نوع مقاله : مقاله پژوهشی
نویسندگان
1 مؤسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
2 گروه فیزیک فضا، مؤسسه ژئوفیزیک دانشگاه تهران، تهران، ایران
3 دانشیار/هیئت علمی مؤسسه ژئوفیزیک
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
The size, chemical composition, and concentration of aerosols significantly impact horizontal visibility. The horizontal visibility can decrease due to the scattering and absorption of visible light by the particles and gases existing in the atmosphere. Horizontal visibility is influenced not only by aerosols but also by meteorological conditions—particularly relative humidity—which can impact visibility both directly and indirectly. Increased relative humidity significantly increases hygroscopic particles, thereby increasing the scattering cross-section and consequently reducing horizontal visibility. In some cases, changes in horizontal visibility are primarily determined by the concentration of PM2.5, meaning visibility can serve as an indicator of air quality. However, when relative humidity dominates changes in visibility, horizontal visibility may not be an accurate indicator of air quality. Overall, horizontal visibility is influenced by both human and natural factors. The concentration of suspended particles and relative humidity are key contributors to visibility changes, each with its own relative proportion. The relationship between PM2.5 concentration and horizontal visibility varies with relative humidity and requires further investigation. In order to examine factors such as the concentration levels of PM2.5 aerosols, and meteorological parameters including relative humidity, on the temporal changes in horizontal visibility in the metropolitan city of Tehran, which has distinct topography, daily average data from the Meteorological Organization's Mehrabad and Geophysics stations for variables like horizontal visibility, wind speed, relative humidity, temperature, and precipitation, as well as daily average pollutant data related to PM2.5 collected from Tehran’s Air Quality Control Center over a 10-year period (2013–2023) for the Sharif University and Tarbiat Modares stations, were collected. Although the Mehrabad and Geophysics stations are geographically close and share similar meteorological conditions, the characteristics of air pollution and atmospheric visibility in the two areas may differ due to different emission properties and levels of urbanization.
The results showed that increased relative humidity intensifies the impact of PM2.5 on visibility reduction, and at humidity levels above 90%, horizontal visibility significantly decreases with increased PM2.5 concentration. Additionally, a nonlinear negative relationship was observed between horizontal visibility, PM2.5 concentration, and relative humidity. Wind speed and average temperature play a positive role in improving visibility. As relative humidity increases, especially for greater values than 20 percent, the relative volumetric growth (VGF) of fine aerosols rises. Furthermore, at lower PM concentrations, VGF shows greater significance and variability, highlighting the role of other factors in moisture absorption by PM2.5 aerosols. It should be emphasized that the particle size distribution, the optical properties, and the chemical composition of aerosols also affect horizontal visibility. Therefore, the impact of aerosols and meteorological conditions on atmospheric visibility is relatively complex. This subject could be explored in future studies using a comprehensive numerical air quality model along with precise meteorological and environmental observations.
کلیدواژهها [English]
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