Investigation of the Optical Properties of Atmospheric Aerosol Particles Using Handheld Sun-Photometer Data (CALITOO): A Case Study of Yazd City

This study examines the optical and physical properties of aerosols in Yazd, Iran, under three distinct atmospheric conditions, clear, dusty, and urban-industrial pollution, using data from the CALITOO handheld sun photometer and MODIS satellite observations. The main metrics analyzed were Aerosol Optical Depth (AOD) and the Ångström Exponent (AE). Aerosol optical depth (AOD) is a measure of the amount of particles present in the vertical column of the Earth's atmosphere. Ångstrom exponent (AE) is a qualitative measure of the dimensions of the dominant aerosols. The research focused on February 15 (clear day), March 25 (dusty day), and November 23, 2021 (urban-industrial pollution day), with data collected at wavelengths of 465, 540, and 619 nm.
On the clear day of February 15, AOD values were low, ranging between 0.03 and 0.05, signifying minimal aerosol concentration and indicating a clean atmosphere dominated by fine particles. The corresponding AE was high at 1.60, supporting the presence of smaller particles. Visibility on this day exceeded 10 km, reflecting the low aerosol load. The concentrations of particulate matter (PM₁₀ and PM_2.5) were recorded at 10 and 6 µg/m³, respectively, well below the permissible limits, confirming the air quality.
In contrast, March 25, a dusty day, exhibited a substantial increase in AOD, with values ranging from 0.56 to 0.57, indicating a dense presence of coarse particles, predominantly dust. The AE for this day was approximately 0.10, which aligns with the dominance of larger particles typically associated with dust events. Visibility was notably reduced to below 5 km, corresponding to the high particle concentration and significant light scattering. On this day, PM₁₀ and PM_2.5 concentrations were 324 and 67 µg/m³, respectively, exceeding acceptable thresholds and highlighting the severe dust pollution event, affecting air quality. MODIS satellite data for this day showed an AOD close to 0.54, aligning well with ground-based CALITOO measurements, demonstrating the strong agreement between satellite and in-situ observations.
The urban-industrial pollution day on November 23 showed moderate AOD values between 0.30 and 0.32. The AE was 1.30, indicating a mixture of fine and medium-sized particles, typical of industrial emissions. Visibility remained below 5 km, pointing to significant airborne pollutants that restricted clarity. The particulate matter concentration for PM₁₀ was recorded at 66 µg/m³, and for PM_2.5 at 58 µg/m³. The concentration of PM₁₀ was below the permissible limit, while the concentration of PM_2.5 exceeded the limit, aligning with the remote sensing results.
Validation through MODIS satellite observations confirmed the ground-based CALITOO data's reliability, with close AOD correlations noted on March 25. The HYSPLIT model identified the Arabian Peninsula as the dust source for March 25, highlighting regional transport's impact on local air quality. Such findings underscore the importance of combining ground-based and satellite data for comprehensive aerosol analysis, aiding in accurate assessments of air quality and supporting informed environmental management.
In summary, clear days showed low AOD and high AE values, indicating fine particles and good visibility. Dusty conditions featured high AOD and low AE, reflecting the presence of coarse particles and reduced visibility. Urban-industrial pollution days had moderate AOD and higher AE, pointing to a mix of smaller particles from human-made sources. The inclusion of particulate matter concentrations (PM₁₀ and PM_2.5) provided additional evidence of air quality variations, confirming severe pollution levels on dusty and industrial polluted days. The consistency between CALITOO and MODIS data, complemented by HYSPLIT modeling, demonstrated the effectiveness of integrated remote sensing approaches in understanding aerosol concentration behavior and guiding air quality management strategies.