COVID-19 disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated in Wuhan, China and spread with an astonishing rate across the world. The transmission routes of SARS-CoV-2 are still debated, but recent evidence strongly suggests that COVID-19 could be transmitted via air in poorly ventilated places. Some studies also suggest the higher surface stability of SARS-CoV-2 as compared to SARS-CoV-1. It is also possible that small viral particles may enter into indoor environments from the various emission sources aided by environmental factors such as relative humidity, wind speed, temperature, thus representing a type of an aerosol transmission. Here, we explore the role of relative humidity in airborne transmission of SARS-CoV-2 virus in indoor environments based on recent studies around the world. Humidity affects both the evaporation kinematics and particle growth. In dry indoor places i.e., less humidity (< 40% RH), the chances of airborne transmission of SARS-CoV-2 are higher than that of humid places (i.e., > 90% RH). Based on earlier studies, a relative humidity of 40-60% was found to be optimal for human health in indoor places. Thus, it is extremely important to set a minimum relative humidity standard for indoor environments such as hospitals, offices and public transports for minimization of airborne spread of SARS-CoV-2.

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[1] The direct radiative forcing (DRF) of dust particles is most uncertain among all the major aerosol species because of the large regional variation in their shapes and composition. The Indian Desert is known to be a source of natural mineral dust of nonspherical shapes. Particle shape and exact mineralogical information are essential for modeling dust optical properties as the latter governs their refractive indices. The realistic dust shapes, namely, sphere, spheroid, Chebyshev, and cylinder, based on Scanning Electron Microscope (SEM) images, have been used to model the mineral dust optics of the Indian Desert using the T-matrix method. The particle radius from 0.1 to 5.0 mm has been considered at wavelengths ranging from ultraviolet to near infrared (0.38-1.2 mm). Using Bruggman's effective medium mixing rule, the refractive index of composite dust particle has been calculated, accounting for both nonmetallic and metallic component (as hematite). Our calculations show that increasing the hematite percentage from 0% to 10% results in reduction of 0.477 and 0.013 in single scattering albedo (SSA) for cylindrical particle of radius 1 mm at 0.38 and 1.02 mm wavelengths, respectively, while the same for volume equivalent spherical particle were 0.484 and 0.022, respectively. The scattering signature of sharp-edged cylindrical particle showed the largest deviation to sphere compared with that of other relatively smooth particles (spheroid and Chebyshev). Changes in dust optical properties because of nonsphericity and varying hematite percentage were estimated for two cases: background dust and dust storm at visible wavelength. The change in SSA between the above two cases was insignificant for particles of radii <0.4 mm for each hematite percentage considered. On the other hand, for particles of size range 0.4-1 mm, the change in SSA increases with increasing hematite percentage. A 6% increase in hematite leads to an SSA reduction of more than 0.2 for particle radius of 1 mm for both background dust and dust storm cases. Optical properties of polydisperse dust distribution at visible wavelength suggest the likely hematite percentage as 0%-4% in the Indian mineral desert dust. The effect of hematite variation on SSA is found to be stronger than particle nonsphericity. The present work will lead to a better estimation of the radiative forcing imposed by dusts as well as their satellite retrieval over the Indian Desert region.

[1] To investigate the probable mixing state of aerosols in the Indo-Gangetic Basin, six different mixing cases, viz. external mixing, internal mixing, and four combinations of core-shell type mixing (black carbon, BC over dust, watersoluble over dust, BC over water-soluble and water-soluble over BC) have been considered. Composite single scattering albedo (SSA) have been computed for six cases for postmonsoon, winter and pre-monsoon seasons and are compared with the Aerosol Robotic Network (AERONET) retrieved SSA values. The most probable mixing state in the post-monsoon season seems either to be external mixing or water-soluble coating over dust and in the winter season, the external mixing seems to be the probable mixing state. However, in the pre-monsoon season, BC coating over dust seems to be the most probable mixing state. This type of mixing leads to enhanced absorption and needs future attention to better understand the aerosol radiative effect in this region.

The aim of this study conducted at Kanpur (26.51°N, 80.23°E), India, was to quantify chemical properties of dust and the intensity of mixing, due to its interaction with various emissions from anthropogenic activities, during its long range transport. Aerosol mass was collected at Indian Institute of Technology, Kanpur (IIT-K) located in the Indo-Gangetic Plain from April-July 2011, a period marked by intense dust storms and onset of monsoon. The sampling days were classified as Dust, Polluted Dust 1 (PD 1 ), Polluted Dust 2 (PD 2 ) and Continental days. PM 10 (coarse mode) and PM 2.5 (fine mode) collected on filter substrates were analysed for chemical composition. Elemental concentrations were measured using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). The results show that crustal elements like Ca, Fe, K, Na and Mg were dominant in coarse mode during dusty days, whereas, elements of anthropogenic origin like Cu, Ni, Se and V were mostly concentrated in fine mode during PD 1 as well as PD 2 . Very low elemental concentrations were found during continental days. SO 4 2-, Cl -and NO 3 -were found to be high during PD 1 and PD 2 days. Very good correlations of NH 4 + with Cl -and SO 4 2-ions in PD 1 days indicate their common sources of origin and formation of ammonium chloride and ammonium sulphate. Water Soluble Inorganic Carbon (WSIC) was found during all dust days, Water Soluble Organic Carbon (WSOC) was found to be highest during PD 1 and PD 2 days.