Lokesh Kumar Sahu

The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O₃) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O₃ build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NO_x-VOC transition regime for higher O₃) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O₃ seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O₃-tracer analysis shows significant contributions of anthropogenic NO_x emissions from South Asia (15–18 %) and other regions (32–37 %) to O₃ levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O₃ over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.

The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O₃) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O₃ build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NO_x-VOC transition regime for higher O₃) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O₃ seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O₃-tracer analysis shows significant contributions of anthropogenic NO_x emissions from South Asia (15–18 %) and other regions (32–37 %) to O₃ levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O₃ over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.

Severe post-monsoon haze in Delhi-NCR poses air-quality and health risks and reflects regional and urban emissions. Sonipat lies along the principal transport corridor linking the agricultural burning regions of Punjab and Haryana with Delhi and serves as an intermediate receptor for regional pollution. However, source apportionment at upwind locations remains limited. To address this gap, we measured composition-based PM _2.5 , defined as non-refractory PM _2.5 plus equivalent black carbon, at Sonipat from 25 October to 15 November 2023 using a Time-of-Flight Aerosol Chemical Speciation Monitor and a multi-wavelength Aethalometer. Positive matrix factorization, black-carbon apportionment, and trajectory and wind analyses were used to identify carbonaceous aerosol sources and examine their evolution during haze, non-haze, and Diwali periods. Two severe haze episodes occurred, with composition-based PM _2.5 exceeding 300 µg m ⁻³ . Organic aerosol contributed 65 % of non-refractory PM _2.5 , and its daily mean reached nearly 140 µg m ⁻³ . Oxygenated organic aerosol accounted for approximately 57 %–60 % of total organic aerosol, with more-oxidized oxygenated organic aerosol reaching 42.8 µg m ⁻³ during peak haze, indicating substantial secondary processing and accumulation. Under the reference absorption Ångström exponent pair, biomass- or solid-fuel-related eBC accounted for approximately 78 % of the campaign-integrated eBC burden, although the quantitative source split was sensitive to the assumed exponents. Northwestern transport signatures and co-variation of eBC _bb with biomass-burning and solid-fuel organic aerosol indicate an important regional combustion influence. These findings show that carbonaceous aerosol was already present at this receptor before further interaction with Delhi emissions and support coordinated regional controls alongside sustained urban emission reductions.

The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O₃) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O₃ build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NO_x-VOC transition regime for higher O₃) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O₃ seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O₃-tracer analysis shows significant contributions of anthropogenic NO_x emissions from South Asia (15–18 %) and other regions (32–37 %) to O₃ levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O₃ over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.

Sonipat, located ~40 km northwest of Delhi, lies along the principal transport corridor linking post-monsoon agricultural burning regions of Punjab–Haryana with Delhi and serves as an intermediate receptor for regional pollution. We conducted intensive high time resolution measurements of composition-based PM₂.₅ (non-refractory PM₂.₅ plus black carbon) from 25 October to 15 November 2023 using a ToF-ACSM and an Aethalometer to characterize carbonaceous aerosol sources during the biomass-burning period. Two severe haze episodes occurred, with PM₂.₅ exceeding 300 µg/m³. Organic aerosol dominated the submicron mass (~65 % of non-refractory PM₂.₅), with daily mean concentrations peaking near 140 µg/m³ during the first haze episode. Positive Matrix Factorization resolved five components including hydrocarbon-like, biomass-burning, and solid-fuel combustion organic aerosol, and two oxygenated fractions representing semi-volatile and low-volatility aged aerosol. Secondary organic aerosol accounted for ~57–60 % of organic aerosol mass, with low-volatility oxygenated organic aerosol reaching 42.8 µg/m³ during peak haze, indicating substantial regional aging and accumulation. Biomass-derived black carbon contributed ~78 % of total black carbon (mean 10.9 µg/m³), far exceeding fossil-fuel contributions (~3.1 µg/m³). Trajectory and wind analyses consistently identified northwestern agricultural regions as dominant sources with minor traffic influence, indicating that extreme carbonaceous aerosol over Delhi-NCR largely forms outside the urban core through regional transport biomass and solid-fuel combustion emissions combined with sustained secondary processing, highlighting the need for coordinated airshed-scale emission reductions across the Indo-Gangetic Plain.

Sonipat, located ~40 km northwest of Delhi, lies along the principal transport corridor linking post-monsoon agricultural burning regions of Punjab–Haryana with Delhi and serves as an intermediate receptor for regional pollution. We conducted intensive high time resolution measurements of composition-based PM₂.₅ (non-refractory PM₂.₅ plus black carbon) from 25 October to 15 November 2023 using a ToF-ACSM and an Aethalometer to characterize carbonaceous aerosol sources during the biomass-burning period. Two severe haze episodes occurred, with PM₂.₅ exceeding 300 µg/m³. Organic aerosol dominated the submicron mass (~65 % of non-refractory PM₂.₅), with daily mean concentrations peaking near 140 µg/m³ during the first haze episode. Positive Matrix Factorization resolved five components including hydrocarbon-like, biomass-burning, and solid-fuel combustion organic aerosol, and two oxygenated fractions representing semi-volatile and low-volatility aged aerosol. Secondary organic aerosol accounted for ~57–60 % of organic aerosol mass, with low-volatility oxygenated organic aerosol reaching 42.8 µg/m³ during peak haze, indicating substantial regional aging and accumulation. Biomass-derived black carbon contributed ~78 % of total black carbon (mean 10.9 µg/m³), far exceeding fossil-fuel contributions (~3.1 µg/m³). Trajectory and wind analyses consistently identified northwestern agricultural regions as dominant sources with minor traffic influence, indicating that extreme carbonaceous aerosol over Delhi-NCR largely forms outside the urban core through regional transport biomass and solid-fuel combustion emissions combined with sustained secondary processing, highlighting the need for coordinated airshed-scale emission reductions across the Indo-Gangetic Plain.

The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O₃) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O₃ build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NO_x-VOC transition regime for higher O₃) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O₃ seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O₃-tracer analysis shows significant contributions of anthropogenic NO_x emissions from South Asia (15–18 %) and other regions (32–37 %) to O₃ levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O₃ over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.

The Trans-Himalaya, located between the Great Himalaya and the Tibetan Plateau, is ecologically fragile and one of the most climate-sensitive regions. However, lack of systematic in-situ measurements have hindered the evaluation of chemistry-climate models and constrained our understanding of the global-to-regional influences over this region. To address this, we present a comprehensive analysis by combining first yearlong (October 2024–September 2025) surface ozone (O3) measurements at Leh (3.4 km above mean sea level) with satellite data and numerical simulations (global model: CAM4-Chem, regional model: WRF-Chem). Our measurements revealed a noontime O3 build-up (~3–7 ppbv), a feature typically absent over high altitude mountains. WRF-Chem suggests that this feature is driven by photochemical production (NOx-VOC transition regime for higher O3) and entrainment. Despite partial compensation of nighttime loss by noontime build-up, the region acts as a net chemical sink. Seasonal cycle peaks in June (~64±6 ppbv; touching air quality threshold), unlike typical premonsoon peaks over Gangatic and Himalayan regions. CAM4-Chem simulation with CAMS-derived stratospheric contribution, captured the O3 seasonality, but overestimated the wintertime levels by 10–16 ppbv. Tagged O3-tracer analysis shows significant contributions of anthropogenic NOx emissions from South Asia (15–18 %) and other regions (32–37 %) to O3 levels. Additionally, stratospheric inputs and biogenic emissions shape the seasonal cycle. This study provides insight into the chemistry and dynamics governing O3 over the Trans-Himalaya across different time scales. These findings are invaluable for designing future field observations, model improvements, and policy planning for this region.