood heating during cold spells and the confinement induced by the topography which favours the accumulation of fine carbonaceous particles in a thin stable layer near the ground.

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Figure 3. Analysed PM10 maps from the National Air Quality Forecast Platform. PM10, daily maximum, France zone, on 05, 06, 07 and 08/12/2016, combining model and observations. [Source: © Prev’air]
The intense episode of fine particulate pollution in Paris in early December 2016 illustrates this phenomenon. Temperatures are low, the weather situation is a “stable anticyclonic” type situation with inversion of low layers (less than 200 m in Ile de France, on 1 December) and light winds over a large northern half of the country. These conditions are conducive to the increase of local emissions induced by heating and limit the atmospheric dispersion processes of pollutants emitted by urban (heating, road traffic) and industrial sources.

In the Paris region, the maximum PM10 concentrations measured by AIRPARIF were 146 µg/m3 on December 1 and 122 µg/m3 on December 2. Such a level ranks this episode among the most important winter episodes of the last ten years. The situation was unblocked, with the arrival of an easterly flow: the wind forces were low but the direction being constant, the accumulation phenomenon was limited.

1.3. Summer pollution

In summer, in the absence of wind with maximum solar radiation, the accumulation of pollutants such as nitrogen oxides (NOx), volatile organic compounds (VOCs), including hydrocarbons, methane and carbon monoxide (CO) leads to the production of ozone in the lower layers. Thus, almost every summer, major cities record episodes of photochemical pollution peaks.

When the wind carries the air mass, pollution occurs downstream of the precursor sources. Indeed, the chemical reaction of ozone formation from nitrogen dioxide NO2 is reversible and located near intense emissions of nitrogen monoxide (due to road traffic). Ozone can then react with the NO monoxide formed during the dissociation of NO2 to give back nitrogen dioxide. As a result, higher ozone concentrations are sometimes observed in peri-urban or rural areas near large cities than in the centre of the agglomeration.

This is an urban ozone plume phenomenon. Upstream of urban pollutant emissions, oxidant concentrations (noted Ox as a whole with  O= O3 + NO2) correspond to the background level. Downstream from cities, low nitrogen oxide emissions no longer allow the chemical destruction of ozone, which returns to its background value, increased by the quantities produced upstream and carried by the wind.

Biogenic VOC emissions. During this season, vegetation also contributes to the emission of VOCs, particularly under the influence of the sun and high temperatures. The production of VOCs by vegetation is dominated by isoprene (C5H8) emissions, mainly from deciduous trees, with conifers emitting more complex terpene compounds. Volatile Organic Compounds play an important role in atmospheric chemistry and contribute to the formation of secondary pollutants such as Ozone and secondary organic aerosols. On a global scale, vegetation is responsible for about 90% of VOC emissions compared to 10% for human activities [3].

Finally, we cannot limit ourselves to the local level to consider episodes of heavy pollution: large-scale transport also explains the horizontal extension of these episodes over several hundred kilometres. Urban ozone pollution episodes are strongly linked to air mass transport and interactions between cities, regions and countries must be taken into account to quantify these phenomena. Here again, the change in weather conditions (change in wind direction, greater cloud cover) determines the end of the episode.

2. Health impacts and economic costs

The high proportion of exposed people makes urban and indoor pollution a major public health problem, as highlighted by numerous studies (e.g. those [4] carried out by the Laboratory of Cytophysiology and Cell Toxicology at the Paris 7 Université). Toxicological studies and epidemiological methods have identified short-term effects, in the form of clinical manifestations occurring within minutes to weeks after exposure, and long-term effects, after several months or years of chronic exposure. These effects result in higher mortality or a reduction in the life expectancy of the populations concerned.

Although awareness of the dangers of air pollutants to human health has existed since antiquity, it became a matter of concern in the first half of the 20th century following dramatic episodes such as the one in London in December 1952, responsible for more than 4000 deaths (see Air Pollution).

Toxicological studies that establish the health consequences of pollutants range from experimentation on cell cultures in the laboratory to exposure to animals or human subjects that are exposed to known and controlled quantities of pollutants. In addition, epidemiological methods link, through statistical tools, the measurements taken of certain pollutants and the pathologies observed in the people subjected to this pollution. Indeed, only the long-term monitoring of cohorts on a large number of subjects makes it possible to compare health data (pathologies, hospitalizations, deaths, etc.) with measurements of the various pollutants.

2.1. Pathologies

partifcules fines sante - effet pollution sante - effet pollution corps - effet pollution air - pollution air problemes respiratoires - encyclopedie environnement - air quality - health effects fines particules
Figure 4. Health effect of fine particles. [Source: © M. Pithon and S. Guidotti]
These different methods agree on the fact that the highly oxidizing and irritating nature of gaseous components or particles causes pulmonary (asthma, respiratory diseases), cardiovascular (arrhythmias, myocardial ischemia), neurological and certain cancers.

The penetration of gaseous pollutants into the respiratory tract depends on their solubility. Pollutants such as ozone (O3) or nitrogen dioxide (NO2) are highly oxidizing and therefore highly irritating. In the case of particles, their deposition in the respiratory tract depends on their size. The smaller the particles, the deeper they penetrate into the lungs and settle in the bronchi, or even the bronchioles, causing exacerbation of asthma, bronchiolitis, lung disease and worsening cardiovascular disease (see Figure 5).

These various studies emphasize the weight of chronic pollution in the resulting pathologies and show that there are no thresholds below which exposure to pollutants is without health risk. As the sensitivity threshold is very different from one individual to another, attention should not be paid only to intense pollution episodes but also measures to reduce background pollution should be widely favored.

2.2. A few figures

In its reports, the World Health Organization (WHO) distinguishes between the effects of ambient air pollution and those of indoor air pollution, while recognizing the combined effects and the difficulty of accurately identifying the two contributions [5]. In the following, only the effects of ambient air pollution will be reported.

The use of new models to assess exposure to fine particulate matter (PM2.5) combined with risk assessment methods has resulted in an estimated of old more than 3 million deaths worldwide from outdoor air pollution per year, confirming that the majority of these deaths occur in low-income countries [6].

In 2011, a three-year European study, APHEKOM [7], used a conventional health impact assessment (HIA) method to estimate the impact of urban pollution on the health of residents in 25 major European cities participating in the project. This study estimates the average life expectancy gain, in months, at age 30 years old, if the average levels of fine particulate matter (PM2.5) were reduced to 10 µg/m3 as recommended by WHO. The annual health costs and associated costs (absenteeism, loss of quality and life expectancy) related to pollution are estimated to reach €30.5 billion annually for Europe.

Results also make it possible to link the occurrence of certain chronic diseases to the residence of people who live near major roads. Thus 15% to 20% of asthma attacks in children and the worsening of chronic obstructive pulmonary disease in adults over 65 years of age could be linked to pollution.

Finally, a recent report published by Santé Publique France [8] presents the latest quantitative health impact assessment (QISA) conducted on the link between chronic exposure to fine particulate matter (PM2.5) and mortality in 2007 and 2008. This study concludes that more than 48,000 deaths per year are estimated to be due to pollution from anthropogenic particulate matter in our country. Results by region are available in the form of maps. Such studies have also been conducted to quantify the impact of pollution on the Ozone. The uncertainties of these assessments are also recalled, uncertainties upstream on the quantification of emissions and exposures of populations, on the causes of the pathologies observed (confounding factors related to lifestyles, diet, smoking, exposure in the workplace,…) and on the methodological choices used. Despite their limitations, these results have the advantage of raising awareness of the urgency of the health problem and the importance of the measures to be implemented to reduce air pollution.

3. Public information at national and European level: the Prev’air and Copernicus Atmosphere systems

As recalled in Marianne Moliner-Dubost’s article (see How does the law protect air quality?), the 1996 Air Act has profoundly modified the national organization around issues related to the air we breathe. This was the starting point for the establishment of a network of air quality monitoring agencies (AASQA). During the 2000s, this system was completed with the creation of the Prev’air system, which was oriented towards modelling.

3.1. The Prev’air system

This platform for predicting the concentrations of the main regulated pollutants is managed on a daily basis by INERIS (Institut National de l’EnviRonnement et des rIsqueS). It was developed in 2004 by the consortium integrating INERIS, Météo France, CNRS and LCSQA (Laboratoire Central de Surveillance de la Qualité de l’air). Today, this consortium continues to develop this system, which is unique in Europe.

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Figure 5. Operation of the Prev’Air Platform. [Source: © Prev’Air]
Prev’air [9] aims to provide regular and reliable information on the concentrations expected over the next three days in France and Europe for the main regulated pollutants and to make these results accessible to the greatest number of people. To carry out these forecasts, two numerical models called “chemistry-transport” models are used: the CHIMERE model, co-developed by INERIS and the Pierre Simon Laplace Institute, and the MOCAGE model developed by Météo-France.

These operating models to provide predictions of pollutant concentrations require the following information:

  • An emissions inventory, now static (not dependent on the weather situation) that details the spatial and temporal distribution of emissions over a period of time. These emissions include those resulting from human activity (anthropogenic emissions) but also those resulting from natural sources (vegetation, soils, etc.). These emissions files are regularly updated to take into account changes in land use and are the subject of research projects to make them dynamic and dependent on the situation (weather conditions, time of year, public holidays, etc.). In particular, we can mention the inventory developed as part of the MACC and MACCII research projects, which are precursors to the Copernicus projects.
  • Weather conditions over the period considered, because as seen in the first part, the weather conditions create favourable conditions for a pollution episode and above all are a major element in the end of the events. The weather forecast will therefore impact the quality of the air quality forecast. This point largely explains the differences in the prediction of pollutant concentrations (or air quality by extension) sometimes observed between different sources of information.
  • Boundary conditions, it is in fact the state of the chemical composition at the edge of the domain under consideration, these pollutants may then interact or move in the modelling domain (transport effect for example).

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Figure 6. TNO MACC – 2009 emission inventory for SO2. [Source: © Copernicus Atmosphere]
The outputs thus produced by Prév’air can then be reused by approved air quality agencies to produce finer-scale forecasts, adapted to their territory, or displayed in cartographic form, allowing the evolution of an episode to be monitored on the time and spatial scale considered.

This system, a pioneer in Europe, is regularly reassessed [10], and is continuously evolving to take into account more chemical phenomena, to increase the resolution in the field, and to correct a bias observed in the forecast. Research work in the field of chemistry and air quality modelling is extensive and contributes to enriching this service to users. Thus, Primequal calls for projects, launched in 2005, aimed to study the links between air quality and agriculture. It should also be noted that since the launch of air quality models in 2004, the models have been enriched in terms of chemical reactions taken into account, particularly through the modelling of secondary aerosols.

3.2. Copernicus Atmosphere services on Europe

An alternative approach has been adopted at European level. The MACC (Monitoring Atmospheric Composition and Climate) research projects have made it possible to develop from 2009 onwards an unparalleled air quality monitoring and forecasting system involving a set of around ten models developed and implemented by various European teams. They brought together 35 European partners, national meteorological services and other research organizations, including Météo-France and INERIS [11]. Since 2015, part of this work has led to the establishment of operational services: the Copernicus Atmopshere services [12].

The operational implementation of these services has been delegated by the European Union to major operators. For the atmospheric monitoring component, the operator is the European Centre for Medium-Range Weather Forecasting (ECMWF/ECMMT). Under its aegis, Météo-France and INERIS coordinate air quality forecasting and analysis services in Europe and services for evaluating air quality management strategies.

Since 2015, on the website www.regional.atmosphere.copernicus.eu, 4-day forecasts of concentrations of 10 pollutants (including pollen species during the pollination seasons) are available, as well as previous day’s analyses (forecasts corrected using spot observations collected by the European Environment Agency) or annual re-analyses that make it possible to assess and monitor trends and therefore the impact of public policies. This data is offered with visualization and download services that comply with standardized data exchange protocols (including WMS/WCS services). These download services are entirely free of access and free of charge, and are intended to be used by downstream expert systems, which will enhance these outputs by providing finer resolution air quality information, cross-referencing these data with other data sources, statistical processing.

Based on the conclusions of the pioneering MACC projects, the multi-model approach has been retained, the CAMS regional forecasts are now being developed by combining the outputs of seven of the best models in Europe and all capable of providing the expected level of service. These models are produced by national meteorological services or organizations (from Finland, France, Norway, Sweden, the Netherlands and the United Kingdom) and university laboratories (such as Aarhus University/Denmark, Rhenish Institute for Environmental Research at University of Cologne/Germany, Warsaw University of Technology/Poland). The flagship forecast produced by this system is called the Ensemble, it is the median of the individual results, it thus benefits from the advantages of each of the models and allows to propose a quality forecast on Europe at a resolution of 0.1°.

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Figure 7. Fine particle concentration, daily average for 18/09/2017, the right scale is graduated at μg. [Source: © S. Guidotti and M. Pithon, Copernicus Atmosphere]
Note that for this production, each individual model uses the same input data, ECMWF weather forecasts, emission data and boundary conditions produced by other Copernicus Atmosphere services. This strengthens the robustness of the system and allows the intrinsic uncertainties of the models to be analyzed and taken into account.

The system, which complements the global system [13], is currently in the expansion phase and continues to offer new services (new pollen species, new reanalyses) as well as controlled and monitored developments at the level of the European Union, including ambitious programs to make available mass data download platforms supplemented with services. This can be done by updating emission inventories or by proposing innovative services such as a fire monitoring system based on satellite data assimilation. The latter system is intended to be taken into account in the context of regional air quality; thus, the impact of a fire outside Europe on air quality at European level can be included in the forecasts.

This European service can also be a major support in the monitoring of air quality at national level, through the interweaving of modelling systems at different geographical scales, as is now the case for the Prev’air and Copernicus Atmosphere systems. Finally, the use of “Sentinel” satellite data should also allow the implementation of new products and a significant improvement of existing products.

 


References and notes

Cover image. Air pollution (smog) in New Delhi. Source: Flickr – License: CC BY-NC]

[1] WHO report of November 2016 “Burden of disease from the joint effects of household and ambient air pollution for 2012″[http://www.who.int/airpollution/data/AP_jointeffect_BoD_results_Nov2016.pdf]

[2] Malardel S. (2009). Fundamentals of Meteorology, Cepadues-Editions

[3] Fuentes et al (2000). Biogenic Hydrocarbons in the Atmospheric Boundary Layer: A Review Bulletin of the American Meteorological Society 81:7, 1537-1575

[4] Auger F. (2006). Involvement of fine atmospheric particles (PM <2.5 μm) in the induction of cardiovascular diseases. In vitro study of the relationships between the epithelium of the respiratory tract and the cells of the vascular endothelium. Thesis University Paris 7.

[5] WHO (Nov 2016). Burden of disease from the joint effects of household and ambient air pollution for 2012[http://www.who.int/phe/health_topics/outdoorair/databases/AP_jointeffect_BoD_results_Nov2016.pdf?ua=1]

[6] WHO (Press release. 27 September 2016). WHO publishes national estimates of exposure to air pollution and health effects. [http://www.who.int/mediacentre/news/releases/2016/air-pollution-estimates/fr/]

[7] The Aphekom Project. Improving knowledge and communication for decision making on air pollution and heath in Europe[http://aphekom.org/web/aphekom.org/home]

[8] Santé Publique France. Impacts of chronic exposure to fine particulate matter on mortality in mainland France and analysis of health gains from several air pollution reduction scenarios

[9] Prevair

[10] Rouil L., Honore C., Vautard R., Beekmann M., Bessagnet B., Malherbe L., Meleux F., Dufour A., Elichegaray C., Flaud J-M., Menut L., Martin D, Peuch A., Peuch V-H., Poisson N., 2009, PREV’AIR : an operational forecasting and mapping system for air quality in Europe, BAMS, DOI: 10.1175/2008BAMS2390.1

[11] Marécal V., Peuch V.-H., Andersson C., Andersson S., Arteta J., Beekmann M., Benedictow A., Bergstrom R., Bessagnet B., Cansado A., Cheroux F., Colette A., Coman A., Curier R. L.,. Denier van der Gon H. A. A. C, Drouin A., Elbern H., Emili E.,. Engelen R. J.,. Eskes H. J., Foret G., Friese E., Gauss M., Giannaros C., Joly M., Jaumouillé E., Josse B., Kadygrov N., Kaiser J. W., Krajsek K., Kuenen J., Kumar U., Liora N.,. Lopez E., Malherbe L., Martinez I., Melas D., Meleux F., Menut L., Moinat P., Morales T., Parmentier J., Piacentini A., Plu M., Poupkou A., Queguiner S., Robertson L., Rouil L., Schaap M., Segers A., Sofiev M., Thomas M., Timmermans R., Valdebenito A., van Velthoven P., van Versendaal R., Vira J., and Ung A., A regional air quality forecasting system over Europe: the MACC-II daily ensemble production, Geosci. Model Dev. Discuss, 8, 2739-2806, 2015, doi:10.5194/gmdd-8-2739-2015

[12] Regional Copernicus

[13] http://atmosphere.copernicus.eu/


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: GUIDOTTI Sylvie, PITHON Marion (July 3, 2019), Outdoor air pollution: keys to understand, inform and prevent, Encyclopedia of the Environment, Accessed December 22, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/air-en/outdoor-air-pollution-understanding-to-inform-and-prevent/.

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室外空气污染:了解、告知和预防的关键

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  室外空气质量是当前社会面临的一大挑战。本文旨在阐明空气污染事件所涉及的现象,并描述其对健康造成影响和对经济带来的冲击。对室外空气质量的关注促使国家当局和欧盟提供空气质量监测和预报服务,如Prev’air和哥白尼大气区域系统。

  根据世界卫生组织和法国公共卫生部的最新数据,2012年,室外和室内空气污染的综合影响导致全球650万人和欧洲60万人过早死亡[1]。环境空气污染是一个古老的现象,是人类集中在城市地区造成的结果(阅读大气污染),如今,它已成为公共卫生和环境面临的一大挑战(见图1)。

环境百科全是-空气-对汽车交通进行的测量
图1. 因颗粒物污染加龙河谷能见度降低,污染当天对汽车交通进行的测量(图卢兹南部A64公路上的标志)。2018年2月25日。

  1996年12月30日的《空气与合理使用能源法》将空气污染定义为“人类直接或间接地将有害物质引入大气和封闭空间,这些物质可能危及人类健康、损害生物资源和生态系统、影响气候变化、破坏物质财产并散发恶臭”。

  污染物排放源可以是人为的,也可以来源于自然。在天气条件的作用下,这些污染物在大气中会产生复杂的现象,从而导致空气质量下降。本文通过实例介绍并说明了有利于污染物的聚集或有利于污染物在大气中稀释或扩散的气象条件。悬浮颗粒物或活性气体对人体的有害影响已成为众多研究的主题。根据这些研究结果,需要实施法规,监测主要污染物的阈值超标情况,为公众和决策者提供参考。为了更好地了解和预测产生污染的因素,数值模拟是运行预警和决策支持系统的重要工具之一。

1.天气条件——影响大气污染物浓度的一个重要因素

  “大气”是指地球周围受重力影响的所有气体。地球的大气层和气体包络层一文描述了大气的组成及其物理和化学特征。污染现象涉及大气的前两层,即对流层(海拔在12公里以下,视纬度而定)和平流层(高达60公里)。影响平流层的污染主要表现为额外的温室效应(参见大气污染),以及特殊天气条件和人类活动产生的卤代化合物(氯氟化合物或氟氯化合物)对保护性臭氧层造成的破坏。

1.1大气边界层

  在不同的天气条件下,污染物浓度变化最大的是对流层,特别是大气边界层。这一边界层是大气中受地球表面影响的部分,其厚度从几百米到几公里不等,取决于较大尺度的大气条件和地表特征[2]。地面摩擦力和温度的昼夜变化对气流和温度有影响。

  与人类活动有关的主要污染物(氮氧化物、硫氧化物、碳氢化合物和细颗粒物)或自然污染物(沙尘、海雾、植被等释放的挥发性有机化合物) 也是在大气边界层释放的。污染物的来源和形成在空气污染一文中有所描述。污染物被排放到大气后,会根据天气状况扩散或积聚(见图2),在下述条件的影响下,污染物会被迅速迁移、稀释、溶解和沥滤:

  风的影响。在风力作用下,污染物横向漂移长达几百公里。风速越高,污染物在排放源附近的累积量就越少。

  垂直湍流的影响。在边界层内出现垂直湍流有两个原因:一是由于下地表加热引起的热力效应,二是受地面障碍物引起的水平和垂直风切变的影响引起的机械效应。这种湍流还有利于与地面、植被和遇到的各种障碍物接触的污染物的稀释和干沉降。

  温度的影响。温度直接影响污染物之间的化学反应速度,从而影响污染物转化为二次污染物。

  降水(雨、雪、雾)的影响。有些气态污染物可溶于水,会溶解在雾滴或云层中。这种现象被称为湿沉降。在云中,气溶胶颗粒可以作为云滴的凝结核,促使云滴形成。降水会将空气中的颗粒物冲刷下来。就颗粒而言,则称为云相中的气溶胶捕获。至于降雨,则会导致颗粒和气体从云层中渗出。

环境百科全是-空气-污染物迁移及其转化的简化图
图 2. 污染物迁移及其转化简图。[版权所有]

  因此,气象条件影响了大气边界层的稳定性。

      因此,在白天晴朗的天空中,地面温度高于地表空气温度时,随着地面升温,不稳定大气层厚度增加。这种不稳定性使得污染物在周围空气中被稀释。然而,到了晚上,地表会因辐射而降温。这时,我们会观察到几百米范围内的逆温现象。在这一层中,空气的稳定性会影响污染物的扩散。

1.2冬季污染

        逆温现象在冬季微风的反气旋条件下尤为明显。由于白天的太阳辐射较弱,持续时间较短,这个季节的温度逆温会持续数小时。

  由人类活动(交通、区域供暖、工业设施等)产生的污染物排放(二氧化氮、 二氧化硫、细颗粒物)会滞留在大气低层并积聚在次。此外,与这些寒冷事件有关的供暖增加了颗粒物排放。只有天气条件发生变化、扰动来临、风力加强或风向改变,才能通过污染物的垂直扩散来结束污染物的滞留现象。

  最后,特殊情况(山谷效应、地形起伏、海风或城市热岛)也可能通过有害化学元素的积累而导致污染现象。法国阿尔沃河谷冬季经常出现颗粒物污染就是一个例证。主要原因是在寒流期间大量使用木材供暖,以及地形造成的封闭性,促使细小的碳质颗粒在近地面的薄稳定层中累积。

环境百科全是-空气-来自国家空气质量预测平台的PM10 图
图3. 国家空气质量预报平台的PM10 图。2016年12月5、6、7、8日,结合模型和观测数据,法国区域的PM10日最大值。
[图片来源:© Prev’air]

  2016年12月初巴黎严重的细颗粒物污染事件说明了这一现象。当时气温偏低,天气状况属于“稳定反气旋”类型,低层逆温(12月1日法兰西岛低于200米),北半部大面积地区风力较小。这些条件有利于增加供暖引起的本地排放,并限制了城市(供热、道路交通)和工业源排放的污染物在大气中的扩散。

  在巴黎地区,巴黎大区空气质量监测协会(AIRPARIF)测得的PM10最高浓度在12月1日为146微克/立方米,12月2日为122微克/立方米。这样的浓度水平在过去十年最重要的冬季天气事件中名列前茅。随着一股东风的到来,情况得以缓解:风力虽小,但风向稳定,累积现象有限。

1.3夏季污染

  夏季,在无风且太阳辐射最强的情况下,污染物如氮氧化物(NOx)、挥发性有机化合物(VOCs)(包括碳氢化合物、甲烷和一氧化碳)的累积,会导致大气低层产生臭氧。因此,几乎每年夏天,大城市的光化学污染都会达到峰值。

  当风携带气团时,污染就会发生在前体污染源的下风向。事实上,二氧化氮形成臭氧的化学反应是可逆的,并且发生在一氧化氮大量排放的地方(如道路)。臭氧可与二氧化氮分解过程中形成的一氧化氮发生反应,生成二氧化氮。因此,有时在大城市附近的城郊地区或农村地区观察到的臭氧浓度高于城市的中心地区。

  这是一种城市臭氧烟羽现象。在城市污染物排放的上游,氧化剂浓度(总体为Ox,Ox=O3+NO2)与本底水平相对应。在城市的下风向,低氮氧化物排放量不再允许臭氧发生化学还原反应,臭氧恢复到其背景值,随着上风向产生的数量和风的携带而增加。

  生物排放的挥发性有机物。夏季,植物也会释放挥发性有机物,尤其是在强烈阳光和高温的影响下。植被释放的挥发性有机物主要是异戊二烯(C5H8),主要来自落叶树木,针叶树则可释放出更复杂的萜烯化合物。挥发性有机物在大气化学中发挥着重要作用,并促进了臭氧和次级有机气溶胶等二次污染物的形成。在全球范围内,植被排放了约90%的挥发性有机化合物,而人类活动排放的挥发性有机化合物仅占10%[3]

  最后,我们不能局限于局地尺度来考虑严重污染事件:大尺度运输解释了污染物横向扩散到几百公里之外的原因。城市臭氧污染事件与气团输送密切相关。要量化这些现象,必须考虑到城市、区域和国家之间的相互作用。

2.健康影响和经济成本

  大量的研究(例如巴黎第七大学细胞生理学和细胞毒理学实验室进行的研究[4])表明,高比例的暴露人群使城市和室内污染成为一个重大的公共健康问题。毒理学研究和流行病学方法确定了短期影响和长期影响。前者表现为接触后几分钟至几周内出现的临床表现,后者表现为长期接触后数月或数年内出现的临床表现。这些影响导致相关人口的死亡率上升或预期寿命缩短。

  尽管人们很早就认识到空气污染物对人类健康的危害,但人们真正开始关注这一问题是在1952年12月伦敦发生了严重污染事件,造成4000多人死亡之后(参见大气污染)。

  确定污染物对健康的影响而进行的毒理学研究包括在实验室中对细胞培养物进行实验,以及让动物或人体接触已知和受控数量的污染物。此外,流行病学方法通过统计工具,将对某些污染物的测量结果与在受污染人群中观察到的病症联系起来。事实上,只有对大量受试者进行长期监测,才有可能将健康数据(病理、住院、死亡等)与各种污染物的测量结果进行比较。

2.1病理学

环境百科全是-空气-细颗粒对健康的影响
图4. 细颗粒物对健康的影响。
[图片来源:©马里昂·皮顿和西尔维·吉多蒂]

  这些不同的方法一致认为,气体成分或颗粒物的高度氧化性和刺激性会导致肺部疾病(哮喘、呼吸系统疾病)、心血管疾病(心律失常、心肌缺血)、神经系统疾病和某些癌症。

  气态污染物能否进入呼吸道取决于其溶解度。臭氧(O3)或二氧化氮(NO2)等污染物具有高度氧化性,因此刺激性很强。至于颗粒物,它们在呼吸道中的沉积取决于其大小。颗粒物越小,进入肺部越深,沉积在支气管甚至细支气管中,导致哮喘、细支气管炎、肺部疾病和心血管疾病恶化(见图5)。

  这些不同研究强调了慢性污染在导致疾病中的重要作用,低于该阈值接触污染物不会对健康造成危害。因为灵敏度阈值在个体之间存在差异。不应仅关注严重污染事件,还应广泛采取措施减少本底污染。

2.2几个数字

  世界卫生组织(WHO)在其报告中区分了环境空气污染的影响和室内空气污染的影响,同时认识到两者的综合影响以及准确识别两种影响的难度[5]。下文将只报告环境空气污染的影响。

  使用新模型评估细颗粒物(PM2.5)的暴露情况,并结合风险评估方法,估计全世界每年有300多万人因室外空气污染而死亡,证实这些死亡大多发生在低收入国家[6]

  2011年,一项为期三年的欧洲研究APHEKOM[7],采用传统的健康影响评估方法,估算了城市污染对参与该项目的25个欧洲主要城市居民健康的影响。该研究估算了如果按照世界卫生组织的建议,将细颗粒物(PM2.5)的平均水平降低到10微克/立方米,30岁人群平均预期寿命增加的月数。据估计,欧洲每年与大气污染有关的健康费用和相关费用(缺勤、质量损失和预期寿命)达305亿欧元

  研究结果还将某些慢性病的发生与居住在主干道附近的居民联系起来。15-20%的儿童哮喘发作和65岁以上成年人慢性阻塞性肺病的恶化可能与污染有关。

  最后,法国公共卫生部[8]发布了一份报告,该报告介绍了2007年和2008年对长期接触细颗粒物(PM2.5)与死亡率之间的联系所做的最新定量健康影响评估(QISA)。这项研究表明,法国每年有超过48,000人死于人为颗粒物造成的污染。各地区的研究结果以地图的形式提供。为量化污染对臭氧的影响,也进行了此类研究。报告还回顾了这些评估的不确定性,上游的不确定性涉及人口的排放和暴露的量化、所观察到的病理原因(与生活方式、饮食、吸烟、工作场所接触等有关的混杂因素……)以及所使用的方法选择。尽管存在局限性,但研究结果提高了人们对健康问题紧迫性的认识,使人们认识到有必要采取措施以减少空气污染。

3.国家和欧盟层面的公共信息——Prev’air系统和哥白尼大气监测服务系统

  正如玛丽安·莫利纳-杜博斯特的文章所述(参见法律如何保护空气质量?),1996年颁布的《空气法》深刻地改变了国家机构对空气相关问题的看法。这是建立空气质量监测机构网络(AASQA)的起点。在2000年代,随着以模型为导向的Prev’air系统的创建,这一系统得以完善。

3.1 Prev’air系统

  该平台用于预测主要受管制污染物的浓度,由法国国家工业环境与风险研究院(INERIS)负责日常管理。该系统于2004年由INERIS、法国气象局、法国国家科学研究中心和空气质量监测中心实验室(LCSQA)联合开发。今天,该联盟仍在继续开发这一欧洲独一无二的系统。

环境百科全是-空气-Prev'air平台的操作
图5. Prev’air平台的运行。
[图片来源:© Prev’air]

  Prev’air[9]旨在定期提供有关法国和欧洲未来三天主要受管制污染物浓度的可靠信息,并让更多人了解这些数据。为了进行预测,使用了两个被称为“化学-输运”的数值模型:由INERIS和皮埃尔-西蒙·拉普拉斯研究所共同开发的CHIMERE模型,以及由法国气象局开发的MOCAGE模型。

  这些预测污染物浓度的运行模型需要以下信息:

  排放清单,是静态的(不依赖于天气状况),详细记录了一段时间内排放量的时空分布。排放包括人类活动产生的排放(人为排放),也包括自然来源(植被、土壤等)产生的排放。考虑到土地使用的变化,这些排放清单会定期更新。依据不同情况(天气条件、一年中的时间点、公共假期等),排放清单会发生动态变化。值得一提的是,清单开发工作是MACC和MACCII研究项目的一部分。该研究项目是哥白尼项目的前身。

  天气条件,如第一部分所示,天气条件为污染事件创造了有利条件,也是污染事件结束的主要因素。因此,天气预报会影响空气质量预测的精度。这一点在很大程度上解释了不同信息来源对污染物浓度(或扩展的空气质量)预测的差异。

  边界条件。实际上是考虑了区域边界的化学成分状态。这些污染物可能会在建模区域内相互作用或移动(例如传输效应)。

环境百科全是-空气-TNOMACC-2009年二氧化硫排放清单
图6. 应用科学研究组织监测大气成分和气候研究项目(TNO MACC)2009年二氧化硫排放清单
[图片来源:© 哥白尼大气监测服务系统]

  Prav’air系统生成的数据可由获批的空气质量机构重新利用,以生成适合其领域的更精细的预报,或以制图形式显示,从而在所考虑的时空尺度内监测污染事件的演变。

  该系统是欧洲的先驱,会定期对其重新评估[10],并不断发展以考虑更多的化学现象,提高现场分辨率,纠正预报中观测到的偏差。化学和空气质量建模领域的研究工作有助于丰富这项面对用户的服务。因此,2005年Primequal呼吁启动项目研究空气质量与农业之间的联系。还应指出的是,2004年推出了空气质量模型,通过对二次气溶胶进行建模,将化学反应考虑在内,进一步对其进行了完善。

3.2欧洲哥白尼大气监测服务系统

  欧洲采用了另一种方法。在监测大气成分和气候研究项目(MACC)的建立,2009年开发一个无以伦比的空气质量监测和预报系统。该系统涉及由不同欧洲团队开发和实施的一组模型,大约有十个。汇集了35个欧洲合作伙伴、国家气象部门和其他研究组织,包括法国气象局和INERIS[11]。2015年,设立了哥白尼大气监测服务系统[12]

       这些服务的业务实施已由欧盟委托给主要运营商。大气监测部分的运营商是欧洲中期天气预报中心(ECMWF/ECMMT)。在该中心的支持下,法国气象局和INERIS协调欧洲的空气质量预报和分析服务以及空气质量管理战略评估服务。

  自2015年以来,欧洲环境网站www.regional.atmosphere.copernicus.eu提供了未来410种污染物(包括授粉季节的花粉物种)浓度的预报前一天的分析(利用欧洲环境署收集的定点观测数据修订预报)或年度再分析,从而可以评估和监测趋势,进而评估和监测公共政策的影响。这些数据提供可视化和下载服务,遵守标准化数据交换协议(包括WMS/WCS服务)。下载服务完全免费,供下游专家系统使用。通过提供分辨率更高的空气质量信息,将这些数据与其他数据源交互参照,统计处理以提升输出质量。

  在MACC先驱项目结论的基础上,保留了多模型方法,CAMS区域预报由欧洲七个最佳模型的输出结果组合而成,所有模型均能达到预期的服务水平。这些模型由国家气象服务机构或组织(来自芬兰、法国、挪威、瑞典、荷兰和英国)及大学实验室(如丹麦奥尔胡斯大学、德国科隆大学莱茵环境研究所、波兰华沙工业大学)制作。该系统生产的旗舰预报产品被称为集成预报,是各个预报结果的中位数。得益于每个模型的优点,可以提供0.1°分辨率的欧洲空气质量预报。

环境百科全是-空气-2017年9月18日的日均细颗粒浓度
图7. 2017年9月18日的日均细颗粒浓度,单位为毫克。
[图片来源:© 马里昂·皮顿和西尔维·吉多蒂,哥白尼大气监测服务系统]

  请注意,每个单独的模型都使用相同的输入数据、ECMWF天气预报数据、排放数据和其他哥白尼大气服务产生的边界条件。这增强了系统的稳健性,可以对模型的内在不确定性加以分析和考量。

  该系统作为全球系统的补充[13],目前正处于扩展阶段,会继续提供新的服务(新的花粉品种、新的再分析),并在欧盟层面对其发展情况进行控制和监测,包括提供大规模数据下载平台并辅以服务的项目。这可以通过更新排放清单或提供创新服务,如基于卫星数据同化的火灾监测系统来实现。火灾监测系统应纳入区域空气质量预报系统;因此,欧洲以外的火灾对欧洲空气质量的影响可应纳入预报之中。

  Prev’air系统和哥白尼大气监测服务系统是两个不同地理尺度的建模系统,两个模型之间的交互作用。这项欧洲天气服务系统也可以为国家层面的空气质量监测提供支持。最后,使用“哨兵”卫星数据还将有助于实施新产品和改进现有产品。

 


参考资料和说明

封面照片:新德里的空气污染(烟雾)。图片来源:Flickr-License:CC BY-NC]

[1] 世卫组织2016年11月报告“2012 年家庭和环境空气污染联合影响造成的疾病负担”[http://www.who.int/airpollution/data/AP_jointeffect_BoD_results_Nov2016.pdf]

[2] Malardel S. (2009). Fundamentals of Meteorology, Cepadues-Editions

[3] Fuentes et al (2000). Biogenic Hydrocarbons in the Atmospheric Boundary Layer: A Review Bulletin of the American Meteorological Society 81:7, 1537-1575

[4] Auger F. (2006). Involvement of fine atmospheric particles (PM<2.5μm)in the induction of cardiovascular diseases. In vitro study of the relationships between the epithelium of the respiratory tract and the cells of the vascular endothelium. Thesis University Paris 7.

[5] 世卫组织(2016年11月)。2012年家庭和环境空气污染共同影响的疾病负担。[http://www.who.int/phe/health_topics/outdoorair/databases/AP_jointeffect_BoD_results_Nov2016.pdf?ua=1]

[6] 世卫组织(新闻稿,2016年9月27日)。世卫组织发布了国家对空气污染暴露和健康影响的估计。[http://www.who.int/mediacentre/news/releases/2016/air-pollution-estimates/fr/]

[7] Aphekom项目。改善欧洲空气污染和健康决策的知识与交流。[http://aphekom.org/web/aphekom.org/home]

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[9] Prevair

[10] Rouil L., Honore C., Vautard R., Beekmann M., Bessagnet B., Malherbe L., Meleux F., Dufour A., Elichegaray C., Flaud J-M., Menut L., Martin D, Peuch A., Peuch V-H., Poisson N., 2009, PREV’AIR : an operational forecasting and mapping system for air quality in Europe, BAMS, DOI: 10.1175/2008BAMS2390.1

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[12] Regional Copernicus

[13] http://atmosphere.copernicus.eu/

 


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: GUIDOTTI Sylvie, PITHON Marion (March 13, 2024), 室外空气污染:了解、告知和预防的关键, Encyclopedia of the Environment, Accessed December 22, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/air-zh/outdoor-air-pollution-understanding-to-inform-and-prevent/.

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