What has been learned from the Indian Ocean Experiment?

A brief summary of results from Pre-INDOEX experiments
by Xu Li-Jones, C4/SIO
June 1998

I. Introduction
II. Impact
III. Summary
IV. References
V. List of Figures

I. Introduction

The Indian winter monsoon region is bordered to the north by the highly populated and rapidly developing Indian sub-continent and to the south by the vast Indian Ocean. INDOEX (Indian Ocean Experiment) [Ramanathan et al., 1995, and 1996] is designed to use this unique natural laboratory to study the large-scale impact of pollutants on marine tropospheric chemistry and cloud properties. However, in this region the pre-existing data on the chemical, physical and radiative properties of the atmosphere are sparse [McFarquhar et al., 1994]. To provide back-ground information for the forthcoming 1999 INDOEX intensive experiment, 6 pre-INDOEX cruises on ships-of-opportunity (4 inter- hemispheric and 2 coastal) have been conducted over the Arabian Sea and Indian Ocean region. The first was carried out on the NOAA R/V Malcolm Baldrige in 1995. Since then, three inter-hemispheric and 2 coastal cruise experiments have been conducted on the R/V Sagar Kanya during winter monsoon seasons in 1996, 1997 and 1998. In addition, continuous measurements of aerosol chemical, physical, and optical properties on Kaashidhoo island (4.97N, 73.47E) were initialized in January of 1998 to provide a climatology record for the pre-, during and after-INDOEX period. Data from the most recent experiments are still under investigation. In this report, we will summarize the progresses that made through these experiments and model simulations, although findings in their developing stage will not be covered.

II. The impact from anthropogenic pollutants: intensity and spatial coverage

Results from pre-INDOEX experiments further confirm the anthropogenic impact from the Indian sub-continent to the Indian Ocean region. The intensity and spatial coverage can be reflected by trace gas concentrations, aerosol properties, and radiation flux measured during these experiments.

1. Impact on the Concentrations of Atmospheric Components
Concentrations of aerosol, O3, CO, CO2 and NOy previously measured during the 1995 NOAA R/V Malcolm Baldrige cruise [Rhoads et al., 1997] have revealed large temporal and spatial variations in the Indian Ocean region (Figure 1). From the Southern Hemisphere extratropical to the Northern Hemisphere tropical continental region, the non- sea-salt (nss) SO4= increased by a factor of 4. Evidence of direct transport of anthropogenic emissions (such as dark grey filter color and high dust loading) was detected 1500 km from southern India. Since then, more evidences of the anthropogenic impact in the Indian Ocean region have been observed. Results from the 1996 pre-INDOEX on the R/V Sagar Kanya has shown that total aerosol mass concentration increased from a low of a few ng/m3 over the remote ocean region to a high of about 80 ng/m3 near the coast of Bombay [Jayaraman et al., 1997]. This large concentration increase is also reflected by each individual chemical component measured during the same cruise (Figure 2). During two coastal cruises, Moorthy et al. [1998] found a significant increase in the columnar loading close to the coast. Meteorological analysis suggests that these high aerosol concentrations could be linked to transport from the Indian subcontinent and also from sources in the Middle East and North Africa [Krishnamurti et al., 1998]. Trajectory calculations show that the northeast monsoonal low level flow can transport sulfates, mineral dust and other aerosols from the Indian sub-continent to the intertropical convergence zone (ITCZ) within 6 to 7 days and the transport of mineral dust in the middle troposphere from the Arabian desert can reach to 4000 km away with a transit time of 2 to 3 days [Krishnamurti et al., 1998].

The stronger concentration gradient in the coastal region for dust compared with that of nss SO4= suggests that the dust source over the Indian sub-continent has a substantial fraction in the large particle size range that are subjected to gravitational settling. The similar distribution pattern among NH4+, NO3- and dust reveals that a large fraction of NO3- and NO3- measured in the coastal region may be associated with the large soil dust particles, possibly from fertilizer NH4NO3. After the initial settling of large particles, different aerosol species exhibit a similar variation pattern (Figures 2 and 3). This implies that the concentrations of these aerosol species were governed by similar transport pattern and removal processes. In Figure 2, the lowest concentrations of all species were observed in the ITCZ zone, where precipitation was encountered. During the 1995 cruise, ratios of trace gases to aerosol concentrations suggest that biomass burning fires did not contribute significantly to the pollutants observed in that study [Rhoads et al., 1997].

The aerosol chemical composition measured during the 1996 cruise (Figure 4) has shown relative contributions to the total aerosol mass (sum of sea salt, dust, nss SO4=, NO3- and NH4+ ) to be about 20% from nss SO4=, 10% from NO3- and 10% from NH4+, even over the open ocean region that is several thousands of km away from the continent. Sea salt aerosol dominates the total aerosol mass during most time periods over the remote Indian Ocean region, with an average of 52%. In the coastal region, however, dust concentration reached about 50% of the total aerosol mass, while sea salt aerosol dropped to about 10%. Because of the high impact of anthropogenic pollutants in the Indian Ocean, biogenic nss SO4= that oxidized from dimethyl sulfide (DMS) has shown a very minor contribution to the total aerosol atmospheric loading. Consequently, the proposed climate feedback from the DMS oxidation (Charlson et al., 1987) will not play a significant role in this region.

2. Impact on the Aerosol Size Distribution
The effect of anthropogenic pollutants from the Indian sub-continent are also clearly reflected in the measured aerosol size properties. Results from the 1996 cruise show that the sub micron size particles increased more than an order of magnitude in the number concentration near the coast than that over the interior ocean [Jayaraman et al., 1997]. The corresponding Angstrom exponent increased from 0.2 over the Indian ocean to about 1.4 near the coast. The impact on the aerosol size distribution is also evident from results from the two coastal cruises performed in 1996 (10 days in March and 8 days in April) [Moorthy et al., 1998; Satheesh and Moorthy, 1997; Satheesh et al., 1998]. Large increases in the submicron particles were also found through the aerosol optical depth wavelength dependence. For wavelengths below 600 nm, values of aerosol optical depth for the near coastal regions are higher than those of the far coastal regions, suggesting an increase in the concentration of submicron particles in the near coastal region. As the wavelength increases, the two distributions gradually become overlapped, suggesting a similar level of large particle concentration in the two regions [Satheesh and Moorthy, 1997]. There is a considerable broadening of the accumulation mode with increase in distance from the coast possibly due to the microphysical processes [Satheesh et al., 1998]. Aerosol size distributions over far coastal and remote ocean locations are bimodal with more stable modes, compared to the coastal regions with mean values of the mode radii at 0.06 and 0.74 um. Over far coastal and remote marine locations, the effective radius becomes larger due to the increase in the relative dominance of larger particles in the size spectrum [Moorthy et al., 1998]. The mass size distribution measured by Jayaramans group revealed large concentration increases in the size ranges of around 0.1 nm and >12.5 nm when approaching from open ocean to the coastal region (data after January 30 in Figure 5). These two size ranges are consistent with the size characteristic of industrial combustion and soil dust aerosols, respectively.

3. Impact on the Atmospheric Optical Properties
The high level of anthropogenic pollutants and more optical effective aerosol size have a large effect on the optical properties in the Indian Ocean region. Aerosol optical depth in the visible wavelength region was found to be in the range of 0.2 to 0.5 over the Arabian Sea and <0.1 over the equatorial Indian Ocean [Jayaraman et al., 1997]. Aerosol optical depth decreases rapidly as the distance from the coast increases. The influence of coastal characteristics are found to prevail up to about 1000 km offshore. In the remote ocean regions the optical depths are generally low, usually in the range 0.1 to 0.2, when the wind speed is low [Moorthy et al., 1997]. Satheesh et al. [1998] found that the aerosol optical depth decreases with an e-1 scaling distance in the range of 1700 to 2400 km under calm to moderate wind conditions. The scaling distance in aerosol optical depth increases with wind speed which was attributed mainly to increased local production of sea-spray aerosols over the ocean at high wind speeds [Satheesh et al., 1998]. Aerosol optical depths measured in prevailing marine air masses are found to increase nearly exponentially with average wind speed, but lack of any dependence with relative humidity [Moorthy et al., 1997]. At higher wind speed, aerosol optical depths tend to increase towards longer wavelengths [Moorthy et al., 1998].

A regression analysis made between the direct solar flux at the surface and aerosol optical depth reveals that the flux decreases by about 42 W/m2 for solar flux and 14 W/m2 for global flux with every 0.1 increase in aerosol optical depth (Figure 6) [Jayaraman et al., 1997]. Krishnamurti et al. [1998] found that transport of pollutants from the Indian sub- continent has result in an increase in the aerosol optical depth at the equator by as much as 0.2. Based on the relationship between the radiation flux and aerosol optical depth [Jayaraman et al., 1997], this level of aerosol optical depth corresponds to a decrease in the solar radiative forcing at the sea surface by about 10 to 20 W/m2 [Krishnamurti et al., 1998].

4. The Role of ITCZ in inter-Hemispheric Exchange
The inter-hemispheric transect measurements suggest that the ITCZ serves as an efficient isolator in blocking the transport of pollutants from the Indian sub-continent to the Southern Hemisphere; across the ITCZ at 6S the nss SO4= increased by a factor of 4 (Figure 1). Nevertheless, the concentrations of aerosol, CO and CO2 are consistently higher in the southern tropical than extratropical region (Figure 1) suggesting the isolation is not absolute. Back trajectories indicate that the remote Indian Ocean is the immediate source region for the pollutants measured south of ITCZ, transferred through cross-ITCZ flow [Rhoads et al., 1997]. These interhemispheric transports are effected by eddies that wrap around the ITCZ; these eddies bring clean Southern Hemisphere air to about 10N in the Indian Ocean and carry polluted continental air into the Southern Hemisphere [Krishnamurti et al., 1998].

Improvement in Understanding the Meteorology Condition
Using a very high-resolution global model (70x70 km, at 6 hourly intervals), Krishnamurti et al., [1997] reanalyzed the global meteorology conditions with the emphasis on the winter monsoon in the Indian Ocean region. Compared with observed data, the model reveals more realistic meteorology patterns over the standard low-resolution model. Using this high-resolution model, they generated fields of wind, temperature, pressure, pressure, humidity, rainfall, clouds and a suite of derived fields related to the heat sources and sinks of the winter monsoon. As an example, Figure 7A to C, shows the monthly mean streamlines and isotachs at 200mb, 850mb and 1000mb for February 1993. These fields show that the winter monsoon flows emanating from the Indian sub-continent in very shallow northerlies that turn rapidly and have a strong westerly component from the sea level to about 2 km. They found that the depth of the monsoonal flow increases in the vicinity of the ITCZ near the equator. The strong diurnal change in the depth of the inversion layer over the land mass of south Asia, coupled with the rapid veering of wind with height has important implications on the diurnal change of pollution transports from the north towards the ITCZ. The converging monsoonal and trade wind systems of the two hemispheres and tropical disturbances to the south of the ITCZ shows a basic mechanism for the inter-hemispheric transport. The cloud fields show a maximum in the middle cloud fractions where air is relatively moist. The heat sources and sinks as a function of latitudes and altitude show the relative importance of the convective and radiative heating. Large values of convective heating reside over the region of the ITCZ (of the order 5 to 6C/day), while non-convective heating is spotty. These meteorology conditions in the Indian Ocean region are important for experimental design and data interpretation for the INDOEX.

5. Ozone
Besides the maxima in the northern continental region from the conventional photochemical reaction, ozone and gaseous reactive nitrogen has shown an additional maxima in the southern extratropical, presumably due to the downward transport from the stratosphere or lightning [Rhoads et al., 1997]. A strong diurnal variation in ozone was observed in the equatorial regions of both hemispheres. The low levels of NO encountered indicate that most of the marine boundary layer over the Indian Ocean is a region of active photochemical ozone destruction [Rhoads et al., 1997].

6. Aerosol Solubility
Results from the most recent cruise suggest that the soluble fraction in anthropogenic aerosol is surprisingly low [Shaw and Cantrell, 1998, preliminary results]. South of ITCZ, the characteristics of the measured aerosol number concentrations in the 20 to 600 nm diameter size range is generally agrees with that of ammonium sulfate, while in the Northern Hemisphere, the measured cloud condensation nuclei (CCN) are consistent with aerosol which have a soluble fraction of only 0.05 (Figure 8). Early results have shown [Rhoads et al., 1997] that after 2 to 6 days of travel, air masses were substantially depleted in soluble species (relative to CO concentration). However, data measured during Sagar Kanya cruise #109 in 1996 does not suggest any selective removal process for soluble and non-soluble aerosol after the initial gravitational settling (Figure 3). The aerosol solubility is an important issue. The low soluble fraction may suggest that a large fraction of the anthropogenic aerosol that reaches ITCZ from the Indian sub-continent may not serve as efficient CCN. This issue should be further investigated during the intensive INDOEX experiment.

III. Summary

Data from all pre-INDOEX experiments revealed findings that the impact of the anthropogenic pollution from the Indian sub-continent on the chemical, physical and optical properties of the atmosphere can be substantial in magnitude and spatial coverage. These pre-INDOEX experiments provided critical first-hand knowledge of the Indian Ocean region which is necessary for the success of the main INDOEX experiment in 1999. Major progress has been made in the following areas:

Provided a better meteorology background for the INDOEX intensive field phase (IFP).

Observed substantially more evidence of the anthropogenic impact on aerosol chemical, physical and radiative properties.

Linked the anthropogenic impact on aerosol properties to the radiation flux in the Indian Ocean region.

Currently, INDOEX research efforts focus on interpreting data from pre-INDOEX experiments and preparing for the 1999 INDOEX (IFP). Modeling simulation is underway to predict radiation flux using the aerosol properties measured through these pre-INDOEX experiments. C4s data integrating and distributing system (CIDS) is up and running and data from different research groups are gradually being integrated into the system for distribution. As we sail into 1999, we will expect more exciting results to be generated from this comprehensive international project.


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List of Figures
Figure 1. Time series showing strong correlation between CO (ppbv) CO2 (ppmv - 385 ppmv) and bulk aerosols including ammonium, non-sea-salt (NSS) sulfate and nitrate (llg m-3) for the cruise
Figure 2. Daily average aerosol concentrations measured in the Indian Ocean region during the pre-INDOEX cruise 109 from Jan 10 to Feb 2, 1996.
Figure 3Daily average aerosol concentration measured in the Indian Ocean region during the pre-INDOEX cruise 109 from Jan 10 to 29, 1996.
Figure 4Aerosol chemical composition measured during the pre-INDOEX cruise 109 from Jan 10 to 29, 1996.
Figure 5Spectral variation of average aerosol optical depths for the near coastal (solid line) and far coastal (dashed line) regions.
Figure 6Relationship between the (a) direct solar flux and (b) global flux at the surface with aerosol optical depth.
figure 7Monthly mean wind field in Feb 1993 at 200, 850 and 1000mb.
Figure 8CCN spectrum taken march 25 in "dirty" Northern Hemisphere air.

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