Aerosol Number Concentrations and Visibility during Dense Fog over a Subtropical Urban Site

Manju Mohan,Swagata Payra

doi:10.1155/2014/495457

Abstract

There is now enough evidence of greater frequencies and extent of fog formation in urban areas. These could easily be linked to rapid increase in aerosol number concentration (ANC) peculiar to polluted urban environments. It is therefore pertinent to study ANC and visibility alongside the meteorological parameters in order to investigate the relationships which may possibly exist between these parameters especially during foggy conditions. This study based on field measurements of ANC for aerosol spectrum varying from 0.3 µm to 20 µm attempts to investigates whether a threshold ANC could be associated with a given visibility range during low visibility conditions including dense fog episodes. Thus, the present work explores relationship between ANC size spectrum and visibility (100 m–4500 m) in a polluted urban environment in India with specific reference to episodes of dense fog during winter period. The study depicts a threshold minimum value of ANC during foggy conditions. A power relationship between ANC and visibility is obtained. Further, aerosol number distribution and size distribution function are also studied and empirical relation is compared with previous studies. Further work is suggested to strengthen the findings presented here.

Highlights

Fog formation occurs typically in aerosol-laden surface air under high relative humidity, ranging from undersaturated to slightly supersaturated conditions [1]
The aim of this study is to explore the relationships that exist between aerosol number concentration (ANC), aerosol size distribution (ASD), and visibility with special attention to low visibility episodes
ANC variations are shown during as well as several hours before and after the dense fog formation. It is clearly revealed in this figure that low visibility conditions are associated with higher values of ANC during the fog initiation and vice versa

Summary

Introduction

Fog formation occurs typically in aerosol-laden surface air under high relative humidity, ranging from undersaturated to slightly supersaturated conditions [1]. It is worth mentioning that only few studies are reported that include aerosol number concentration (ANC) and visibility during fog [4, 5]. There are some recent studies related to fog over Delhi [6,7,8,9,10] Amongst these studies, Mohan and Payra, 2009 [9], explore ANC and visibility related issues during fog. Mohan and Payra, 2009 [9], explore ANC and visibility related issues during fog They studied visibility and ANC during winter season of 2004 and 2006 in a qualitative manner where air pollutant concentrations were shown to be correlated with the daily episodes of fog. Atmospheric aerosols in the accumulation mode efficiently scatter the solar radiation because their size is of the same order as that of the wavelength of radiation and accumulation mode aerosols can reduce visibility to a greater extent due to increased scattering and exacerbate the process of fog formation

Objectives

Results

Conclusion