Heterogeneity in aerosol characteristics at the semi-arid and island AERONET observing sites in India and Maldives

ABSTRACT

Multi-year Aerosol Robotic Network (AERONET) direct Sun retrieved and inversion algorithm derived aerosol products at a semi-arid, urban site, Jaipur (26.90° N, 75.80° E) and island observing site, Maldives Climate Observatory-Hanimaadhoo (MCO-Hanimaadhoo, 6.74° N, 73.17° E) are analysed to investigate heterogeneity in aerosol optical and microphysical properties. Results reveal the existence of a large seasonal diversity in the frequency distributions of aerosol optical depth (AOD_500 nm, AOD_1020 nm) and Ångström exponent (AE_{440–870 nm}) during different seasons at Jaipur and MCO-Hanimaadhoo. These are indicative of the advection of different aerosol types (viz., black carbon (BC) aerosol, organic aerosol, sulfate particle, dust, sea salt, nitrate particle, and mixtures thereof) from a variety of production mechanisms influenced by strong seasonal changes of anthropogenic activities as well as modulations induced by the climatic condition. The cumulative frequency analysis of the single scattering albedo (SSA) difference (i.e. ΔSSA = SSA_440 nm – SSA_1020 nm) shows that at Jaipur ΔSSA is predominantly negative (around 88% days) while at MCO-Hanimaadhoo it is positive (around 74% days). The positive and negative values of ΔSSA are respectively linked to a stronger absorption by BC mixed anthropogenic pollution aerosols at 1020 nm and to a stronger absorption by mineral dust containing iron oxide at 440 nm. The spectral behaviour of SSA, thus, facilitates investigation of the existence of iron oxide or BC in aerosols. The ‘Bivariate Kernel density’ plots of SSA versus fine-mode fraction (FMF) of AOD_440 nm/AE_{440–870 nm} reveal that at Jaipur the aerosol ensemble consists of coarse-mode particles (AE and FMF cluster in the range 0.2–0.4), a dominant category along with significant fine-mode and much less mixed category. At MCO-Hanimaadhoo fine-mode particle category (with FMF and AE cluster in the range 0.90–0.95 and 1.2–1.6 respectively) is the only dominant category. The persisting log-normal bimodal feature in aerosol volume size distribution (AVSD) is observed both at Jaipur and MCO-Hanimaadhoo. The modal volume concentration of coarse-mode aerosol decreases from FMF of AOD_675 nm = 0.25 (inherently belonging to the coarse-mode regime) to FMF of AOD_675 nm = 0.95 (inherently belonging to the fine-mode regime). This transformation in coarse-to fine-mode volume concentration is associated with a steady rise in AE_{440–874 nm} supporting this changeover.     

Oxygen transport in zeolite Y measured by quenching of encapsulated tris(bipyridyl)ruthenium

This study deals with emission quenching of zeolite encapsulated trisbipyridyl ruthenium (II) (Ru(bpy)₃²⁺) by oxygen. Oxygen saturated solutions of Ru(bpy)₃²⁺ typically show about 70% quenching (I₀/I=3.3), where I₀ and I are the peak intensities of the emission in N₂ and O₂, respectively. However, an aqueous suspension of Ru(bpy)₃²⁺-zeolite Na–Y (Si/Al = 2.5) (abbreviated as Ru–Na–Y) showed no quenching at all. This observation motivated us to analyze how the transport of O₂ is occurring in the zeolite. Upon exposure of solid Ru–Na–Y (99% of intrazeolitic water) to N₂/O₂ dry gases, quenching in oxygen was found to be 5% (I₀/I=1.07). Partial dehydration at room temperature with loss of 33% of the water molecules from the zeolite led to 66% (I₀/I=2.96) quenching. Dehydration of Ru–Na–Y at 250 °C under vacuum overnight led to complete loss of intrazeolitic water and increased quenching to 90% (I₀/I=10.7). Nanocrystalline Ru(bpy)₃²⁺-zeolite Y upon vacuum dehydration lost 55% of the intrazeolitic water and showed 96% (I₀/I=25.3) quenching. The extent of quenching of Ru(bpy)₃²⁺ in zeolites by O₂ is by far the largest as compared to previously studied matrices, and is being attributed to confinement of O₂ in the supercages, which leads to increase in number of collisions with Ru(bpy)₃²⁺ and enhanced quenching. However, these samples showed complete lack of sensitivity (I₀/I=1) to oxygen upon exposure to water saturated gas or dissolved gas. Dealumination of zeolite framework by treatment with (NH₄)₂SiF₆ produced a framework of Si/Al = 9.5, and with SiCl₄ a framework of Si/Al > 100. With increasing dealumination, the extent of quenching by dissolved O₂ increased.