Variability in chemical and stable isotopic characteristics of bulk aerosols at Goa (a coastal station along the west coast of India)

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, 3 CSIR-National Institute of Oceanography, Regional Center, 176 Lawsons Bay ColonyVisakhapatnam – 530 017, India

*Corresponding author email: rajagni9@gmail.com; agnir@nplindia.org

Paper No. A21A-0009

2012 AGU Fall Meeting

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.

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

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 (15.46N, 73.8E; 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.

1. Introduction

References

8. Conclusions-

3. Study Region-

Fig. 1: Location and air sampler used for bulk aerosol sampling at the roof of National Institute of Oceanography Goa (India).

Acknowledgements-

Aggregate(CaCO3 rich particle)

Layered structure(Calcite and quartz rich particle)Triangular flake structure

Flattened, rounded, crust like structure

Flattened particle Rounded particle

Bent structure with channelsAngular structure( O, C, N and Si rich )

Crust particles

Rounded particle (bio-aerosol)

Flattened particle

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.

4. Instruments

6. Results-

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)

R denotes 13C/12C and 15N/14N ratio in sample and standard. Schematics of isotopic analyses of aerosol samples are shown in Fig.2.

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.

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).

Fig. 2: Schematic presentation of aerosol filter stable isotopic analyses.

Sample filter

Hole Puncher

Sample aliquot

Tin/Silver cup

Fig.4: Temporal variability in isotopic values (d13CTC, d13CTOC and d15NTN) of bulk aerosols over Goa.

Fig.6: Prevailing wind-directions during sampling days of this study are depicted by different symbols.

Fig.7: Observed variability in meteorological parameters (wind speed, ambient air temperature and relative humidity) on the aerosol sampling days of this study.

The coastal locale Goa despite being known for a temperate climate throughout the year, clearly reveals seasonal influence of wind circulation pattern on the chemical and isotopic composition atmospheric aerosols. 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.

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.

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).

d15N values of bulk aerosols during summer are significantly lower than that of source 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.

Ambient aerosols over Goa appear to be significantly influenced by seasonal reversal 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.

9. Implications-

This study further underscores utility of stable isotopes of biogenic elements C and N 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).

10. Future work- 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.

Fig.5: End-member isotopic values (d15NTN, d13CTC) of typical source emissions (adapted from various published reports and research papers).