Tempo-spatial Characteristics Of PM_2.5 At Multi-scales And Its Influencing Mechanisms In The Core Of The Yangtze River Delta

In recent years,haze pollution caused by high concentration of particulate matter often impacts many cities in China,which has aroused growing concerns.Especially,fine particulate matter(PM_2.5)posed a serious threat to human health,climate and living environment.The coupling effects of human activities,underlying surface,and synoptic conditions drive particulate matters occurring,gathering and then cause haze pollution episode.For haze pollution governance,it is very important to track the tempo-spatial characteristics of PM_2.5,reveal its affecting factors and influencing mechanisms.In this paper,based on multi-disciplinary theories such as landscape ecology,geography,environmental science and meteorology,we selected the core region of the Yangtze River Delta?YRD?where PM_2.5 is very serious as the sample area to examine the multi-scale variations of PM_2.5 and their links with various factors using remote sensing,GIS and computer technology from the special perspective of pollution-episode scale.Our conclusions are useful for guiding regional air pollution control and urban ecological planning.The main conclusions are as follows:?1?The tempo-spatial variations of PM_2.5 in the study area were clarified from conventional multiple scales.In 2014-2016,the YRD witnessed three annual PM_2.5 concentrations all exceeding the China's Ambient Air Quality Standard Grade ??35?g/m3?in China.Although PM_2.5 decreased gradually in three years,the region was still one of three severe regional haze-pollution areas in our country.With respect to a seasonal change,the PM_2.5 concentration was highest in winter,followed by spring and autumn,and was lowest in summer.From January to December,the monthly PM_2.5 concentration showed a U-shaped curve,with high values at January,February and December,and low ones at July,August and September.Regarding the spatial variations,the PM_2.5 concentration was higher in the northwest,decreasing from the northwest to the southeast in the study area.?2?Based on the definition of PM_2.5 pollution-episode scale and three pollution processing indexes?i.e.,the max increment?Mlep?,the daily increase rate?DIep?and the accumulation degree?ADep??,we explored the change and accumulation characteristics of PM_2.5 at this scale.The results indicated that,in 2014-2016,the YRD suffered more than 50 regional pollution episodes,most of which appeared in winter,early spring and late autumn.PM_2.5 pollution episodes in winter were characterized by "a long duration and a wide impact in pollution stage",and by "a large and fast increase in PM_2.5 concentration".While PM2 5 pollution in summer is characterized by "coming fast and disappearing fast".On the spatial difference,PM_2.5 was easier to accumulate in heavily-polluted cities than lightly-polluted ones.However,particulate matter in big cities without severe pollution was also likely to gather due to the concentration of human activities,the large urban area and its microclimate.?3?At pollution-episode scale,human activities play a decisive role in regional haze pollution.The contribution ratio to PM_2.5 pollution from first particles?PM10?,secondary formation?SO₂,NO₂ and CO?and meteorological factors was about 6:2:1,showing that transformations from SO₂ and NO₂ to sulfate and nitrate were also responsible for PM_2.5 pollution besides PM10.In cities with higher socioeconomic development,the contribution of first particles was larger,while the one of secondary formation was smaller;and vice versa.Among the five major pollution emissions,industrial and energy consumption had a largest explanatory power?PD?for PM_2.5 pollution level,while population density,energy consumption per unit area and urban size had the greatest PD for PM_2.5 accumulation.Thus,controlling pollutants discharged by human activities is the essential strategy in mitigating the PM pollution in the YRD.?4?Landscape structures and patterns of the underlying surface of the study area had a significant impact on the spatial variation of PM_2.5,and the influence of the former was obviously greater than the later.In four landscape types:? Containing many human emissions,construction land had the highest PM_2.5 concentration.Moreover,the urban underlying environment was conductive to the accumulation of particles.Therefore,this landscape mainly had an increased effect,which would be stronger in the bigger and more integration patches,on PM_2.5 pollution;? With little human disturbance in forest land,it had the lowest PM_2.5 concentration.At the same time,the vegetation can restrain the dust and adsorb the particulate matter,forest land mainly played a role in reducing PM_2.5,which usually increased with the forest patch expanding;? Farmland and wetland had a complex effect on PM_2.5.They had both increase and reduction effects changing in the different season and environment on PM_2.5.For the contribution ratio on the spatial difference of PM_2.5,the value of forest land was largest,followed by construction land and wetland,and farmland was lowest.Thus,the significant spatial difference of PM_2.5 in the study area is mainly due to the large differences in landscape structure.In the north of the study area,the main three land-use types are farmland,construction land and wetland,which have an increased effect on PM_2.5,but in the south,the dominant landscape is forest land reducing PM_2.5.Consequently,increasing the green land?especially forest land?area,dividing the large construction patches,and making the construction land more fragmented or be "satellite city" are necessary to reduce the cumulative intensity of PM2,5,and strengthen the reduction effect of vegetation on particulate.All of these are uesful for the regional haze pollution control.?5?The influences of meteorological factors on PM_2.5 were compared from the whole-year and the pollution-episode scales.In the whole year,PM_2.5 had significant negative relationships with atmospheric temperature?T?,relative humidity?RH?,precipitation?PRE?,pressure?P0?and wind speed?WS?,but this correlation was weakened or even disappeared in pollution episode.It is supposed that PM_2.5 pollution episodes mainly occurred in weak synoptic conditions.When we select the pollution episode scale to analyze,the great effect of heavy rainfall and large WS was avoided,and then the impacts of weather factors significantly reduced.Moreover,during PM_2.5 episodes,increasing T?accelerating the secondary particle formation?,low WS?causing regional pollution by diffusing pollutants to the surrounding area?,and rising RH?increasing the transformation rate from precursors gas to particles by moisture absorption?may also aggravate haze pollution.In general,temperature had a largest contribution to daily PM_2.5 concentration,and the other factors had similar contribution rates.The contribution of RH varied greatly at different scales,indicating it had uncertain effects on PM_2.5.It was not ignored that RH sometimes caused PM_2.5 increasing.?6?On basis of the relationship between RH and each pollutant,change process and influencing mechanism of PM_2.5 pollution were synthetically evaluated.First,various pollutants discharged by human activities gradually accumulate in a certain space-time scope under the very dry conditions?RH<45%?.At this time,most of gaseous precursors have not yet transformed into fine particles,so fine particulate matters mainly come from local first emission or long-distance transport.PM_2.5 concentration is increasing and fluctuating at this stage,which is before a PM_2.5 pollution episode or its early phase.Second,when RH is 45-70%,an increase in RH can accelerate the moisture absorption and the "gas-particle" transformation rate,and then PM_2.5 concentration begins rising and gradually reaches the peak.Last,when RH>80%,increasing RH can drive particles sinking by its gravity or with rainfall,and PM_2.5 concentration begins decreasing until the pollution process ends.