Measuring and Modeling Ozone-vegetation Interactions in Hong Kong: Implications for Ecosystem Services Provided by Subtropical Vegetation

Tropospheric ozone pollution is a major environmental concern due to the tremendous impacts of ozone on both human and vegetation health. The impacts on subtropical vegetation such as that found in Hong Kong are particularly less well examined. Here we present the first city-wide ultra-high-resolution model simulations of ecosystem services and ozone damage to plants, with a focus on Hong Kong, one of the most densely populated cities in the world, using the Terrestrial Ecosystem Model in R (TEMIR). We combined high-resolution local measurements and satellite observations with TEMIR to find that local vegetation produces a 1.58 ± 0.39 Tg C yr-1 carbon uptake (NPP) and a 4.56 ± 0.29 Gg O3 yr-1 ozone uptake (Fd) over the entire Hong Kong, which represents a large contribution to the regulation of atmospheric composition. The two uptake fluxes are further reduced by 4.95 % ± 4.63 % and 1.96 % ± 1.37 %, respectively, due to ozone damage to plants. Our simulations also showed that surface ozone pollution regulates itself through the overall negative ozone-vegetation feedback via two major pathways, where the feedback to ozone levels due to reduced biogenic emissions (Δ[O3]E = −0.31 ± 0.25 ppb) dominates over that due to reduced ozone dry deposition (Δ[O3]vd = +0.08 ± 0.07 ppb) over the entire Hong Kong. The uncertainties represent one standard deviation of the resulting values from different experimental simulations with varying plant photosynthetic activities and varying extents of ozone damage to plants. Our study aims to illustrate the vital role of local vegetation, especially evergreen broadleaf forests, in regulating climate and air quality through carbon and pollutant uptake quantitatively, and help devise optimal strategies for ozone control. Our study provides evidence of the importance of vegetation management, which is thus far an often neglected but potentially highly useful approach, in mitigating air pollution in Hong Kong, beyond the conventional ways of controlling vehicular and power plant emissions. Our technical development and scientific findings also provide a valuable basis for the Hong Kong Government and related agencies to tackle air quality issues, including the worsening problem of ozone pollution in Hong Kong, through adopting new methods related to local vegetation.Moreover, to construct a database of photosynthetic capacity and foliage density specific to plant function type (PFT) and species, we carried out in-situ measurements of the maximum rate of carboxylation scaled to 25°C (Vcmax25) and the leaf area index (LAI) in local rural areas and hiking trails. It was found that Vcmax25 is quite varied among local tree species from 30.2 to 72.7 μmol CO2 m-2 s-1. We estiamted a representative Vcmax25 of 50.7 ± 15.4 μmol CO2 m-2 s-1 for the photosynthetic capacity of the local broadleaf evergreen trees (BET), which is slightly larger than the published Vcmax25 values of BET in other regions. We also measured leaf area index (LAI) in local rural woodlands following standard methods. The measured LAI values vary regionally with a range between 2.3 to 5.3 m2 m-2, which is close to the LAI of local plants in Hong Kong found in literature. We compared the values of satellite-derived LAI and measured LAI for investigating the plant type variation within any 500-m model grid cell areas and the specific LAI for non-BET plant types. Generally consistent differences between the measured LAI and satellite LAI match the land use characteristics, thus supporting the importance of utilizing high-resolution land surface representation. The data can provide a better representation of plant photosynthetic capacity and canopy properties specific to local vegetation in numerical models and increase the predictive capacity of ecosystem models in general. With enhanced understanding of the local vegetation profile and atmosphere-vegetation interactions, the Hong Kong government and the other agencies might be able to devise appropriate forest management and climate change mitigation policies.對流層內臭氧為人類及植被的健康帶來嚴重的負面影響，使對流層臭氧污染成為主要的環境污染問題。臭氧對亞熱帶植被的影響甚少被研究。我們在此發佈首個用於分析香港本地的臭氧污染對植物的傷害及其對本地生態系統的影響的高解析度的生態生理模型實驗。該實驗使用了「在R 運行的陸地生態系統模型」（TEMIR）。本研究同時使用了本地監測站的高解析度數據及衛星觀測數據來計算了香港本地生態系統的全年碳沉降（淨生產力）達1.53 ± 0.30 Tg C yr-1，以及本地植被的全年總臭氧乾沉降達4.50 ± 0.21 Gg O3 yr-1，此兩者對調節大氣成分作出很大貢獻。上述兩項數據因為地面臭氧對植被的傷害而分別減少4.95 % ± 4.63 % 和 1.96 % ± 1.37 %。本研究的模型實驗亦展示了本地地面臭氧污染通過兩條主要路俓達至整體性負值的臭氧植被反饋並進行自我調節。兩條臭氧植被反饋的主要路俓分別為本地植被受到臭氧造成的瞬時傷害和累積傷害並通過降低生物源有機物排放對本地地面空氣臭氧濃度的直接單向反饋 （Δ[O3]E = −0.31 ± 0.25 ppb），及本地植被受到臭氧造成的瞬時傷害和累積傷害並通過引起植被葉片氣孔閉合對本地地面空氣臭氧濃度的直接單向反饋 （Δ[O3]vd = +0.08 ± 0.07 ppb）。上述數值的不確定值表示多個模型實驗結果的一個標準差，模型實驗包括改變植物光合能力及臭氧對植被造成的傷害程度。本研究的目標是以數據展示香港本地植被（尤其是常綠闊葉林）通過碳和臭氧沉降來調節空氣質素的重要作用，並幫助設計控制臭氧污染的更佳方法。此外，到目前為止，植被管理經常被忽視，但其實它是能非常有效地對抗空氣污染的潛在方法。本研究提供數據證明除傳統上控制排放的方式外，植被管理在改善香港的空氣污染方面的重要性。我們的技術發展和科學發現將為香港政府和有關機構通過採用與本地植被有關的新方法來解決空氣污染問題（包括香港日益嚴重的臭氧污染問題）提供寶貴的基礎。為了建立包含本地植物物種特有的光合活性和綠葉密度的資料庫，本研究團隊在香港本地郊野區域及郊遊徑對附近植物的最大羧化速率及葉面積指數進行了現場測量。本文發現各種本地樹種特有的最大羧化速率（調整至25°C）之間有明顯差異，介乎於30.2 到72.7 μmol CO2 m-2 s-1 之間。本文整合不同樹種的數據並計算了代表本港所有常綠闊葉樹種的最大羧化速率（調整至25°C）達50.7 ± 15.4 μmol CO2 m-2 s-1，比常見研究發表的其他地區的常綠闊葉樹種的最大羧化速率略高。現場測量的葉面積指數介乎2.3 到 5.3 m2 m-2 並展現出明顯區域性差異，此數字接近已有文本記錄的香港本地葉面積指數。本文通過比較衛星觀測的葉面積指數及現場測量的葉面積指數探究在半公里範圍內的植物功能型的分佈和差異及非常綠闊葉樹種的特有葉面積指數。衛星觀測的葉面積指數及現場測量的葉面積指數之間的差異與土地用途大致符合，此結果支持使用高解析度的地面表達數據的必要性。新的數據可以在數值模型中更有效地表達本地植被特有的光合作用能力及冠層特性，並提高生態模型的預測能力。隨著對本地植被特質及大氣與植被關係有了更全面的理解，本地政府及其他機構或能設計更適合的林木管理及氣候變化政策。TAO, Ka Chuen.M.Phil. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves )Abstracts in English and Chinese.Title from PDF title page (viewed on ...