Atmospheric Environment (v.125, #PB)

South Asian aerosols in perspective: Preface to the special issue by K. Krishna Moorthy; S.K. Satheesh; M.M. Sarin; Arnico K. Panday (307-311).

Seasonal variation of vertical distribution of aerosol single scattering albedo over Indian sub-continent: RAWEX aircraft observations by S. Suresh Babu; Vijayakumar S. Nair; Mukunda M. Gogoi; K. Krishna Moorthy (312-323).
To characterize the vertical distribution of aerosols and its seasonality (especially the single scattering albedo, SSA) extensive profiling of aerosol scattering and absorption coefficients have been carried out using an instrumented aircraft from seven base stations spread across the Indian mainland during winter 2012 and spring/pre-monsoon 2013 under the Regional Aerosol Warming Experiment (RAWEX). Spatial variation of the vertical profiles of the asymmetry parameter, the wavelength exponent of the absorption coefficient and the single scattering albedo, derived from the measurements, are used to infer the source characteristics of winter and pre-monsoon aerosols as well as the seasonality of free tropospheric aerosols. The relatively high value of the wavelength exponent of absorption coefficient over most of the regions indicates the contribution from biomass burning and dust aerosols up to lower free tropospheric altitudes. A clear enhancement in aerosol loading and its absorbing nature is seen at lower free troposphere levels (above the planetary boundary layer) over the entire mainland during spring/pre-monsoon season compared to winter, whereas concentration of aerosols within the boundary layer showed a decrease from winter to spring. This could have significant implications on the aerosol heating structure over the Indian region and hence the regional climate.
Keywords: Aerosol absorption; Scattering coefficients; Single scattering albedo;

Airborne and ground based CCN spectral characteristics: Inferences from CAIPEEX – 2011 by Mercy Varghese; Thara V. Prabha; Neelam Malap; E.A. Resmi; P. Murugavel; P.D. Safai; Duncan Axisa; G. Pandithurai; K. Dani (324-336).
A first time comprehensive study of Cloud Condensation Nuclei (CCN) and associated spectra from both airborne and ground campaigns of the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) conducted over the rain shadow region of Western Ghats during September and October 2011 is illustrated. Observations of CCN spectra during clean, polluted and highly polluted conditions indicated significant differences between airborne and ground observations. Vertical variation of CCN concentration is illustrated from airborne observations in the clean, polluted and highly polluted conditions with different air mass characteristics. The cloud base CCN number concentrations are three times less than that of the surface measurements at different supersaturations. Diurnal variations of the ground based CCN number concentration and activation diameter showed bimodality. Atmospheric mixing in the wet conditions is mainly through mechanical mixing. The dry conditions favored convective mixing and were dominated by more CCN than the wet conditions. New particle formation and growth events have been observed and were found more often on days with convective mixing. The average critical activation diameter (at 0.6% SS) observed at the ground is approximately 60 nm and availability of a large number of particles below this limit was due to the new particle formation. Observations give convincing evidence that the precipitable water and liquid water path is inversely proportional to surface CCN number concentration, and this relationship is largely dictated by the meteorological conditions.
Keywords: CAIPEEX; CCN spectra; Mixing; IGOC; Activation diameter; Nucleation;

Meridional gradients in aerosol vertical distribution over Indian Mainland: Observations and model simulations by S.S. Prijith; S. Suresh Babu; N.B. Lakshmi; S.K. Satheesh; K. Krishna Moorthy (337-345).
Multi-year observations from the network of ground-based observatories (ARFINET), established under the project ‘Aerosol Radiative Forcing over India’ (ARFI) of Indian Space Research Organization and space-borne lidar ‘Cloud Aerosol Lidar with Orthogonal Polarization’ (CALIOP) along with simulations from the chemical transport model ‘Goddard Chemistry Aerosol Radiation and Transport’ (GOCART), are used to characterize the vertical distribution of atmospheric aerosols over the Indian landmass and its spatial structure. While the vertical distribution of aerosol extinction showed higher values close to the surface followed by a gradual decrease at increasing altitudes, a strong meridional increase is observed in the vertical spread of aerosols across the Indian region in all seasons. It emerges that the strong thermal convections cause deepening of the atmospheric boundary layer, which although reduces the aerosol concentration at lower altitudes, enhances the concentration at higher elevations by pumping up more aerosols from below and also helping the lofted particles to reach higher levels in the atmosphere. Aerosol depolarization ratios derived from CALIPSO as well as the GOCART simulations indicate the dominance of mineral dust aerosols during spring and summer and anthropogenic aerosols in winter. During summer monsoon, though heavy rainfall associated with the Indian monsoon removes large amounts of aerosols, the prevailing southwesterly winds advect more marine aerosols over to landmass (from the adjoining oceans) leading to increase in aerosol loading at lower altitudes than in spring. During spring and summer months, aerosol loading is found to be significant, even at altitudes as high as 4 km, and this is proposed to have significant impacts on the regional climate systems such as Indian monsoon.
Keywords: Aerosols; Vertical distribution; Boundary layer;

Investigations of aerosol black carbon from a semi-urban site in the Indo-Gangetic Plain region by Hema Joshi; Manish Naja; K.P. Singh; Rajesh Kumar; P. Bhardwaj; S. Suresh Babu; S.K. Satheesh; K. Krishna Moorthy; H.C. Chandola (346-359).
Long-term (2009–2012) data from ground-based measurements of aerosol black carbon (BC) from a semi-urban site, Pantnagar (29.0°N, 79.5°E, 231 m amsl), in the Indo-Gangetic Plain (IGP) near the Himalayan foothills are analyzed to study the regional characterization. Large variations are seen in BC at both diurnal and seasonal scales, associated with the mesoscale and synoptic meteorological processes, and local/regional anthropogenic activities. BC diurnal variations show two peaks (morning and evening) arising from the combined effects of the atmospheric boundary layer (ABL) dynamics and local emissions. The diurnal amplitudes as well as the rates of diurnal evolution are the highest in winter season, followed by autumn, and the lowest in summer-monsoon. BC exhibits nearly an inverse relation with mixing layer depth in all seasons; being strongest in winter (R2 = 0.89) and weakest (R2 = 0.33) in monsoon (July–August). Unlike BC, co-located aerosol optical depths (AOD) and aerosol absorption are highest in spring over IGP, probably due to the presence of higher abundances of aerosols (including dust) above the ABL (in the free troposphere). AOD (500 nm) showed annual peak (>0.6) in May–June, dominated by coarse mode, while fine mode aerosols dominated in late autumn and early winter. Aerosols profiles from CALIPSO show highest values close to the surface in winter/autumn, similar to the feature seen in surface BC, whereas at altitudes > 2 km, the extinction is maximum in spring/summer. WRF-Chem model is used to simulate BC temporal variations and then compared with observed BC. The model captures most of the important features of the diurnal and seasonal variations but significantly underestimated the observed BC levels, suggesting improvements in diurnal and seasonal varying BC emissions apart from the boundary layer processes.
Keywords: Aerosols; Black carbon; Aerosol optical depth; Boundary layer; Indo-Gangetic Plain; Himalayas;

The mass absorption efficiency (MAE) of light absorbing water-soluble organics, representing a significant fraction of brown carbon (BrC), has been studied in fine mode aerosols (PM2.5) from a source region (Patiala: 30.2 °N, 76.3 °E) of biomass burning emissions (BBEs) in the Indo-Gangetic Plain (IGP). The mass absorption coefficient of BrC at 365 nm (b abs-365), assessed from absorption spectra of aqueous extracts, exhibits significant linear relationship with water-soluble organic carbon (WSOC) for day (R2 = 0.37) and night time (R2 = 0.77) samples; and slope of regression lines provides a measure of MAE of BrC (daytime: ∼0.75 m2 g−1 and night time: 1.13 m2 g−1). A close similarity in the temporal variability of b abs-365 (for BrC) and K+ in all samples suggests their common source from BBEs. The b abs-365 of BrC follows a power law (b abs-λ ≈ λ−α; where α = angstrom exponent) and averages around 5.2 ± 2.0 M m−1 (where M = 10−6). A significant decrease in the MAE of BrC from the source region (this study) to the downwind oceanic region (over Bay of Bengal, Srinivas and Sarin, 2013) could be attributed to relative increase in the contribution of non-absorbing WSOC and/or photo-bleaching of BrC during long-range atmospheric transport. The atmospheric radiative forcing due to BrC over the study site accounts for ∼40% of that from elemental carbon (EC).
Keywords: Brown carbon; Biomass burning; WSOC; Carbonaceous aerosols; India; South Asia;

Atmospheric fine-mode particulate matter (PM2.5), collected during January to December 2007 at a high-altitude site (Mt. Abu, 24.6°N, 72.7°E, 1680 m asl) in the western India, have been analysed for carbonaceous and inorganic species to assess their temporal variability and the influence of emissions and transport processes, particularly from the Indo-Gangetic Plain (IGP). The mass concentrations (μg m−3) of elemental carbon (EC), organic carbon (OC) and water-soluble OC (WSOC) are observed to be the highest during winter months, and found to vary between 0.2 and 2.8, 0.5–9.8 and 0.4–8.7, respectively. This variation in mass concentration is attributed to the synoptic scale long-range transport from the emission source regions in the Indo-Gangetic Plain (IGP). Relatively lower fractional contribution of carbonaceous species to PM2.5 mass is found during March–September. This is mainly attributed to the reduction in strength of biomass burning emissions near the study site, increase in boundary layer height and simultaneous increase in mineral dust concentration via transport from the Thar Desert and Arabian Peninsula. However, the OC/EC ratio is nearly uniform throughout the year (5.5 ± 1.6, n = 71) and indicate towards the dominance of biomass burning emissions at Mt Abu. This is further supported by a significant correlation between nss-K+ and OC (R2 = 0.71) with nss-K+/OC ratio averaged at 0.10 ± 0.04. The higher WSOC/OC ratio (av: 0.65 ± 0.16; 1σ) compared to those in IGP, suggested formation of secondary organic aerosols during the transport, which is further attested by the observed strong correlation between secondary OC (SOC) and WSOC concentrations. A back-trajectory-assisted analysis has been performed to estimate the levels of OC, EC, WSOC, OC/EC and WSOC/OC in two distinct air-masses classified as the marine air mass and the IGP influenced air mass. This analysis shows significantly higher concentration of OC, EC and WSOC in the IGP influenced air masses by 64.9%, 121.4% and 113.2% respectively, as compared to the marine air masses. This important observation of carbonaceous aerosols at Mt Abu highlights the role of synoptic scale transport particularly from the biomass burning sources in the IGP.
Keywords: Carbonaceous aerosols; Elemental carbon; Organic carbon; Water –soluble organic carbon; Indo-Gangetic Plain; High-altitude site;

The oxidation efficiency of atmospheric SO2, measured as a molar ratio of SO 4 2 − to total SOx (SOx = SO2 +  SO 4 2 − ), referred as S-ratio, have been studied from a high altitude site (Gurushikhar, Mt. Abu: 24.6° N, 72.7° E, 1680 m ASL) in a semi-arid region of western India. A global 3-dimensional Chemical Transport Model (CTM), GEOS-Chem (v8-03-01), is employed to interpret the observed patterns. The S-ratios derived from time series SO2 and SO 4 2 − measurements exhibited a pronounced seasonality, with relatively low ratios in Feb–Mar 2010, high ratios in Nov–Dec 2009 and intermediate values in Sep–Oct 2009. The lower S-ratios for Feb ‘10 and Mar ‘10 (median values 0.10 and 0.08 respectively) have been attributed to the relatively high planetary boundary layer (PBL) heights – to reduce the SO2 loss from the atmosphere via dry deposition – as well as the lower OH radical levels and low ‘aged air mass influx’ during these months. On the other hand, low PBL heights and significant long range transport contributions are projected to be the possible causes for the higher S-ratios during Nov ‘09 and Dec ‘09 (median values 0.30 and 0.28 respectively). The seasonal patterns for the S-ratios predicted by the CTM for the GEOS-Chem 4° × 5° grid cell containing the sampling site showed highest ratios in Jul–Aug, and the lowest in Apr. The model has been employed further to study the contributions from various parameters to the S-ratios such as PBL, OH, RH, dust load, transport pattern and dry deposition. Sensitivity simulations showed the S-ratios enhancing with dust load with the peak in May (∼4.7% (median)). Similarly, the ‘dry deposition’ is seen to boost the S-ratios with the peak in August (∼66.3% (median)). Also, model simulations to assess the ‘altitudinal dependence of S-ratios’ have revealed a pronounced seasonal behaviour.
Keywords: Sulphur dioxide; Sulphate; Atmospheric oxidation; GEOS-Chem; OH radical; Dry deposition;

The ambient particulate matter injected from biomass burning emissions (BBEs) over northern India has been a subject of major debate in the context of regional air quality and atmospheric chemistry of several organic and inorganic constituents. This necessitates an observational approach over a large spatial and temporal scale. We present an extensive data set on PM2.5 samples (n = 147) collected for one full year from a sampling site (Patiala: 30.2°N, 76.3°E) in the source region of BBEs in northern India. During the sampling period from October 2011 to September 2012, PM2.5 mass concentration varied from ∼20 to 400 μg m−3. Among the major constituents, contribution of total carbonaceous aerosols (OC + EC) ranged from 8 to 60%. The average OC/EC and K+/EC ratio, varying from 3.2 to 12 and 0.26 to 0.80, respectively, emphasizes the dominance of BBEs over the annual seasonal cycle. The average secondary organic matter (SOM) accounts for ∼10–40% of PM2.5 mass in different seasons; whereas contribution of secondary inorganics was maximum (∼40%) during the winter. The pronounced temporal variability in SOM suggests its contribution from varying sources, their emission strength and process of secondary organic formation. Diurnal differences in the chemical constituents are attributable to regional meteorological factors and boundary layer dynamics. The emerging data set from this study is important to understand feedback mechanism from anthropogenic activities to the regional climate change scenario.
Keywords: Carbonaceous aerosols; Paddy-residue burning; Wheat-residue burning; Tropical region; Water-soluble aerosols;

Black carbon and the Himalayan cryosphere: A review by Charles G. Gertler; Siva Praveen Puppala; Arnico Panday; Dorothea Stumm; Joseph Shea (404-417).
The Himalayan cryosphere borders global hotspots for emissions of black carbon (BC), a carbonaceous aerosol with a short atmospheric lifespan and potentially significant impacts on glaciers and snow cover. BC in the atmosphere absorbs radiation efficiently, leading to localized positive climate forcing. BC may also be deposited onto snow and ice surfaces, thereby changing their albedo. This review presents up-to-date observational data of BC in the atmosphere and in snow and ice, as well as its effects on the cryosphere in the Hindu-Kush-Himalayan (HKH) region along the northern edge of South Asia. Significant spatial variation exists in the measured concentrations of BC in the atmosphere and cryosphere. A strong seasonal pattern exists, with highest concentrations in the pre-monsoon and lowest during the monsoon. Existing observations show bias towards certain areas, with a noticeable lack of measurements on the south side of the Himalaya. Significant uncertainty persists in the emissions estimates of BC in the HKH region, with a standard deviation of regional emissions from various emission inventories of 0.5150 × 10−9 kg m−2 s−1, or 47.1% of the mean (1.0931 × 10−9 kg m−2 s−1). This and other uncertainties, including poor model resolution, imprecision in deposition modeling, and incongruities among measurement types, propagate through simulations of BC concentration in atmosphere and cryosphere. Modeled atmospheric concentrations can differ from observations by as much as a factor of three with no systematic bias, and modeled concentrations in snow and ice can differ from observations by a factor of 60 in certain regions. In the Himalaya, estimates of albedo change due to BC range from about 2 to 10%, estimates of direct radiative forcing due to BC in the atmosphere from (−2)–7 W m−2, and surface forcing estimates from 0 to 28 W m−2, though every forcing estimate uses its own definition, with varying degrees of complexity and numbers of feedbacks. We find the most important course of further study to be model verification, enabled by increasing observational data and in this region and consistent measurement protocol.
Keywords: Black carbon; Hindu-Kush-Himalaya; Cryosphere; Glaciers; Albedo; Climate forcing;

Short wave Aerosol Radiative Forcing estimates over a semi urban coastal environment in south-east India and validation with surface flux measurements by K. Aruna; T.V. Lakshmi Kumar; B.V. Krishna Murthy; S. Suresh Babu; M. Venkat Ratnam; D. Narayana Rao (418-428).
The short wave direct Aerosol Radiative Forcing (ARF) at a semi urban coastal location near Chennai (12.81 °N, 80.03 °E, ∼45 m amsl), a mega city on the east coast of India has been estimated for all the four seasons in the year 2013 using the SBDART (Santa Barbara Discrete ordinate Atmospheric Radiative Transfer) model. As inputs to this model, measured aerosol parameters together with modeled aerosol and atmospheric parameters are used. The ARF in the atmosphere is found to be higher in the pre-monsoon and winter seasons compared to the other seasons whereas at the surface, it is found to be higher in the south-west (SW) monsoon and winter seasons. The estimated ARF values are compared with those reported over other locations in India. The effect of Relative Humidity on ARF has been investigated for the first time in the present study. It is found that the ARF increases with increasing RH in the SW monsoon and winter seasons. An unique feature of the present study is the comparison of the net surface short wave fluxes estimated from the model (SBDART) and measured fluxes using CNR 4 net radiometer. This comparison between the estimated and measured fluxes showed good agreement, providing a ‘closure’ for the estimates.
Keywords: Aerosol Radiative Forcing; Aerosol optical depth; Black carbon; Single scattering albedo; Relative humidity; Radiative fluxes;

Atmospheric abundances of black carbon aerosols and their radiative impact over an urban and a rural site in SW India by M.P. Raju; P.D. Safai; K. Vijayakumar; P.C.S. Devara; C.V. Naidu; P.S.P. Rao; G. Pandithurai (429-436).
Observations on black carbon (BC) aerosols over an urban site (Pune) and a rural, high altitude site (Sinhagad) during summer and winter seasons over the period of 2009–2013 are reported. Apart from the temporal variation of BC over both the sites, its mass fraction to total suspended particulates (TSP) is studied. Finally, using the chemical composition of TSP and BC in the OPAC model, season-wise optical properties of aerosols are obtained which are further used in the SBDART model to derive the aerosol radiative forcing (ARF) at surface and top of the atmosphere and thereby the atmospheric forcing and heating rates in each season over both the sites. BC mass concentration and its mass fraction to TSP (Mf BC) were higher at Pune than at Sinhagad, indicating impact of more anthropogenic sources. At both the sites winter season witnessed higher BC concentrations than summer as well as higher Mf BC which is due to the prevailing favorable meteorological conditions in winter. Diurnal variation of BC showed different patterns at Pune and Sinhagad in terms of strength and occurrence of high and low values that could be attributed to varying local boundary layer conditions and source activities at both the sites. Negative ARF indicated cooling at top of the atmosphere and at surface leading to warming of the atmosphere at both the sites. However, surface cooling and atmospheric warming was more dominant at Pune leading to higher atmospheric heating rates, underlining the impact of absorbing BC aerosols which were about three times more at Pune than Sinhagad.
Keywords: Urban site; Rural site; Black carbon; Temporal variations; Aerosol radiative forcing; Atmospheric heating rates;

Aerosol chemical characterization and role of carbonaceous aerosol on radiative effect over Varanasi in central Indo-Gangetic Plain by S. Tiwari; U.C. Dumka; D.G. Kaskaoutis; Kirpa Ram; A.S. Panicker; M.K. Srivastava; Shani Tiwari; S.D. Attri; V.K. Soni; A.K. Pandey (437-449).
This study investigates the chemical composition of PM10 aerosols at Varanasi, in the central Indo-Gangetic Plain (IGP) during April to July 2011, with emphasis on examining the contribution of elemental carbon (EC) to the estimates of direct aerosol radiative effect (DARE). PM10 samples are analysed for carbonaceous aerosols (Organic Carbon, OC and EC) and water-soluble ionic species (WSIS: Cl, SO4 2−, NO3 , PO4 2− NH4 +, Na+, K+, Mg2+ and Ca2+) and several diagnostic ratios (OC/EC, K+/EC, etc) have been also used for studying the aerosol sources at Varanasi. PM10 mass concentration varies between 53 and 310 μg m−3 (mean of 168 ± 73 μg m−3), which is much higher than the National and International air quality standards. The OC mass concentration varies from 6 μg m−3 to 24 μg m−3 (mean of 12 ± 5 μg m−3; 7% of PM10 mass), whereas EC ranges between 1.0 and 14.3 μg m−3 (4.4 ± 3.9 μg m−3; ∼3% of PM10 mass). The relative low OC/EC of 3.9 ± 2.0 and strong correlation (R2 = 0.82) between them suggest the dominance of primary carbonaceous aerosols. The contribution of WSIS to PM10 is found to be ∼12%, out of which ∼57% and 43% are anions and cations, respectively. The composite DARE estimates via SBDART model reveal significant radiative effect and atmospheric heating rates (0.9–2.3 K day−1). Although the EC contributes only ∼3% to the PM10 mass, its contribution to the surface and atmospheric forcing is significantly high (37–63% and 54–77%, respectively), thus playing a major role in climate implications over Varanasi.
Keywords: PM10; Chemical composition; Carbonaceous aerosols; Ionic species; EC radiative effect; Varanasi;

First observations of light non-methane hydrocarbons (C2–C5) over a high altitude site in the central Himalayas by Tapaswini Sarangi; Manish Naja; S. Lal; S. Venkataramani; Piyush Bhardwaj; N. Ojha; R. Kumar; H.C. Chandola (450-460).
This study presents observations of methane (CH4) and light non-methane hydrocarbons (NMHCs) for the first time from a high altitude site Nainital (29.4°N, 79.5°E, 1958 m amsl) in the central Himalayas. The whole air samples collected with a frequency of 3 samples per week during April 2009–December 2011 are analyzed using a Gas Chromatograph equipped with Flame Ionization Detector (GC-FID). Additionally, samples were collected from two semi-urban sites (Haldwani and Pantnagar) in the adjoining Indo Gangetic plain region. CH4 and NMHCs show a distinct seasonal cycle over this region with more frequent observations of higher levels during winter (DJF) and late autumn (SON) and lower levels during the summer–monsoon (JJA). Different NMHCs exhibit better correlations during autumn/winter as compared to the summer–monsoon season. The annual mean mixing ratios of methane, ethane, ethene, propane, propene, i-butane, n-butane, acetylene, and i-pentane at Nainital are measured to be 1.9 ± 0.1 ppmv, 1.8 ± 1.0, 0.7 ± 0.9, 0.6 ± 0.8, 0.6 ± 0.7, 0.6 ± 0.7, 0.5 ± 0.6, 1.0 ± 0.8, and 0.5 ± 0.6, respectively (all in ppbv). The seasonal cycle of CH4 at Nainital is found to be similar to other global high altitude sites (Jungfraujoch and Mauna Loa) but somewhat different than a high altitude site Mt. Abu in India. NMHCs, other than ethane and propane, are found to be higher over this central Himalayan region than other sites. Additionally, composition of NMHCs is shown to be different over the study region when compared with other sites in the IGP region. A correlation study between ln((n-butane)/(ethane)) and ln((i-butane)/(ethane)) showed that oxidation by the OH radical is the main removal mechanism of these species over the central Himalaya and dilution maintains the ratios of these species. The lowest slope of propane and acetylene with CO during summer and spring are indicating absence of fresh air mass over this region. This study fills a major gap in observational data for light NMHCs in the Himalayas and has implications for better understanding of tropospheric chemistry over this region.
Keywords: NMHCs; CH4; Central Himalaya; Regional pollution; IGP;

Aerosol characteristics in north-east India using ARFINET spectral optical depth measurements by B. Pathak; T. Subba; P. Dahutia; P.K. Bhuyan; K. Krishna Moorthy; M.M. Gogoi; S. Suresh Babu; L. Chutia; P. Ajay; J. Biswas; C. Bharali; A. Borgohain; P. Dhar; A. Guha; B.K. De; T. Banik; M. Chakraborty; S.S. Kundu; S. Sudhakar; S.B. Singh (461-473).
Four years (2010–2014) of spectral aerosol optical depth (AOD) data from 4 Indian Space Research Organisation's ARFINET (Aerosol Radiative Forcing over India) stations (Shillong, Agartala, Imphal and Dibrugarh) in the North-Eastern Region (NER) of India (lying between 22–30°N and 89–98°E) are synthesized to evolve a regional aerosol representation, for the first time. Results show that the columnar AOD (an indicator of the column abundance of aerosols) is highest at Agartala (0.80 ± 0.24) in the west and lowest at Imphal (0.59 ± 0.23) in the east in the pre-monsoon season due to intense anthropogenic bio-mass burning in this region aided by long-range transport from the high aerosol laden regions of the Indo-Gangetic Plains (IGP), polluted Bangladesh and Bay of Bengal. In addition to local biogenic aerosols and pollutants emitted from brick kilns, oil/gas fields, household bio-fuel/fossil-fuel, vehicles, industries. Aerosol distribution and climatic impacts show a west to east gradient within the NER. For example, the climatological mean AODs are 0.67 ± 0.26, 0.52 ± 0.14, 0.40 ± 0.17 and 0.41 ± 0.23 respectively in Agartala, Shillong, Imphal and Dibrugarh which are geographically located from west to east within the NER. The average aerosol burden in NER ranks second highest with climatological mean AOD 0.49 ± 0.2 next to the Indo-Gangetic Plains where the climatological mean AOD is 0.64 ± 0.2 followed by the South and South-East Asia region. Elevated aerosol layers are observed over the eastern most stations Dibrugarh and Imphal, while at the western stations the concentrations are high near the surface. The climate implications of aerosols are evaluated in terms of aerosol radiative forcing (ARF) and consequent heating of the atmosphere in the region which follows AOD and exhibit high values in pre-monsoon season at all the locations except in Agartala. The highest ARF in the atmosphere occurs in the pre-monsoon season ranging from 48.6 Wm−2 in Agartala to 25.1 Wm−2 in Imphal. Winter radiative forcing follows that in pre-monsoon season at these locations. The heating rate is high at 1.2 K day−1 and 1.0 K day−1 over Shillong and Dibrugarh respectively in this season. However, Agartala experiences higher surface forcing (−56.5 Wm−2) and consequent larger heating of the atmosphere of 1.6 K day−1 in winter.
Keywords: Aerosol optical depth; Multiwavelength solar Radiometer; ARFINET; Ångström exponent; Extinction coefficient; HYSPLIT; Aerosol radiative forcing;

The Indo Gangetic Plain (IGP) has been identified from back-trajectory analyses, as one of the most potential region affecting the species transport to the Northeastern region of India (NER). The continental export efficiency (εε) of BC, NOx and SO2 within the boundary layer is estimated in order to examine how efficiently these chemical species are transported towards the NER. For this the measurements carried out at Dibrugarh, a wet tropical location in NER during 2012–2013 have been used as the references in the estimation of the species enhancements above their background. CO is used as a passive tracer of transport due to its longer lifetime in the atmosphere. The emission estimates of BC, NOx, SO2 and CO in the IGP region are adopted from the emission inventories REAS and INTEX-B. The estimated export efficiency is highest in winter (DJF) for BC and NOx, whereas SO2 shows maximum efficiency in monsoon (JJAS). BC due to efficient transportation/removal from the IGP region exhibits highest εε values compared to the other species. NOx and SO2 on the other hand get transformed to other chemical species shortly after emission into the atmosphere and hence are less efficiently transported towards the study region. The export of BC, CO, NOx and SO2 are expected to supplement the chemical atmosphere in NER, which is further studied through the annual variability in their distribution in Dibrugarh. Pearson correlation analyses of BC, NOx and SO2 with CO is carried out to examine the similarity or dissimilarity among the sources.
Keywords: Export efficiency; BC; NOx; SO2; CO; CWT;

Inter-comparison and performance evaluation of chemistry transport models over Indian region by Gaurav R. Govardhan; Ravi S. Nanjundiah; S.K. Satheesh; K. Krishna Moorthy; Toshihiko Takemura (486-504).
Aerosol loading over the South Asian region has the potential to affect the monsoon rainfall, Himalayan glaciers and regional air-quality, with implications for the billions in this region. While field campaigns and network observations provide primary data, they tend to be location/season specific. Numerical models are useful to regionalize such location-specific data. Studies have shown that numerical models underestimate the aerosol scenario over the Indian region, mainly due to shortcomings related to meteorology and the emission inventories used. In this context, we have evaluated the performance of two such chemistry-transport models: WRF-Chem and SPRINTARS over an India-centric domain. The models differ in many aspects including physical domain, horizontal resolution, meteorological forcing and so on etc. Despite these differences, both the models simulated similar spatial patterns of Black Carbon (BC) mass concentration, (with a spatial correlation of 0.9 with each other), and a reasonable estimates of its concentration, though both of them under-estimated vis-a-vis the observations. While the emissions are lower (higher) in SPRINTARS (WRF-Chem), overestimation of wind parameters in WRF-Chem caused the concentration to be similar in both models. Additionally, we quantified the underestimations of anthropogenic BC emissions in the inventories used these two models and three other widely used emission inventories. Our analysis indicates that all these emission inventories underestimate the emissions of BC over India by a factor that ranges from 1.5 to 2.9. We have also studied the model simulations of aerosol optical depth over the Indian region. The models differ significantly in simulations of AOD, with WRF-Chem having a better agreement with satellite observations of AOD as far as the spatial pattern is concerned. It is important to note that in addition to BC, dust can also contribute significantly to AOD. The models differ in simulations of the spatial pattern of mineral dust over the Indian region. We find that both meteorological forcing and emission formulation contribute to these differences. Since AOD is column integrated parameter, description of vertical profiles in both models, especially since elevated aerosol layers are often observed over Indian region, could be also a contributing factor. Additionally, differences in the prescription of the optical properties of BC between the models appear to affect the AOD simulations. We also compared simulation of sea-salt concentration in the two models and found that WRF-Chem underestimated its concentration vis-a-vis SPRINTARS. The differences in near-surface oceanic wind speeds appear to be the main source of this difference. In-spite of these differences, we note that there are similarities in their simulation of spatial patterns of various aerosol species (with each other and with observations) and hence models could be valuable tools for aerosol-related studies over the Indian region. Better estimation of emission inventories could improve aerosol-related simulations.
Keywords: WRF-Chem; SPRINTARS; BC; AOD; Dust;

Risk assessment of bioaccessible trace elements in smoke haze aerosols versus urban aerosols using simulated lung fluids by Xian Huang; Raghu Betha; Li Yun Tan; Rajasekhar Balasubramanian (505-511).
Smoke-haze episodes, caused by uncontrolled peat and forest fires, occur almost every year in the South-East Asian region with increased concentrations of PM2.5 (airborne particulate matter (PM) with diameter ≤ 2.5 μm). Particulate-bound trace elements (TrElems), especially carcinogenic and toxic elements, were measured during smoke haze as well as non-haze periods in 2014 as they are considered to be indicators of potential health effects. The bioaccessibilities of 13 TrElems were investigated using two types of simulated lung fluids (SLFs), Gamble's solution and artificial lysosomal fluid (ALF), instead of the commonly used leaching agent (water). The dissolution kinetics was also examined for these TrElems. Many TrElems showed higher solubility in SLFs, and were more soluble in ALF compared to the Gamble's solution. Cu, Mn and Cd were observed to be the most soluble trace elements in ALF, while in Gamble's solution the most soluble trace elements were Cu, Mn and Zn. The dissolution rates were highly variable among the elements. Health risk assessment was conducted based on the measured concentrations of TrElems and their corresponding toxicities for three possible scenarios involving interactions between carcinogenic and toxic TrElems and SLFs, using the United States Environmental Protection Agency (USEPA) human health risk assessment model. The cumulative cancer risks exceeded the acceptable level (1 in a million i.e. 1 × 10−6). However, the estimation of health quotient (HQ) indicated no significant chronic toxic health effects. The risk assessment results revealed that the assessment of bioaccessibility of particulate-bound TrElems using water as the leaching agent may underestimate the health risk.
Keywords: Metal bioaccessibility; Simulated lung fluids; Smoke haze; Health risk assessment; Aerosols;

Seasonal differences in aerosol abundance and radiative forcing in months of contrasting emissions and rainfall over northern South Asia by P. Sadavarte; C. Venkataraman; R. Cherian; N. Patil; B.L. Madhavan; T. Gupta; S. Kulkarni; G.R. Carmichael; B. Adhikary (512-523).
A modeling framework was used to examine gaps in understanding of seasonal and spatial heterogeneity in aerosol abundance and radiative forcing over northern South Asia, whose glimpses are revealed in observational studies. Regionally representative emissions were used in chemical transport model simulations at a spatial resolution of 60 × 60 km2, in April, July and September, chosen as months of contrasting emissions and rainfall. Modeled aerosol abundance in northern South Asia was predominantly found to be dust and carbonaceous in April, dust and sulfate in July and sulfate and carbonaceous in September. Anthropogenic aerosols arose from energy-use emissions (from industrial sources, residential biofuel cooking, brick kilns) in all months, additionally from field burning in April, and incursion from East Asia in September. In April, carbonaceous aerosols were abundant from open burning of agricultural fields even at high altitude locations (Godavari), and of forests in the eastern Gangetic Plain (Kolkata). Direct radiative forcing and heating rate, calculated from OPAC-SBDART, using modeled aerosol fields, and corrected by MODIS AOD observations, showed regionally uniform atmospheric forcing in April, compared to that in other months, influenced by both dust and black carbon abundance. A strong spatial heterogeneity of radiative forcing and heating rate was found, with factor of 2.5–3.5 lower atmospheric forcing over the Tibet plateau than that over the Ganga Plain and Northwest in July and September. However, even over the remote Tibet plateau, there was significant anthropogenic contribution to atmospheric forcing and heating rate (45% in Apr, 75% in Sep). Wind fields showed black carbon transport from south Asia in April and east Asia in September. Further evaluation of the transport of dust and anthropogenic emissions from various source regions and their deposition in the Himalaya and Tibet, is important in understanding regional air quality and climate change over this ecosystem.
Keywords: Ganga plain; Tibet plateau; Aerosol emissions; Surface concentration; Radiative forcing; Atmospheric heating;