Abstract
A campaign was conducted at Kalas Island, Sundarban to address fog-induced changes in optical and physical properties of aerosols during the winter period (11–16 January 2014). Being an isolated remote island in the northern coastal region of Bay of Bengal, the measurement site provides a unique opportunity to investigate aerosol properties and foggy conditions during transported aerosol plumes from the Indo-Gangetic Basin (IGB). Two fog events were observed over Sundarban during the campaign increasing Aerosol Optical Depth (AOD) by almost a factor of three (1.6 ± 0.4) compared to “background” AOD of 0.52 ± 0.2 on a normal winter day. Back-trajectory analysis shows that aerosols mostly originate from the IGB contributing to higher fine-mode as well as coarse-mode aerosol concentrations during the foggy days. Black Carbon (BC), known as a tracer for anthropogenic sources, is found to be about 15.2 ± 1.3 µg/m3 at such a remote region and increased by 30% during foggy days indicating strong influence from transported anthropogenic aerosols from nearby urban regions. Similar enhancement is also observed in aerosol absorption coefficient, especially in the UV region. Low ventilation due to calm and cool atmosphere with a shallow boundary layer during foggy days could have ‘trapped’ BC over Sundarban and resulted in such an enhancement. On the other hand, the absorption angstrom exponent reduced indicating dominance of weakly spectral dependent aerosols during foggy period, mostly associated with fossil-fuel combustion. However, this could also be the result of BC coating with water-soluble species under high RH conditions. Enhancement of absorbing aerosols during foggy period reduces the incoming solar radiation, causing large perturbation in the radiation budget over the site.
Highlights
Fog is a common natural phenomenon in the IndoGangetic Basin (IGB) during winter (December–February) reducing visibility to less than 1 km, causing serious deleterious effects in transportation, delay of flights, car accidents and human loss (Goloub et al, 2001)
On the normal day, Relative Humidity (RH) levels were similar to those on the foggy day, wind speed was significant and the transported aerosol plumes moved further into the Bay of Bengal (BoB) resulting in lower Aerosol Optical Depth (AOD) and higher atmospheric transparency
The comparison of AOD values and its spectral distribution between the current and previous studies conducted at different environments over the Indo-Gangetic Basin (IGB) shows a great similarity in aerosol loading and optical properties suggesting the presence of a homogeneous fog-aerosol layer that extends over the entire region (Goloub et al, 2001)
Summary
Fog is a common natural phenomenon in the IndoGangetic Basin (IGB) during winter (December–February) reducing visibility to less than 1 km, causing serious deleterious effects in transportation, delay of flights, car accidents and human loss (Goloub et al, 2001). This supports that fog can cause accumulation of transported aerosols over remote regions due to low ventilation that is generally observed over urban regions in the IGB during foggy periods (Ganguly et al, 2006; Das et al, 2008; Safai et al, 2008, etc.).
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