key: cord-1024909-oi8iuiky
authors: Girach, Imran A; Ojha, Narendra; Babu, S Suresh
title: Ozone chemistry and dynamics at a tropical coastal site impacted by the COVID-19 lockdown
date: 2021-08-09
journal: J Earth Syst Sci
DOI: 10.1007/s12040-021-01666-3
sha: 6f5f30d93d4b48814053aeba5d0ef360d77f3b7e
doc_id: 1024909
cord_uid: oi8iuiky

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°N, 76.9°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 O(3) over this tropical coastal environment. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12040-021-01666-3.

After the outbreak of Coronavirus disease 2019 pandemic, lockdowns were imposed in various countries to minimize the spread. The lockdowns have been shown to drastically reduce the anthropogenic emissions improving the air quality significantly (Bauwens et al. 2020; Le et al. 2020) . Large reductions in nitrogen dioxide (NO 2 ) over major cities have been observed worldwide (Dantas et al. 2020; Liu et al. 2020) . Globally, the reduction in tropospheric O 3 burden by about 6 Tg (*2%) is suggested to have altered the radiative forcing significantly (Miyazaki et al. 2020) . The major source of NO 2 is fossil fuel burning in vehicles, industries, power generators, etc. NO 2 is one of the major air pollutants and also the main precursor of ozone (O 3 ; Seinfeld and Pandis 1998) . O 3 plays a key role in tropospheric chemistry as the major source of hydroxyl radical (OH), which is known as detergent for removal of many pollutants from the atmosphere. O 3 also controls the oxidation capacity of the troposphere, besides its role as a greenhouse gas. O 3 is not directly emitted in the atmosphere, rather it is formed through photochemical reactions, involving the precursor gases such as nitrogen oxides (NO x = NO + NO 2 ), CO, and hydrocarbons (Seinfeld and Pandis 1998) . Two competing chemical processes, O 3 production via photolysis of NO 2 vs. O 3 destruction by NO, govern O 3 variability in urban and rural environments. While higher O 3 levels during daytime are due to dominance of photochemical production, the titration reaction dominates during night-time causing lower O 3 levels (Seinfeld and Pandis 1998) . Nevertheless, the dependence of O 3 on the concentrations of the precursors is complex and nonlinear. Therefore, accurate measurements over distinct environment are necessary to characterise the chemical regimes and to understand role of various atmospheric processes.

A nationwide lockdown was also imposed by the Government of India starting since 25 March 2020 (https://mha.gov.in/sites/default/files/PR˙Nation alLockdown˙26032020˙0.pdf). A 14-hr voluntary 'Janata (i.e., public) curfew' was also observed on 22 March 2020. The lockdown was implemented in a phased manner, during phase-1 (25 March-14 April 2020) and phase-2 (15 April-03 May 2020), the imposed guidelines restricted movements of all vehicles, industrial activities, and other services, except the essential and emergency services. Thus, anthropogenic emissions of trace gases and aerosols from various sources such as vehicles, industries, etc., largely came to a halt. Since phase-3 (04 May 2020 onwards), certain relaxations were implemented which included vehicular movements and a few other anthropogenic activities. However, lockdown restrictions remained stringent over the hotspot regions (https://www.mha.gov.in/notifications/circularscovid-19). Recently, a few studies have reported the reduction of air pollutants near source regions over megacities in India (Kumari and Toshniwal 2020; Mahato et al. 2020; Sharma et al. 2020) and synoptic changes over the Indian region (Shehzad et al. 2020) . Black carbon concentrations showed reductions in the range of 16-60% with respect to climatological mean over the Indian region (Gogoi et al. 2021) . Observations from 134 monitoring sites of Central Pollution Control Board across India showed reduction of *40-70% in PM 2.5 , *40-60% in PM 10 ; *30-70% in NO 2 ; and *20-40% in CO, but the changes in surface O 3 were observed to be site-speciBc (Dhaka et al. 2020; Singh et al. 2020; Soni et al. 2021 ). However, O 3 chemistry over tropical coastal environments away from strong sources but impacted by land-sea breeze systems, has not yet been explored. In this regard, we have investigated the changes in surface O 3 as well as NO 2 over a tropical coastal station by analysing in-situ measurements during the Brst week of lockdown (25-30 March 2020). In addition, measurements available over India's capital, Delhi (28.7°N, 77.1°E) were used to infer the heterogeneity in surface O 3 chemistry.

The observational site Thumba, Thiruvananthapuram (8.5°N, 76.9°E, *3 m amsl), is situated on the southwest coast of India, approximately 500 m away from the Arabian Sea coast. This tropical coastal site experiences typical mesoscale circulation comprising of sea-breeze (SB) during daytime and land-breeze (LB) during night-time almost throughout the year (Narayanan 1967). Thiruvananthapuram district has a population of 3.3 million as per census 2011.

In-situ measurements of surface O 3 and NO x were performed using ultraviolet (UV) photometric O 3 analyser (model 49 C) and chemiluminescence NO x analyser (Model 42i) of Thermo Electron Corporation, USA, respectively. Based on absorption of UV radiation at *253.7 nm by O 3 molecules, O 3 mixing ratios in sample air were derived using the Beer-Lambert law. NO x analyser works on the principle of chemiluminescence property of NO 2 molecules. The oxidation of NO by O 3 molecules forms excited NO 2 molecules. The measurement of luminous radiation emitted by these excited NO 2 molecules is a measure of NO concentration. NO 2 is measured by converting it into NO using molybdenum converter under high temperature. Sample air was drawn from *3 m above the ground using a TeCon tube and pre-conditioned air (dehumidiBed and Bltered for dust particles) was fed to the analysers for these measurements. O 3 analyser was calibrated with an in-built O 3 generator and a zero air generator. The uncertainty in O 3 mixing ratio, lower detection limit, and linearity are ±5%, 1 ppbv and 1%, respectively. NO x analyser was calibrated using standards traceable to the National Institute of Standards and Technology diluted with custom-made calibrator. Lower detection limit, linearity, and uncertainty for NO x measurements are 0.05 ppbv, 1% and ±5%, respectively (Tanimoto et al. 2007 ). Data were recorded at 5 min intervals. Further details of instruments as well as site description can be found elsewhere (Girach et al. 2012; Nair et al. 2018) . The observations during 25-30 March 2020 (6 days), besides observations during pre-lockdown period and previous years are used in the present study.

In addition, surface wind speed and direction used in the study were measured using the automatic weather station (AWS), Dynalab Weathertech Pvt. Ltd, India, having accuracy of ±0.5 ms À1 and ±3°, respectively.

For the purpose of comparison of O 3 changes, measurements at 14 different sites across Delhi (28.7°N; 77.1°E) made under the National Air Quality Monitoring Programme (NAMP) by Central Pollution Control Board (CPCB) under Ministry of Environment, Forest and Climate Change (MoEFCC) of India are also used. Hourly averaged O 3 data falling beyond 3-sigma standard deviations were Bltered prior to estimation of daily averages. Details on measurements and Bltering criteria can be found in the CPCB reports (CPCB 2011) and previous studies (Schnell et al. 2018) .

Diurnal patterns of surface O 3 is characterised by higher mixing ratios during daytime and lower mixing ratios during night-time, opposite to NO 2 diurnal variations, over Thumba. These variations in O 3 and NO 2 are significantly inCuenced by photochemistry, SB and LB activity, besides the impact of boundary layer changes (Nair et al. 2002; David and Nair 2011; Girach et al. 2012) . Daytime (12-17 hrs) and night-time (23-06 hrs) average NO 2 variations are shown in Bgure 1 during 19-30 March 2020 and during the same period of previous 3 years (during 2016-2019). Generally, night-time NO 2 level is higher than that of daytime (Bgure 1) due to absence of NO 2 photolysis, titration of O 3 with NO producing NO 2 , shallow boundary layer height limiting the vertical mixing, and prevalence of LB transporting NO x rich air to the study site Girach et al. 2012) . Daytime NO 2 during Janata curfew day showed some reduction, and daytime NO 2 value of the previous day is comparable to that of the mean value of this day of previous years. However, reduction in nighttime NO 2 was beyond 1-sigma variability due to reduced emission during daytime under the curfew restrictions. A quick response to reduction in emission is mainly due to a shorter lifetime of NO 2 (2-8 hrs; Liu et al. 2016) . Although unexpected, NO 2 remained lower on subsequent day. Daytime and night-time NO 2 during lockdown period of 25-30 March 2020 were observed to be significantly lower (beyond 1-sigma standard deviation) with respect to mean values in previous years. Average values during this period are shown by horizontal dashed lines in Bgure 1. While NO 2 was reduced by 0.8 ppbv (40%) during daytime, reduction during night-time is seen to be 2.3 ppbv (35%). Larger reduction in absolute magnitude of night-time NO 2 is expected as LB transports the relatively polluted airmass from the land. Since night-time NO 2 levels are associated with daytime O 3 (David and Nair 2011), impact on daytime O 3 is expected to be significant as discussed subsequently. The observations from Tropospheric Monitoring Instrument (TROPOMI) aboard Sentinel-5 Precursor satellite showed 1.0910 15 ± 3.0910 14 molecules cm À2 tropospheric NO 2 during 24-30 March 2020 over the study location, which was lower by *40% as compared to the level (1.7910 15 ± 1.3910 14 molecules cm À2 ) during same period of previous year. This reduction was also beyond the 1-sigma standard deviation and in line with the percentage reduction seen in daytime surface NO 2 . Figure 2(b) shows the mean diurnal variations of O 3 where suppression in O 3 production is clearly evident during lockdown period. Considering the O 3 during morning hours (07-10 hrs), estimated slope from linear regression analysis is a measure of O 3 production rate. O 3 production rate which was 9.8 ppbv h À1 during previous years is found to be reduced to 3.3 ppbv h À1 , i.e., by a factor of 3 during the lockdown. Figure 2 (c) shows the mean diurnal variations of NO 2 where eAective suppression of fumigation peak during morning hours is clearly evident, besides lower values during day and night. Since O 3 production rate depends on precursor concentrations, lower levels of NO 2 (and other precursors) in the morning hours (Bgure 2c) could result in reduced production rate and lower daytime O 3 . It is worth to mention here that NO 2 dispersed over the sea during LB (night-time) gets recirculated, after chemical losses, during SB which prevails during daytime over the site Lower NO x during night could have enhanced O 3 by reducing its titration with NO during nighttime (23-06 hrs). However, since O 3 level itself was reduced during daytime, inCuence of reduced titration is compensated and significant change in night-time O 3 was not observed. However, the slower evening loss in O 3 was observed as discussed subsequently.

SB component (SBC) is calculated as [wind speed 9 sin(325°-wind direction)] as the coastline of Thiruvananthapuram is aligned along *145-325°. Figure 3 shows diurnal variations of O 3 for three individual days (25-27 March 2020) along with SB component. SBC is used to demarcate SB and LB, as positive and negative values denote SB and LB, respectively. Crossing from SB to LB denote the onset of LB as marked by red dashed line in Bgure 3. In normal conditions, it has already been illustrated that O 3 remains high till the onset of LB Girach et al. 2012) . O 3 started decreasing with reduction in solar radiation (sunset time is marked with black dashed line in Bgure 3) . Interestingly, a hump of 5-10 ppbv around the LB onset is observed over three days. This feature was observed consistently during 1997-1998 over the site as reported by Nair et al. (2002) . However, it has not been observed regularly in the recent years, as reported by David and Nair (2011) .

In order to explain the O 3 -hump with onset of LB, we have performed simulations using a photochemical box model. We used Master Mechanism modelversion 2.5, developed at National Center for Atmospheric Research (NCAR), which simulates chemical evolution of air parcel with time considering the detailed photochemistry in absence of dynamics or further emissions. The detailed description of the model can be found elsewhere (Madronich 2006; Nair et al. 2011; Ojha et al. 2012; Soni et al. 2021) . The environmental conditions such as latitude, longitude, date, parcel elevation, total column ozone, SO 2 column, NO 2 column, aerosol optical depth, single scattering albedo, aerosol angstrom coefBcient, temperature, air density were set to typical values of 8.5°N, 76.9°E, 26 March 2020, 10 m, 280 DU, 0.12 DU, 0.11 DU, 0.50, 0.82, 1.2, 300 K, 2.45910 19 molecules cm À3 , respectively as obtained from Ozone Monitoring Instrument (https://disc.gsfc.nasa.gov/), groundbased observations at the study site and literature (Babu et al. 2007; David and Nair 2011) . The initial/background concentrations of carbon monoxide, methane, isoprene, formaldehyde, ethane, ethene, propane, propene, n-butane, and i-butane were included as 300, 2000, 1, 0.7, 2.5, 5, 1.5, 3, 5, and 0.1 ppbv, respectively, which represent the typical coastal/rural conditions. The background value of O 3 has been set as free tropospheric value of 40 ppbv (Ajayakumar et al. 2019) . The crucial diurnal constraints used in the simulation were the observed nitric oxide (NO) and NO 2 (supplementary Bgure S2). In order to account for the overestimation in the measurement of NO 2 due to molybdenum convertor (Winer et al. 1974) , observed NO 2 were used in the simulation after giving an oAset of 35% based on sensitivity simulations. In addition, variation of boundary layer height and entrainment velocity were assumed to their typical values of 0.2-1.5 km and 0.05-20 cm s À1 , respectively (Bremaud et al. 1998; Girach et al. 2012) .

We performed Brst simulation for airmasses sampled at Thumba site based on above-mentioned model conBguration. As SB airmass travels over the land during daytime, it gets enriched by precursors (i.e., NO 2 ) due to, for instance, vehicular emission. Thus, by giving a small increment of 35% in NO 2 , we performed another simulation for airmasses over land. As Thumba site experiences airmasses from land during night-time, we have combined the two simulations in Bgure 4. The model reproduced the observed diurnal feature of O 3 , including the O 3 -hump around onset of LB. However, magnitude of simulated O 3 -hump is *5 ppbv, smaller than the observed value, which could be partly due to discount of transport. Diurnal variation of O 3 simulated for the airmass over land showed higher levels of O 3 during daytime, which is in agreement with the observations at an inland station Kariavattom (8.56°N; 76.89°E, *4 km from the coast), which is close to Thumba (supplementary Bgures S3 and S4). Significant O 3 level sustained in the evening hours due to lower NO caused by reduced emission under lockdown. This slow losses of O 3 after sunset, results in significant O 3 even in evening hours, similar to that over a rural site in India . Thus, in nutshell, this feature of O 3 -hump is attributed to the weaker O 3 titration with NO over land during lockdown. This allowed a significant amount of O 3 to sustain over land and get transported to the observational site with the onset of LB. It is worth mentioning that under normal condition (unlike lockdown), higher NO in the evening hours titrates O 3 eAectively after sunset and O 3 levels in the LB are much lower and hence O 3hump is not seen under normal condition. The above mechanism would also explain the O 3 -hump reported earlier (Nair et al. 2002) due to lower NO x at that time (Nair et al. 2018) .

To compare the O 3 behaviour in a contrasting environment, we utilised the O 3 observations over NO x -rich environment, Delhi. Figure 5 shows the daytime O 3 averaged over 14 monitoring stations across Delhi during lockdown period of 25 March-04 April 2020. After the implementation of lockdown surface O 3 showed reduction by *25 ppbv; however, after a few days, O 3 started increasing slowly converging to reference curve of 2019 afterwards. The increase in O 3 across Delhi (Mahato et al. 2020 ) and over another urban location, Ahmedabad (Soni et al. 2021) was mainly attributed to stronger diminish of NO which titrates O 3 , non-linear chemistry and changes in meteorology. Thus, overall daytime O 3 reduction is insignificant over Delhi specifically few days after lockdown. This feature shows the nonlinear and complex O 3 chemistry for urban locations where plenty of volatile organic compounds are present besides heterogeneous chemistry under significant aerosol loading. This also points to the contrasting behaviour of chemistry and dynamics of O 3 as observed over a coastal station.

Present study highlights the impact of lockdown on surface O 3 chemistry over a tropical coastal station, Thiruvananthapuram, during Brst week of lockdown (25-30 March 2020). O 3 precursor, NO 2 reduced significantly (2.3 and 0.8 ppbv during night-time and daytime, respectively) which led to suppression of observed O 3 production rate by a factor of 3. Suppression of fumigation peak in NO 2 contributed in lowering the O 3 production rate. Consequently, daytime (12-17 hrs) surface O 3 reduction of 15 ppbv (36%) was seen. Interestingly, surface O 3 showed a hump (5-10 ppbv) around the onset of land breeze. The O 3 -hump was reproduced by the simulations using a photochemical box model. Reappearance of this feature which was seen during 1997-1998, is due to a weaker titration slowing the evening time O 3 losses over the land and sustaining significant O 3 levels till the onset of land breeze. This study shows that the lockdown has inCuenced the O 3 chemistry and dynamics dramatically over this tropical coastal region. comments and suggestions from two anonymous reviewers are acknowledged.

Imran A Girach: Conceptualization, measurements, analysis, modelling, manuscript preparation; Narendra Ojha: analysis, visualisation, writing, review & editing; S Suresh Babu: guidance and supervision. 

Dynamical nature of tropospheric ozone over a tropical location in Peninsular India: Role of transport and water vapour

Temporal heterogeneity in aerosol characteristics and the resulting radiative impacts at a tropical coastal station -Part 2: Direct short wave radiative forcing

Impact of coronavirus outbreak on NO 2 pollution assessed using TROPOMI and OMI observations

Ozone nighttime recovery in the marine boundary layer: Measurement and simulation of the ozone diurnal cycle at Reunion Island

The impact of COVID-19 partial lockdown on the air quality of the city of Rio de Janeiro, Brazil

Diurnal and seasonal variability of surface ozone and NO x at a tropical coastal site: Association with mesoscale and synoptic meteorological conditions

diminution and haze events over Delhi during the COVID-19 lockdown period: An interplay between the baseline pollution and meteorology

The changes in near-surface ozone and precursors at two nearby tropical sites during annular solar eclipse of 15

Impact of lockdown measures during COVID-19 on air quality: A case study of India

Unexpected air pollution with marked emission reductions during the COVID-19 outbreak in China

NO x lifetimes and emissions of cities and power plants in polluted background estimated by satellite observations

Chemical evolution of gaseous air pollutants down-wind of tropical megacities: Mexico City case study

EAect of lockdown amid COVID-19 pandemic on air quality of the megacity Delhi

Global tropospheric ozone responses to reduced NOx emissions linked to the COVID-19 world-wide lockdowns

Decadal changes in surface ozone at the tropical station Thiruvananthapuram (8.542°N, 76.858°E), India: EAects of anthropogenic activities and meteorological variability

Temporal variations in surface ozone at Thumba (8.6°N, 77°E): A tropical coastal site in India

Ozone in the marine boundary layer of Bay of Bengal during postwinter period: Spatial pattern and role of meteorology

Surface ozone and precursor gases at Gadanki (13.5°N, 79.2°E), a tropical rural site in India

Variabilities in ozone at a semi-urban site in the Indo-Gangetic Plain region: Association with the meteorology and regional processes

A study on variation of atmospheric pollutants over Bhubaneswar during imposition of nationwide lockdown in India for the COVID-19 pandemic

Exploring the relationship between surface PM2.5 and meteorology in Northern India

Atmospheric chemistry and physics: From air pollution to climate change

EAect of restricted emissions during COVID-19 on air quality in India

The impact of COVID-19 as a necessary evil on air pollution in India during the lockdown

Diurnal and temporal changes in air pollution during COVID-19 strict lockdown over different regions of India

Impact of COVID-19 lockdown on surface ozone build-up at an urban site in western India based on photochemical box modelling

Direct assessment of international consistency of standards for ground-level ozone: Strategy and implementation toward metrological traceability network in Asia

Response of commercial chemiluminescent nitric oxidenitrogen dioxide analyzers to other nitrogen-containing compounds

We gratefully acknowledge the Central Pollution Control Board, Ministry of Environment, Forest and Climate Change (MoEFCC) for the groundbased measurement of O 3 over Delhi obtained from website, https://app.cpcbccr.com/ccr/#/caaqmdashboard/caaqm-landing. We thank S Madronich and co-workers for freely available Master Mechanism model through the website of NCAR ACOM. We also acknowledge the use of tropospheric NO 2 column data from the TROPOMI as obtained from https://disc.gsfc.nasa.gov/datasets? keywords=tropomi%20no2&page=1. Constructive