| Air Pollution has been a global problem over the past decades.Besides anthropogenic emissions,meteorological conditions and climate change have been found to play very important roles in the inter-annual variability and long-term trend of air quality.Based on the air pollutants data from ground-based measurements,using several statistical methods(i.e.wavelet analysis,ensemble empirical mode decomposition,weather typing technique),our research aimed to understand the impact of meteorological conditions and climate change on air quality in three most developed regions of China.Specially,we focus on the response of local weather patterns and meteorological conditions to climate change and their impact on the inter-annual/inter-decadal variability and spatial variation of potential air quality over Beijing-Tianjin-Hebei(BTH)and Yangtze River Delta(YRD)regions of China.Light extinction coefficients(bext)were calculated from visibility data to partly represent air quality.Using wavelet analysis and Ensemble empirical mode decomposition,regional averaged bext in three research areas were decomposed into three sub-components with different periods.The short-period/high frequency component showed a negative correlation between bext in autumn and bext in the following winter in all the three selected areas,with a correlation up to-0.7 in the BTH region.The inter-annual shifting of the dominating period of East Asian winter monsoon was found to be the dominant factor.Component with longer period showed that the inter-decadal variability of bext in the BTH region was different from that in other two regions of China.The BTH bext showed an increase from 1980 to early 1990s and a decrease afterwards,while the bext was increased during the entire study period in the other two regions of China.However,in order to understand the relative contribution of meteorological conditions and the climate change to inter-annual/inter-decadal variability of air quality over the three regions of China,we need to eliminate the influence of anthropogenic emissions.Thus based on the ground-based O3 observation data,we modified a method to reconstruct "synoptic air pollution index",which was the sub-component only influenced by meteorological conditions.Synoptic weathers over Pearl River Delta(PRD)region of China(Hong Kong)during the time period of 1999-2001 were classified into seven typical patterns using a semi-objective weather typing technique.The highest O3 concentrations were found to be associated with the influence of Northwest Pacific typhoons,whereas the lowest average concentrations were linked to southerly flow introduced by summer monsoons.Taking into account inter-annual variabilities in frequency and intensity of circulation patterns,reconstructed "synoptic O3" captured up to 50%of the observed inter-annual variability and 36%of the increasing trend.This study highlights an important linkage between weather/climate and air quality.In the following work,we analyzed the air pollution index(API)based on a modified reconstruction method over the most polluted regions in China:the Beijing-Tianjin-Hebei and Yangtze River Delta regions."Synoptic API" was reconstructed based on the frequencies of the occurrence of synoptic weather patterns and the evolution process of API under the continuous influence of certain weather patterns,based on which,we further analyzed the inter-annual and inter-decadal variation of air pollution over these two regions from the climatological perspective.The results are as follows:API in BTH and YRD was also closely connected with the weather types.When mainland China was under the influence of a weak anti-cyclonic system,these two regions were suffering from more frequent air pollution.API in North China was found to increase with an averaged rate of 8.5/day as a result of stable meteorological conditions and regional transport from the southern industrial area.Same increasing pattern was observed in East China with the averaged rate up to 12/day.The strong high pressure system over the mainland China with associated strong northwesterly was found to have a strong sweeping effect of air pollutants.As a result,API value was relatively lower than normal.Time series of climate-induced variabilities over 34 years were extracted from 10-year API using Comrie’s discriminating method to extrapolate this component to climatic time scales.The day-to-day variation of "synoptic API" was found to be significantly correlated with the observed API with correlation coefficients R=0.38(spring),R=0.39(autumn),R=0.44(winter)in BTH and R=0.4(summer),R=0.38(autumn)in YRD,which suggests that day-to-day variation of API was partly modulated by the evolution of weather types.The seasonal averaged reconstructed API in winter over the BTH was somehow in phase with observed API,however no significant correlation coefficient could be obtained.In autumn,seasonal averaged reconstructed API in YRD was found to be significantly correlated with observed one with a coefficient of R =0.46.Reconstructed API was found to be significantly correlated with bext aver the time period of 1980-2013 both in the inter-annual(R=0.50)and in the inter-decadal(R=0.52)scale,which suggests that the inter-annual and inter-decadal variability of bext in BTH was greatly influenced by the change of meteorological conditions induced by climate change.With reference to climatic index,reconstructed API in BTH in winter and spring was significantly correlated with NPO/WP index with the correlation coefficients R=0.58 and R=0.38.While reconstructed API in YRD was found to be significantly negatively correlated with NO index in autumn and winter with the correlation coefficients R=0.-0.57 and R=-0.58.Four combinations of NPO/WP and NO were found to modulate the spatial distribution of air quality over North and East China.When NPO/WP and NO were in-phase combinations,a negative correlation between air quality over BTH and YRD was significant.In other words,when NPO/WP and NO were both in positive(negative)phase,the Siberian High and Aleutian Low were weakened(intensified)leading to weaker(stronger)northerlies or northwesterlies,resulting in worse(better)potential air quality over BTH,at the same time,a low(high)pressure anomaly dominated over tropical west Pacific/Indian Ocean introducing more(less)clean oceanic air mass to YRD resulting in better(worse)potential air quality over this region.Under the out-of-phase combinations of NPO/WP and NO,when positive-phase(negative-phase)NPO/WP occurs with negative-phase(positive-phase)NO,northwesterlies over the BTH was weakened(strengthened)while YRD was under the influence of less(more)oceanic flow,worse(better)air quality controlled both BTH and YRD regions of China. |
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