Aerosol-boundary layer dynamics and its effect on aerosol radiative forcing and atmospheric heating rate in the Indian Ocean sector of Southern Ocean

The study examines the thermodynamic structure of the marine atmospheric boundary layer (MABL) and its effect on the aerosol dynamics in the Indian Ocean sector of Southern Ocean (ISSO) between 30°S-67°S and 57°E-77°E. It includes observations of aerosols and meteorology collected during the Xth Southern Ocean Expedition conducted in December 2017. The results revealed the effect of frontal-region-specific air-sea coupling on the thermodynamic structure of MABL and its role in regulating aerosols in ISSO. The MABL over the subtropical front was unstable and formed a well-evolved mixed layer (≈2400 m) capped by low-level inversions (≈660 m). Convective activities in the Sub-Antarctic Frontal region were associated with the Agulhas Retroflection Current, which supported the formation of a well-developed mixed layer (≈1860 m). The mean estimates of aerosol optical depth (AOD) and black carbon (BC) mass concentrations were 0.095 ± 0.006 and 50 ± 14 ng m^-3, respectively, and the resultant clear sky direct shortwave radiative forcing (DARF) and atmospheric heating rate (HR) were 1.32 ± 0.11 W m^-2 and 0.022 ± 0.002 K day^-1, respectively. In the polar front (PF) region, frequent mid-latitude cyclones led to highly stabilized MABL, supported low-level multi-layered clouds (>3-layers) and multiple high-level inversions (strength > 0.5 K m^-1 > 3000 m). The clouds were mixed-phased with temperatures less than -12 °C at 3000 m altitude. Interestingly, there was higher loading of dust and BC aerosols (276 ± 24 ng m^-3), maximum AOD (0.109 ± 0.009), clear sky DARF (1.73 ± 0.02 W m^-2), and HR (0.029 ± 0.005 K day^-1). This showed an accumulation of long-range advected anthropogenic aerosols within baroclinic-boundaries formed over the PF region. Specifically, in the region south of PF, weak convection caused weakly-unstable MABL with a single low-level inversion followed by no clouds/single-layer clouds. Predominant clean maritime air holding a small fraction of dust and BC accounted for lower estimates of AOD (0.071 ± 0.004), BC concentrations (90 ± 55 ng m^-3) and associated clear sky DARF and HR were 1.16 ± 0.06 W m^-2 and 0.019 ± 0.001 K day^-1, respectively.

Carbonaceous aerosols and their light absorption properties over the Bay of Bengal during continental outflow

The marine atmosphere of the Bay of Bengal (BoB) is prone to get impacted by anthropogenic aerosols from the Indo-Gangetic Plain (IGP) and Southeast Asia (SEA), particularly during the northeast monsoon (NEM). In this study, we quantify and characterize carbonaceous aerosols and their absorption properties collected in two cruise campaigns onboard ORV Sindhu Sadhana during the continental outflow period over the BoB. Aerosol samples were classified based on the air mass back trajectory analyses, wherein samples were impacted by the continental air parcel (CAP), marine air parcel (MAP), and mix of both (CAP + MAP). Significant variability in the PM₁₀ mass concentration (in μg m^-3) is found with a maximum value for MAP samples (75.5 ± 36.4) followed by CAP + MAP (58.5 ± 27.3) and CAP (58.5 ± 27.3). The OC/EC ratio (>2) and diagnostic tracers i.e. nss-K⁺/EC (0.2-0.96) and nss-K⁺/OC (0.11-1.32) along with the absorption angstrom exponent (AAE: 4.31-6.02) and MODIS (Moderate Resolution Imaging Spectroradiometer) derived fire counts suggest the dominance of biomass burning emission sources. A positive correlation between OC and EC (i.e. r = 0.86, 0.70, and 0.42 for CAP, MAP, and CAP + MAP, respectively) further confirmed the similar emission sources of carbonaceous species. Similarly, a significant correlation between estimated secondary organic carbon (SOC) and water-soluble organic carbon (WSOC; r = 0.99, 0.96, and 0.97 for CAP, MAP, and CAP + MAP, respectively) indicate their similar chemical nature as well as dominant contribution of SOC to WSOC. The absorption coefficient (b_abs-365) and mass absorption efficiency (MAE_BrC-365) of the soluble fraction were estimated at 365 nm wherein, b_abs-365 showed a linear relationship with WSOC and nss-K⁺, signifying the contribution of water soluble brown carbon from biomass burning emissions. The estimated MAE_BrC-365 (0.30-0.93 m² g^-1), during this study, was consistent with the earlier observations over the BoB, particularly during the continental outflow season.

Decadal variations in natural and anthropogenic aerosol optical depth over the Bay of Bengal: the influence of pollutants from Indo-GangeticPlain

Perennial increase in atmospheric pollution over the Bay of Bengal (BoB) and South China Sea is reported due to increase in human population and industrial activity in South and Southeast Asia. Based on total aerosol optical depth (AOD) derived from MODIS (moderate resolution imaging resolution imaging spectroradiometer), natural and anthropogenic fractions were derived. The seasonality and spatial variability in rate of increase in total, natural, and anthropogenic AOD fractions were examined over the BoB using data collected between 2001 and 2019. Both total and anthropogenic AOD displayed statistically significant rate of increase in the northwest BoB (NWB) and western coastal BoB (WCB) regions during 2001 to 2019 whereas the long-term changes are insignificant in the other regions of BoB. Significant increase in AOD in the NWB and WCB regions is mainly contributed by dominant outflow of anthropogenic emissions from Indo-Gangetic Plain (IGP) area of Indian subcontinent. The magnitude of AOD decreased by half from northern BoB to equatorial region due to increase in distance from the source region. The contribution of anthropogenic AOD was >70% to total AOD with higher contribution during winter and lower during summer. The rate of increase in both total and anthropogenic AOD was close to 0.104 and 0.099 per decade in the NWB and 0.069 and 0.059 per decade in the WCB region between 2001 and 2019. The rate of increase in total and anthropogenic AOD decreased from 2001-2009 (0.164 and 0.115 per decade respectively) to 2010-2019 (0.068 and 0.076 per decade respectively) in the NWB region. Significant increase in anthropogenic AOD by 50 and 30% was observed during El Niño and La Niña periods respectively than normal year in both northwest BoB (NWB) and western coastal (WCB) regions due to change in strength and direction of winds. Although some fraction of anthropogenic AOD is found over the entire BoB, significant rate of increase in anthropogenic AOD is found only about 23% of the area of BoB than hitherto reported as entire BoB. The impact of atmospheric deposition of anthropogenic aerosols on biogeochemical processes, such as primary production and ocean acidification, needs further evaluation.

Climatology and trends of aerosol optical depth with different particle size and shape in northeast China from 2001 to 2018

Aerosol generated from the economic development and extensive urbanization in northeast China (NEC) could influence aerosol optical properties and affect the regional air quality. The level 3 aerosol optical depth (AOD) of different particle size and shape (spherical or nonspherical) obtained by Multiangle Imaging Spectroradiometer (MISR) version 23 were used to estimate their seasonal, annual, and decadal distribution and contribution in NEC from 2001 to 2018. The highest AOD of approximately 0.3 was found in the central Liaoning urban agglomeration, and the lowest AOD occurred in the mountainous area of NEC; the proportion of spherical AOD in NEC region was more than 90%. The contribution of large AOD was higher in spring, ranging from 28.8% to 29.8%. In spring and summer, small and medium AODs were concentrated in central Liaoning (approximately 0.2-0.3 and 0.06-0.08, respectively). The annual variation in the AOD of different particle size was significantly higher in Liaoning than in Jilin and Heilongjiang. The annual proportions of small and spherical AODs were approximately 60% and 90%, respectively. The annual occurrence of clean conditions with AOD < 0.05 was most common in northern Heilongjiang (approximately 20%). In NEC, the annual occurrence frequencies of 0.05 < AOD < 0.15 and AOD > 0.6 were the highest (approximately 50%) and the lowest (less than 1%), respectively. Interdecadal AOD revealed a positive trend from 2001 to 2008 and a negative trend from 2009 to 2018. The frequency of occurrence trend at different AOD levels also changed from positive to negative between these two periods. The findings in this study are based on the first aerosol retrieval of the newly released MISR in NEC. The results provide a comprehensive understanding of the regional and climatological aerosol extinction with different AOD of size and shape as well as various level bins in NEC.

Recent Increase in Winter Hazy Days over Central India and the Arabian Sea

Indian subcontinent is greatly vulnerable to air pollution, especially during the winter season. Here, we use 15 years (2003-2017) of satellite and model reanalysis datasets over India and adjoining Seas to estimate the trend in hazy days (i.e. days with high aerosol loading) during the dry winter season (November to February). The number of hazy days is increasing at the rate of ~2.6 days per year over Central India. Interestingly, this is higher than over the Indo-Gangetic Plain (~1.7 days/year), a well known global hotspot of particulate pollution. Consistent increasing trends in absorbing aerosols are also visible in the recent years. As a result, the estimated atmospheric warming trends over Central India are two-fold higher than that over Indo-Gangetic Plain. This anomalous increment in hazy days over Central India is associated with the relatively higher increase in biomass burning over the region. Moreover, the trend in aerosol loading over the Arabian Sea, which is located downwind to Central India, is also higher than that over the Bay of Bengal during the dry winter season. Our findings not only draw attention to the rapid deteriorating air quality over Central India, but also underline the significance of increasing biomass burning under the recent climate change.

Brown carbon in the continental outflow to the North Indian Ocean

In this paper, we synthesize the size distribution and optical properties of the atmospheric water-soluble fraction of light-absorbing organic carbon (brown carbon; BrC) in the continental outflow from the Indo-Gangetic Plain (IGP) in South Asia to the North Indian Ocean. A comparison of the mass absorption coefficient of water-soluble BrC (babs-WSBrC-365nm) in PM2.5 with that in PM10 sampled over the Bay of Bengal reveals the dominance of BrC in fine mode. Furthermore, the babs-BrC-365nm shows a significant linear relationship with mass concentrations of airborne particulate matter, water-soluble organic carbon and non-sea-salt-K+ in the continental outflow from the IGP. This observation emphasizes the ubiquitous nature and significant contribution of water-soluble BrC from biomass burning emissions (BBEs). Comparing the absorption properties from this study with global datasets, it is discernible that BBEs dominate BrC absorption. Furthermore, the imaginary refractive index of water-soluble BrC (kWSBrC-365nm) in marine aerosols sampled over the North Indian Ocean during November is significantly higher than during December to January. Thus, significant temporal variability is associated with crop-residue burning emissions in the IGP on the composition of BrC over the North Indian Ocean. Our estimates show that the babs-WSBrC-365nm and kWSBrC-365nm from post-harvest crop-residue burning emissions in the IGP are much higher than the BBEs from the southeastern United States and Amazonian forest fires. Another major finding of this study is the lack of significant relationship between kWSBrC-365nm and the mass ratio of elemental carbon to particulate organic matter, as previously suggested by chamber experiments to model varying BrC absorption properties in ambient aerosols. Therefore, considerable spatio-temporal variability prevails among emission sources (wood burning vs. crop-residue burning), which needs to be considered when assessing the regional radiative forcing of BrC relative to major absorbing elemental carbon.

Estimation and inter-comparison of dust aerosols based on MODIS, MISR and AERONET retrievals over Asian desert regions

This study presents detailed analysis of spatiotemporal variations and trend of dust optical properties i.e., Aerosol Optical Depth (AOD) and Angstrom component over Asian desert regions using thirteen years of data (i.e., 2001-2013) retrieved from Aerosol Robotic Network (AERONET), Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR). These regions include Solar Village, Dunhuang and Dalangzadgad and are considered as origin of desert aerosols in Asia. Mann-Kendall trend test was used to show the trend of AOD. The relationship of AOD with weather parameters and general AOD trend over different wavelengths has also been shown. AOD's trend has been observed significant throughout the year in Solar Village, while in Dunhuang and Dalanzadgad the significant trend has been found only in peak period (March-June). Analysis show high values of AOD and low values of angstrom in Solar Village during peak period. In Chinese desert regions, high values of AOD have been found during peak period and low values in pre-peak period. Significant relationship has been observed between AOD and average temperature in Solar Village and Dalanzadgad whereas rainfall and wind speed showed no significant impact on AOD in all desert regions.

Characterization of particulate matter and black carbon over Bay of Bengal during summer monsoon: results from the OMM cruise experiment

Total and size-segregated particulate matter (PM) and black carbon (BC) concentrations over the Bay of Bengal (BoB) have been measured in the summer monsoon (August-September 2014) onboard a scientific cruise conducted as a part of the Ocean Monsoon and Mixing (OMM) experiment. Role of long-range transport and prevailing meteorology in producing the observed spatio-temporal features is inferred by synthesizing the results of in situ observations in conjunction with the wind components from Modern Era Retrospective Analysis for Research and Applications (MERRA), rainfall data from Tropical Rainfall Measuring Mission (TRMM), surface BC concentration and BC Aerosol Optical Thickness (AOT_BC) from MERRA2 and HYSPLIT back trajectory and dispersion model analysis. Mean values of total PM and BC mass concentrations are observed to be ~ 21.4 μgm^-3 and ~ 393ngm^-3 respectively. The study has revealed significant influence of monsoon rainfall (over the measurement locations and regions through which transport occurred) on the concentration of both PM and BC over northern BoB. Results also indicate transport of aerosols with significant anthropogenic fraction, from the land regions at west to the BoB. A comprehensive analysis showed that while an eastward wind (westerly) from the Indian mainland produced an increase in PM_2.5 over northern BoB, a southerly wind, mostly from the Indian Ocean, caused a decrease in concentration of PM_2.5. Spectral variation of absorption coefficients of aerosols reveals that most of the BC over BoB is associated with fossil fuel combustion. Prevailing strong surface-level convergence (associated with a low-level anticyclone) resulted in accumulation and consequent enhancement of aerosol concentration over central and northern BoB during the study period. In addition, horizontal flow rates estimated across western boundary of BoB using AOT_BC from MERRA2 for 10 years revealed an increasing trend in BC transport from the mainland leading to a gradual buildup in BC concentration over the regions of BoB.

Ultraviolet solar radiation in the tropical central Andes (12.0°S)

Ultraviolet (UV) solar irradiance measurements performed in the central Andes, Huancayo, Peru (12.0°S, 75.3°W, 3313 m asl) between January 2003 and December 2006 were used to analyse daily, monthly, and annual cycles of UV solar irradiance.

Chemical characterization of PM1.0 aerosol in Delhi and source apportionment using positive matrix factorization

Fine aerosol fraction (particulate matter with aerodynamic diameter <= 1.0 μm (PM)_1.0) over the Indian Institute of Technology Delhi campus was monitored day and night (10 h each) at 30 m height from November 2009 to March 2010. The samples were analyzed for 5 ions (NH₄⁺, NO₃^-, SO₄^2-, F^-, and Cl^-) and 12 trace elements (Na, K, Mg, Ca, Pb, Zn, Fe, Mn, Cu, Cd, Cr, and Ni). Importantly, secondary aerosol (sulfate and nitrate) formation was observed during dense foggy events, supporting the fog-smog-fog cycle. A total of 76 samples were used for source apportionment of PM mass. Six factors were resolved by PMF analyses and were identified as secondary aerosol, secondary chloride, biomass burning, soil dust, iron-rich source, and vehicular emission. The geographical location of the sources and/or preferred transport pathways was identified by conditional probability function (for local sources) and potential source contribution function (for regional sources) analyses. Medium- and small-scale metal processing (e.g. steel sheet rolling) industries in Haryana and National Capital Region (NCR) Delhi, coke and petroleum refining in Punjab, and thermal power plants in Pakistan, Punjab, and NCR Delhi were likely contributors to secondary sulfate, nitrate, and secondary chloride at the receptor site. The agricultural residue burning after harvesting season (Sept-Dec and Feb-Apr) in Punjab, and Haryana contributed to potassium at receptor site during November-December and March 2010. The soil dust from North and East Pakistan, and Rajasthan, North-East Punjab, and Haryana along with the local dust contributed to soil dust at the receptor site, during February and March 2010. A combination of temporal behavior and air parcel trajectory ensemble analyses indicated that the iron-rich source was most likely a local source attributed to emissions from metal processing facilities. Further, as expected, the vehicular emissions source did not show any seasonality and was local in origin.

An integrated campaign for investigation of winter-time continental haze over Indo-Gangetic Basin and its radiative effects

An outflow of continental haze occurs from Indo-Gangetic Basin (IGB) in the North to Bay of Bengal (BoB) in the South. An integrated campaign was organized to investigate this continental haze during December 2013-February 2014 at source and remote regions within IGB to quantify its radiative effects. Measurements were carried out at three locations in eastern India; 1) Kalas Island, Sundarban (21.68°N, 88.57°E) - an isolated island along the north-east coast of BoB, 2) Kolkata (22.57°N, 88.42°E) - an urban metropolis and 3) Siliguri (26.70°N, 88.35°E) - an urban region at the foothills of eastern Himalayas. Ground-based AOD (at 0.5 μm) is observed to be maximum (1.25±0.18) over Kolkata followed by Siliguri (0.60±0.17) and minimum over Sundarban (0.53±0.18). Black carbon concentration is found to be maximum at Kolkata (21.6±6.6 μg·m(-3)) with almost equal concentrations at Siliguri (12.6±5.2 μg·m(-3)) and Sundarban (12.3±3.0 μg·m(-3)). Combination of MODIS-AOD and back-trajectories analysis shows an outflow of winter-time continental haze originating from central IGB and venting out through Sundarban towards BoB. This continental haze with high extinction coefficient is identified up to central BoB using CALIPSO observations and is found to contribute ~75% to marine AOD over central BoB. This haze produces significantly high aerosol radiative forcing within the atmosphere over Kolkata (75.4 Wm(-2)) as well as over Siliguri and Sundarban (40 Wm(-2)) indicating large forcing over entire IGB, from foothills of the Himalayas to coastal region. This winter-time continental haze also causes about similar radiative heating (1.5 K·day(-1)) from Siliguri to Sundarban which is enhanced over Kolkata (3 K·day(-1)) due to large emission of local urban aerosols. This high aerosol heating over entire IGB and coastal region of BoB can have considerable impact on the monsoonal circulation and more importantly, such haze transported over to BoB can significantly affect the marine hydrological cycle.