The dual carbon isotope signatures and optical properties of carbonaceous aerosols have been investigated simultaneously for the first time in the South Asian outflow during an intensive campaign at ...the Maldives Climate Observatory on Hanimaadhoo (MCOH) (February and March 2012). As one component of the Cloud Aerosol Radiative Forcing Dynamics Experiment, this paper reports on the sources and the atmospheric processing of elemental carbon (EC) and water‐soluble organic carbon (WSOC) as examined by a dual carbon isotope approach. The radiocarbon (Δ14C) data show that WSOC has a significantly higher biomass/biogenic contribution (86 ± 5%) compared to EC (59 ± 4%). The more 13C‐enriched signature of MCOH‐WSOC (−20.8 ± 0.7‰) compared to MCOH‐EC (−25.8 ± 0.3‰) and megacity Delhi WSOC (−24.1 ± 0.9‰) suggests that WSOC is significantly more affected by aging during long‐range transport than EC. The δ13C‐Δ14C signal suggests that the wintertime WSOC intercepted over the Indian Ocean largely represents aged primary biomass burning aerosols. Since light‐absorbing organic carbon aerosols (Brown Carbon (BrC)) have recently been identified as potential contributors to positive radiative forcing, optical properties of WSOC were also investigated. The mass absorption cross section of WSOC (MAC365) was 0.5 ± 0.2 m2 g−1 which is lower than what has been observed at near‐source sites, indicating a net decrease of WSOC light‐absorption character during long‐range transport. Near‐surface WSOC at MCOH accounted for ~1% of the total direct solar absorbance relative to EC, which is lower than the BrC absorption inferred from solar spectral observations of ambient aerosols, suggesting that a significant portion of BrC might be included in the water‐insoluble portion of organic aerosols.
Key Points
Fossil fuel contribution to PM2.5 EC and WSOC is 41% and 14% respectivelyAerosol LRT leads to 13C enrichment and a less absorbing WSOC aerosolWSOC accounts for only ~ 1% of direct solar absorbance relative to EC
Light-absorbing organic aerosols, known as brown carbon (BrC), counteract the overall cooling effect of aerosols on Earth's climate. The spatial and temporal dynamics of their light-absorbing ...properties are poorly constrained and unaccounted for in climate models, because of limited ambient observations. We combine carbon isotope forensics (δ
C) with measurements of light absorption in a conceptual aging model to constrain the loss of light absorptivity (i.e., bleaching) of water-soluble BrC (WS-BrC) aerosols in one of the world's largest BrC emission regions-South Asia. On this regional scale, we find that atmospheric photochemical oxidation reduces the light absorption of WS-BrC by ~84% during transport over 6000 km in the Indo-Gangetic Plain, with an ambient first-order bleaching rate of 0.20 ± 0.05 day
during over-ocean transit across Bay of Bengal to an Indian Ocean receptor site. This study facilitates dynamic parameterization of WS-BrC absorption properties, thereby constraining BrC climate impact over South Asia.
Particulate matter with size less than or equal to 2.5 μm (PM
2.5
) samples were collected from an urban site Pune, India, during April 2015 to April 2016. The samples were analyzed for various ...chemical constituents, including water soluble inorganic ions, organic carbon (OC), and elemental carbon (EC). The yearly mean total mass concentration of PM
2.5
at Pune was 37.3 μg/m
3
, which is almost four times higher than the annual WHO standard (10 μg/m
3
), and almost equal to that recommended by the Central Pollution Control Board, India (40 μg/m
3
). Measured (OC, EC) and estimated organic matter (OM) were the dominant component (56 ± 11%) in the total particulate matter which play major role in the regional atmospheric chemistry. Total measured inorganic components formed about 35% of PM
2.5
. Major chemical contributors to PM
2.5
mass were OC (30%), SO
4
2−
(13%), and Cl
−
and EC (9% each). The high ratios of OC/EC demonstrated the existence of secondary organic carbon. The air mass origin and correlations between the various components indicate that long range transport of pollutants from Indo-Gangetic Plain (IGP) and Southern part of the Arabian Peninsula might have contributed to the high aerosol mass during the dry and winter seasons. To our knowledge, this is the first systematic study that comprehensively explores the chemical characterization and source apportionment of PM
2.5
aerosol speciation in Pune by applying multiple approaches based on a seasonal perspective. This study is broadly applicable to understanding the differences in anthropogenic and natural sources in the urban environment of particle air pollution over this region.
Abstract
Fine particulate-matter is an important component of air pollution that impacts health and climate, and which delivers anthropogenic contaminants to remote global regions. The complex ...composition of organic molecules in atmospheric particulates is poorly constrained, but has important implications for understanding pollutant sources, climate-aerosol interactions, and health risks of air pollution exposure. Here, comprehensive nontarget high-resolution mass spectrometry was combined with in silico structural prediction to achieve greater molecular-level insight for fine particulate samples (
n
= 40) collected at a remote receptor site in the Maldives during January to April 2018. Spectral database matching identified 0.5% of 60,030 molecular features observed, while a conservative computational workflow enabled structural annotation of 17% of organic structures among the remaining molecular dark matter. Compared to clean air from the southern Indian Ocean, molecular structures from highly-polluted regions were dominated by organic nitrogen compounds, many with computed physicochemical properties of high toxicological and climate relevance. We conclude that combining nontarget analysis with computational mass spectrometry can advance molecular-level understanding of the sources and impacts of polluted air.
This study presents and evaluates the most comprehensive set to date of chemical, physical and optical properties of aerosols in the outflow from South Asia covering a full winter (Nov. 2014 - March ...2015), here intercepted at the Indian Ocean receptor site of the Maldives Climate Observatory in Hanimaadhoo (MCOH). Cluster analysis of air-mass back trajectories for MCOH, combined with AOD and meteorological data, demonstrate that the wintertime northern Indian Ocean is strongly influenced by aerosols transported from source regions with three major wind regimes, originating from the Indo-Gangetic Plain (IGP), the Bay of Bengal (BoB) and the Arabian Sea (AS). As much as 97 ± 3% of elemental carbon (EC) in the PM
10
was also found in the fine mode (PM
2.5
). Other mainly anthropogenic constituents such as organic carbon (OC), non-sea-salt (nss) -K
+
, nss-SO
4
2−
and NH
4
+
were also predominantly in the fine mode (70-95%), particularly in the air masses from IGP. The combination at this large-footprint receptor observatory of consistently low OC/EC ratio (2.0 ± 0.5), strong linear relationships between EC and OC as well as between nss-K
+
and both OC and EC, suggest a predominance of primary sources, with a large biomass burning contribution. The particle number-size distributions for the air masses from IGP and BoB exhibited clear bimodal shapes within the fine fraction with distinct accumulation (0.1 μm < d < 1 μm) and Aitken (0.025 μm < d < 0.10 μm) modes. This study also supports that IGP is a key source region for the wider South Asia and nearby oceans, as defined by the criteria that anthropogenic AODs exceed 0.3 and absorption AOD > 0.03. Taken together, the aerosol pollution over the northern Indian Ocean in the dry season is dominated by a well-mixed long-range transported regime of the fine-mode aerosols largely from primary combustion origin.
Aerosol emissions in South Asia are large. The emitted aerosols can travel significant distances and, during the Asian southwest monsoon especially, are prone to modification through cloud processing ...and wet scavenging while being transported. The scale of emissions and transport means that the global climate impact of these aerosols are sensitive to modification en route, but the process-level understanding is still largely lacking. In this study, we analyse long-term aerosol data measured at an observatory established in Hanimaadhoo, Republic of Maldives, to investigate the long-term properties of aerosols over the Indian Ocean as well as to understand the effect of precipitation on the aerosol particle size distribution during long-range transport. The observatory location is ideal because it is a receptor site with little local influence, and, depending on the season, receives either polluted air masses coming from the Indian subcontinent or clean marine air masses from the Indian Ocean. We analysed the sub-micron particle number size distribution measured during the years 2004-2008, and 2014-2017, and this is the first inter-seasonal long-term study of the sub-micron aerosol features in the region. The aerosol origin and its relative exposure to wet scavenging during long-range transport were analysed using back-trajectory analysis from HYSPLIT. By comparing aerosol measurements to precipitation along its transport, this study shows that there is a substantial change in particle number size distributions and concentrations depending on the amount of rainfall during transport. During the southwest monsoon season, the aerosol size distribution was notably bi-modal and total particle concentrations clearly reduced in comparison with the prevailing aerosol size distribution during the northeast monsoon season. Precipitation during transport usually corresponded with a greater reduction in accumulation mode concentrations than for smaller sizes, and the shape of the median size distribution showed a clear dependence on the trajectory origin and route taken.
The properties of the atmospheric aerosols depend on the source region and on the modifications that occur during their transport in the air. We have studied physical and chemical properties of ...aerosols along with their sink mechanism over two locations in southwest India, an urban site (Pune) and well-established climate observatory at Sinhagad (SINH), which represents rural and high altitude site. The ground-based measurements of aerosols, together with their radiative properties in this study have provided means to understand the observed variability and the impact on the aerosol radiative properties effectively over this region. The annual mean elemental carbon concentration (3.4 µg m
− 3
) at Pune was observed about three times higher compared to SINH (1.3 µg m
− 3
), indicating strong emissions of carbon-rich aerosols at the urban location. Aerosol optical properties were derived using the OPAC model which were used to compute the Aerosol radiative forcing (ARF) over both stations calculated using SBDART (Santa Barbara DISORT Atmospheric Radiative Transfer) model shows pronounced seasonal variations due to changes in aerosol optical depth and single scattering albedo at both locations. The year-round ARF was 4–5 times higher over Pune (31.4 ± 3.5 Wm
− 2
) compared to SINH (7.2 ± 1.1 Wm
− 2
). The atmospheric heating rate due to aerosols shows a similar pattern as ARF for these locations. The heating was higher in the wintertime, ~ 0.9–1.6 K day
− 1
at Pune, and ~ 0.3–0.6 K day
− 1
at SINH. The estimated scavenging ratio was found high for NO
3
−
and Ca.
2+
. The wet deposition fluxes of Cl
−
, SO
4
2−
, Na
+
, Mg
2+
were observed higher for SINH as compared to Pune, due to the high amount of rain received at SINH.
Multi-year Aerosol Robotic Network (AERONET) direct Sun retrieved and inversion algorithm derived aerosol products at a semi-arid, urban site, Jaipur (26.90° N, 75.80° E) and island observing site, ...Maldives Climate Observatory-Hanimaadhoo (MCO-Hanimaadhoo, 6.74° N, 73.17° E) are analysed to investigate heterogeneity in aerosol optical and microphysical properties. Results reveal the existence of a large seasonal diversity in the frequency distributions of aerosol optical depth (AOD
500
nm
, AOD
1020
nm
) and Ångström exponent (AE
440-870
nm
) during different seasons at Jaipur and MCO-Hanimaadhoo. These are indicative of the advection of different aerosol types (viz., black carbon (BC) aerosol, organic aerosol, sulfate particle, dust, sea salt, nitrate particle, and mixtures thereof) from a variety of production mechanisms influenced by strong seasonal changes of anthropogenic activities as well as modulations induced by the climatic condition. The cumulative frequency analysis of the single scattering albedo (SSA) difference (i.e. ΔSSA = SSA
440
nm
- SSA
1020
nm
) shows that at Jaipur ΔSSA is predominantly negative (around 88% days) while at MCO-Hanimaadhoo it is positive (around 74% days). The positive and negative values of ΔSSA are respectively linked to a stronger absorption by BC mixed anthropogenic pollution aerosols at 1020 nm and to a stronger absorption by mineral dust containing iron oxide at 440 nm. The spectral behaviour of SSA, thus, facilitates investigation of the existence of iron oxide or BC in aerosols. The 'Bivariate Kernel density' plots of SSA versus fine-mode fraction (FMF) of AOD
440
nm
/AE
440-870
nm
reveal that at Jaipur the aerosol ensemble consists of coarse-mode particles (AE and FMF cluster in the range 0.2-0.4), a dominant category along with significant fine-mode and much less mixed category. At MCO-Hanimaadhoo fine-mode particle category (with FMF and AE cluster in the range 0.90-0.95 and 1.2-1.6 respectively) is the only dominant category. The persisting log-normal bimodal feature in aerosol volume size distribution (AVSD) is observed both at Jaipur and MCO-Hanimaadhoo. The modal volume concentration of coarse-mode aerosol decreases from FMF of AOD
675
nm
= 0.25 (inherently belonging to the coarse-mode regime) to FMF of AOD
675
nm
= 0.95 (inherently belonging to the fine-mode regime). This transformation in coarse-to fine-mode volume concentration is associated with a steady rise in AE
440-874
nm
supporting this changeover.
Black carbon (BC) aerosols impact climate and air quality. Since BC from fossil versus biomass combustion have different optical properties and different abilities to penetrate the lungs, it is ...important to better understand their relative contributions in strongly affected regions such as South Asia. This study reports the first year-round 14C-based source apportionment of elemental carbon (EC), the mass-based correspondent to BC, using as regional receptor sites the international Maldives Climate Observatory in Hanimaadhoo (MCOH) and the mountaintop observatory of the Indian Institute of Tropical Meteorology in Sinhagad, India (SINH). For the highly-polluted winter season (December-March), the fractional contribution to EC from biomass burning (fbio) was 53 5% (n = 6) at MCOH and 56 3% at SINH (n = 5). The fbio for the non-winter remainder was 53 11% (n = 6) at MCOH and 48 8% (n = 7) at SINH. This observation-based constraint on near-equal contributions from biomass burning and fossil fuel combustion at both sites compare with predictions from eight technology-based emission inventory (EI) models for India of (fbio)EI spanning 55-88%, suggesting that most current EI for Indian BC systematically under predict the relative contribution of fossil fuel combustion. A continued iterative testing of bottom-up EI with top-down observational source constraints has the potential to lead to reduced uncertainties regarding EC sources and emissions to the benefit of both models of climate and air quality as well as guide efficient policies to mitigate emissions.
Precipitation chemistry measurements are essential to estimate the deposition of ionic components that are beneficial or harmful to the ecosystems and quantify the wet scavenging of the atmospheric ...constituents. A long-term (2006 to 2016) database of the water-soluble inorganic components in the rainwater (RW) at Pune (an urban location) and Sinhagad (a high-altitude rural location) in southwest India was examined to understand the trends in measured anthropogenic components and the factors controlling the chemistry of RW in this region. We observed the long-term increasing trends in NH
4
+
at Pune and SO
4
2−
, NO
3
¯, and NH
4
+
at Sinhagad. About 10% of rain samples were acidic (below 5.6). Low pH values were generally measured at Pune (8%) and Sinhagad (9%) stations when the air mass back trajectories (AMBTs) originated from land. This study suggests that NO
3
¯ plays a vital role in acidifying the rain in both the sampling stations, though SO
4
2−
remains the dominant factor. The high concentrations of Ca
2+
and NH
4
+
have played a significant role in neutralizing the acidity of RW. The cluster analysis of AMBTs showed that at Pune 74% and Sinahagd 65% of rain events were associated with oceanic air masses from the Arabian Sea.
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