Vertical profiles of aerosol black carbon in the atmospheric boundary layer over a tropical coastal station: Perturbations during an annular solar eclipse

doi:10.1016/j.atmosres.2010.11.019

Atmospheric Research

Volume 99, Issues 3–4, March 2011, Pages 471-478

https://doi.org/10.1016/j.atmosres.2010.11.019 Get rights and content

Abstract

Altitude profiles of aerosol black carbon (BC) in the atmospheric boundary layer (ABL) over a tropical coastal station, Trivandrum have been examined on two days using an aethalometer attached to a tethered balloon. One of these days (15th January, 2010) coincided with a (annular) solar eclipse, the longest of this century at this location, commenced at 11:05 local time and ended by 15:05, lasting for 7 min and 15 s (from 13:10:42), with its maximum contact occurring at ~ 13:14 IST with ~ 92% annularity, thereby providing an opportunity to understand the eclipse induced perturbations. Concurrent measurements of the ABL parameters such as air temperature, relative humidity and pressure were also made on these days to describe the response of the ABL to the eclipse. BC profiles, in general, depicted similar features up to an altitude of ~ 200 m on the eclipse day and control day, above which it differed conspicuously with profiles on eclipse day showing increasingly lower concentration as we moved to higher altitudes. Examination of the meteorological profiles showed that the altitude of maximum convection rapidly fell down during the eclipse period compared to that on control day indicating a rather shallow convection on eclipse day. Comparison of diurnal variations of BC at the surface level showed that the rate of decrease in BC during daytime on the eclipse day was smaller than that on the control day due to the reduced convection, shallow ABL and consequent reduction in the ventilation coefficient. Moreover the time of the nocturnal increase has advanced by ~ 1:30 h on the eclipse day, occurred at around 19:30 IST in contrast to all the other days of January 2010, where this increase usually occur well after 20:30 IST, with a mean value of 21:00 IST. This is attributed to the weak sea-breeze penetration during the eclipse day, which led to an early onset of the land breeze.

Research Highlights

► Effects of a noontime solar eclipse on altitude profile of aerosol BC were investigated. ► Large reduction in convection during eclipse led to a shallow sea breeze cell. ► Weakening of sea breeze led to an early onset of land breeze on the eclipse day. ► Consequently lower BC concentrations occurred at altitudes above the TIBL. ► Rate of dispersion of BC at the surface was far less on the eclipse day.

Introduction

Information on the vertical distribution of aerosols is important not only from the environmental perspective, but vital as well in the accurate estimation of aerosol radiative impacts (Satheesh et al., 2008, Moorthy et al., 2009). Notwithstanding this, the limited knowledge on the vertical distribution of aerosols in the atmosphere and its large variability, result in one of the largest sources of uncertainties in aerosol radiative forcing estimation (Kaufman et al., 2002). Also, the impact of aerosols, particularly the absorbing species such as BC (black carbon) and dust, on climate forcing is significantly different depending on the abundance of these species above or below clouds (Chand et al., 2009, Moorthy et al., 2009). This makes the knowledge of the vertical distribution of BC all the more important.

In contrast to the inland or oceanic environments, coastal environments are distinct due to the prevalence of the meso-scale processes such as sea and land breeze circulation, which would produce significant short-term changes in the characteristics of aerosols (Moorthy et al., 1993, Pillai and Moorthy, 2001). During daytime, these circulations lead to the formation of a shallow thermal internal boundary layer (TIBL) near the coast much below the inland boundary layer (BL), thereby partly confining the aerosols within a shorter vertical extent. The depth of the TIBL increases strongly with the inland penetration of the sea breeze, until it eventually merges with the inland BL. This would lead to structures in the vertical distribution of aerosols depending on the atmospheric boundary layer (ABL) characteristics (such as depth and inland fetch) and the return flow in the opposite direction at the top of the ABL (Beegum et al., 2008). The development of TIBL associated with onshore flow has great significance in the dispersion of air pollutants (Kunhikrishnan et al., 1993). In such cases the altitude distribution becomes time dependent and also depends on factors that may affect the ABL depth. In this paper we examine the vertical profiles of BC aerosols over a tropical coastal site, Trivandrum during two clear days of January 2010, one of which coincided with the annular solar eclipse of 15th January 2010. The results are used to examine the changes brought about in the vertical distribution of BC associated with the changes in the ABL characteristics caused by the solar eclipse.

Section snippets

Profiling site, general meteorology and experimental set-up

The experiment was carried out from Thumba, a coastal location in Trivandrum (TVM, 8.5°N, 76.9°E), the actual site being ~ 500 m due east of the Arabian Sea coast as shown in Fig. 1a. The city of TVM is a sparsely industrialized, semi-urban, tropical location with a population of ~ 1 million and is located ~ 6 km due southeast of the site. The profiling site, being inside the rocket launching station, is well separated from large-scale anthropogenic activities associated with the urban center.

Being a

Data analysis

The data analysis involved in correcting the aethalometer measured BC to change in ambient pressure and temperature to bring it down to the STP as the aethalometer flow was maintained by a standard flow meter under standard temperature (T_o, 293 K) and pressure (P_o, 1017 hPa conditions) following Moorthy et al. (2004) $M_{B} = M_{B}^{*} {[\frac{P_{o} T}{P T_{o}}]}^{- 1}$ where M_B^⁎ are the raw concentrations of BC mass; M_B are the true concentrations at STP; P_o and T_o correspond to standard pressure and temperature while P and T are the

Results and discussion

The experiment was conducted on two clear days, 15th and 28th January 2010, both being clear sunny days. The profiling on both days spanned over a period of 3:30 h from 10:30 to 14:00 IST. This period typically corresponds to the well-developed convective activity in the atmospheric boundary layer (ABL) (the local sunrise in January being ~ 07:00 IST). However, the first day (15th January, hereafter termed as eclipse day) coincided with a (annular) solar eclipse, the longest of this century at

Summary

Altitude distributions of aerosol BC were made on two clear days from a tropical coastal station, Trivandrum, in which the first day coincides with an annular solar eclipse which was on the 15th January 2010. It is found that the

1.
BC profiles in general depicted similar features up to an altitude of 200 m on both days, above which it differed conspicuously with profiles on eclipse day showing increasingly lower concentration as we moved to higher altitudes.
2.
Examination of the meteorological

Acknowledgements

The work was carried out under the ARFI project of ISRO Geosphere-Biosphere Program. The authors wish to thank the TIFR Balloon Facility, Hyderabad for providing Kytoon for the experiment and TERLS facilities and operation division headed by Mr. Koshy Mammen and Meteorological facility headed by Dr. Shambhu Namboodiri K. V. for the ground support and necessary clearances.

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View full text

Vertical profiles of aerosol black carbon in the atmospheric boundary layer over a tropical coastal station: Perturbations during an annular solar eclipse

Abstract

Research Highlights

Introduction

Section snippets

Profiling site, general meteorology and experimental set-up

Data analysis

Results and discussion

Summary

Acknowledgements

Sci. Total Environ.

J. Atmos. Sol. Terr. Phys.

Aerosol black carbon over a tropical coastal station in India

Geophys. Res. Lett.

Impact of a mountain grassland fire on the concentration of aerosol black carbon and carbon monoxide near the surface at a remote coastal location

Atmos. Res.

Satellite-derived direct radiative effect of aerosols dependent on cloud cover

Nature Geosci.

Real-time measurement of the absorption coefficient of aerosol particles

Appl. Opt.

A satellite view of aerosols in the climate system

Nature

Sodar echo patterns and study of thermal plumes over Thumba

Mausam

Study on thermal internal boundary layer structure over Thumba

Ann. Geophys.