Effects of future climatic and land use changes on biogenic and biomass burning emissions: implications for future global air quality

Climatic and land use changes have been found to affect atmospheric composition and air quality worldwide, but the mechanisms and pathways involved are not fully understood or quantified. Moreover, chemical transport models that attempt to quantify atmospheric composition vary drastically in terms of reaction mechanisms and parameterization approaches. Understanding the effects of future climatic and land use changes on different types of air pollutants while continuously improving the science behind chemical transport models will enable better air quality management in the future, especially with a rapidly changing climate and land cover.In the first study, we use a global coupled chemistry-climate-land model, the Community Earth System Model (CESM) with Community Atmosphere Model 4 (CAM4) and Community Land Model 4 (CLM4) being the atmospheric and land component, respectively, to investigate how global aerosols would respond to future climate and land use change via changes in biogenic and biomass burning emissions. Time-sliced simulations are conducted for the base year (2000) and two future years (2050 and 2100). For the future years, two future projected scenarios driven by both climate and land use projections following the Representative Concentration Pathways RCP4.5 and RCP8.5 are conducted. These two scenarios are selected to represent realistic cases where RCP4.5 represents the regeneration of forest cover and strong implementation of CO2 emission control measures, while RCP8.5 represents widespread deforestation and business-as-usual CO2 emissions. Biogenic emissions are actively simulated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) version 2. Biomass burning emissions are simulated in a separate simulation using a CLM4 fire module that accounts for biomass burning emissions by predicting land area burned. Our results suggest an increase in total aerosol burden of 1.4 Tg for the RCP4.5 scenario by year 2100. The enhanced biogenic emissions due to larger forest cover and temperature increase in RCP4.5 contribute significantly to global aerosol burden (55% of the increase in aerosol burden). In the RCP8.5 scenario, the total aerosol burden increases by 1.3 Tg by year 2100. The increased biomass burning emissions in the RCP8.5 scenario contribute significantly to aerosol burden (65% of the increase in aerosol burden). Results from all simulations also show that surface aerosol concentrations are largely shaped by biomass burning emissions in the future.In the second study, we use CESM version 2, with CAM6 and CLM5 to investigate how atmospheric oxidizing power, specifically ozone and hydroxyl (OH) radical concentrations would respond to future changes in climate, land use, and emissions in year 2050 according to the RCP8.5 scenario. Time-sliced simulations are conducted for the base case and each driving factor (climate, land use, anthropogenic, biogenic and biomass burning emissions) separately. This scenario is selected since it represents a business-as-usual scenario in order to examine the effects of climate inactions. Biogenic emissions are simulated using MEGAN2. Biomass burning is simulated by CLM5. Our results suggest that both ozone and hydroxyl radical concentrations are most significantly controlled by anthropogenic emissions, resulting in a 9% increase in tropospheric ozone burden and 16% increase in global tropospheric mean OH concentrations. Climate change is the second most important factor in affecting ozone concentrations, reducing tropospheric ozone burden by 4.3% and is the third most important for OH concentrations, increasing global tropospheric mean OH concentration by 5.4%. Biogenic emissions are the second most important factor affecting OH concentrations, reducing it by 8.9%. Since anthropogenic emissions is the most important factor in affecting ozone and OH concentrations, attention should be placed on anthropogenic emission control.Secondary organic aerosols (SOAs) make up 90% of the aerosols in forested regions. However, the large number of different species and different reaction pathways in the formation and removal processes of SOAs make accurate modeling of global SOA concentrations a difficult endeavor. Recent advances in SOA modeling focus on formation pathways while neglecting removal processes. In the third study, we introduce a novel, experimentally based removal mechanism into the GEOS-Chem global chemical transport model with full SOA chemistry, namely, the heterogeneous oxidation of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA), the most common SOA in rural regions. This scheme uses a humidity dependent heterogeneous oxidation rate coefficient measured at ambient conditions to calculate the mass loss of IEPOX-SOA. Modeled results show a decrease in SOA concentration globally. However, this proposed scheme reduces the already underestimated IEPOX-SOA concentrations. Although including heterogeneous oxidation scheme improves the science behind the modeling of SOAs, the results do not necessarily improve the accuracy of the modeled atmospheric composition.All in all, both biogenic and biomass burning emissions are affected by climatic and land use changes and increasing their emissions will increase global aerosol burden significantly. Surface aerosol concentrations are largely sensitive to biomass burning emissions. In the gaseous atmosphere, ozone and OH concentrations are most affected by anthropogenic emissions in the 2050-RCP8.5 scenario. For model improvements, modeling heterogeneous oxidation using ambient reaction rate coefficients can be a good way to improve model representations of SOA removal processes if we require model predictions at one focused site. Such findings call for greater integration between forest, agricultural and air quality management, in order to enable better air quality mitigation policies in the future.气候和土地利用的变化会影响全世界的大气成分和空气质量，但是所涉及的机制和途径尚未得到充分理解或量化。而且，试图量化大气成分的化学传输模型在反应机理和参数化方法方面有很大的不同。了解未来的气候和土地利用变化对不同类型的空气污染物的影响，同时不断改进化学迁移模型背后的科学知识，将能够在未来更好的管理空气质量，尤其是在气候和土地覆盖迅速变化的情况下。在第一个研究中，我们使用全球耦合的化学-气候-土地模型（CESM），其中CAM4和CLM4分别是大气和土地成分，调查全球气溶胶和大气氧化能力如何通过改变生物和生物质燃烧排放来应对未来的气候和土地利用变化。我们对基准年（2000年）和两个未来年份（2050年和2100年）进行了时间分段的模拟。在未来年份，根据代表浓度路径RCP4.5和RCP8.5，我们进行了两个由气候和土地利用预测驱动的未来预测方案。选择这两种方案来代表现实情况，其中RCP4.5代表森林覆盖的再生和强有力的CO2排放控制措施的实施，而RCP8.5代表广泛的森林砍伐和常规的CO2排放。MEGAN2积极模拟了植物排放。CLM4火灾模块在单独的模拟中模拟生物量燃烧排放，该模块通过预测燃烧的土地面积来说明生物量燃烧排放。我们的结果表明，在2100年，RCP4.5情景的总气溶胶负荷将增加1.4Tg。在RCP4.5情景中，更大的森林覆盖面积和温度升高而导致的生物排放增加对全球气溶胶负担做出了重大贡献（占气溶胶负担增加的55％）。在RCP8.5情景中，在2100年，总气溶胶负荷增加1.3Tg。在RCP8.5情景中，增加的生物质燃烧排放量大大增加了气溶胶负担（占气溶胶负担增加量的65％）。所有模拟的结果也表明，未来表面气溶胶的浓度很大程度上受生物质燃烧排放的影响。在第二项研究中，我们使用了带有CAM6和CLM5的CESM版本2，研究了大气氧化能力，特别是臭氧和羟基自由基浓度将如何响应根据RCP8.5的2050年气候，土地使用和排放的未来变化。对基准年和每个驱动因素（气候，土地利用，人为，生物和生物质燃烧排放）分别进行了时间片模拟。选择此方案是因为它代表了通常的方案，以便检查气候不作为的影响。。生物排放由MEGAN2模拟。生物质燃烧由CLM5模拟。我们的结果表明，人为排放最显着地控制了臭氧和羟基自由基的浓度，导致对流层臭氧负担增加了9％，对流层平均羟基自由基浓度增加了16％。气候变化是影响臭氧浓度的第二重要因素，对流层臭氧负担减少4.3％，对羟基自由基浓度而言第三重要，对流层平均羟基自由基浓度增加5.4％。生物排放是影响羟基自由基浓度的第二重要因素，可将其降低8.9％。由于人为排放是影响臭氧和羟基浓度的最重要因素，因此应注意人为排放控制。二次级有机气溶胶（SOA）占森林地区气溶胶的90％。但是，在SOA的形成和去除过程中存在大量不同的物种和不同的反应途径，因此难以对全球SOA浓度进行精确建模。SOA建模的最新进展集中在形成路径上，而忽略了去除过程。在第三项研究中，我们将基于实验的新型去除机理引入具有完整SOA化学性质的GEOS-Chem全球化学迁移模型中，即IEPOX-SOA的异质氧化。该方案使用在环境条件下测得的湿度相关的异质氧化速率系数来计算IEPOX-SOA的质量损失。建模结果显示全球SOA浓度降低。但是，该提议的方案降低了已经被低估的IEPOX-SOA浓度。尽管包括异质氧化方案可以改善SOA建模背后的科学，但结果不一定能提高建模的大气成分的准确性。综上所述，在我们大气的气溶胶部分，生物和生物质燃烧排放均受到气候和土地利用变化的影响，而增加排放将大大增加全球气溶胶负担。表面气溶胶浓度对生物质燃烧排放非常敏感。在气态大气中，在2050-RCP8.5情景中，人为排放对臭氧和羟基的浓度影响最大。至于模型改进，如果我们需要在一个重点站点进行模型预测，则使用环境反应速率系数对异质氧化进行建模可能是一种改进SOA去除过程的模型表示的好方法。这些发现要求在森林，农业和空气质量管理之间进行更大的整合，以便在未来制定更好的空气质量缓解政策。Lim, Wei Lun Alan.Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves ).Abstracts also in Chinese.Title from PDF title page (viewed on …)