Articles written in Journal of Earth System Science

    • Variations of trace gases over the Bay of Bengal during the summer monsoon

      I A Girach Narendra Ojha Prabha R Nair Yogesh K Tiwari K Ravi Kumar

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      In situ measurements of near-surface ozone (O₃), carbon monoxide (CO), and methane (CH₄) were carried out over the Bay of Bengal (BoB) as a part of the Continental Tropical Convergence Zone (CTCZ) campaign during the summer monsoon season of 2009. O₃, CO and CH₄ mixing ratios varied in the ranges of 8–54 ppbv, 50–200 ppbv and 1.57–2.15 ppmv, respectively during 16 July–17 August 2009. The spatial distribution of mean tropospheric O₃ from satellite retrievals is found to be similar to that in surfaceO₃ observations, with higher levels over coastal and northern BoB as compared to central BoB. The comparison of in situ measurements with the Monitoring Atmospheric Composition & Climate (MACC) global reanalysis shows that MACC simulations reproduce the observations with small mean biases of1.6 ppbv, –2.6 ppbv and 0.07 ppmv for O₃, CO and CH4, respectively. The analysis of diurnal variation of O₃ based on observations and the simulations from Weather Research and Forecasting coupled with Chemistry (WRF-Chem) at a stationary point over the BoB did not show a net photochemical build up during daytime. Satellite retrievals show limitations in capturing CH₄ variations as measured by in situ sample analysis highlighting the need of more shipborne in situ measurements of trace gases over thisregion during monsoon.

    • Evaluation of ambient air quality in Dehradun city during 2011–2014

      Amar Deep Chhavi P Pandey Hemwati Nandan Purohit K D Narendra Singh Jaydeep Singh Srivastava A K Narendra Ojha

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      The variations in the ambient concentrations of particulate matter (SPM and PM$_{10}$) and gaseous pollutants (SO$_{2}$ and NO$_{2}$) at Clock tower (CT), Rajpur road (RR) and Inter State Bus Terminal (ISBT) station in Dehradun city, Uttarakhand, India are analysed for the period of 2011–2014. Mean concentrations are observed to be higher during pre-monsoon season as compared to the winter and monsoon. PM$_{10}$ and SPM concentrations with maximum values of 203 $\pm$ 23 and 429 $\pm$ 49 $\mu$g m$^{-3}$, respectively, during winter, are found to exceed the national standards by factors of 2 and 3. Winter-time elevated pollution in Dehradun is attributed to the lower ventilation coefficient (derived from Era interim model fields) and minimal precipitation. Nevertheless, the SO$_{2}$ and NO$_{2}$ levels are observed to be within the criteria notified by the Central Pollution Control Board (CPCB), India. Correlation analysis shows profound impacts of the meteorology and local dynamics on the observed variations in observed trace species. Additionally, the stronger inter-species correlation variations (r=0.79 for SO$_{2}$ with NO$_{2}$, and r=0.89 for PM$_{10}$ with SPM), which may suggest their origin from common sources. Analysis of ‘Air Quality Index (AQI)’ variations indicates unhealthy atmospheric conditions near the major city centers and bus station. More observations in the region are highly desirable to understand the dispersion of the enhanced pollution in the Dehradun valley.

    • Ozone chemistry and dynamics at a tropical coastal site impacted by the COVID-19 lockdown


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      The nationwide lockdown in India to curb the spread of Coronavirus disease 2019 (COVID-19) led to colossal reduction in anthropogenic emissions. Here, we investigated the impact of lockdown on surface ozone (O$_{3}$) and nitrogen dioxide (NO$_{2}$) over a tropical coastal station – Thumba, Thiruvananthapuram (8.5$^{\circ}$N, 76.9$^{\circ}$E). Daytime as well as night-time NO$_{2}$ showed reduction by 0.8 (40%) and 2.3 (35%) ppbv, respectively during the lockdown period of 25–30 March 2020 as compared with the same period of previous 3 years. Unlike many urban locations, daytime surface O$_{3}$ is found to be dramatically reduced by 15 ppbv (36%) with O$_{3}$ production rate being lower by a factor of 3 during the lockdown. Interestingly, a feature of O$_{3}$-hump during the onset of land breeze typically observed during 1997–1998 has reappeared with magnitude of 5–10 ppbv. A photochemical box model, capturing this feature, revealed that significant O$_{3}$ sustained till onset of land breeze over the land due to weaker titration with NO$_{x}$ during lockdown. It is suggested that the transport of this O$_{3}$ rich air with onset of land breeze led to the observed hump. Our measurements unravel a remarkable impact of the COVID-19 lockdown on the chemistry and dynamics of O3 over this tropical coastal environment.

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