Air Stagnation Index Research Articles

Reduction in European anthropogenic aerosols and the weather conditions conducive to PM2.5 pollution in North China: a potential global teleconnection pathway

Frequent and severe PM2.5 pollution over China seriously harms natural environment and human health. Changes in meteorological conditions in recent decades have been recognized to contribute to the long-term increase in PM2.5 pollution in North China (NC). However, the dominant climatic factors driving the interdecadal changes of the weather conditions conducive to PM2.5 pollution remain unclear. Here we identify a potential global teleconnection mechanism: the decadal reduction in European aerosol emissions since the 1980s may have partially contributed to the interdecadal increase in weather conditions conducive to PM2.5 pollution in NC, measured by an Emission-weighted Air Stagnation Index (ASIE) that increases at a rate of 6.2% decade−1 (relative to the 1981–1985 level). By regression analysis, we show that the decreased European aerosol loadings can warm the lower atmosphere and induce anomalous ascending motion in Europe, which potentially stimulates two anomalous Rossby wave trains in the upper troposphere travelling eastward across Eurasia. The teleconnection patterns project on NC by weakening the near-surface horizontal dispersion, which may be favorable to the increase in local ASIE and air pollution build-up. The suggested mechanism is further supported by the results from a set of large-ensemble simulations, showing that the European aerosol emission decline since the 1980s excites similar local heating and ascending motion and leads to increasing trends of 0.1–0.5 μg m−3 (38 year)−1 in surface sulfate concentrations over most of NC. This proposed ‘West-to-East Aerosol-to-Aerosol’ teleconnection mechanism helps resolve opposite views on the impact of global versus local aerosol forcing on PM2.5 pollution weather in NC. The policy implication is that the sustained decline in European aerosol emissions in coming decades, in conjunction with unabated global and regional warming, could further exacerbate air pollution in NC, thus imposing stronger pressure to reduce local emission sources quicker and deeper.

Spatial Characteristics of PM2.5 Pollution among Cities and Policy Implication in the Northern Part of the North China Plain

In the recent decade, the North China Plain (NCP) has been among the region’s most heavily polluted by PM2.5 in China. For the nonattainment cities in the NCP, joint pollution control with related cities is highly needed in addition to the emission controls in their own cities. However, as the basis of decision-making, the spatial characteristics of PM2.5 among these cities are still insufficiently revealed. In this work, the spatial characteristics among all nonattainment cities in the northern part of the North China Plain (NNCP) region were revealed based on data mining technologies including clustering, coefficient of divergence (COD), network correlation model, and terrain and meteorology analysis. The results indicate that PM2.5 pollution of cities with a distance of less than 180 km exhibits homogeneity in the NCP region. Especially, the sub-region, composed of Xinxiang, Hebi, Kaifeng, Zhengzhou, and Jiaozuo, was strongly homogeneous and a strong correlation exists among them. Compared with spring and summer, much stronger correlations of PM2.5 between cities were found in autumn and winter, indicating a strong need for joint prevention and control during these periods. All nonattainment cities in this region were divided into city-clusters, depending on the seasons and pollution levels to further helping to reduce their PM2.5 concentrations effectively. Air stagnation index (ASI) analysis indicates that the strong correlations between cities in autumn were more attributed to the transport impacts than those in winter, even though there were higher PM2.5 concentrations in winter. These results provided an insight into joint prevention and control of pollution in the NCP region.

Long-term trends and variations in haze-related weather conditions in north China during 1980–2018 based on emission-weighted stagnation intensity

Abstract Recently, climatological and environmental researchers have paid significant attention to the long-term trends and variations in haze-related weather conditions in North China (NC). This study investigates this topic using a quantified air stagnation index (ASIE) that combines stagnation intensity with fixed emission information, given that haze occurrence depends strongly on the rate of emission. ASIE has a close spatial and temporal relationship with observed PM2.5 concentrations, and a strong sensitivity to haze occurrence in NC. The annual ASIE increased by 18.2% over the period 1980–2018 due to significant decreases in planetary boundary layer height and ventilation. However, there was an apparent drop during 2013–2018, which suggests that lower stagnation intensity may take effect on the improved air quality in NC reported in recent years. Such low-frequency oscillation occurred twice during 1980–2018. Hence, if the current trend of decreasing stagnation intensity reverses, haze events may become more common in the future. In addition, the interannual variations in stagnation intensity were very significant. The percentage change of ASIE was as high as 50%–70% in some years. Finally, using the year-to-year growth ratio of ASIE, we highlight the difficulty of the “clean air challenge” due to the variations in stagnation in NC. The results suggest that the enforcement of the emission reduction plan should be tailored according to the stagnation conditions in the region and period of interest.

An Air Stagnation Index to Qualify Extreme Haze Events in Northern China

Stagnation weather affects atmospheric diffusion ability, and hence causes the occurrence of haze events, which have been happening frequently in northern China (NC). This work puts forward an air stagnation index (ASITS) to characterize the stagnation weather in NC, in which the processes of ventilation, vertical diffusion, and wet deposition potency are concerned. ASITS can be applied to analyze air stagnation conditions with daily to monthly time scale. It is shown that the ASITS and particulate matter smaller than 2.5 μm in diameter (PM2.5) concentrations own similar lognormal probability distribution functions on both daily and monthly time scales. And the correlation analyses between the ASITS and PM2.5 concentrations indicate that the ASITS can reflect the monthly and daily variations in PM2.5 concentrations in NC. In addition, ASITS could be used as a leading predictor of haze events since correlation coefficients of ASITS leading PM2.5 concentrations by 1 day were significant and were larger than simultaneous correlation coefficients in almost all areas in NC. The robust relationship between ASITS and PM2.5 concentrations exists possibly because the index can reflect the activities of synoptic systems. ASITS could be a useful statistical indicator for variations in PM2.5 concentrations and haze events, and a good tool in analyzing the relationship between climate change and long-term variations in haze in NC.

Air stagnation in Europe: Spatiotemporal variability and impact on air quality

This paper characterizes the spatiotemporal variability of air stagnation over the Euro-Mediterranean area for the 1979–2016 period by using a simplified air stagnation index (ASI) based on daily precipitation as well as near-surface and upper wind speed data. We have also undertaken the first comparison of stagnation as derived from meteorological reanalysis and observations, finding a reasonably good agreement between both datasets. The main differences arise from the surface wind speed, as this field depends on the local setting of the observational sites and imperfect parameterizations within the reanalysis model. Since air stagnation has considerable spatial heterogeneity over the region, we have regionalized the monthly frequency of stagnant days, resulting five regions with consistent temporal patterns: Scandinavia (SCAN), Northern-Europe (NEU), Central-Europe (CEU), South-West (SW) and South-East (SE). The northern regions (SCAN and NEU), which are affected by moderately strong near-surface winds and ample precipitation, present low frequency and temporal variability in stagnation compared to the southern regions (SW and SE). The winters and summers with the highest stagnation frequency often concur with positive 500 hPa geopotential height anomalies over the regions, with the exception of negative anomalies and a displacement of the extratropical jet to the south in the case of SCAN and NEU during winter. Air stagnation exerts a clear influence on air quality (AQ), with anomalies above 10% for summer ozone (O3) and 30% for winter PM10 (particulate matter ≤10 μm in diameter) on stagnant vs. non-stagnant days over most of the regions. These values exceed 20% and 50%, respectively, in the case of CEU, where air stagnation also drives significant changes in the frequency distributions of these pollutants and increases the likelihood of AQ exceedances. Moreover, persistent and widespread stagnation events favour the build-up of both O3 and PM10 over most of the continent.

Connections between summer air pollution and stagnation

The body of literature on ambient air pollution suggests that atmospheric stagnation events trigger high levels of air pollution. In this paper we use fifteen years (2000–2014) of summertime in situ air quality measurements together with meteorological reanalysis data to examine the temporal correlation of pollutants with the Air Stagnation Index (ASI) on daily timescales. We find that while the direction of the relationship between the ASI and summertime PM2.5 and O3 ranges from near-zero to positive throughout regions comprising the contiguous United States (US), the strength of the relationship is very weak (e.g. in the Northeast the correlation coefficient between the ASI and PM2.5 is 0.09). Moreover, similar to our analysis of the correlation of day-to-day variations of the ASI and pollutants, the percentage of co-occurring extreme pollution and stagnation events is small (e.g. days with a high coverage of stagnation only co-occur with extreme pollution events about one-third of the time in the Northeast). The southern US is an exception to our overall findings as the strength of the relationship between the ASI and pollution is stronger and the percentage of co-occurring events is higher compared with other regions. The results of this study suggest a reevaluation of the ASI as an index to assess meteorological and climatic impacts to air quality.

Climatological study of the Boundary-layer air Stagnation Index for China and its relationship with air pollution

Abstract. The Air Stagnation Index (ASI) is a vital meteorological measure of the atmosphere's ability to dilute air pollutants. The original metric adopted by the US National Climatic Data Center (NCDC) is found to be not very suitable for China, because the decoupling between the upper and lower atmospheric layers results in a weak link between the near-surface air pollution and upper-air wind speed. Therefore, a new threshold for the ASI–Boundary-layer air Stagnation Index (BSI) is proposed, consisting of daily maximal ventilation in the atmospheric boundary layer, precipitation, and real latent instability. In the present study, the climatological features of the BSI are investigated. It shows that the spatial distribution of the BSI is similar to the ASI; that is, annual mean stagnations occur most often in the northwestern and southwestern basins, i.e., the Xinjiang and Sichuan basins (more than 180 days), and least over plateaus, i.e., the Qinghai–Tibet and Yunnan plateaus (less than 40 days). However, the seasonal cycle of the BSI is changed. Stagnation days under the new metric are observed to be maximal in winter and minimal in summer, which is positively correlated with the air pollution index (API) during 2000–2012. The correlations between the BSI and the concentration of fine particulate matter (PM2.5) during January 2013 and November to December in 2015–2017 of Beijing are also investigated. It shows that the BSI matches the day-by-day variation of PM2.5 concentration very well and is able to catch the haze episodes.

Occurrence and persistence of future atmospheric stagnation events.

Poor air quality causes an estimated 2.6 to 4.4 million premature deaths per year1–3. Hazardous conditions form when meteorological components allow the accumulation of pollutants in the near-surface atmosphere4–8. Global warming-driven changes to atmospheric circulation and the hydrological cycle9–13 are expected to alter the meteorological components that control pollutant build-up and dispersal5–8,14, but the magnitude, direction, geographic footprint, and public health impact of this alteration remain unclear7,8. We utilize an air stagnation index and an ensemble of bias-corrected climate model simulations to quantify the response of stagnation occurrence and persistence to global warming. Our analysis projects increases in stagnation occurrence that cover 55% of the current global population, with areas of increase affecting 10 times more people than areas of decrease. By the late-21st century, robust increases of up to 40 days per year are projected throughout the majority of the tropics and subtropics, as well as within isolated mid-latitude regions. Potential impacts over India, Mexico, and the western U.S. are particularly acute due to the intersection of large populations and increases in the persistence of stagnation events, including those of extreme duration. These results indicate that anthropogenic climate change is likely to alter the level of pollutant management required to meet future air quality targets.

Response of air stagnation frequency to anthropogenically enhanced radiative forcing

Stagnant atmospheric conditions can lead to hazardous air quality by allowing ozone and particulate matter to accumulate and persist in the near-surface environment. By changing atmospheric circulation and precipitation patterns, global warming could alter the meteorological factors that regulate air stagnation frequency. We analyze the response of the National Climatic Data Center (NCDC) air stagnation index (ASI) to anthropogenically enhanced radiative forcing using global climate model projections of late-21st century climate change (SRESA1B scenario). Our results indicate that the atmospheric conditions over the highly populated, highly industrialized regions of the eastern United States, Mediterranean Europe, and eastern China are particularly sensitive to global warming, with the occurrence of stagnant conditions projected to increase by 12–25% relative to late-20th century stagnation frequencies (3–18 + days yr−1). Changes in the position/strength of the polar jet, in the occurrence of light surface winds, and in the number of precipitation-free days all contribute to more frequent late-21st century air mass stagnation over these high-population regions. In addition, we find substantial inter-model spread in the simulated response of stagnation conditions over some regions using either native or bias corrected global climate model simulations, suggesting that changes in the atmospheric circulation and/or the distribution of precipitation represent important sources of uncertainty in the response of air quality to global warming.

Air pollution and meteorological processes in the growing dryland city of Urumqi (Xinjiang, China)

Seven years (2000–2006) of monthly PM 10 (particulate matter, d ≤ 10 μm), SO 2, and NO 2 concentrations are reported for Urumqi, the capital of Xinjiang in NW China. Considerably high mean annual concentrations have been observed, which ranged between 150 and 240 μg m − 3 (PM 10), 31 and 50 μg m − 3 (NO 2), and 49 and 160 μg m − 3 (SO 2). The shapes of seasonal variation of all pollutants were remarkably similar; however, winter/summer ratios of concentrations were quite different for PM 10 (2–3) and NO 2 (≈ 4) compared to SO 2 (up to 30). Very high consumption rates of fossil fuels for energy generation and domestic heating are mainly responsible for high annual pollution levels, as well as the (very) high winter/summer ratios. Detailed analysis of the 2000–2006 records of Urumqi's meteorological data resulted in inter-annual and seasonal frequency distributions of (a) (surface) inversion events, (b) heights of surface inversions, (c) stability classes of Urumqi's boundary layer, and (d) the “Air Stagnation Index (ASI)”. Urumqi's boundary layer is shown to be characterized by high mean annual and seasonal frequencies of (surface) inversions and by the dominance of stable dispersion classes. A further outcome of the meteorological analysis is the proof of Urumqi's strong diurnal wind system, which might have particularly contributed to the stabilization of the nocturnal boundary layer. Annual and seasonal variations of pollutant's concentrations are discussed in the context of occurrences of inversions, boundary layer, stability classes, and ASI. The trend of Urumqi's air pollution indicates a strong increase of mean annual concentrations 2000–2003, followed by a slight increase during 2003–2006. These are in strong contrast to (a) the growth of Urumqi's fleet of motor vehicles and (b) to the growing number of stable regimes of Urumqi's boundary layer climate during same period. It is concluded that the (regional and) local administrative technical countermeasures have efficiently lowered Urumqi's air pollution levels.

Some meteorological aspects of air pollution in Athens, Greece

The purpose of this paper is to examine some meteorological aspects of air pollution in the Athens area, Greece. Over recent years much attention has been directed to the importance of spells of weather in providing conditions that give rise to inversion of temperature at or near ground level, and hence to risks of severe incidents of air pollution. So, emphasis is given to the importance of anticyclonic conditions, associated with pronounced stability during ground level radiation inversions, when the same sampling sites show high concentration of both SO2 and smoke. Fifteen years of surface and radiosonde data obtained from Hellinikon Airport Athens are used for a climatological base. The meteorology of high concentration episodes has also been examined and the synoptic characteristics for the onset and end of such episodes have been identified. Finally, trends of pollution episodes and calculation of an “air stagnation index” for Athens have been obtained, which give a predicting indication about the occurrence of high pollution concentration.