Effects Of Regional Afforestation On Air Pollution In Northern China

Since the early 1980s,rapid industrialization and urbanization have been imposing serious adverse effects on air quality and environment,and posing increasing threat to human health across China.Particularly in Northern China,where as the pillar of national economic development,many heavy industries and energy-related industries in China are located.The fossil fuels combustion and vehicle exhaust generated emissions of air pollutants,such as sulfur dioxide?SO₂?,nitrogen oxide?NO_x?,ozone?O₃?and particulate matter(PM_2.5).It has been widely known that plants can uptake air pollutants,and plays a crucial role in terrestrial ecosystem health and safety.The uptake of air pollutants by vegetation via dry deposition and other environmental process are important pathway to remove air pollutants from the atmosphere,improving air quality and ecosystem health.It is worth noting that,while trees and plants can efficiently remove pollutants from the atmosphere,they are also involved in atmospheric chemistry by releasing large quantities of reactive biogenic volatile organic compounds?BVOCs?into the atmosphere as a significant product of biosynthetic activities of trees and plants.BVOCs play important roles in tropospheric chemistry,carbon budget,and global climate change.The three northern regions shelter forest?TNRSF?program in China,also known as“the Green Great Wall”,began in 1978,including 11 provinces and two megacities,Beijing and Tianjin.It is estimated that the program will increase China's forest coverage from 5.05%in the 1970s to 15.95%by 2050.By the end of the fourth phase of the program in 2010,the vegetation coverage over the TNRSF had already reached12.4%.As the largest afforestation activity in human history,the TNRSF in northern China has improved significantly the local ecological environment.As the main construction area of the TNRSF project,vegetation cover in the North China Plain?NCP?region has shown a significant increasing trend in recent decades,particularly along Mount Taihang and the northeast of Hebei Province.This study estimated dry deposition fluxes of two criteria air pollutants?SO₂ and NO_x?across the TNRSF region in Northern China and their long-term trends from1982 to 2010 using a dry deposition model.The results show that,dry deposition velocities of SO₂ and NO_x increased in many places of the TNRSF up to 118.2%for SO2 and 112.1%for NO_x over the last three decades due to the increasing vegetation coverage over the TNRSF.The highest atmospheric deposition fluxes of SO₂ and NO_x were found in the Central-North China region of the TNRSF,followed by the Northeast and the Northwest China regions of the TNRSF.A total of 820,000 t SO₂and 218,000 t NO_x was estimated to be removed from the atmosphere through dry deposition process over the TNRSF from 1982 to 2010.About 50%of the total removal occurred in the Central-North China region.By comparison,the estimated total SO₂ and NO_x dry deposition fluxes from 1982 to 2010 between a TNRSF site in this region and an adjacent farmland outside the TNRSF showed that the fluxes of these two chemicals at the TNRSF site were the factors of 2-3 greater than their fluxes in the farmland,indicating the important effect of TNRSF on the removal of atmospheric pollutants.Atmospheric removal of PM_2.5 by the TNRSF through dry deposition process was also assessed using a bulk big-leaf model and a vegetation collection model in this study.The dry deposition flux of PM_2.5 and their decadal trend from 1999 to 2010were calculated from modeled dry deposition velocity and air concentration retrieved from the satellite remote sensing.Modeling results show that dry deposition velocities of PM_2.5 calculated using the two deposition models increased in many places of the TNRSF over the last decade due to increasing vegetation coverage of the TNRSF.Both increasing deposition velocity due to forest expansion and PM_2.5 atmospheric level contributed to the increasing deposition flux of PM_2.5 during the time period of1999-2010.The highest atmospheric deposition flux of PM_2.5 was again found in the Central-North China region of the TNRSF covering the most parts of Beijing-Tianjin-Hebei area,followed by the Northwestern and the Northeastern China regions of the TNRSF.Whereas,greater collection of PM_2.5 by vegetation was identified in the Northeastern China region of the TNRSF,due to higher forest coverage over this region.The most significant incline of the PM_2.5 atmospheric removal due to vegetation collection was discerned in the Central-North China region because of the most rapid increase in the vegetation coverage in this region.A total mass of 2.85×10⁷ t PM_2.5 was estimated to be removed from the atmosphere through dry deposition process over the TNRSF region from 1999 to 2010.The results also suggest that the TNRSF plays a moderate role in PM_2.5 uptake,but enhances PM_2.5atmospheric removal by 26%in 2010 compared with 1980.To further assess the long-term trends of isoprene emissions in northern China and the impact of the TNRSF on these trends,the historical biogenic isoprene emissions from 1982 to 2010 was generated in this region using a biogenic emission model for gases and aerosols?MEGAN v2.1?.It is estimated that the total amount of the biogenic isoprene emissions during the three decades was 4.4 Tg in northern China and 1.6 Tg in the TNRSF,with annual emissions ranging from 132,000 to176,000 t yr^-1 in northern China and from 45,000 to 70,000 t yr^-1 in the TNRSF,respectively.Isoprene emission fluxes have increased substantially in many areas of the TNRSF due to the growing trees and vegetation coverage from 1982 to 2010,especially in the Central-North China region where the highest emission incline reached to 58%over the last three decades.Biogenic isoprene emissions produced from anthropogenic forests tended to surpass those produced from natural forests,such as boreal forests in northeastern China.More importantly,the estimated isoprene emissions suggest that the TNRSF has altered the long-term emission of isoprene trend in north China from a decreasing?slope=-0.533?to an increasing trend for during 1982 to 2010?slope=0.347?,providing strong evidence for the change in the emissions of BVOCs induced by the human activities on a decadal or longer time scale.Surface ozone?O₃?in the North China Plain?NCP?region was simulated using a coupled weather research and forecasting model and atmospheric chemistry model WRF-Chem for the warm seasons?May to September?in four selected years at a10-year interval?1980,1990,2000 and 2010?to examine the effects of increasing forest cover on surface ozone formation and budget.Model simulations were performed under two biogenic emission scenarios of precursor gases with the first one using year-specific annual emissions,which increased from 1980 to 2010 associated with growing vegetation cover,and the second one using the fixed annual emissions of 1980,which assumed no change in vegetation cover across the NCP for a sensitivity test purpose.In both scenarios,year-specific annual anthropogenic emissions were used,which had increased during the three decades due to economic development.Increasing anthropogenic and biogenic emissions together caused markedly increasing surface O₃ concentrations in most areas of the NCP during the time period of 1980-2010.Anthropogenic emissions of O₃ precursors generally dominated over that of biogenic emissions in ozone formation.The contributions of biogenic emissions were significant in certain areas of NCP with extensive afforestation activities from 1980 to 2010.Afforestation alone contributed 1-2%to ozone formation in Beijing metropolitan area and 4-5%to low populated cities with high forest cover.The increase of vegetation cover also reduced the surface concentration of O₃ through the dry deposition process,leading to an increase of removal rate about 6.6%-13.1%,especially in the forest covered and mountainous regions.The biogenic VOC emission made greater contribution to the O₃ budget as compared to the atmospheric removal by the dry deposition,resulting in a net increase in O₃ level over the NCP.In general,in recent decades,with the continuous expansion of the TNRSF project,major air pollutants such as SO₂,NO_x,O₃,PM_2.5 and isoprene in northern China responded,to some extent,to growing vegetation coverage.It is expected that,with continuous expansion of the TNRSF in next several decades,and taking other air pollutants(NH₃,PM₁₀,and so on)into consideration,TNRSF will make increasing contributions to the atmospheric removal of air pollutants and the improvement of air quality in northern China.Investigations in these aspects are underway.