Characteristics, Causes And Impacts Of Biomass Burning Aerosol Changes In Spring In Indochina Peninsul

Biomass burning activities in the Indochina Peninsula are significantly regulated by the weather and climate systems,and the aerosol from biomass burning can also pose important impacts on the air quality,weather and climate in the surrounding regions and East Asia,which draws widespread attention.Based on satellite observations,reanalysis data and numerical model,from the perspective of weather and climate scales,this paper presents a systematic study of the spatial and temporal variation of biomass burning aerosols in the Indochina Peninsula,to provide a scientific and theoretical basis for air pollution control,weather forecasting and climate change response in Southeast Asia and East Asia.The main conclusions are as follows:(1)The weather and interannual time-scale variation characteristics of biomass burning aerosols in the Indochina Peninsula were investigated.Based on the satellite observations and reanalysis data,it is found that the biomass burning activities in the Indochina Peninsula mainly occur during January-April each year,showing obvious inter-month variation and spatial distribution differences.In January-February(winter),biomass burning occurs mainly in the southern part of the Indochina Peninsula,where the intensity of aerosol emissions is relatively weak.In contrast,biomass burning in March-April(spring)mainly occurred in the northern part of the Indochina Peninsula,the aerosol emission intensity was most prominent in March.Biomass burning aerosols in spring show significant 5–12 days weather-scale variability and ～3 years interannual variability,playing a dominant role in the interannual-scale variability of East Asian aerosols.(2)The main weather patterns that dominate the weather-scale variation of spring biomass burning aerosols in the Indochina Peninsula and their influence mechanisms were analyzed.Choosing 64 high biomass burning aerosol events in the Indochina Peninsula,three types of dominant weather patterns were obtained by using the objective weather typing method based on oblique rotated T-type principal component analysis(T-PCA)to classify the850 h Pa geopotential height field over Southeast and East Asia.The first type is the cold highpressure southward type(the frequency is 25%).In the early stage,the subtropical high(SH)over the Bay of Bengal delivers dry air to the northern part of The Indochina Peninsula,the fire activities were active and the aerosol emission increased;in the later stage,the precipitation and cooling brought by the cold high pressure southward from the Asian continent are suppressive to the burning activity,thus the aerosol decreases.The second type is the SH belt break type(48.44% frequency).In the first period,the Indochina Peninsula was under the control of the east-west penetrating SH with heat and rainless,thus the biomass burning aerosol increased rapidly;in the later period,the high-pressure belt broke in the west of the Indochina Peninsula,and the precipitation in the Indochina Peninsula increased,the aerosol concentration decreased.The third type is the two high-pressure confrontation type(20.31% frequency).In the early stage,similar to the first type,the high-pressure in the Bay of Bengal enhanced the biomass burning aerosol emissions in the northwestern part of the Indochina Peninsula.In the later period,with the convergence of weak cold air from Asia,the West Pacific SH located over the South China Sea enhanced and moved southward,causing more precipitation and fewer aerosols to the north of the Indochina Peninsula.(3)It is revealed that the interannual variation of springtime biomass burning aerosols in the Indochina Peninsula was regulated by El Ni(?)o-Southern Oscillation(ENSO)and dominated the variation of East Asian aerosols.Synthetic analysis shows that in the spring following the previous El Ni(?)o(La Ni(?)a)event,more(less)carbonaceous aerosols can be emitted due to the decrease(increase)in precipitation and drier(wetter)air,and more(lower)biomass burning emissions in the northern part of the Indochina Peninsula.Meanwhile,the anomalous anticyclone(cyclone)in the northwest Pacific Ocean inspired by El Ni(?)o(La Ni(?)a)enhances(weakens)the low-level southwesterly airflow from the northern part of the Indochina Peninsula to southern Japan,which transports more(less)carbonaceous aerosols to East Asia,and thus dominates the change of East Asian aerosols.Overall,the contribution of biomass burning aerosols to the interannual variability of East Asian aerosols exceeds 60%during El Ni(?)o and 80% during La Ni(?)a.(4)It is revealed that the March biomass burning aerosols in the Indochina Peninsula showed distinct instant and delayed effects on local precipitation,and the underlying mechanisms were clarified.Both statistical and numerical simulations showed that March biomass burning aerosols have a suppressive effect on precipitation in the same period(March),while turning to a facilitative effect on precipitation in the later period(early-to mid-April).The physical mechanism is as follows: on the one hand,biomass burning aerosols heat the lower and middle troposphere and cool the boundary layer through radiative forcing,thus enhancing atmospheric stability and suppressing convective precipitation;on the other hand,the diabatic heating anomaly caused by aerosol radiative forcing stimulates the atmospheric circulation response,forcing low-pressure anomalies in the lower troposphere and enhancing water vapor conditions in the Indochina Peninsula through Ekman pumping.During the same period(March),the convective inhibition caused by the increased stability is stronger than the precipitation promotion brought by the increased water vapor conditions,so the overall performance is to inhibit rainfall;while in the later period(April),the convective inhibition effect triggered by the stability diminishes rapidly due to the large reduction of aerosols,but the circulation field response does not disappear immediately.As the monsoon advances,it can provide more water vapor and enhance the convective instability above 850 h Pa,resulting in enhanced precipitation in the Indochina Peninsula,northern South China Sea and southern China.Thereafter,latent condensation heating gradually replaces aerosol radiative heating as the main driving force to maintain the circulation anomaly,allowing the delayed effect to persist until mid-April.Additionally,the aerosol-induced large-scale circulation anomalies will also cause a decrease in precipitation in south and southwest China during March and an increase in precipitation along the outheast coast of China during early and mid April.