This paper investigates the Impact of relative humidity, varying the concentrations of water-soluble aerosol particle concentrations (WASO), Mineral Nuclei Mode Aerosols Particle Concentration (MINN), mineral accumulation mode, nonspherical (MIAN) aerosol particles concentrations and Mineral Coarse Mode Aerosols Particle Concentration (MICN) on the visibility and particles size distribution of desert aerosols based on microphysical properties of desert aerosols. The microphysical properties (the extinction coefficients, volume mix ratios, dry mode radii and wet mode radii) were extracted from Optical Properties of Aerosols and Clouds (OPAC 4.0) at eight relative humidities, RHs (00 to 99%) and at the spectral visible range of 0.4-0.8mm, the concentrations were varied to obtain five different models for each above-mentioned component. Regression analysis of some standard equations were used to determine the Angstrom exponent (α), the turbidity coefficient (β), the curvature (α2), humidification factor (), the mean exponent of aerosol growth curve (µ) and the mean exponent of aerosol size distributions (n). The values of angstrom exponent (α) were observed to be less than 1 throughout the five models at all RHs for the four studied components, and this signifies the dominance of coarse mode particles over fine mode particles. But the magnitude of the angstrom exponent (α) fluctuates all through the studied components except for WASO which increased with the increase in RH across the models and this also signifies the dominance of coarse mode particles with some traces of fine mode particles. The investigation also revealed that the curvature (α2) has both monomodal (negative signs) and bimodal (positive signs) types of distributions all through the five models and this also signifies the dominance of coarse mode particles with some traces of fine mode particles across the individual models for all the studied components. it was also found that the visibility decreased with the increase in RH and increased with the increase in wavelength. The investigation further revealed that the turbidity coefficient (β) fluctuates with the increase in RH and the particles concentrations, and this might be due to major coagulation and sedimentation. The analysis further found that there is a direct inverse power relation between the humidification factor and the mean exponent of aerosols size distribution with the mean exponent of aerosols growth curve. It was also found that as the magnitude of µ increased for MIAN, MINN and MICN, the effective hygroscopic growth decreased. For WASO, it was found that as the magnitude of µ decreased, the effective hygroscopic growth increased with the increase in particles concentrations and RH. The decreased in the magnitude of µ for WASO might be due to the fact that as we increase the non-hygroscopic particles, we decrease the deliquescence. The mean exponent of aerosol size distribution (n) being less than 3 shows foggy condition of the desert atmosphere the four investigated components and five studied models.
In this work, data extracted from OPAC 4.0 was compared and validated with 11 years data record from MEERA-2 model (average Angstrom Exponent and average Extinction Coefficient). The 11 years MEERA-2 model data for 10 selected deserts was extracted at an average relative humidity (RH) condition of 78%RH while OPAC considers eight different (RH) levels (00% to 99%RH). Based on the investigation, MEERA-2 model has the highest angstrom exponent (a) values for (Arabian, Danakil, Ferlo, Lompoul, Patagonian, Registan and Syrian deserts), which is greater than 1, and this signifies the dominance of fine mode particles over coarse mode particles when compared to OPAC 4.0. It can be seen that Chalbi (0.845), Karoo (0.482) and Sahara (0.417) deserts have an a values which is less than 1(indicating the dominance of coarse mode particles over fine mode particles). It can also be said that, MEERA-2 (a) is still greater than that of OPAC and this shows that the deserts atmosphere can be dominated by fine mode particles. The angstrom exponent (a) for OPAC 4.0 fluctuate all through the four studied components except for WASO which increases with the increase in RH and particles concentration (and this signifies that the particles dissolved as they uptake water and reach their saturation level). Based on the results of the investigation (time series analysis), it was found that the model’s significance (b) level are very high, and this shows that the aerosols distribution fluctuates around a reasonable stable, which signifies that the models are very significant. The analysis further found that, the significance (d) from the seasonal aspect is very poor except for Ferlo and Lompoul deserts, this signifies that season effect is consistent over time. It was also found that, the MEERA-2 has the highest visibility (km) over (OPAC 4.0). The investigation further revealed that the Arabian, Syrian and Patagonian deserts have the highest visibility (km) in the months of January, February, June, November and December. OPAC model underestimated the visibility when compared to MEERA-2 model.
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