Paper No. A21A-0009
Variability in chemical and stable isotopic characteristics of bulk aerosols at Goa (a coastal station along the west coast of India)
2012 AGU Fall Meeting
Rajesh Agnihotri1*, S.G. Karapurkar2, V.V.S.S. Sarma3, P. Praveen3, M.D. Dileep Kumar2
1CSIR-National
Physical Laboratory, New Delhi, 110012, India,
2
CSIR-National Institute of Oceanography, Dona Paula Goa 403004, India,
Visakhapatnam – 530 017, India
3
CSIR-National Institute of Oceanography, Regional Center, 176 Lawsons Bay Colony
*Corresponding author email: rajagni9@gmail.com; agnir@nplindia.org
4. Instruments
3. Study Region-
1. Introduction
Atmospheric particles are regarded as transitional geological repositories, integrating imprints of all emissions from local as
well as remotely located source regions including gas to solid phase conversions occurring in ambient conditions. Despite having
shorter atmospheric life time (~7-10 days), they are considered to be capable of influencing regional climate through their direct and
indirect effects. In tropics carbonaceous fraction of bulk aerosols is thought to be governing their overall influence on regional radiation
budget, air quality, human health and net climate. In coastal areas such as Goa, removal of bulk aerosols from atmosphere may provide
biologically important elements to nutrient limited coastal surface waters and hence can influence coastal productivity during lean
periods.
Mass concentrations of basic building blocks of carbonaceous fraction (e.g. Carbon and Nitrogen) added with their isotopic
values (d13C and d15N) have been shown to be providing important clues about primary sources and degree of secondary processing of
primary particles (aging effect) (Pavuluri et al., 2011; Agnihotri et al., 2011 and references therein). To gain further insights into
carbonaceous fraction of bulk atmospheric particles over Goa, we collected bulk aerosols from the roof of National Institute of
Oceanography (15.46N, 73.8E; at a height ~55.8m MASL; Fig.1) from December 2009 to January 2011. A total of 22 aerosol samples
were analyzed for measuring mass concentrations of total carbon (TC), total organic carbon (TOC) and total nitrogen (TN) and their
isotopic ratios d13CTC, d13CTOC, d15N. We discuss here variability observed in the measured parameters in terms of changes in source of
winds and other meteorological variables.
 Ambient atmospheric particles (TSPM) were collected using a high volume air
sampler (APM 430, Envirotech) with typical air flow rate ~1.1-1.7 m3.min-1.
 The isotopic composition of collected particulate were analyzed using Elemental
Analyzer (EA) coupled with a Delta V plus stable isotope mass spectrometer
(Thermo®) in a continuous flow mode.
Tin/Silver cup
Hole Puncher
5. Methodology
From the collected filters contained bulk atmospheric aerosols, sub-aliquots were cut
of 1 cm diameter. For measurements of TC, TN contents and their isotopic ratios (d13C
and d15N) one or two aliquots were taken out and placed in pre-clean Tin cups and
made round pellets which were then combusted in an Elemental Analyzer connected
to a Delta V Stable isotope ratio mass-spectrometer (Thermo). 13C/12C and 15N/14N
ratios of sample gases are then measured with respect to automated injections of
standard (calibrated) CO2 and N2 gases. Same procedure was performed for TOC
measurements on pre-acidified (with 10% supra-pure HCl). Final results are expressed
as d13C and d15N relative to PDB and atmospheric N2 standards respectively, and
defined as:
d13C and d15N = [(Rsample/Rstandard)-1]×1000----------------------- (1)
2. Rationale of the study Agnihotri et al. (2011) revealed that during inter-monsoon period bulk aerosols over the coastal Arabian Sea including Goa were
characterized by significantly lower d15N compared to those over Indian cities and the Bay of Bengal. This observation motivated us to
investigate temporal changes in chemical and isotopic characteristics of bulk aerosols over Goa in response to seasonally reversing
winds (monsoonal activity).
Sample aliquot
Fig. 1: Location and air sampler used for bulk aerosol
sampling at the roof of National Institute of
Oceanography Goa (India).
Sample filter
Fig. 2: Schematic presentation of aerosol filter stable isotopic analyses.
R denotes 13C/12C and 15N/14N ratio in sample and standard.
Schematics of isotopic analyses of aerosol samples are shown in Fig.2.
6. Results8. Conclusions The coastal locale Goa despite being known for a temperate climate throughout the
Aggregate
(CaCO3 rich particle)
year, clearly reveals seasonal influence of wind circulation pattern on the chemical and
isotopic composition atmospheric aerosols.
Crust particles
 Atmospheric aerosols during summer months (pre-monsoon period) (a period when
ambient temperatures and relative humidity are higher) are characterized by lower
mass concentrations of TC, TOC, TN, significantly lower d15N and slightly lighter d13C.
Rounded particle
(bio-aerosol)
 Overall d13C values of TC or TOC measured in this study appear to be an intermediate
value of particles emitted from C-3 type biomass burning (~ -27‰), fossil fuel
combustion (~ -26‰) and burning of coal (average ~ -22‰; Agnihotri et al., 2011).
 Winter aerosols (when ambient temperatures and relative humidity are lower) show
significantly higher TC, TOC, TN mass concentrations, significantly higher d15N values
and slightly heavier d13CTC.
Layered structure
(Calcite and quartz rich
particle)
Triangular flake structure
Flattened, rounded,
crust like structure
 d15N values of bulk aerosols during summer are significantly lower than that of source
Flattened particle
Fig.6: Prevailing wind-directions during sampling days of
this study are depicted by different symbols.
emissions of continental India indicating mixing of lighter N species from adjacent
marine realm (coastal Arabian Sea) undergoing denitrification.
 In contrast, d15N values during winter are significantly higher than that of plausible
biomass –biofuel burning source emissions of northern continental India indicating
significant degree of atmospheric processing during the long range transport of aerosols
coming from north-northeast of India.
9. ImplicationsAngular structure
( O, C, N and Si rich )
Fig.4: Temporal variability in isotopic values
d13CTOC and d15NTN) of bulk aerosols over Goa.
(d13CTC,
 Aerosol sampling has to be done on much higher temporal resolution like sub-weekly basis to investigate local meteorological
influences (if any) on chemical and isotopic characteristics of carbonaceous fraction of ambient aerosols.
 Secondary processing or aging of aerosols has to be studied, in detail, by multi-tracer approach to gain further insights of local scale
lower atmospheric processes and long range transport of aerosols in winter and their plausible influence on aerosol hydro-chemistry,
radiation budgets and net climate.
of winds, inventory of mineral dust during pre-monsoon months, mixing of carbonaceous
aerosols emitted from biomass –biofuel burning mainly occurring in northern parts of
India and variable degree of atmospheric processing during the long range transport of
aerosols.
Fig.5: End-member isotopic values (d15NTN, d13CTC) of typical
source emissions (adapted from various published reports
and research papers).
Fig.3: Variability observed in TC, TOC, TN and TSP mass
concentrations of bulk aerosols over Goa. No aerosol samples were
collected during monsoon period.
10. Future work-
 Ambient aerosols over Goa appear to be significantly influenced by seasonal reversal
Bent structure with channels
Flattened particle
Rounded
particle
 This study further underscores utility of stable isotopes of biogenic elements C and N
Fig.7: Observed variability in meteorological parameters (wind
speed, ambient air temperature and relative humidity) on the
aerosol sampling days of this study.
together with their mass concentrations (TC, TOC, TN) of atmospheric particles for
providing important clues about complex lower atmospheric chemistry especially
information about primary sources and extent of secondary atmospheric processing
(aging effect).
References
Acknowledgements-
Agnihotri, R., Mandal, T. K., Karapurkar, S., Naja, M., Gadi, R., Ahammed, Y. N., Kumar, A., Saud, T., and Saxena, M. (2011) Stable carbon and nitrogen isotopic composition of bulk aerosols over
India and Northern Indian Ocean, Atmos. Environ., 45, 2828–2835, 2011
Pavuluri, C. M., K. Kawamura, T. Swaminathan, and E. Tachibana (2011), Stable carbon isotopic compositions of total carbon, dicarboxylic acids and glyoxylic acid in the tropical Indian aerosols:
Implications for sources and photochemical processing of organic aerosols, J. Geophys. Res., 116, D18307, doi:10.1029/2011JD015617
Turekian, V. C., S. A. Macko, and W. C. Keene (2003), Concentrations, isotopic compositions, and sources of size-resolved, particulate organic carbon and oxalate in near-surface marine air at Bermuda
during spring, J. Geophys. Res., 108, 4157, doi:10.1029/2002JD002053
Authors are thankful to Directors CSIR-NPL
(New Delhi) and CSIR-NIO (Goa) for support
and encouragement of this study. Dr. P. Mehra
(NIO) kindly provided contemporaneous
meteorological data measured by automated
weather station (AWS) installed at NIO-Goa.