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.
Atmospheric aerosol concentrations have been found to change constantly due to the influence of source, winds and human activities over short time periods. This has proved to be a constraint to the study of varied aerosol concentrations in urban atmosphere alongside changing relative humidity and how it affects visibility and aerosol particle size distribution. In this research simulation was carried out using Optical Properties of Aerosols and Clouds (OPAC 4.0) average concentration setup for relative humidity (RH) 0-99% at visible wavelength 0.4-0.8 μm to vary the concentrations of three aerosol components: WASO (Water-soluble), INSO (Insoluble) and SOOT. The Angstrom exponents (α), the curvatures (α2) and atmospheric turbidities (β) were obtained from the regression analysis of Kaufman’s first and second order polynomial equations for visibility. The research determined the mean exponent of the aerosol size growth curve (µ) from the effective hygroscopic growth (geff) and the humidification factors (γ) from visibility enhancement f (RH, λ). The mean exponent of aerosol size distributions (υ) was determined from µ and γ. The results showed that with varied WASO, INSO and SOOT concentrations respectively at different RH, aerosol particle size distributions showed bimodal characteristics with dominance of fine mode particles. Hazy atmospheric conditions prevailed with increasing turbidity.
Well validated simulation software and models are meant to add value through a more precise representation of different atmospheric features and climate characteristics that are physically out of reach. In this work, comparison and validation was carried out using the Modern Era Retrospective Analysis for Research and Applications (MERRA) averaged Angstrom Exponents (α) and averaged visibilities for ten urban countries against simulated Optical Properties of Aerosols and Clouds (OPAC) urban components. The simulations of OPAC were performed at 0%, 50%, 70%, 80%, 90%, 95%, 98% and 99% relative humidities (RH) while the MERRA satellite data were extracted at an average of 78% relative humidity. results based on α values showed that OPAC was only able to simulate well the insoluble and soot aerosol particle size distributions of India’s urban atmosphere such that it had approximately the same insoluble and soot aerosol particle size distributions as India. For the rest of the nine countries (USA, Brazil, Indonesia, China, Japan, Mexico, Nigeria, Pakistan and Ethiopia), OPAC was seen to either overestimate or underestimate water soluble, insoluble and soot aerosol particle size distributions. For visibility and water soluble or insoluble aerosol concentrations, OPAC showed good simulated values that were approximately the same as those found within Ethiopia’s and Japan’s urban atmosphere. OPAC also had visibility value and insoluble aerosol concentration that were approximately the same as those found within China’s atmosphere. For all other countries (India, USA, Brazil, Indonesia, Mexico, Nigeria and Pakistan) OPAC either overestimated their visibilities and underestimated their water soluble, insoluble and soot aerosol concentrations or vice versa. With both OPAC and MERRA, the presence of fine mode aerosol particles in an urban atmosphere was established with α values > 1. But MERRA also showed otherwise for three countries (Nigeria, Ethiopia and Pakistan) that had α values < 1 which implied the presence of coarse mode aerosol particles. The relationship between visibility and α satisfied the direct power law for OPAC, for MERRA the relationship approximately satisfied the power law for most of the countries.
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