Characteristics And Future Temporal-Spatial Distribution Of Pollutants Released From Forest Fires In Fujian Province

In recent years,the frequency and burned area of forest fires increased significantly with the climate change and the intensification of human activities,which has become one of the most severe natural disasters with widely global influence.Forest fires can release a large number of gaseous and particulate pollutants,which have a serious impact on the atmospheric environment,ecology and human health.Among them,high concentration of CO₂ and NO_x released from forest fire can reduce the stomatal conductance of plant leaves,and affect photosynthesis of aboveground parts and physiological activities of underground roots.Meanwhile,solid particles in smoke can promote photochemical reaction,affect light scattering and absorption,aggravate haze formation and endanger human health.In addition,forest fires release a large number of volatile organic compounds,such as non-methane hydrocarbons（NMHCs）,an important precursor to form photochemical smog,with high photochemical activity,which is extremely harmful to the environment and human health under light.Fujian Province has the highest forest coverage in China with high frequency of forest fire.According to the satellite fire product,the annual average number of forest fire hotspots in the whole province is nearly1000,in which more than 95%are caused by human activities.High frequency forest fires can accumulate and release a large amount of smoke pollutants,significantly affecting the regional atmospheric environment and human health.Therefore,it is of great significance to study the emission characteristics of pollutants from forest fire in Fujian and to analyze their spatial and temporal distribution in the future.The study will be benifical for revealing the impact of forest fires on the atmospheric environment and human health in Fujian and also southern subtropical regions of China.In view of this,a quantitative study was performed on the eight dominant tree species in Fujian,including Masson pine（Pinus massoniana）,Chinese fir（Cunninghamia lanceolata Hook）,Camphor（Cinnamomum camphora（L.）Presl）,Eucalyptus（Eucalyptus robusta Smith）,Keteleeria tree（Keteleeria fortunei（Murr.）Carr）,Slash pine（Pinus elliottii）,Acacia tree（Acacia confusa Merr.）,and Phoebe tree（Phoebe bournei（Hemsl.）Yang）,to reveal the emission characteristics of CO,CO₂,NO_x,PM_2.5,and NMHCs from the combustion of leaves and branches of these tree species under two different combustion conditions（smoldering and flaming）by using a self-designed biomass combustion device.Meanwhile,combining with the topography,meteorology,soil texture,infrastructure and socio-economic factors,the future fire density and spatial distribution of pollutants(CO,CO₂,NO_x,PM_2.5 and NMHCs)of Fujian province in the next 20 years（2020-2039）were predicted based on the geographically weighted regression（GWR）model.The main conclusions of the study are as follows:（1）The emission characteristics of inorganic gaseous pollutants and PM_2.5.5 were strongly affected by the differences of tree species and combustion state.The combustion state has the most significant effect on the emission factors of all inorganic gases and PM_2.5 pollutants.Under smoldering condition,the CO emission of leaves and branches of different tree species was higher than that of flaming condition,while CO₂ emission was opposite,flaming condition is higher than smoldering.The emission of PM_2.5 from branches and leaves of different tree species was higher in smoldering than in flaming condition.（2）Under different combustion conditions,the average emission factors of inorganic pollutants,including CO,CO₂,NO_x and PM_2.5,released from burning of leaves of 8 tree species were 251.69±23.293g/kg and 135.803±16.665 g/kg（smoldering and flaming）,969.900±76.422 and 1272.532±85.195g/kg（smoldering and flaming）,1.716±0.197 and 2.130±0.426 g/kg（smoldering and flaming）,20.230±4.865and 9.584±3.604g/kg（smoldering and flaming）,respectively.The average emission factors of various pollutants in branches under different combustion conditions were 243.165±21.171 and 131.747±16.472 g/kg（smoldering and flaming）,944.346±66.568 and 1269.243±102.786g/kg（smoldering and flaming）,0.752±0.164 and 1.015±0.171g/kg（smoldering and flaming）,18.294±4.918 and 9.613±2.650g/kg（smoldering and flaming）,respectively.The emission factors of all kinds of inorganic pollutants in the leaves of trees were higher than branches under different combustion conditions.（3）Leaves and branches of 8 tree species can release a total of 48kinds of NMHCs,including 19 kinds of alkanes,15 kinds of alkenes and14 kinds of aromatics.The total average emission factors of NMHCs in leaves of different tree species were 9.92g/kg and 7.86 g/kg under smoldering and flaming conditions,respectively.Under smoldering and flaming conditions,the total average emission factors of NMHCs in branches were 14.07g/kg and 6.48 g/kg,respectively.The average emission factor of NMHCs in leaves and branches of trees was higher under smoldering condistion than flaming conditions.（4）Fourteen factors are identified as drivers of forest fire density of Fujian province,including elevation,aspect,slope,monthly mean temperature,monthly mean relative humidity,vegetation coverage,density of highway,grid density of railway,grid density of rivers,grid density of population,GDP,silty soil percentage,sandy soil percentage and clay soil percentage.There was significant spatial heterogeneity between the factors and forest fire density in Fujian.The R² value of the fire density prediction model was 0.83,and the coefficient of variation of the overall model was less than 0.001,indicating a goodness of fit of the fire model fitting of Fujian Province.（5）The density of forest fires in Fujian province will shift from west to northwest in the next 20 years（2020²039）,and the density of forest fires in southern Fujian will gradually increase with an increased rate of13%.In the next 20 years,the burned area caused by forest fire in Fujian province will reach about 1813.14 km² with an average annual area of about 81.39 km².In the next 20 years,forest fires in Fujian province are expected to consume about 1.06×10⁴ kt of forest biomass with an average annual number of 500 kt.（6）The forest fires in Fujian province are expected to release 1.39×10⁴ kt of pollutants in the next 20 years with an average annual amount of131.98 kt,in which the emissions of CO,CO₂,NO_x,PM_2.5,and NMHCs are 2008.89,11677.66,15.22,172.68 and 12.25kt respectively.In addition,the total emission of atmospheric pollutants of coniferous and broad-leaved species will be 1.12×10⁴ kt and 2.73×10³ kt,respectively,accounting for 71.94%and 28.06%,respectively.It is shown that the central area of Fujian is the high concentration area of pollutants released by forest fires according to the results of spatial distribution of pollutants.In the next 20 years,the spatial distribution of pollutants will show a radial diffusion trend,and the emission intensity of pollutants in northwest Fujian and southern Fujian will increase significantly.