Wintertime Variation in Carbonaceous Components of PM 10 in the High Altitudes of Himalayas

: Carbonaceous aerosols play a significant role in the Earth’s atmospheric system by affect-ing the visibility, hydrological cycle, climate, radiative forcing and human health. The present study analyses the PM 10 samples collected at three distinct urban locations (Mohal-Kullu, Nainital and Darjeeling) over the Himalayan region of India during winter 2019. The mass concentration of PM 10 were recorded as 51 ± 16 μg m −3 , 38 ± 9 μg m −3 and 52 ± 18 μg m −3 for Mohal-Kullu, Nainital and Darjeeling, respectively. Organic carbon (OC) dominated over elemental carbon (EC) and was found to be 50.2%, 42.8 and 47% of total carbon (TC) at Mohal-Kullu, Nainital and Darjeeling, re-spectively. The respective mass concentrations of carbonaceous species were higher at Mohal-Kullu (OC: 11.1 ± 5.3, EC: 4.2 ± 1.9, WSOC: 5.3 ± 1.3 μg m −3 and TCA: 22.1 ± 10.4 μg m −3 ) followed by Darjeeling (OC: 5.4 ± 2.0, EC: 2.7 ± 1.0, WSOC: 3.9 ± 1.3 μg m −3 and TCA: 22.1 ± 10.4 μg m −3 ) and Nainital (OC: 2.9 ± 1.0, EC: 1.3 ± 0.6, WSOC: 2.1 ± 0.6 μg m −3 and TCA: 6.7 ± 2.4 μg m −3 ). The OC/EC and WSOC/OC ratio at Mohal- Kullu (2.6 ± 0.3, 0.6 ± 0.2), Nainital (2.0 ± 0.4, 0.7 ± 0.2) and Darjeeling (2.3 ± 0.5, 0.7 ± 0.2), respectively, indicates the dominance of fossil fuel combustion (coal and vehic-ular exhaust), with signified additional contribution from secondary organic carbon (SOC).


Introduction
Atmospheric aerosols have been well known as a major pollutant worldwide due to their complex composition, effects on visibility, heat balance of the earth [1,2]. A high loading of aerosol poses severe implications to human health, global climate change and the Earth's radiation budget [3,4]. Carbonaceous aerosols (CAs) are the key component of particulate matter (PM) constituting 20-70% of coarse particulate matter [4]. Carbonaceous components of PM are classified as organic carbon (OC), and elemental carbon (EC) in terms of their optical and physical properties [4,5]. OC is emitted from combustion or indirectly from heterogeneous oxidation of volatile organic compounds (VOCs), OC have wide variety of organic compounds originating from various sources, and is further classified as primary organic carbon (POC) and secondary organic carbon (SOC) in terms of their formation. EC is emitted primarily from incomplete combustion of Biomass (BB) and fossil fuel combustion (FFC) [6][7][8]. As the CAs play crucial role in atmospheric chemistry, earth's radiation budget, human health and air quality of the region, it becomes very important to measure the concentrations of carbonaceous particles to understand their transport, sources and deposition. The Himalayan region is regarded pristine and vulnerable to environmental change due to regional and global change [6]. There has been few research on carbonaceous particles, their sources, movement, and climatic effects [7][8][9][10][11][12][13]. Recent studies on CAs, sources identification and transport processes [7][8][9][10][11][12][13] fill this gap to some extent.
The present study estimates the concentration of CAs at different locations (Mohal-Kullu, Nainital and Darjeeling) of the Himalayan region of India during winter season (January-February 2019). We chose the winter season for the study because of increased BB and CC activity for heating purposes, as well as steady and stagnant air conditions that can lead to large concentrations of CAs. Using ratios, we identify the origins and their contribution to CAs across different study locations. ) is located in the eastern Himalayas. The study site is geographically adjacent to the IGP region (a global hotspot for heavy aerosol loadings) [10]. A detailed description of the sampling location is discussed in previous studies [7][8][9][10]15,16].

Sample Collection and Analysis
PM10 samples were collected during winter season (January-February 2019) twice every week for 24h using respirable dust sampler with a flow rate of 1.2 m 3 min −1 on prebaked (550 °C for 5 h) Pall flex Tissuequartz filters (20 × 25 cm 2 ) at three urban locations of Mohal-Kullu, Nainital and Darjeeling.
Measurement of carbonaceous species (OC, EC) in PM10 samples collected over the study locations were carried out using OC/EC analyzer. This instrument is working on thermal-optical approach. WSOC in PM10 samples were quantified using TOC analyzer (Model: TOC LCPH/CPN, Shimadzu) which works on the principle of catalytic-oxidation combustion at high temperatures of 680 °C. WD-XRF (wave length dispersive X-Ray fluorescence) spectrometer was used for the quantification of elements in PM10 ranging from Barium to Uranium. Details of OC/EC, TOC analyzer and WD-XRF are available in reference therein [9][10][11][12].

Conclusions
CAs associated with PM10 has been studied during the winter season at different sites of the IHR i.e., Mohal-Kullu, Nainital and Darjeeling. The mean concentration of PM10 was 51 ± 16 μg m −3 , 38 ± 9 μg m −3 and 52 ± 18 μg m −3 at Mohal-Kullu, Nainital and Darjeeling, respectively. The major results of the study are summarised below:

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In the present study, the winter concentrations of OC Overall, the diagnostic ratios of OC with EC and WSOC with OC and SOC with OC, showed their positive association with carbonaceous components and major influence of BB as a source of carbonaceous species over the IHR. Linear regression analysis was performed among carbon components OC, EC, WSOC and SOC for more information on sources of CAs. OC and EC exhibited significant correlation throughout the study period which is attributed to common nature of their sources.

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Long range transported aerosols from IGP and surrounding areas contribute to carbonaceous species along with local emissions.

Funding:
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The datasets developed during the current study are available from the corresponding author on reasonable request.