مطالعه‌ي كيفيت هوا در مناطق غرب و جنوب غرب كشور براساس محصولات ضخامت نوري سنجنده‌ي MODIS

Study of Air Quality over West and South West Iran using Aerosol Optical Thickness Products of MODIS

رخداد گرد و غبار به‌عنوان يكي از پديده هاي بارز در مناطق خشك و نيمه‌خشك تاثير بسزايي در كيفيت هوا دارد. در اين پژوهش به‌منظور تخمين شدت رخداد گرد و غبار از محصولات ضخامت نوري سنجنده‌ي ماديس استفاده شده است. ضخامت نوري آلاينده ها براساس شاخص هاي مبتني بر تفاضل بازتابش سطحي و اختلاف دماي درخشندگي در طول ‌موج‌هاي 11 و12 ميكرومتر محاسبه شده است. اين شاخص ها به‌منظور تخمين وسعت و بارزسازي گرد و غبار، پس از تعيين آستانه هاي دمايي محاسبه شده‌اند. همبستگي منفي بين پارامتر ديد افقي و ضخامت نوري هواويزها و نيز رابطه‌ي همبستگي مثبت خطي قابل ملاحظه ‌يي بين غلظت ذرات معلق PM10 و ضخامت نوري هواويز ها در طول‌ موج 550 نانومتر برقرار است. تحليل آماري، مبين توانايي روش مذكور در تفكيك گرد و غبار از آلاينده هاي پيش‌زمينه و كاربرد آن در بهينه سازي بارزسازي گرد و غبار و شدت آن در منطقه است.

Dust, as an aerosol, significantly impacts air quality in arid and semi arid regions. Dust events in Iran, as a result of the semiarid climate, and its location in the global dust belt neighboring the deserts of Arabian countries, occur frequently. In recent years, the intensity of dust phenomena has increased, most especially in west and southwest Iran. In this study, the Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosols Optical Thickness (AOT) product is applied in order to estimate dust intensity. Since Aerosol Optical Thickness at 550nm has a close relationship with the brightness temperature of MODIS bands 31 and 32, and Normalized Difference Dust Index (NDDI), the calculated AOT is proposed through quantitative analysis of MODIS data for major dust events over west and southwest Iran during years 2000-2009. The proposed calculated AOT matches MODIS AOT very well, with a squared correlation coefficient of 0.740. Moreover, based on MODIS measurements, sequential separation of the dust cloud from the bright underlying surface and water cloud is accomplished through the Brightness Temperature Difference (BTD) of suspended particle matter in 11 and 12 micrometer wavelengths of MODIS, NDDI and refined cloud threshold utilization. By statistical analysis of MODIS measurements, thresholds are determined over west and south west regions of Iran. Validations with ground meteorological observations over the region revealed good agreement of the proposed method in separating dust from the bright surface and cloud, which obviate the deficiency of remote-sensing data products of dust particles near the source as a result of bright surface radiance contributions. In addition, there is considerable correlation between MODIS AOT at 550 nm and Ahwaz station PM10, while there is a negative correlation between calculated AOT and horizontal visibility. The statistical analysis and case studies reveal the accuracy of this method in extracting dust from underlying aerosols, and the usefulness of this technique in dust enhancement optimization.