بررسي نقش شاخص‌هاي پوشش گياهي و مؤلفه‌هاي جغرافيايي مكان بر عمق نوري هواويزهاي فصلي ايران

Investigating the role of vegetation indices and geographic components on seasonal aerosol optical depth over Iran

عمق نوري هواويزها (AOD) كميتي بي‌بعد است كه ميزان عبوردهي پرتو نور در جو را نشان مي‌دهد. شناخت AOD براي درك تأثيرات آن بر كيفيت هوا و ارائه راهكاري‌هاي مقابله با آن ضروري است. هدف از اين پژوهش بررسي نقش مؤلفه‌هاي جغرافيايي مكان و شاخص‌هاي پوشش گياهي بر عمق نوري هواويزهاي فصلي (AOD550nm) موجود در جو ايران است. در اين پژوهش از فرآورده 6 سنجنده MODIS ماهواره Aqua براي ارزيابي مقادير AOD، NDVI و EVI به‌ شكل فصلي طي دوره آماري 2017-2003 استفاده شد. جنوب غرب، شرق و سواحل خليج‌فارس بالاترين و شمال غرب كشور و زاگرس كمترين مقدار AOD را به خود اختصاص داده‌اند. دليل بالا بودن مقدار AOD در ايران بار ورودي گردوغبار، احتراق سوخت‌هاي فسيلي و كاهش ارتفاع لايه‌مرزي (BLH) در دوره سرد سال است چرا‌كه اكثر آلاينده‌هاي جوي در لايه مرزي منتشر مي‌شود و از راه فرآيندهاي تلاطمي لايه آميخته همگن مي‌شود. همبستگي AOD با ارتفاع و عرض جغرافيايي منفي و در سطح 5 درصد معني‌دار مي‌باشد. همبستگي منفي معني‌دار بين شاخص‌هاي EVI، NDVI و AOD در فصول تابستان و انتقالي سال وجود دارد به‌طوري‌كه كاستي پوشش گياهي با افزايش AOD در هر فصل با يكديگر متناظرند. رگرسيون وزن‌دار جغرافيايي (GWR) نشان داد كه مقدار پوشش گياهي توانايي بالايي در كنترل هواويزهاي وردسپهر پاييني و ارتفاعات نقش سد‌كنندگي مسير ترابرد ذرات را دارند و در نتيجه بر انتشار گردوغبار در حالت محلي و منطقه‌اي نيز تأثير خواهند داشت.

Investigation of the role of vegetation indices and geographic components on seasonal aerosol optical depth (AOD) over the Iranian region is carried out. Aerosols are suspended particles in an air that have diameters between 0.001 and 100 micrometers. Aerosols play an important role in the radiation properties of atmosphere and hence affect the earth climate system. Vegetation cover can impede surface erosion by wind and hence, has a close relationship with the emission. Dust emission leading to dust events in urban area can have an adverse effect on human health as well as human activities, for example by reduction in visibility. This research aims to seasonally evaluate the roles of geographical locations and vegetation indices on AOD over Iran, based on satellite data. This includes the evaluations the role of each of these components in AOD550 nm variations. In this study, the daily data of the 6-level 3 products (MYD08_M3_6) including AOD550 nm, Deep Blue Algorithm, MODIS sensor data, Aqua satellite data, are used. Pixel data were downloaded over the Iranian region from 2003 to 2017 with a spatial resolution of 1 × 1 arc. Two indicators, namely the Enhanced Vegetation Index (EVI) and Normalized Difference Vegetation Index (NDVI) of the Aqua Satellite, for the study period with AOD data were used. The optical sensitivity of aerosols data was verified using the Aqua Satellite data from the Aerosol Robotic Network (AERONET). The GWR and OLS methods were used to find the spatial relationships of aerial photo sensor optical depths with geographic location and vegetation indices. The average values of AOD over Iran, based on the data of the Aqua, are between (0.11 for spring) and (0.16 for autumn) respectively. The average AOD value in the spring indicates the enhancement of dust events in the region. In winter, the average AOD value over Iran is 0.12, with the lowest standard deviation. In the summer, according to Aqua satellite data, this value is 0.133, with the maximum scatters and deviation from the largest mean observed value. Based on the EVI and NDVI indexes, the maximum statistical values, including the range of changes, maximum, average, scatter level and deviation from the typical values of both indicators were observed in the warm season of the year. The maximum EVI index peaked in the summer with 0.478 and the lowest of 0.043 in the winter. The maximum NDVI index, like the EVI index, was obtained for the summer with 0.777 and its lowest value is -0.69 for the spring. The maximum correlation between the atmospheric optical depth and geographic components of the area is for the altitude and then the latitude and then the longitude. The correlation between the AOD with the altitude and latitude of location of the area is negative and significant, and the correlation of the AOD with longitude is not significant in any seasons. There is a negative correlation between AOD and NDVI, and also EVI index in all seasons, although it is 0.039 in winter, which is relatively low. The results of the AOD assessment show that the maximum spring and autumn has the lowest average AOD over the Iranian area. This is due to the combination of dry conditions and relatively strong wind speeds in the spring those results in dust storms that increase the amount of AOD. In contrast, the maximum AOD over Iran is for the spring with a value of 0.48 that occurs in southwestern part of Iran. The second largest focal point, highlighted in all AOD seasons, is for the Persian Gulf coast area between Bushehr and Bandar Abbas. AOD over this coastal area can be associated with favorable wind conditions in mineral dust deposition which transported to the area and sea salt. Other areas with high AOD can be found in the Makran coastal area in the southeast of Iran, between the plain of Lut and the Mangrove plain, as pervious AOD study in southeastern Iran indicted. Based on the climate distribution of the EVI and NDVI vegetation indices and the seasonal spatial variation of aerosols, it is shown that vegetation factor in dust emission efficiency varies from one region to another with season. This regional disparity is due to the variation of vegetation-humus-release and the coupling of two or more of these factors; therefore, vegetation can significantly improve the treatment of dusty storm areas with the internal sources in the country. The maximum correlations with the geographic components of the location with the optical depth over the Iranian area are for the elevation and then the latitude and then the