key: cord-0860837-h1mb11n1
authors: Mor, Suman; Kumar, Sahil; Singh, Tanbir; Dogra, Sushil; Pandey, Vivek; Ravindra, Khaiwal
title: Impact of COVID-19 lockdown on air quality in Chandigarh, India: Understanding the emission sources during controlled anthropogenic activities
date: 2020-08-18
journal: Chemosphere
DOI: 10.1016/j.chemosphere.2020.127978
sha: e3c64e982187fc7d41bd555c9aae1898faf09729
doc_id: 860837
cord_uid: h1mb11n1

The variation in ambient air quality during COVID-19 lockdown was studied in Chandigarh, located in the Indo-Gangetic plain of India. Total 14 air pollutants, including particulate matter (PM(10), PM(2.5)), trace gases (NO(2), NO, NO(x), SO(2), O(3), NH(3), CO) and VOC's (benzene, toluene, o-xylene, m,p-xylene, ethylbenzene) were examined along with meteorological parameters. The study duration was divided into four parts, i.e., a) 21 days of before lockdown b) 21 days of the first phase of lockdown c) 19 days of the second phase of lockdown d) 14 days of the third phase of lockdown. The results showed significant reductions during the first and second phases for all pollutants. However, concentrations increased during the third phase. The concentrations of SO(2), O(3,) and m,p-xylene kept on increasing throughout the study period, except for benzene, which continuously decreased. The percentage decrease in the concentrations during consecutive periods of lockdown were 28.8 %, 23.4 % and 1.1 % for PM(2.5) and 36.8 %, 22.8 % and 2.4 % for PM(10) respectively. The Principal Component Analysis (PCA) and characteristic ratios identified vehicular pollution as a primary source during different phases of lockdown. During the lockdown, residential sources showed a significant adverse impact on the air quality of the city. Regional atmospheric transfer of pollutants from coal-burning and stubble burning were identified as secondary sources of air pollution. The findings of the study offer the potential to plan air pollution reduction strategies in the extreme pollution episodes such as during crop residue burning period over Indo-Gangetic plain.

increasing throughout the study period, except for benzene, which continuously decreased. 23

The percentage decrease in the concentrations during consecutive periods of lockdown were 24 28.8 %, 23.4 % and 1.1 % for PM 2.5 and 36.8 %, 22.8 % and 2.4 % for PM 10 respectively. 25

The Principal Component Analysis (PCA) and characteristic ratios identified vehicular 26 pollution as a primary source during different phases of lockdown. During the lockdown, 27 residential sources showed a significant adverse impact on the air quality of the city. 28

Regional atmospheric transfer of pollutants from coal-burning and stubble burning were 29 identified as secondary sources of air pollution. The findings of the study offer the potential 30 to plan air pollution reduction strategies in the extreme pollution episodes such as during crop 31 lockdown. A decrease of 28.8 % and 36.8 % were seen for PM 2.5 and PM 10 during this phase 137 as compared to the pre-lockdown period. A major fraction of these reductions in PM 138 concentrations can be attributed to the massive decrease in the vehicular traffic, halting of 139 industries and stopping of all construction activities (chdcovid19, 2020) which are the 140 significant sources of the particulate matter in the urban ambient air. 141

During the second phase, the average PM 2.5 and PM 10 concentrations were recorded as 15.4 142 µg/m 3 and 43.9 µg/m 3, respectively. The levels increased by 7.7 % and 22.3 % for PM 2.5 and 143 PM 10, respectively, as compared to the first phase of lockdown. There was not much 144 difference in the regulations for human activities between the first and second phase of 145 lockdown. But the regulation for movement of residents for essential services was 146 implemented after the completion of few days of total lockdown during the first phase, while 147 in the second phase, this movement occurred daily throughout the stage of lockdown 148 (chdcovid19, 2020). This might be the primary reason for a little increase in emissions, which 149

can be attributed to automobiles and, consequently, which lead to higher concentrations in the 150 ambient air. 151

The third phase of lockdown was implemented with few relaxations like reopening of 152 government offices, private offices, and few essential industrial manufacturing units 153 (chdcovid19, 2020). The average concentrations of PM 2.5 and PM 10 were 19.8 µg/m 3 and 55.5 154 µg/m 3 respectively, during this phase of the lockdown period. A very slight decrease of 1.1 % 155 and 2.4 % for PM 2.5 and PM 10 was seen as compared to the before lockdown period. This can 156 be attributed to the emissions from increased vehicular traffic and emissions from industrial 157 units Lawrence et al., 2013; Sawyer, 2010) . 158

The 21 days average concentrations of NO 2 , NO, NO x, and NH 3 before lockdown were 13.9 161 µg/m 3 , 7.2 µg/m 3 , 13.0 ppbv, and 68.0 µg/m 3 respectively, which were well under the 162 national ambient air quality standards (NAAQS). During the first phase of lockdown period 163 concentrations of these gases, NO 2 , NO, NO x , NH 3 reduced to 10.7 µg/m 3 , 1.9 µg/m 3 , 7.0 164 ppbv, and 38.3 µg/m 3 respectively. NO 2 reduced by 23 %, whereas NO, NO x and NH 3 165 showed a significant decrease of 74.1 %, 46.4 %, and 43.7 %, respectively. During the second 166 phase of the lockdown, the average concentrations of these gases were recorded as 11.6 167 µg/m 3 , 2.4 µg/m 3 

The concentration of SO 2 kept on increasing from 9.9 µg/m 3 before lockdown period to 10.0 184 µg/m 3 during the first phase to 11.4 µg/m 3 during the second phase and to 11.8 µg/m 3 during 185 the third phase of lockdown (Table 1) during the study periods of before lockdown, the first phase, second  202 phase and third phase of lockdown respectively. This can be due to the following two 203 reasons: 1) Continuous increase in the intensity of solar radiation as the study period 204 proceeded, which helped to the continuous rise of photochemical reactions and O 3 formation. 205

2) The decrease in the concentration of NO during the lockdown period as NO helps in the 206 breakdown of O 3 to O 2 and comparatively less decrease in the concentrations of CO and 207

VOCs, which helps in the formation of O 3 in the ambient air. Diurnal analysis of PM concentrations before the lockdown period showed that PM 2.5 had its 240 peak concentrations during the midnight as 24.5 µg/m 3 at 1:00 a.m., while the lowest 241 concentration was found as 16.6 µg/m 3 at 4:00 p.m., The PM 2.5 concentrations remain high 242 around midnight followed by the peaks during day time between 10 a.m. to 1 p.m.. During 243 the first phase of lockdown, the peak was recorded as 20.7 µg/m 3 at 11:00 a.m., and lowest 244 concentrations was recorded as 11.5 µg/m 3 at 7:00 & 8:00 p.m. During the second phase, the 245 highest peaks for PM 2.5 were found at midnight time with a peak of 18.8 µg/m 3 at around 12 246 a.m. followed by a second peak during day time between 12 p.m. and 5 p.m., and the lowest 247 concentrations was recorded as 12.2 µg/m 3 at 9 a.m. The highest peaks for all the parts of the 248 study period were found during midnight except for the first phase of lockdown. The reason 249 for the accumulation of PM 2.5 during night time seems to associated with poor dispersion and 250 accumulation of pollutants due to lower mixing height, lower wind speed, and favorable 251 relative humidity, as also reported by (Lou et al., 2017). The peak of PM 2.5 during the first 252 phase of lockdown was observed around 11 p.m., whereas for subsequent lockdown phases 253 the PM 2.5 peaks were observed during evening time. These peaks during day time can be 254 attributed to the increase in automobile movement as relaxations were given to the residents 255 of the city for movement between 10 a.m. and 3 p.m. (chdcovid19, 2020). During the third 256 phase, again, the highest concentrations was 23.8 µg/m 3 at 12 a.m., followed by lower peaks 257 at 12 p.m. and 1 p.m. (Fig. 2) . 258

In the case of PM 10 , the peak was 71.7 µg/m 3 at 11:00 p.m., and the lowest levels were 259 recorded (43.2 µg/m 3 ) at 6:00 a.m. before the lockdown, and the second lowest peaks were 260 observed between 10 a.m. to 2 p.m. During the first phase of lockdown, the highest and 261 lowest concentrations were found as 44.4 µg/m 3 at 11:00 a.m. and 28.7 µg/m 3 6:00 a.m., 262 respectively. During the second phase of lockdown, the maximum concentration was found to 263 be 53.5 µg/m 3 at 12 p.m., and the minimum was 31.2 µg/m 3 at 7 a.m. In the third phase, the 264 highest concentration was 74.5 µg/m 3 at 12 p.m., and the lowest level was 43. while for the study period of before lockdown it was found as 54.4 µg/m 3 at 6 a.m. (Fig.3) . 285

The diurnal average of SO 2 for the study period before the lockdown showed two peaks of 286 concentration 12.1 µg/m 3 at 11:00 a.m. in the morning and 7:00 p.m. in the evening. The least 287 concentration for the same period was recorded at 6:00 a.m. in the morning as 6.9 µg/m 3 . The 288 analysis showed the maximum and minimum concentration during the first phase of the 289 lockdown period was 14.5 µg/m 3 at 9 p.m. and 7.8 µg/m 3 at 6:00 a.m., respectively. During 290 the second phase, the maximum and minimum concentrations were 16.2 µg/m 3 at 9 p.m. and 291 9 µg/m 3 at 2 p.m. During the third phase, the maximum and minimum concentrations were 292 recorded as 16.2 µg/m 3 at 8 a.m. and 8.4 µg/m 3 at 12 p.m. & 4 p.m., respectively (Fig. 3) . 293

Diurnal averages of carbon monoxide didn't show significant differences in the time of 294 highest and lowest concentrations throughout the study period except for maximum 295 concentration during the third phase of lockdown which was found to be 0.76 mg/m 3 at 6 296 a.m. In contrast, the maximum concentrations for the study period (before lockdown, first 297 phase and the second phase of lockdown) were found to be 0.79 mg/m 3 at 10 p.m. 0.56 298 J o u r n a l P r e -p r o o f mg/m 3 at 11 p.m. and 0.65 mg/m 3 at 10 p.m., respectively. The lowest average concentrations 299 were 0.39 mg/m 3 at 4 p.m., 0.33 mg/m 3 at 4 p.m., 0.36 mg/m 3 at 5 p.m. and 0.35 mg/m 3 at 4 300 p.m. for the study periods of before lockdown, the first phase, second phase and third phase 301 of lockdown respectively (Fig. 3) . 302

The Benzene also showed similar diurnal patterns across all phases of the study period with the 313 peak concentration during the midnight time and lower concentration during day time. This 314 pattern could be due to the higher dispersion of benzene during the daytime and accumulation 315 in the ambient air during midnight. The maximum average concentrations were found as 5.78 316 µg/m 3 at 10 p.m., 3.8 µg/m 3 at 10 p.m., 3.65 µg/m 3 at 12 a.m., and 3.7 at 8 p.m. for all the 317 consecutive parts of the study as the phases of lockdown progressed. The minimum 318 concentrations were 3.53 µg/m 3 at 4 p.m., 2.45 µg/m 3 at 3 p.m., 2.24 µg/m 3 at 2 p.m. and 319 1.31 µg/m 3 at 5 p.m. for the periods of before lockdown, the first phase, second phase and 320 third phase of lockdown respectively (Fig. 3) . 321

Rain scavenging is a very crucial phenomenon for the removal of several air pollutants 323 during the peaks of wind speed during the whole study period (Fig. 1 & Fig. 2 found somewhat similar relationships of relative humidity with particulate matter. During the 360 study period, relative humidity went on decreasing except few peaks, which were 361 accompanied by rainfall events (Fig. 2) . The average value of 75.3 % before the lockdown 362 study period decreased to the average value of 59.7 % during the first phase, 52.1 % during 363 the second phase, and 50.1 % during the third phase of lockdown. The relative humidity 364 remained in the range favorable to the PM 2.5 accumulations throughout the study period 365 except for few peaks during the before lockdown period. This also partly explains the lower 366 variations in the PM 2.5 concentrations during the study period. By comparing the daily 367 average relative humidity and daily average PM 10 , SO 2, and NO 2 concentration, it was found 368 that these pollutants showed a lot of fluctuation throughout the study period, which signifies 369 rapid reductions and rapid accumulations due to addition and reduction of pollutant loads 370 from sources. (Table 4) , Characteristic Ratios (Table 3) 

Correlations and HYSPLIT backwind trajectories (Fig. 4) were applied to identify the 385 possible sources of pollutants before and during the lockdown period in Chandigarh. 386

To identify the sources of air pollutants, PCA was applied to the dataset using SPSS software. showed 23.5 % variance with higher factor loadings for benzene, SO 2 , PM 2.5, and PM 10, 420 which can be attributed to the emissions from power plants burning coal (Wang et al., 2013) . 421

Factor 3 showed an 11.7 % variance with a significant factor loading for NH 3 . The source of 422 NH 3 can be the agricultural and rural residential sources in the vicinity of the city. 423

During the third phase, Factor 1 showed a 41.6 % variance with higher factor loadings for 424 ethylbenzene, NO, o-xylene, toluene, benzene, m,p-xylene, and NO x . Again, the source of 425 these emissions can either be fuel burning in vehicles or regional transport of these pollutants 426 occurring due to stubble burning from upwind areas or can be the combination of both 427 sources. Factor 2 explained a 27 % variance with significant factor loadings for PM 2.5 , PM 10 (Table 3) , which also indicates the regional transport of 480 VOCs and aged air masses. This shows regional transport of VOCs from coal-burning 481 thermal power plants and biomass burning in the nearby upwind states were also contributing 482 to the city's VOC pollution load (Fig. 4, Ravindra et al., 2020) . coming from residential areas also adversely affect the air quality of the city. This finding 527

gives vital evidence that local household air pollution also affects the overall air quality of 528

Chandigarh. The study helps to identify and understand the possible sources of air pollution 529 during controlled anthropogenic activities. Though, to assign specific sources contribution, a 530

proper source apportionment study is recommended. However, in the absence of a particular 531 source apportionment study, the finding of the current study will be useful to plan proper 532 policy interventions to reduce the air pollution originating from local and regional sources. 533

The authors would like to thank CPCC and HCWH for financial support for carrying out 535 

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April. PM 10 , toluene, and CO also showed their second-highest peaks on 6 th and 5 th April. 490 NH 3 and NO showed their highest peaks on 7 th April. All these peak concentrations can be 491 (Table 3 ) also indicated the regional transport of aged air parcels to the study area. 505The third phase of lockdown introduced many relaxations in the city, like reopening of the 506 government, commercial offices, and essential manufacturing industrial units. The 507 concentrations during this phase increased as compared to the other stages of lockdown. They 508 were slightly less than the before lockdown period for almost all pollutants except for 509 nitrogen oxides and NH 3 . The reason for lower concentrations of trace gases seems to be the 510 enhanced photochemical reactions with increased solar radiation and the formation of 511 secondary compounds like O 3 , ammonium sulfate, and ammonium nitrate.