Winter Haze Days Research Articles

Impacts of East Asian winter monsoon circulation on interannual variability of winter haze days in Guangdong Province

Haze days in Guangdong Province (GD) occur mainly in winter, with the highest number of moderate/severe haze days occurring in January. The interannual variability of winter haze days in GD is closely related to the Siberian High, the Aleutian Low, the East Asian Trough, the 850-hPa meridional wind over eastern China, and the 200-hPa East Asian Jet. We construct a winter comprehensive circulation index (WCIDX) to express the joint effects of the above regional-scale circulations on haze days in GD. The positive phase of WCIDX is closely linked to the Eurasian teleconnection pattern, which induces the abnormal southerly winds, high temperatures, and enhanced precipitation over GD by modulating the regional atmospheric circulations, localized meteorological conditions (i.e., enhanced lower specific humidity, reduced planetary boundary layer height, decreased surface wind speed, and enhanced low-level atmospheric inversion) conductive to a higher haze in GD. In addition, the positive phase of the WCIDX favors enhancement of the positive 850-hPa Eady growth rate (EGR) over the mid–high-latitude areas of East Asia, which enhances atmospheric baroclinity and results in a northward shift of the East Asian Jet. Pollutants accumulated over the Indochina Peninsula are advected to GD by the southwesterly airflows. It was found that enhanced horizontal warm advection, adiabatic heating, and diabatic heating provide conditions favorable for low-level atmospheric inversion.

Impacts of Sea Surface Temperature on the Interannual Variability of Winter Haze Days in Guangdong Province

Impacts of Sea Surface Temperature on the Interannual Variability of Winter Haze Days in Guangdong Province

Contribution of climate/meteorology to winter haze pollution in the Fenwei Plain, China

AbstractIn recent years, anthropogenic emissions in the Fenwei Plain (FWP) have decreased; however, haze pollution remains a serious issue. This study explored the possible reasons for this enduring problem in terms of climate and meteorology. Firstly, the contribution of climate and meteorology to haze pollution in the FWP was quantified using a best fit model and differences in key meteorological parameters were analysed over several time periods. Key climate factors were identified using a relative importance test and correlation analysis, and the adjusted optimal subset model (AOSM) was used to predict the number of winter haze days for the entire winter and for individual winter months, respectively. Results showed that the average minimum contribution (CONave) of climate/meteorology to winter haze pollution was 24.3% from 1984 to 2016 and the modulating role of climate and meteorological conditions increased significantly after 2010 (CONave = 55.8%). This was attributed to a significant decrease (increase) in sea level pressure and surface wind speed (500 hPa geopotential height and surface temperature) in the FWP. The explained variance (R2) and mean absolute error (MAE) of the models for the entire winter period were 88% and 3.54 days, respectively. For predictions in November, December, January, and February, the R2 was above 71% and the MAE was below 1.83 days. Furthermore, independent predictions for 2018 showed that the bias in the monthly models was lower by 5 days compared to the entire winter model. The results of a recycling independent test indicated that all the models evaluated had excellent stability, proving that the climate factors chosen by the AOSM do affect haze pollution in the FWP.

A teleconnection between sea surface temperature in the central and eastern Pacific and wintertime haze variations in southern China

Haze pollution in recent decades varies largely with both pollutant emissions and meteorological conditions. Using the discrete wavelet transform (DWT) method, we separate these two influences on haze variations in southern China in the time series of haze observations from 1981 to 2011. This helps us to identify the meteorological influence on interannual variation in haze occurrences in southern China and thus observe a teleconnection between the thermal forcing of sea surface temperature (SST) in the central and eastern Pacific and wintertime haze occurrences in southern China (R = − 0.51, p < 0.05). The total haze days in winter is highest among all seasons over southern China and the climotological mean of number of winter haze days is 7.5 days for the region. Compared with the normal winters, the regional mean of the number of haze days in southern China is reduced by ~ 5 days in the winters with above-normal Niño3.4 SST (during El Niño phases), but increased by ~ 4 days in the winters with below-normal Niño3.4 SST (during La Niña phases). In the warm SST winters, the cumulative consequences of strong winds, more precipitation, and a more unstable atmosphere with an “upper colder and lower warmer” vertical pattern leading to more ascendance can all hinder haze formation, whereas in the cold SST winters, opposite meteorological conditions are favorable to haze formation. These meteorological conditions induced by anomalous SST make wintertime haze pollution in southern China vary from year to year to a large extent. This study suggests a strong sensitivity of winter haze occurrences in southern China to the viability of the SST in the central and eastern Pacific.

Associations between winter atmospheric teleconnections in drought and haze pollution over Southwest China

In the early 21st century, Southwest China (SWC) frequently experienced extreme droughts and severe haze pollution events. Although the meteorological causes of these extreme droughts have been widely investigated, previous studies have yet to understand the causes of haze pollution events over SWC. Moreover, the associations between winter atmospheric teleconnections during drought and haze pollution event across SWC has received negligible attention and therefore warrants investigation. This study examines the associations between the atmospheric teleconnections with respect to winter droughts and winter haze pollution over SWC. Our main conclusions are as follows. (1) Winter precipitation and winter haze days (WHD) over SWC had three major fluctuations from 1959 to 2016. (2) The atmospheric circulation pattern over the Eurasian (EU) continent associated with WHD over SWC resembled that of winter droughts over SWC, where both can be characterized by an EU teleconnection pattern. The Arctic Oscillation (AO) mainly induced the atmospheric circulation pattern over the EU continent that is associated with WHD over SWC. (3) The sea surface temperature (SST) and low circulation anomalies in the Pacific and north Atlantic associated with WHD were similar to those associated with winter droughts over SWC. La Niña events and negative phases of the North Atlantic Oscillation (NAO) may induce winter drought and increase the WHD over SWC. (4) Compared with winter drought over SWC, the variation in the WHD was more complex and the factors affecting WHD were more diverse, and winter drought and its related atmospheric circulations were important factors that induced haze pollution over SWC. Overall, this study not only fills a gap in the literature with respect to the associations between the atmospheric teleconnections of winter drought and winter haze pollution over SWC, but also provides an important scientific basis for the development of potential predictions of local monthly haze pollution, which improves the forecast accuracy of local short-term haze pollution and enriches the theoretical understanding of the meteorological causes of local haze pollution.

Contrasting impacts of two types of El Niño events on winter haze days in China's Jing-Jin-Ji region

Abstract. El Niño events differ widely in their patterns and intensities. The regional climate anomalies caused by different types of El Niño events likely lead to various impacts on winter haze pollution in China. Based on long-term site observations of haze days in China from 1961 to 2013, this study explores the effects of eastern Pacific (EP) and central Pacific (CP) types of El Niño events on the number of winter haze days (WHDs) in China's Jing-Jin-Ji (JJJ) region and the physical mechanisms underlying WHD changes. The results show statistically significant positive and negative correlations, respectively, between WHDs in the JJJ region and EP and CP El Niño events. At most sites in the JJJ region, the average WHD increased in all EP El Niño years, with the maximum change exceeding 2.0 d. Meanwhile, the average WHD decreased at almost all stations over this region in all CP El Niño years, with the largest change being more than −2.0 d. The changes in large-scale circulations indicate obvious positive surface air temperature (SAT) anomalies and negative sea level pressure (SLP) anomalies over North China, as well as southerly wind anomalies at the middle to low troposphere over eastern China in the winters of EP El Niño years. These anomalies are conducive to increases in WHDs in the JJJ region. However, there are significant northerly and northwesterly wind anomalies at the middle to low troposphere over eastern China, as well as stronger and wider precipitation anomalies in the winters of CP El Niño years, which contribute to decreased WHDs over the JJJ region. Changes in local synoptic conditions indicate negative SLP anomalies, positive SAT anomalies, and weakened northerly winds over the JJJ region in the winters of EP El Niño years. The total occurrence frequency of circulation types conducive to the accumulation (diffusion) of aerosol pollutants is increased (decreased) by 0.4 % (0.2 %) in those winters. However, the corresponding frequency is decreased (increased) by 0.5 % (0.6 %) in the winters of CP El Niño years. Our study highlights the importance of distinguishing the impacts of these two types of El Niño events on winter haze pollution in China's JJJ region.

Association of Climate-related Total Atmospheric Energy Anomalies in the Tibetan Plateau with Haze in Eastern China

ABSTRACT As climate warming is widespread over the world, the Tibetan Plateau is particularly sensitive to climate warming impacts owing to its high elevations and complex topography, receiving worldwide attention for its significant feedbacks through affecting the atmosphere circulation, and thus the Asia climate system even to global. In this study, the new metric “total atmospheric energy”, is considered to investigate the association of thermal anomalies on the Tibetan Plateau with haze events in China. The total atmospheric energy (TPE) of the Tibetan Plateau, including sensible heat energy, potential energy, kinetic energy, and latent heat energy, was calculated using US National Center for Environmental Prediction (NCEP) meteorological reanalysis data from 1980 to 2016 to characterize the atmospheric thermal forcing of the large topography of the Tibetan Plateau. Results show that TPE has displayed periods of increase levels in recent decades, and inter-annual variations of TPE are significantly positively correlated with winter haze days over northern China but are negatively associated with haze events in southern China. Further diagnostic analysis and simulated results using the Weather Research and Forecasting (WRF)-Chem model indicate that the anomalous increase of TPE leads to enhanced thermal stability of the lower atmosphere, a weakened East Asian winter monsoonal wind in northern China, and increased near-surface wind speed in southern China, which are conductive to positive PM2.5 anomalies in northern China and negative PM2.5 anomalies in southern China. This implies that meteorological changes induced by TPE anomalies may play an important role in wintertime haze pollution over different regions of eastern China. Understanding the climate-related TPE modulating the favourable meteorological conditions for winter haze pollution over Eastern China could contribute to long-term planning for air pollution control.

Severe winter haze days in the Beijing–Tianjin–Hebei region from 1985 to 2017 and the roles of anthropogenic emissions and meteorology

Abstract. We applied a global 3-D chemical transport model (GEOS-Chem) to examine the variations in the frequency and intensity in severe winter haze days (SWHDs) in Beijing–Tianjin–Hebei (BTH) from 1985 to 2017 and quantified the roles of changes in anthropogenic emissions and/or meteorological parameters. Observed SWHDs were defined as the days with daily mean PM2.5 concentration exceeding 150 µg m−3, and simulated SWHDs were identified by using the same threshold but with adjustment on the basis of simulation biases. Comparisons between the simulated SWHDs and those obtained from the observed PM2.5 concentrations and atmospheric visibility showed that the model can capture the spatial and temporal variations in SWHDs in China; the correlation coefficient between the simulated and observed SWHDs is 0.98 at 161 grids in China. From 1985 to 2017, with changes in both anthropogenic emissions and meteorological parameters, the simulated frequency (total severe haze days in winter) and intensity (PM2.5 concentration averaged over severe haze days in winter) of SWHDs in BTH showed increasing trends of 4.5 d per decade and 13.5 µg m−3 per decade, respectively. The simulated frequency exhibited fluctuations from 1985 to 2017, with a sudden decrease from 1992 to 2001 (29 to 10 d) and a rapid growth from 2003 to 2012 (16 to 47 d). The sensitivity simulations indicated that variations in meteorological parameters played a dominant role during 1992–2001, while variations in both emissions and meteorological parameters were important for the simulated frequency trend during 2003–2012 (simulated trends were 27.3 and 12.5 d per decade owing to changes in emissions alone and changes in meteorology alone, respectively). The simulated intensity showed a steady increase from 1985 to 2017, which was driven by changes in both emissions and meteorology. Process analysis on all SWHDs during 1985–2017 indicated that transport was the most important process for the formation of SWHDs in BTH with a relative contribution of 65.3 %, followed by chemistry (17.6 %), cloud processes (−7.5 %), dry deposition (−6.4 %), and planetary boundary layer (PBL) mixing (3.2 %). Further examination showed that SWHDs exhibited large interannual variations in frequency and intensity, which were mainly driven by changes in meteorology. The results of this study have important implications for the control of SWHDs in BTH.

High time resolution observation of PM2.5 Brown carbon over Xi'an in northwestern China: Seasonal variation and source apportionment

There is growing evidence suggesting the enhancement of brown carbon (BrC) in severe haze episodes. In this study, hourly measurements of BrC in PM2.5 were conducted in Xi'an, a typical city in northwestern China during winter and summer. The absorption coefficient for methanol exacts at 365 nm (babs365, methanol, which is typically used as a proxy of methanol-soluble BrC) in the winter sampling period was over 7 times than that in summer. The mass absorption cross-section for methanol extracts (MAC365, methanol, normalized by babs365, methanol to organic carbon, OC) in winter sampling period was nearly 1.5 times of that in the summer. During the winter haze days, the average babs365,methanol peaked at midnight and the lowest values in the morning, in contrast to high levels in afternoon and low levels at night in non haze days. Unlike the diurnal patterns in winter, summer babs365, methanol diurnal variation presented high midday and low afternoon levels in haze days. However, in non haze days, the pattern showed high morning levels and night low levels. Haze and non haze variations of chemical species levels, babs365, methanol, and MAC365, methanol during winter and summer sampling time showed that the effects of atmospheric aging were complex and could either enhance or reduce light absorption of BrC. Source apportionment based on positive matrix factorization receptor model and multiple linear regressions showed that primary emission was an important contributor to BrC emissions during the winter sampling period, whereas secondary formation played an important role in summer.

Climate modulation of Niño3.4 SST-anomalies on air quality change in southern China: Application to seasonal forecast of haze pollution

Air stagnation modulates the frequency and duration of haze events. Based on meteorological and environmental observation data during 1980 to 2013, the present study analyzed the relationship between the interannual variations of Sea Surface Temperature (SST) over tropical central-eastern Pacific and the number of winter haze days (WHD) over the Southern China (SC) region. The potential preceding signal of Sea Surface Temperature anomalies (SSTAs) in Niño3.4 region associated with ENSO can be used as a predictor of haze occurrences in winter. Results indicate that the detrended WHD in the SC is significantly correlated (r = −0.55) with the contemporary SSTAs in Niño3.4 region. The winters with warm Niño3.4 SSTAs in (El Niño) tend to be accompanied with less haze events in the SC, resulting from more local precipitation and enhanced mid-level winds, which helps to build an unstable condition that is conductive for the decrease of haze occurrence. The precursory signal of WHD variability can be detected in the tropical central-eastern Pacific SST, which is amplified since August to winter season. Based on the August–October mean SSTAs in Niño3.4 and three other identified predictors, we developed a seasonal prediction model of the WHD using RF regression method. The model accounted for 90% of the total variance of the WHD in the SC and ranked the SSTAs in Niño3.4 as the most important predictor. This implies that the SSTAs tropical Pacific play a significant role in the variability of WHD in the SC. Since the predictors can be readily monitored in real time, the model provides a real time forecast tool and could brighten the prospects for seasonal forecast of haze anomalies in vulnerable regions such as the SC.

Mercury stable isotope compositions of Chinese urban fine particulates in winter haze days: Implications for Hg sources and transformations

Atmospheric pollution by fine particulates and mercury (Hg) in emerging economies is a serious environmental concern. Here, we present Hg concentrations and isotope compositions of 24-hour integrated fine particulate matter (PM2.5) samples collected during January 2014 at four large Chinese cities (three inland cities: Beijing, Changchun and Chengdu; one coastal city: Hong Kong), with an aim of identifying the potential Hg sources and transformation processes. Mean concentrations of PM2.5 (171 ± 62 μg m−3) and PM2.5-bound Hg (1.3 ± 1.1 ng m−3) in Chengdu were the highest. Overall, PM2.5 samples in Chinese inland cities were characterized by moderately negative δ202Hg (−1.08 ± 0.64‰, 1σ), slightly negative Δ199Hg (−0.13 ± 0.28‰, 1σ) and insignificant Δ200Hg (0.03 ± 0.05‰, 1σ). On average, δ202Hg of PM2.5 was the highest in Chengdu (−0.74 ± 0.67‰, 1σ), followed by Beijing (−1.11 ± 0.26‰, 1σ) and Changchun (−1.60 ± 0.45‰, 1σ). PM2.5 from Beijing showed the most negative Δ199Hg (−0.31 ± 0.40‰, 1σ) that was significantly lower than Changchun (−0.12 ± 0.21‰, 1σ) and Chengdu (−0.02 ± 0.15‰, 1σ). Coal combustion and cement production were identified to be the dominant sources of PM2.5-bound Hg in these cities, with additional Hg sources from non-ferrous metal smelting in Chengdu. Besides, Hg emissions from biomass burning were evident in specific days for each city. We found that source materials and isotope fractionation during emission processes could not fully explain the observed Hg isotope compositions in PM2.5, especially the large negative Δ199Hg values (<−0.3‰) in Beijing. The near-unity slope of Δ199Hg vs. Δ201Hg in PM2.5 samples from each studied city indicates that HgII in the fine aerosols was likely photo-reduced to different degrees in the atmosphere following emission from sources.

Impacts of different types and intensities of El Niño events on winter aerosols over China

El Niño is a strong signal of interannual climate variability. The occurrence of an El Niño event may have an important impact on atmospheric pollutant concentrations in China through changes to the regional climate. This study quantitatively examines the differences among the effects of different types and intensities of El Niño events on winter mean aerosol concentration and severe haze days in China. It is found that Central Pacific (CP) El Niño events lead to larger increases in the aerosol burden over southern China relative to the same intensity of Eastern Pacific (EP) events. For El Niño events of the same type but with different intensities, moderate events induce large increases in surface aerosol concentrations over the entire eastern China, while strong and weak events lead to obvious decreases in the surface aerosol concentration over northern China. These differences are mainly attributed to changes in atmospheric circulation, with the resulting changes in aerosol mass transport occurring in response to the different types of El Niño event. Except for moderate CP events, all kinds of El Niño events lead to an increase in severe haze days in winter over northern China. Strong and weak EP El Niño events and weak CP El Niño events lead to an increase in severe haze days in winter over southern China. A change in the south-north transport of aerosols caused by El Niño events also play a dominate role in changing the occurrence of winter haze days in China.

An observation-based perspective of winter haze days in four major polluted regions of China.

An observation-based approach is used to examine key characteristics of winter haze days in four major polluted regions in China. Major findings in this study are: first, there was no significant trend in the number of winter haze days in most provinces and districts in eastern China from 1973 to 2012, contrary to the 2.5-fold increase in the emissions of particulate matter and its precursors (PM emissions) in the same period of time. Second, meteorological and climate conditions rather than PM emissions are in control of the interannual variabilities and trends of winter haze days. These interannual variabilities (ranging from 24 to 125%) pose a substantial masking effect that must be overcome by any control of PM emissions before its impact becomes statistically detectable. Finally, we find that global warming may have contributed significantly to the trend of winter haze days in eastern China.

How does El Niño-Southern Oscillation modulate the interannual variability of winter haze days over eastern China?

The haze pollution over eastern China has evident interannual variability. Based on the observed daily visibility at the meteorological stations from 1980 to 2018, the interannual variability of winter haze days (WHD) and its relation to El Niño-Southern Oscillation (ENSO) are investigated. ENSO has a significant negative modulation on the WHD over southern China (south of 30°N) and an insignificant positive modulation on the WHD over North China (north of 35°N). Based on the analyses on the haze-prone weather condition and climate variability conducive to excessive WHD anomaly at interannual time scale, anomalous subsidence with suppressed precipitation plays a dominant role for haze over southern China where wet deposition is the most important, while southerly wind anomaly plays a dominant role for haze over North China. An anomalous anticyclone (cyclone) over western North Pacific is stimulated by El Niño (La Niña) during its peak phase in winter, and the anomalous ascending (descending) motion on the northwestern flank of this anomalous anticyclone (cyclone) induces a significant excessive (deficient) precipitation over southern China, conducive to less (more) WHD over southern China during El Niño (La Niña) winters via wet deposition. The insignificant southerly (northerly) wind anomaly associated with weakened (enhanced) winter monsoon during El Niño (La Niña) is responsible for the slightly more (less) haze days over North China during El Niño (La Niña) winters.

Variability of winter haze over the Beijing-Tianjin-Hebei region tied to wind speed in the lower troposphere and particulate sources

This study analyzes the variability of winter haze days and visibility in the Beijing-Tianjin-Hebei (BTH) region, in relation to the regional average wind speed changes in the lower troposphere and emissions for 1961–2014. Winter mean surface meteorological data, NCEP/NCAR atmospheric reanalysis data, and fossil fuel emission data are used. The results reveal a significant increase in the haze days of +0.8 days decade−1 (p < .01), and a subsequent decline in visibility of −1.56 km decade−1 (p < .01). Most interestingly, an accelerated increase in the number of haze days was observed for the last 11-year period (+8.3 days decade−1, p < .01) of the study period (2004–2014). Regression analysis indicates that the increase of haze occurrence and decrease in visibility are partly due to the significant (p < .01) declining trend of the mean wind speed in the lower troposphere (−0.19 m s−1 decade−1 at 10 m, (−0.23 m s−1 decade−1 at 925 hPa), and − 0.21 m s−1 decade−1 at 850 hPa), and partly due to the declining (dust storm frequency as a proxy, −0.41 days dec−1) surrounding particulate sources and increasing fossil fuel emissions (total carbon emission as a proxy, +48,206.8 thousand metric tons dec−1). Specifically, wind speed changes in the lower troposphere explain 41.3% of the interannual varation of the winter haze days and 71.2% of the visibility variance. These are extended to 51.7% and 81.6% respectively when combined with information about the natural (dust storm frequency) and anthropogenic (fossil fuel emissions) particulate sources. Therefore, the analyses show that wind speed changes in the lower troposphere, together with the varied natural and anthropogenic sources of particulates, play a key role in modulating winter haze and visibility conditions in the BTH area.

Changes in Haze Trends in the Sichuan-Chongqing Region, China, 1980 to 2016

This study analyzed the long-term variations and trends of haze pollution and its relationships with emission and meteorological factors using the haze days (HDs) data derived from surface observation stations in Sichuan-Chongqing (SCC) region during 1980–2016. The results showed that the multi-year mean number of HDs were 68.7 and 4.9 days for the Sichuan-Basin (SCB) and the rest of SCC region, respectively. The seasonally averaged HDs over SCB reached its maximum in winter (34.7 days), followed by autumn (17.0 days) and spring (11.6 days), and with the minimum observed in summer (5.5 days). The inter-annual variations of HDs in 18 main cities revealed that Zigong, Neijiang, and Yibin, which are located in the southern of SCB, have been the most polluted areas over the SCC region in the past decades. A notable increasing trend in annual HDs over the majority of SCC region was found during 1980–1995, then the trend sharply reversed during 1996–2005, while it increased, fluctuating at some cities after 2006. Seasonally, the increased trend in spring and autumn seems to be the strongest during 1980–1995, whereas the decreased trend in spring and winter was stronger than other seasons during 1996–2005. In addition, a remarkable increasing trend was found in winter since 2006. Using correlation analysis between HDs and emission and meteorological factors during different periods, we found that the variability of local precipitation days (PDs), planetary boundary layer height (PBLH), near-surface wind speed (WS), and relatively humidity (RH) play different roles in influencing the haze pollution change during different historical periods. The joint effect of sharp increase of anthropogenic emissions, reduced PDs and WS intensified the haze pollution in SCB during 1980–1995. In contrast, decreased HDs during 1996–2005 are mainly attributable to the reduction of PM2.5 emission and the increase of PDs (especially in winter). In addition, the decrease of PDs is likely to be responsible for the unexpected increase in winter HDs over SCB in the last decade.

The effects of El Niño–Southern Oscillation on the winter haze pollution of China

Abstract. It has been reported in previous studies that El Niño–Southern Oscillation (ENSO) influenced not only the summer monsoon, but also the winter monsoon over East Asia. This contains some clues that ENSO may affect the winter haze pollution of China, which has become a serious problem in recent decades, through influencing the winter climate of East Asia. In this work, we explored the effects of ENSO on the winter (from December to February) haze pollution of China statistically and numerically. Statistical results revealed that the haze days of southern China tended to be fewer (more) than normal in El Niño (La Niña) winter, whereas the relationships between the winter haze days of northern and eastern China and ENSO were not significant. Results from numerical simulations also showed that ENSO influenced the winter atmospheric anthropogenic aerosol content over southern China more obviously than it did over northern and eastern China. Under the emission level of aerosols for the year 2010, winter atmospheric anthropogenic aerosol content over southern China was generally greater (less) than normal in El Niño (La Niña) winter. This was because the transport of aerosols from South and Southeast Asia to southern China was enhanced (weakened), which masked the better (worse) scavenging conditions for aerosols in El Niño (La Niña) winter. The frequency distribution of the simulated daily surface concentrations of aerosols over southern China indicated that the region tended to have fewer clean and moderate (heavy) haze days, but more heavy (moderate) haze days in El Niño (La Niña) winter.

A comparison of the effects of interannual Arctic sea ice loss and ENSO on winter haze days: Observational analyses and AGCM simulations

This study compares the impacts of interannual Arctic sea ice loss and ENSO events on winter haze days in mainland China through observational analyses and AGCM sensitivity experiments. The results suggest that (1) Arctic sea ice loss favors an increase in haze days in central–eastern China; (2) the impact of ENSO is overall contained within southern China, with increased (reduced) haze days during La Nina (El Nino) winters; and (3) the impacts from sea ice loss and ENSO are linearly additive. Mechanistically, Arctic sea ice loss causes quasi-barotropic positive height anomalies over the region from northern Europe to the Ural Mountains (Urals in brief) and weak and negative height anomalies over the region from central Asia to northeastern Asia. The former favors intensified frequency of the blocking over the regions from northern Europe to the Urals, whereas the latter favors an even air pressure distribution over Siberia, Mongolia, and East Asia. This large-scale circulation pattern favors more frequent occurrence of calm and steady weather in northern China and, as a consequence, increased occurrence of haze days. In comparison, La Nina (El Nino) exerts its influence along a tropical pathway by inducing a cyclonic (anticyclonic) lower-tropospheric atmospheric circulation response over the subtropical northwestern Pacific. The northeasterly (southwesterly) anomaly at the northwestern rear of the cyclone (anticyclone) causes reduced (intensified) rainfall over southeastern China, which favors increased (reduced) occurrence of haze days through the rain-washing effect.

Seasonal Variation in Water-Soluble Ions in Airborne Particulate Deposition in the Suburban Nanjing Area, Yangtze River Delta, China, During Haze Days and Normal Days.

To investigate the seasonal variation and characterization of water-soluble ions (WSIs) present in airborne particle deposition (APD) during Haze Days (visibility ≤7.5km) and Normal Days (visibility >7.5km) in suburban Nanjing area, 151 filter samples were collected from 18 May 2013 to 26 May 2014. Ten different WSIs from the samples were determined by Ion Chromatography. The results indicated that secondary WSIs (NH4+, NO3-, and SO42-) were the main ions in the WSIs, averaging 17.2, 18.5, and 17.1μg/m3, respectively, and accounting respectively 20.9, 22.5, and 20.8% of the total WSIs. On Haze Days, the concentration of WSIs increased dramatically in fine size (particle size <2.1μm), especially for NH4+, NO3-, and SO42- (increased by 52.6, 71.3, and 73.1%, respectively), whereas the concentrations of WSIs increased slowly in coarse size (2.1μm<particle size<10μm), in which NH4+, NO3-, and SO42- increased by 14.7, 27.2, and 54.5%, respectively. According to the backward trajectories and the principal component analysis analysis, Nanjing APD were mainly derived from the soil dust in northern China (35%) in the spring, from ocean air masses (61 and 55%) in the summer and the autumn, and from local air masses (73%) in the winter. On summer Haze Days, secondary components in PM2.1 consisted mainly of (NH4)2SO4 and NH4NO3, whereas secondary components in PM2.1-10 consisted mainly of (NH4)2SO4, NH4Cl, and NH4NO3. The increasing concentrations of secondary components increase the light extinction coefficients of aerosol on winter and autumn Haze Days. The concentrations of WSIs in fine size rose sharply on Haze Days, leading the visibility to exponential decline. Differently, the concentrations of WSIs in coarse size were not the main cause in the change of the visibility.

Statistical Prediction of Winter Haze Days in the North China Plain Using the Generalized Additive Model

AbstractWinter (December–February) haze days in the North China Plain (WHDNCP) have recently dramatically increased. In addition to human activities, climate change and variability also contributed to the severe situation and supported the possibility of seasonal predictions. In this study, using the generalized additive model (GAM), the sea surface temperature around the Alaska Gulf and sea ice area of the Beaufort Sea were selected as the predictors to establish a statistical prediction model (SPM). The difference between the current and previous year of WHDNCP (WDY) was predicted first and was then added to the observation of the previous year to obtain the final predicted WHDNCP. For WDY prediction, the root-mean-square error of the SPM using GAM was 3.01 days. In addition to the annual variation, the tropospheric biennial oscillation features and the dramatically increasing trend after 2010 were both captured successfully. Furthermore, for the final predicted WHDNCP anomalies, the long-term trend and turning points were simulated well, and the percentage of the same mathematical sign was 91.7%. Independent prediction tests were performed for 2014 and 2015, and the forecast bias was 0.86 and 0.19 days, respectively. To assess the predictive ability, recycling independent tests (including real-time hindcasts for the period 2005–15) were also applied, and the percentage of the same sign was 100%.