Comprehensive study on the amount of CO2 absorbed by vegetation: A case study in Ho Chi Minh city, Vietnam

The city Ho Chi Minh (HCMC) is one of the largest cities in Vietnam with the most dramatically economic development rate. Along with the economic development, the urbanization process in this city is also taking place very fastly. Due to the rapid urbanization and development, the emission rate from the industry and transportation leads to the increase in the amount of carbon dioxide (CO2) which has been worsening the climate change. Protecting forests and conducting afforestation so that CO2 is transformed to nutrition through photosynthetic conversion is one of the most effective ways to mitigate the effects of climate change. As a result, the accumulation of CO2 emissions has become a global concern. Vegetation absorbs carbon dioxide, helps to conserve the environment, produces oxygen, reduces noise, and helps to stabilize subsurface water. This paper highlights the results of ENVI software which was used to interpret remote sensing images and Arcgis to evaluate the amount of carbon dioxide absorbed by vegetation in each administrative unit: district in HCMC and ward. According to the obtained results, the amount of CO2 absorbed in urban districts “District 1”, “District 3”, “District 4”, “District 5”, “Phu Nhuan District” is immensely low due to the high population density in the center of city. The population is mainly concentrated in the center districts but land area for vegetation is low. Regarding the suburban area, with mangrove forests, Can Gio District has the highest amount of CO2 absorbed of 35,894.075 tons/day and followed by Cu Chi District with 21,548.48 tons/day. It can be indicated that Can Gio and Cu Chi districts improtantly function like the greenhouse gas sinks for the whole HCMC. The success of this study could contribute to climate change mitigation and support in urban and land planning, as well as resettlement policies. Aside from that, CO2 emission and absorption assessment and evaluation in large–scale cities like HCMC has become a crucial, urgent, and practical issue nowadays.


Introduction
The increase in the concentration of CO 2 in the atmosphere is the cause of the problem of climate change because the earth cannot absorb all the CO 2 and other greenhouse gases [1][2][3][4][5]. According to the World Meteorological Organization's (WMO) Greenhouse Gas Report, CO 2 emissions from 2015 to 2016 increased sharply from 400 ppm to 403.3 ppm. Climate change according to the worst scenario of the Ministry of Natural Resources and Environment (inundated area accounts for 23% of the natural land area), many industrial zones will be seriously affected. Most of the industrial zones are heavily flooded, the lowest is over 10% of the area, the highest is up to 67% of the area. Hiep Phuoc Industrial Park in Nha Be District is located next to Nha Be River, in the area strongly influenced by the East Sea tide, so the possibility of flooding is very high. Other significantly affected industrial zones such as. Phong Phu Industrial Park, Le Minh Xuan Industrial Park, Hi-Tech Park. The impact of climate change on the agricultural production model in Ho Chi Minh City is that most of the agricultural land area is flooded and saline intrusion leads to a reduction in agricultural production. Crops and livestock developed according to the urban agriculture model are affected, typically Binh Chanh District is planned to have the largest perennial ornamental plant area in the city with a scale of 550 hectares by 2025. According to Japanese experts, 1 hectare of forest, dense plantation, or equivalent area absorbs 1000 kg of CO 2 per day and emits 730 kg of O 2 . A hectare of grass, on the other hand, may absorb 360 kg of CO 2 and exhale 240 kg of O 2 every day [6][7][8][9]. The researcher [10] have applied the GIS model to predict the ability to reduce CO 2 through afforestation. The main goal of the study was to inventory CO 2 emissions in Ohio, thereby assessing the efficiency in CO 2 sequestration of plantations. The study suggests different mitigation levels from different forest planning policies. CO 2 emissions in 1996 were 214,038,081 tons. The amount of CO 2 absorbed from the existing and newly planted forests is about 89,227,483 tons. Price of applying GIS and remote sensing to create a hypothetical buffer zone with an area of 806,374 hectares capable of absorbing 9,106,650 tons of CO 2 , equivalent to 4,246% of greenhouse gas. Fangmeier et al. [11] have studied the application of high-resolution satellite imagery to estimate carbon emissions from deforestation in 2001. The work uses landsat ETM + image, combining field survey and linear regression method and unverified classification. Evaluate changes of plant mulch in the period 1992-2001. In addition, the result of research shows the conversion of primary forests is the main cause of carbon emissions. Acacia has the best carbon capture capacity, around 2.78 tons/ha per year. Meanwhile, bamboo forest can absorb up to 18 tons/ha and the secondary forest is 15 tons/ha. Recently, the integration of GPS and GIS technology with remote sensing has shown to be an efficient tool for monitoring changes in vegetation and the ability of a specific region to absorb greenhouse gases. In this paper, we look at how much CO 2 is absorbed by vegetation in HCMC, Vietnam, and how this can help to mitigate the effects of climate change by supporting urban planning, land use planning, and resettlement strategies. Furthermore, one of the most important, pressing, and practical challenges today is the calculation and measurement of CO 2 emissions and removals in major cities such as Ho Chi Minh City.

Materials
The Department of Statistics and the Ho Chi Minh City statistical yearbook were used to create attribute data. It includes the information about population, area of districts, administrative division of HCMC in 2016 and studied location. The administrative border of HCMC and landsat 8 are the spatial data. Landsat picture data with a 30 m resolution was obtained from two major US websites, Earth Explorer and NASA, using photos taken in 2016.

Methods
Software used in the study is ArcGIS 10.1, ENVI 5.3 software along with supporting softwares such as Google Earth and Excel software. The steps included are data collection, data analysis and data processing, calculation of the potential of CO 2 absorbed from plants and commenting and evaluating the results. It can be summarized into 3 steps [12][13][14]: Step 1: Collect baseline data for the calculation including attribute data on population, area of districts and communes, spatial data such as 2016 Landsat 8 image data and data administrative maps of Ho Chi Minh City at district and commune levels. The above data is collected from sources such as the University of Natural Resources and the Environment, NASA's URSI website in partnership with Geodesy and government regulators.

Fig.2. Methodology
Step 2: From the collected data, remote sensing image processing, NDVI value calculation, NDVI value classification and vegetation cover mapping were established. After the data processing process is completed, the satellite image interpretation combined with field survey will be performed and the results are presented in the form of a map of the total CO 2 absorbed potential by each district and district. HCMC wards and communes and tables.
Step 3: The calculated results are analyzed, compared and proposed solutions for environmental planners and managers.

Image Calibration
The remote sensed imaged is combined and enhanced the quality, then cut by region. Landsat 8 is combined, enhance the quality then cut by region.
The acquired spectral reflectance observations in red/visible region (red) and near-infrared region (NIR) are used in the formula of Normalized Difference Vegetation Index (NDVI): (1)

Application of GIS
Remote sensing data has a spatial resolution of 30m with the coordination of WGS 84. The adsorption of the pixel having NDVI value from 0.2 to 0.6 is 32.4 kg CO 2 /day. Whereas the figure of the pixel having NDVI more than 0.6 is 90. The total CO 2 adsorption for each administrative unit is analyzed by Zone statistic tool as Table with Sum formula. After that, the data of total CO 2 adsorption is contained in a shapefile. The tool Joint was used to assign the aforementioned shapefile to the administrative data layer. Table of ArcGis 10.3 contains data on population, land size, and absorbed CO 2 amount, which is used to map the total absorption potential CO 2 in HCMC. Additional information such as grid systems, projection, annotation, scale ruler, and needle pointing direction are required to complete the map.   [15]. Fig. 4 shows the information of Ho Chi Minh city with 3 NDVI value thresholds. Firstly, the locations having NDVI value lower than 0.2 are considered as a hydrological system or an empty land. Secondly, the areas having NDVI value from 0.2 to 0.6 are considered grassland and greenery. Thirdly, the areas having NDVI value higher than 0.6 are mainly mangrove forests. Their areas are 577,197 km 2 , 71044,486 km 2 , 460,707 km 2 and account for 27.7%, 50.15% and 22.15% of the total area respectively. The study used the coordinates of survey locations found on Google Earth or recorded fields to determine the sort of object being surveyed to assess the accuracy of the aforesaid classification method. Then compare them to the value on the classified image to determine the classification method's accuracy. The result shows that combining NDVI method with threshold image classification gives quite high accuracy level, but there are still errors. The spectral disturbance of the image, the influence of the shooting angle and the blur of the terrai are inevitable during processing images. However, it is possible to use this method to analyze and interpretate image, especially the vegetation cover [15].    6 presents a map of the total CO 2 potential uptake from green trees (vegetation) in HCMC in 24 districts of HCMC and Fig. 7 show a comparison chart of CO 2 adsorption potential between urban and suburban districts in HCMC. The results show that the potential CO 2 absorption capacity of green trees (vegetation) in HCMC is about 84,732.77 tons of per day. In general, urban areas such as District 1, District 3, District 4, District 5, District 6, District 7, District 8, District 10, District 11 have a relatively low total potential for CO 2 absorption from plants due to the eclipse. Sparse objects, "scarce" land areas where the population is concentrated. Meanwhile, suburban districts such as Hoc Mon District, Binh Chanh District, Cu Chi District, Can Gio District have high CO 2 absorption reserves. In which, Can Gio district has the highest total potential amount of CO 2 absorbed from the vegetation cover in 24 districts, HCMC. (iii) District 9 is located in the east of the city, bordering District 2, Thu Duc and Dong Nai and Binh Duong provinces with the natural boundary of Dong Nai River and (iv) Location District 9 is bordered by provinces and far from the city center, so it has not really attracted the population. These results predict that District 9 will become a new ecological city with a natural canal system and a high proportion of greenery. For suburban districts, CO 2 reserves are highest absorbed from mangroves in Can Gio district with absorption capacity of about 35,894,075 tons CO 2 /day. The area has good air quality, sparsely populated, and a fresh environment with good conditions such as a total area of 704.45 km2 -the largest area in HCMC with population in 2016 of Can Gio District is 75,022 people -The least population Can Gio is a biosphere reserve area recognized by UNESCO. Moreover, its geographical location bordered with the South China Sea is thus favorable for mangrove development. Besides, Can Gio district is a new district merged into HCMC, so the population is still not concentrated.

Conclusions
Ho Chi Minh City's vegetation coverage and daily CO 2 absorption capability were explored in this study. HCMC's vegetation is thought to absorb around 84,732.77 tons every day. District 4 has the smallest vegetation cover and the lowest CO 2 absorption rate of 16,807 tons per day, compared to District 1, District 3, District 4, District 5, District 10, Phu Nhuan, and Binh Thanh. District 9 has the maximum amount of CO 2 absorbed by vegetation, with about 3,167.19 tons per day, among Districts 2, 6, 7, 8, 9, District 11, District 12, Tan Binh, Thu Duc, Go Vap, Binh Tan, and Tan Phu. With 35,514.22 tons of CO 2 absorbed per day, the mangroves of Can Gio District have the highest amount of CO 2 absorbed among suburban districts. As a result, the dense vegetation of the suburb districts, particularly Can Gio and Cu Chi District, is regarded as a greenhouse gas sink for Ho Chi Minh City.