Morphology, Chemical Composition and Mixing State of Atmospheric Aerosols from Two Contrasting Environments in Southern India

Abstract

Abstract. The state of mixing of aerosols significantly influence their transformation, deposition, radiative forcing and health effects. We report the realistic (as in the atmosphere) state of mixing and morphology of aerosols at two nearby but contrasting environments; the urban region Bengaluru and a remote region Challakere. Ambient aerosols are collected on filter substrates using a High-Volume Sampler and analysed using Scanning Electron Microscope 15 (SEM) equipped with Energy Dispersive X-Ray spectroscopy (EDX). The results show that the prevailing state of mixing of aerosols is 'core-shell' with SiO2 as the core and Carbon-SiO2-others (Calcium-Magnesium-Aluminium and other dust origin elements) combinations as the shell. On an average, for Bengaluru (Challakere), 66% (51%) of the core surface is coated. The sample-wise mean Carbon content of the composite particles reaches as high as ~26% (~9%) at Bengaluru (Challakere). The ambient black carbon (BC) mass concentration and the amount of rainfall that 20 occurs just prior to the end of sampling are found to significantly influence the Carbon content of the particles. To the best of our knowledge, this is a first-of-its kind study over Indian region, coupling realistic aerosol observations and spectroscopy along with advanced image processing techniques to investigate the state of mixing and morphology of atmospheric aerosols at single particle resolution. This knowledge regarding the real-life state of mixing of aerosols would be useful in constraining the models studying aerosol-radiation interactions. 25 1 Introduction:

Simulations of Black Carbon Over Indian Region: Improvements and Implications of Diurnality in Emissions

“…In a stricter sense, the diurnal variation in emissions of BC as well as the adjustment factor would vary from station to station depending upon the nature of sources and mesoscale meteorology running over the synoptic scale (Govardhan et al, 2016). Thus, the modified emissions may not necessarily improve the model's performance 5 homogeneously throughout the study domain. This is especially clear from disagreements that still persists between model (DIEM+AF configuration) and the observations at a few stations ( fig.2d Chennai May, fig.2i.…”

“…Another important parameter is the assumptions about state of mixing of aerosol in general and BC in particular which significantly governs the resulting optical and radiative effects 15 of aerosol (Jacobson, 2001;Chandra et al, 2004;Peng et al, 2016). In these regards, the realistic state of mixing of aerosol over the Indian region (as revealed by a recent study (Hariram et al, 2018) could be examined and prescribed in the models to reveal its role in unrealistic simulations of AOD. This study has also revealed an important information about response of NSBC mass concentration to the changes in emissions.…”

“…It has been seen in our earlier studies (Govardhan et al, 2015(Govardhan et al, , 2016 that the model simulations show a good comparison with observations when the ambient aerosol loading is less. Thus, keeping in mind such 5 a behaviour of the model we chose to carry out the model simulations and further evaluations only for those seasons which depict relatively higher aerosol burden over the Indian region (i.e. pre-monsoon, post-monsoon and winter).…”

Responses to the comments of Reviewer #1

“…It is also reported that the largest volume fraction (34.216 %) of particles captured is in the outer layer. Moreover, it is also estimated that about 65 % of the volume fraction of particles is found in the top two layers of the filter (Beatriz et al 2016). Additionally, it is reported that, the inner layers are found to trap only the fine particles of mean particle diameter 0.533 µm.…”

“…We, on the other hand, are interested to study the morphology and state of mixing of dust-BC composite aerosol system (dust particles with BC depositions), which mainly falls under coarse mode aerosols. A recent study (Beatriz et al 2016) has reported that quartz fiber filters efficiently collect particles of different size ranges in the top layer ranging from diameter of 0.199 µm to 10.272 µm. It is also reported that the largest volume fraction (34.216 %) of particles captured is in the outer layer.…”