Integrated Campaign For Aerosols, Gases And Radiation Budget Research Articles

Assessment of submicron aerosol liquid water content and mass-based growth factors in South Asian outflow over the Indian Ocean

Aerosol-bound water, a ubiquitous and abundant component of atmospheric aerosols, has an impact on regional climate, visibility, human health, the hydrological cycle, and atmospheric chemistry. Yet, the intricate relationship between aerosol liquid water (ALWC) and chemical composition and relative humidity (RH) was not well understood. The present study explores ALWC derived from the ISORROPIA II model using real-time, high-resolution data of non-refractory submicron chemical species and meteorological parameters (temperature and RH) collected over the Indian Ocean as part of the ICARB (Integrated Campaign for Aerosols, Gases, and Radiation Budget)-2018 experiment. Results show that ALWC values over the South Eastern Arabian Sea (SEAS) were found to be higher by 4–6 times than those observed over the Equatorial Indian Ocean (EIO) due to a large decrease in aerosol loading from SEAS to EIO. ALWC peaked in the early morning hours (4:00–7:00), with greater values during the nighttime and lower values during the daytime across SEAS, which is comparable with RH variation. While the ratio of organics-to-SO42− mass fraction linearly decreased with increasing mass-based growth factors (MGFs) over EIO, such a scenario was not observed over SEAS. The latitudinal gradient of mass fraction of ALWC had shown a decrease towards EIO, consistent with organic fraction. The extinction coefficient of the dry mass of submicron particles is noticeably increased by 40 % by ALWC over SEAS and EIO. Moreover, ALWC could enhance the aerosol negative forcing by an average of 66 % (64 %) over SEAS (EIO) at the top of the atmosphere during the cruise period. These inferences imply that ALWC is the key factor in assessing the role of aerosols on atmospheric radiative forcing. Overall, the present study highlights the serious need to consider the ALWC in climate forcing simulations, particularly in moist tropical environments where their effect can be significant.

Influence of south Asian outflow on secondary organic aerosol formation over the Indian Ocean: Inferences from water-soluble low molecular weight dicarboxylic acids and related organic compounds during ICARB 2018 experiment

Low molecular weight dicarboxylic acids constituting a pivotal fraction of organic aerosols impact radiative forcing of Earth's atmosphere. However, a significant knowledge gap exists associated with their sources and formation pathways in the atmosphere. In this study, we report on water-soluble components such as dicarboxylic acids, oxoacids, and α-dicarbonyls as well as carbonaceous and ionic components in the continental outflow from the South Asian region to the Arabian Sea and the equatorial Indian Ocean during ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) 2018 experiment. Non-sea-salt sulfate (nss SO42−) was found to be the predominant component, followed by ammonium (NH4+) and organic carbon (OC) in fine-sized (<1.1 μm) aerosols. Total diacids carbon content accounted for 17% (on average) of total carbon (TC). Oxalic acid (C2) was the most abundant organic acid, contributing ~41% to total diacids, followed by terephthalic acid (tPh) and succinic acid (C4). Of these results, together with selected organic and inorganic mass ratios and linear relationships among the organic acids, we highlight the sources of organic acids over Indian Ocean. Over continental outflow regions in Indian Ocean, dicarboxylic acids are majorly derived from the emissions of biomass burning and combustion related sources, that underwent transformation in the atmosphere through photochemical reactions during the long range transport. However, over the deep oceanic environment of Indian Ocean, photo-oxidation of biogenic unsaturated fatty acids were found to be the important sources. The findings from this study will be helpful to improve further our understanding of the sources and formation processes of organic aerosols and their climate effects over the South Asian outflow regions.

Cloud condensation nuclei properties of South Asian outflow over the northern Indian Ocean during winter

Abstract. Extensive measurements of cloud condensation nuclei (CCN) and condensation nuclei (CN) concentrations in the South Asian outflow to the northern Indian Ocean were carried out on board an instrumented research vessel, as part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during the winter season (January–February 2018). Measurements include a north–south transect across the South Asian plume over the northern Indian Ocean and an east–west transect over the equatorial Indian Ocean (∼2∘ S), which is far away from the continental sources. South Asian outflow over the northern Indian Ocean is characterized by the high values of CCN number concentration (∼5000 cm−3), low CCN activation efficiency (∼25 %) and a steep increase in CCN concentration with the increase in supersaturation. In contrast, low CCN concentration (∼1000 cm−3) with flat supersaturation spectra was found over the equatorial Indian Ocean. The CCN properties exhibited significant dependence on the geometric mean diameter (GMD) of the aerosol number size distribution, and CCN activation efficiency decreased to low values (&lt;20 %) at the time of new-particle formation events over near-coastal and remote oceanic regions. The analysis of the activation efficiencies for the “similar” aerosol size distributions over the northern Indian Ocean indicated the primary role of aerosol number size distribution on CCN activation efficiency. The dependence of CCN properties and activation efficiency on size-segregated aerosol number concentration, especially during the ultrafine (&lt;100 nm) particle events, is investigated in detail for the first time over the region.

Winter time chemical characteristics of aerosols over the Bay of Bengal: continental influence.

As part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) conducted under the Geosphere Biosphere Programme of Indian Space Research Organisation, ship-based aerosol sampling was carried out over the marine environment of Bay of Bengal (BoB) during the northern winter months of December 2008 to January 2009. About 101 aerosol samples were collected, covering the region from 3.4° to 21° N latitude and 76° to 98° E longitude-the largest area covered-including the south east (SE) BoB for the first time. These samples were subjected to gravimetric and chemical analysis and the total aerosol loading as well the mass concentration of the ionic species namely F(-), Cl(-), Br(-), NO2 (-), NO3 (-), PO4 (2-), SO4 (2-), NH4 (+), etc. and the metallic species, Na, Mg, Ca, K, Al, Fe, Mn, Zn, and Pb were estimated for each sample. Based on the spatial distribution of individual chemical species, the air flow pattern, and airmass back trajectory analysis, the source characteristics of aerosols for different regions of BoB were identified. Significant level of continental pollution was noticed over BoB during winter. While transport of pollution from Indo-Gangetic Plain (IGP) contributed to aerosols over north BoB, those over SE BoB were influenced by SE Asia. A quantitative study on the wind-induced production of sea salt aerosols and a case study on the species dependent effect of rainfall are also presented in this paper.

Sources of black carbon aerosols in South Asia and surrounding regions during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB)

Abstract. This study examines differences in the surface black carbon (BC) aerosol loading between the Bay of Bengal (BoB) and the Arabian Sea (AS) and identifies dominant sources of BC in South Asia and surrounding regions during March–May 2006 (Integrated Campaign for Aerosols, Gases and Radiation Budget, ICARB) period. A total of 13 BC tracers are introduced in the Weather Research and Forecasting Model coupled with Chemistry to address these objectives. The model reproduced the temporal and spatial variability of BC distribution observed over the AS and the BoB during the ICARB ship cruise and captured spatial variability at the inland sites. In general, the model underestimates the observed BC mass concentrations. However, the model–observation discrepancy in this study is smaller compared to previous studies. Model results show that ICARB measurements were fairly well representative of the AS and the BoB during the pre-monsoon season. Elevated BC mass concentrations in the BoB are due to 5 times stronger influence of anthropogenic emissions on the BoB compared to the AS. Biomass burning in Burma also affects the BoB much more strongly than the AS. Results show that anthropogenic and biomass burning emissions, respectively, accounted for 60 and 37% of the average ± standard deviation (representing spatial and temporal variability) BC mass concentration (1341 ± 2353 ng m−3) in South Asia. BC emissions from residential (61%) and industrial (23%) sectors are the major anthropogenic sources, except in the Himalayas where vehicular emissions dominate. We find that regional-scale transport of anthropogenic emissions contributes up to 25% of BC mass concentrations in western and eastern India, suggesting that surface BC mass concentrations cannot be linked directly to the local emissions in different regions of South Asia.

Black carbon and carbon monoxide over Bay of Bengal during W_ICARB: Source characteristics

The ship borne measurements of near-surface black carbon (BC) and carbon monoxide (CO) were carried out over Bay of Bengal (BoB) during the winter period of 2009 under W_ICARB, the second phase of ‘Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)’. The CO mixing ratio and BC mass concentration varied in the ranges of 80–480 ppbv and 75–10,000 ng m−3, respectively over this marine region. The BC and CO showed similar variations over northern BoB where airmass from Indo-Gangetic Plain (IGP) region prevailed during the observations period leading to a very strong positive correlation. The association of BC and CO was poor over the eastern and southern part of BoB could be due to the removal of BC aerosols by rain and/or processes of dilution and mixing while transported over to BoB. The highest value of CO observed over eastern BoB was partially due to biomass burning over East Asia. The BC/CO ratio for IGP airmass found to be 20.3 ng m−3 ppb−1 and ∼16 ng m−3 ppb−1 during winter and pre-monsoon, respectively which indicate the role of biomass burning as the source of BC over the region. Based on the emission flux of CO from various inventories and observed BC/CO ratios during pre-monsoon and winter, the BC emission for India is estimated to be in the range of 0.78–1.23 Tg year−1. The analysis of scavenging of BC revealed the loss rate of BC due to relative humidity 0.39 ± 0.08 ng m−3 ppb−1 RH (%)−1 over northern BoB and 0.53 ± 0.04 ng m−3 ppb−1 RH (%)−1 over the southern-BoB during winter.

Spatial Gradients in Aerosol-Induced Atmospheric Heating and Surface Dimming over the Oceanic Regions around India: Anthropogenic or Natural?

Abstract Making use of the extensive shipboard and aircraft measurements of aerosol properties over the oceanic regions surrounding the Indian peninsula, under the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) field experiment during the premonsoon season (March–May), supplemented with long-term satellite data and chemical transport model simulations, investigations are made of the east–west and north–south gradients in aerosol properties and estimated radiative forcing, over the oceans around India. An eastward gradient has been noticed in most of the aerosol parameters that persisted both within the marine atmospheric boundary layer and above up to an altitude of ~6 km; the gradients being steeper at higher altitudes. It was also noticed that the north–south gradient has contrasting patterns over the Bay of Bengal and the Arabian Sea on the either side of the Indian peninsula. The aerosol-induced atmospheric heating rate increased from a low value of ≤0.1 K day−1 in the southwestern Arabian Sea to as high as ~0.5 K day−1 over the northeastern Bay of Bengal. The simulations of species-resolved spatial gradients have revealed that the observed gradients are the result of the strong modulations by anthropogenic species over the natural gradients, thereby emphasizing the role of human activities in imparting regional forcing. These large spatial gradients in aerosol forcing induced by mostly anthropogenic aerosols over the oceanic regions around the Indian peninsula can potentially affect the regional circulation patterns.

Aerosol characterization: comparison between measured and modelled surface radiative forcing over Bay of Bengal

Under the aegis of Indian Space Research Organisation's ongoing climate change research programme, Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) was undertaken from 27 December 2008 to 30 January 2009 over the Bay of Bengal (BoB). Since measurements of this sort have not been made near the eastern-most coastline of BoB, a cruise was planned to have observations along the boundaries of the BoB. Here we report the columnar aerosol optical thickness (AOT), total water vapour (TWV), total ozone column (TOC) and aerosol direct surface forcing calculations by using a pair of handheld Microtops II (Ozonometer and Sunphotometer) and a gimbal-mounted short-wave (SW) pyranometer, with high temporal resolution (5 minutes). The presence of high aerosol loading and absorbing aerosols over the coastal and Andaman regions resulted in a diurnal mean aerosol surface radiative forcings of −24 and −35 W m−2, respectively. These results are in good agreement with radiative transfer model estimations.

Vertical distribution of ozone in the lower troposphere over the Bay of Bengal and the Arabian Sea during ICARB-2006: Effects of continental outflow

[1] Measurements of vertical distributions of ozone and meteorological parameters were made over the Bay of Bengal (BOB) and Arabian Sea (AS) from ocean research vessel Sagar Kanya during the period of 18 March to 10 May 2006 as a part of the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB). The observations showed a highly polluted layer (average ozone ∼68.0 ± 10.1 ppbv) over the northern BOB as compared to the northern AS (51.5 ± 7.6 ppbv), southern BOB (42.7 ± 12.8 ppbv), and southern AS (40.9 ± 9.5 ppbv) in the altitude range of 1–3 km. In this altitude range, specific humidity was lower by about 2–6 g/kg, and temperature was higher by 1°C–3°C over the northern BOB and northern AS as compared to their southern counterparts. This comparison and the total potential source contribution function analysis indicate that the observations over the northern BOB and the northern AS were influenced by the transport of continental air masses. The outflow from the polluted atmosphere over northern India, particularly over the Indo-Gangetic Plain, resulted in higher mixing ratios of ozone over the northern BOB. The air masses from the northern Indian region contributed an enhancement of about 13 ± 6 ppbv to the mixing ratio of ozone over the BOB in the altitude range of 0.75–3.0 km. The mixing ratios of ozone in the unperturbed marine air masses were found to be 11 ± 6 and 5 ± 2 ppbv lower than the average ozone over the BOB and AS, respectively. These results clearly show the outflow of continental air containing high ozone and possibly other pollutants over the northern BOB, which can have significant implications in the chemistry and climate of relatively cleaner regions.

Source identification of aerosols influencing atmospheric extinction: Integrating PMF and PSCF with emission inventories and satellite observations

The relative influence of source categories of aerosols that affect atmospheric extinction is analyzed by evaluating the potential source contribution function (PSCF) based source regions of the positive matrix factorization (PMF) estimated factors against satellite retrievals of aerosol index and active fires and combining with emission inventory information. This approach has been applied to aerosol chemical data obtained from the integrated campaign undertaken during March–May 2006: Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB). Four source categories were identified: dust, nitrate‐and‐dust, biomass‐and‐fossil combustion, and secondary species. The relative influence of dust and nitrate‐and‐dust was higher during north–south transport from west Asia over the Arabian Sea during the period of campaign, coincident with highs in the spatial distribution of Ultraviolet Aerosol Index (UVAI) from the Ozone Monitoring Instrument (OMI), implying dust‐nitrate association in the outflow from west Asia. The relative influence of anthropogenic sources (biomass‐and‐fossil combustion and secondary species) was higher over Bay of Bengal during March–April 2006. High fire frequency from the Moderate Resolution Imaging Spectroradiometer (MODIS), coincident with the probable source regions identified in the central Indo‐Gangetic plain and central India (south of 27°N), indicates influence of biomass burning source. The biomass‐and‐fossil combustion factor arising from biofuel, crop residue, and forest fires is evident by their large emission flux rather than from industrial sources in the probable source regions associated with this factor. In contrast, thermal power plant and industries largely influenced the secondary species factor. This approach provides verification of source categories identified through PMF against active sources from satellite remote sensing and provides an estimate of their relative strength based on emission inventory information.

Spatial distribution of near-surface CO over bay of Bengal during winter: role of transport

As part of Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB), cruise-based measurements of near-surface CO were carried out over Bay of Bengal (BoB) covering the latitude–longitude sector 3.5°N–21.0°N and 76.0°E–98.0°E, during winter months of December 2008 to January 2009. These in-situ measured CO mixing ratio varied in the range of 80–480 ppbv over this marine environment with the distinct spatial pattern. The highest mixing ratios were measured over southeast-BoB with mean value of 379±58 ppbv. CO mixing ratios were high over north-BoB compared to southern BoB. These in-situ measurements were compared with the satellite-measured surface CO obtained from Measurements of Pollution in the Troposphere (MOPITT) onboard TERRA and found to be in good agreement over most of the regions, except at southeast-BoB. Surface CO and column CO from MOPITT data showed a similar spatial pattern. Based on the analysis of airmass back-trajectories, satellite-based spatial map of CO distribution over Asian region and Potential Source Contribution Function analysis, different pathways of transport of CO were identified. Transport from northern landmass as well as from south-east Asia has a significant influence in the spatial variation of CO over BoB. Winter-time mixing ratio of CO was found to be higher compared to those measured during other campaigns conducted during February–March 1999, 2001 (pre-monsoon) and September–October, 2002 (post-monsoon).

Observations of the thermodynamic structure of marine atmospheric boundary layer over Bay of Bengal, Northern Indian Ocean and Arabian Sea during premonsoon period

This paper presents the observations of the thermodynamic structure of the Marine Atmospheric Boundary Layer (MABL) over Bay of Bengal (BoB), Northern Indian Ocean (NIO) and Arabian Sea (AS) regions, using radiosonde observations carried out as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) during March–May 2006. Most of the profiles show the general structure of tropical MABL which consists of surface layer, sub-cloud layer (mixed layer), transition layer, cloud layer and trade wind inversion (TWI). However, in few soundings over BoB and AS, it was observed that the cloud layer is remarkably well-mixed, forming a double mixed layer (DML) structure. This feature is seen to be closely associated with the low level anticyclonic system over these sectors. The average profiles of vertical wind (corresponding to the region of observation) obtained from NCEP showed that there is an increase in the vertical velocity on the days when such a double mixed layer was observed and the altitude region of this high ω coincides with that of the DML. The study also reveals some important differences in the MABL structure during convectively active and weak periods. The spatial variation of the parameters like mixed layer (ML) depth, height of lifting condensation level (LCL), level of free convection (LFC), and trade wind inversion (TWI) are presented and discussed in detail. The TWI observations are first of their kind from the BoB region. The TWI top is found to be at a higher height compared to the observations over other oceanic regions, and the relationship between the TWI altitude and lapse rate of specific humidity is used to explain why the TWI was formed at a higher height. The observations of the MABL parameters are also compared with those reported from the previous campaigns over these regions.

Vertical and Horizontal Gradients in Aerosol Black Carbon and Its Mass Fraction to Composite Aerosols over the East Coast of Peninsular India from Aircraft Measurements

During the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) experiment of ISRO‐GBP, altitude profiles of mass concentrations of aerosol black carbon (MB) and total (composite) aerosols (MT) in the lower troposphere were made onboard an aircraft from an urban location, Chennai (13.04 ∘N, 80.17 ∘E). The profiling was carried out up to 3 km (AGL) in eight levels to obtain higher resolution in altitude. Besides, to explore the horizontal gradient in the vertical profiles, measurements were made at two levels [500 m (within ABL) and 1500 m (above ABL)] from ~10 ∘N to 16 ∘N and ~80 ∘E to 84 ∘E. The profiles showed a significant vertical extent of aerosols over coastal and offshore regions around Chennai with BC concentrations (~2 μg m−3) and its contribution to composite aerosols remaining at the same level (between 8 to 10% for FBC) as at the surface. Even though the values are not unusually high as far as an urban location is concerned, but their constancy throughout the vertical column will have important implications to climate impact of aerosols.

Spatial and vertical heterogeneities in aerosol properties over oceanic regions around India: Implications for radiative forcing

AbstractThe influence of atmospheric aerosols on Earth's radiation budget and hence climate, though well recognized and extensively investigated in recent years, remains largely uncertain mainly because of the large spatio‐temporal heterogeneity and the lack of data with adequate resolution. To characterize this diversity, a major multi‐platform field campaign ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) was carried out during the pre‐monsoon period of 2006 over the Indian landmass and surrounding oceans, which was the biggest such campaign ever conducted over this region. Based on the extensive and concurrent measurements of the optical and physical properties of atmospheric aerosols during ICARB, the spatial distribution of aerosol radiative forcing was estimated over the entire Bay of Bengal (BoB), northern Indian Ocean and Arabian Sea (AS) as well as large spatial variations within these regions. Besides being considerably lower than the mean values reported earlier for this region, our studies have revealed large differences in the forcing components between the BoB and the AS. While the regionally averaged aerosol‐induced atmospheric forcing efficiency was 31 ± 6 W m−2 τ−1 for the BoB, it was only ∼18 ± 7 W m−2 τ−1 for the AS. Airborne measurements revealed the presence of strong, elevated aerosol layers even over the oceans, leading to vertical structures in the atmospheric forcing, resulting in significant warming in the lower troposphere. These observations suggest serious climate implications and raise issues ranging from the impact of aerosols on vertical thermal structure of the atmospheric and hence cloud formation processes to monsoon circulation. Copyright © 2009 Royal Meteorological Society

Identification of aerosol type over the Arabian Sea in the premonsoon season during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB)

A discrimination of the different aerosol types over the Arabian Sea (AS) during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB‐06) is made using values of aerosol optical depth (AOD) at 500 nm (AOD500) and Ångström exponent (α) in the spectral band 340–1020 nm (α340–1020). For this purpose, appropriate thresholds for AOD500 and α340–1020 are applied. It is shown that a single aerosol type in a given location over the AS can exist only under specific conditions while the presence of mixed aerosols is the usual situation. Analysis indicates that the dominant aerosol types change significantly in the different regions (coastal, middle, and far) of AS. Thus the urban/industrial aerosols are mainly observed in coastal AS, the desert dust particles occur in the middle and northern AS, while clear maritime conditions mainly occur in far AS. Spectral AOD and Ångström exponent data were analyzed to obtain information about the adequacy of the simple use of the Ångström exponent and spectral variation of α for characterizing the aerosols. Using the least squares method, α is calculated in the spectral interval 340–1020 nm along with the coefficients a1 and a2 of the second‐order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength. The results show that the spectral curvature can effectively be used as a tool for their discrimination, since the fine mode aerosols exhibit negative curvature, while the coarse mode particles exhibit positive curvature. The correlation between the coefficients a1 and a2 with the Ångström exponent, and the atmospheric turbidity, is further investigated.

Aerosol optical depths at Mohal-Kullu in the northwestern Indian Himalayan high altitude station during ICARB

First time observations of spectral aerosol optical depths (AODs) at Mohal (31.9°N, 77.11°E; altitude 1154 m amsl) in the Kullu valley, located in the northwestern Indian Himalayan region, have been carried out during Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB), as a part of the Indian Space Research Organisation-Geosphere Biosphere Program (ISRO-GBP). AODs at six wavelengths are obtained using Microtops-II Sunphotometer and Ozonometer. The monthly mean values of AOD at 500 nm are found to be 0.27 ± 0.04 and 0.24 ± 0.02 during March and April, 2006 respectively. However, their monthly mean values are 0.33 ± 0.04 at 380 nm and 0.20 ± 0.03 nm at 870 nm during March 2006 and 0.31 ± 0.3 at 380 nm and 0.17 ± 0.2 at 870 nm during April 2006, showing a gradual decrease in AOD with wavelength. The Ångstrom wavelength exponent ‘α’ had a mean value of 0.72 ± 0.05, implying reduced dominance of fine particles. Further, the afternoon AOD values are higher as compared to forenoon values by ∼ 33.0% during March and by ∼ 9.0% during April 2006 and are attributed to the pollutant lifted up from the valley by the evolving boundary layer. Besides the long-range transportation of aerosol particles by airmass from the Great Sahara and the Thar Desert regions to the observing site, the high values of AODs have also been influenced by biomass burning and frequent incidents of forest fire at local levels.

Spatial distribution of atmospheric carbon monoxide over Bay of Bengal and Arabian Sea: Measurements during pre-monsoon period of 2006

Indian Space Research Organization (ISRO) conducted the ‘Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB)’ for a two-month pre-monsoon period in 2006 with the ocean segment covering Bay of Bengal and Arabian Sea. During this campaign, carbon monoxide (CO) was continuously monitored using a non-dispersive IR analyser. Quantifying CO in ambient air is vital in determining the air quality of a region. Being toxic, CO is a criteria pollutant, but it is a weak green house gas. Globally, very few measurements exist over marine atmospheres to study its temporal pattern; particularly in situ CO measurements are few over the Bay of Bengal and Arabian Sea for comparison. Present measurements indicate: (i) predominant single peak in the diurnal pattern of CO over the marine atmosphere in contrast to the double peak over the continent, (ii) the mean diurnal CO over the marine atmosphere showing an increasing trend towards evening hours, (iii) the amplitude of the AN peaks over the marine atmosphere was ∼ 100 ppbv, while at a remote island site in the Indian Ocean it was ∼ 5 ppbv and (iv) high CO values were observed close to continent and the long range transport by wind also caused CO highs.

Influence of circulation parameters on the AOD variations over the Bay of Bengal during ICARB

MODIS (Moderate Resolution Imaging Spectroradiometer) level-3 aerosol data, NCEP (National Centers for Environmental Prediction) reanalysis winds and QuikSCAT ocean surface winds were made use of to examine the role of atmospheric circulation in governing aerosol variations over the Bay of Bengal (BoB) during the first phase of the ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) campaign (March 18–April 12, 2006). An inter-comparison between MODIS level-3 aerosol optical depth (AOD) data and ship-borne MICROTOPS measurements showed good agreement with correlation 0.92 (p 850 hPa) indicated that direct transport of aerosols from central India was inhibited by the presence of a high pressure and a divergence over BoB in the lower altitudes. On the other hand, in the upper atmospheric levels, winds from central and northern India stretched south eastwards and converged over BoB with a negative vorticity indicative of a downdraft. These wind patterns pointed to the possibility of aerosol transport from central India to BoB by upper level winds. This mechanism was further confirmed by the significant correlations that AOD variations over BoB showed with aerosol flux convergence and flux vorticity at upper atmospheric levels (600–500 hPa). AOD in central and southern BoB away from continental influences displayed an exponential dependence on the QuikSCAT measured ocean surface wind speed. This study shows that particles transported from central and northern India by upper atmospheric circulations as well as the marine aerosols generated by ocean surface winds contributed to the AOD over the BoB during the first phase of ICARB.

Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB): An overview

During March–May 2006, an extensive, multi-institution, multi-instrument, and multi-platform integrated field experiment ‘Integrated Campaign for Aerosols, gases and Radiation Budget’ (ICARB) was carried out under the Geosphere Biosphere Programme of the Indian Space Research Organization (ISRO-GBP). The objective of this largest and most exhaustive field campaign, ever conducted in the Indian region, was to characterize the physico-chemical properties and radiative effects of atmospheric aerosols and trace gases over the Indian landmass and the adjoining oceanic regions of the Arabian Sea, northern Indian Ocean, and Bay of Bengal through intensive, simultaneous observations. A network of ground-based observatories (over the mainland and islands), a dedicated ship cruise over the oceanic regions using a fully equipped research vessel, the Sagar Kanya, and altitude profiling over selected regions using an instrumented aircraft and balloonsondes formed the three segments of this integrated experiment, which were carried out in tandem. This paper presents an overview of the ICARB field experiment, the database generated, and some of its interesting outcomes though these are preliminary in nature.The ICARB has revealed significant spatio-temporal heterogeneity in most of the aerosol characteristics both over land and ocean. Observed aerosol loading and optical depths were comparable to or in certain regions, a little lower than those reported in some of the earlier campaigns for these regions. The preliminary results indicate: low (< 0.2) aerosol optical depths (AOD) over most part of the Arabian Sea, except two pockets; one off Mangalore and the other, less intense, in the central Arabian Sea at ∼18°N latitude High Ångström exponent in the southern Arabian Sea signifying steep AOD spectra and higher abundance of accumulation mode particles in the southern Arabian Sea and off Mangalore Remarkably low Ångström exponents signifying increased concentration of coarse mode aerosols and high columnar abundance in the northern Arabian Sea Altitude profiles from aircraft showed a steady BC level up to 3 km altitude with structures which were associated with inversions in the atmospheric boundary layer (ABL) A surprisingly large increase in the BC mass fraction with altitude Presence of a convectively mixed layer extending up to about 1 km over the Arabian Sea and Bay of Bengal A spatial off shore extent of <100 km for the anthropogenic impact at the coast; and Advection of aerosols, through airmass trajectories, from west Asia and NW arid regions of India leading to formation of elevated aerosol layers extending as far as 400 km off the east coast.

Airborne measurements of submicron aerosols across the coastline at Bhubaneswar during ICARB

Airborne measurements of the number concentration and size distribution of aerosols from 13 to 700 nm diameter have been made at four vertical levels across a coastline at Bhubaneswar (20°25′N, 85°83′E) during the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB) programme conducted in March–April 2006. The measurements made during the constant-level flights at 0.5, 1, 2 and 3 km altitude levels extend ∼100 km over land and ∼150km over ocean. Aerosol number concentrations vary from 2200 to 4500 cm−3 at 0.5 km level but are almost constant at ∼ 6000 cm−3 and ∼ 800 cm−3 at 2 and 3 km levels, respectively. At 1km level, aerosol number concentration shows a peak of 18,070 cm−3 around the coastline. Most of the aerosol size distribution curves at 0.5 km and 1 km levels are monomodal with a maxima at 110nm diameter which shifts to 70 nm diameter at 2 and 3 km levels. However, at the peak at 1 km level, number concentration has a bimodal distribution with an additional maximum appearing in nucleation mode. It is proposed that this maxima in nucleation mode at 1 km level may be due to the formation and transport of new particles from coastal regions.