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Mehmood, T.;  Hassan, M.A.;  Lodhi, E.;  Bilal, M.;  Dar, A.A.;  Liu, J. Impact of Lockdown on PM Charecterisitcs. Encyclopedia. Available online: (accessed on 29 November 2023).
Mehmood T,  Hassan MA,  Lodhi E,  Bilal M,  Dar AA,  Liu J. Impact of Lockdown on PM Charecterisitcs. Encyclopedia. Available at: Accessed November 29, 2023.
Mehmood, Tariq, Muhammad Azher Hassan, Ehtisham Lodhi, Muhammad Bilal, Afzal Ahmed Dar, Junjie Liu. "Impact of Lockdown on PM Charecterisitcs" Encyclopedia, (accessed November 29, 2023).
Mehmood, T.,  Hassan, M.A.,  Lodhi, E.,  Bilal, M.,  Dar, A.A., & Liu, J.(2022, October 25). Impact of Lockdown on PM Charecterisitcs. In Encyclopedia.
Mehmood, Tariq, et al. "Impact of Lockdown on PM Charecterisitcs." Encyclopedia. Web. 25 October, 2022.
Impact of Lockdown on PM Charecterisitcs

Air is a diverse mixture of gaseous and suspended solid particles. Several new substances are being added to the air daily, polluting it and causing human health effects. Particulate matter (PM) is the primary health concern among these air toxins. The World Health Organization (WHO) addressed the fact that particulate pollution affects human health more severely than other air pollutants. The spread of air pollution and viruses, two of the millennium’s most serious concerns, have been linked closely. 

COVID-19 health effects lockdown particulate matter (PM)

1. Inference of Lockdown on Emission Sources

The lockdown-based reduction of PM pollution showed complex phenomena, and many studies showed contradictory results. The lockdown decreased human activity by up to 90%, plus environmental emissions in Spain, the US, Italy, and Wuhan by nearly 30% [1]. Reducing economic activity increased air quality worldwide [2]. The change was dramatic in developed nations such as Europe and the US [3]. A reduction in NO2, SO2, and PM was observed during the lockdown process due to strict lockdowns in most affected countries. During the COVID-19 lockdown time, the concentrations decreased by more than half. However, the reduction achieved is not expected to be maintainable  [4].
According to some studies, the COVID-19 pandemic has raised emissions compared to last year [5][6]. However, a lower PM concentration in some Western European cities is less significant since the residential heating system was the main contributor to PM  [7]. There is also evidence that PM concentrations increased during the lockdown phase. This was attributed to increased domestic heating and industrial activity in peripheral regions and some areas of northeast China, thereby compensating for the disruption of manufacturing activities in major cities  [8]. PM concentration can also increase due to the long-term transport phenomena of PM from adjacent agricultural and industrial zones, as demonstrated in Brazil and Morocco  [9][10] Additionally, these studies indicate that traffic-related policy interventions are inadequate to resolve air quality issues, and other relevant departments must be taken into account  [8]. Furthermore, essential steps are needed concerning agricultural burning or the search for ideal sites for industrial activities.
Black carbon (BC) concentrations were higher all day in the pre-COVID stage than in other stages. Meanwhile, BC concentrations had few variations between lockdown, secondary, and tertiary reaction cycles, indicating a significant source of BC in Suzhou’s industrial processes. Persistent precipitation triggered the lower Spring Festival and tertiary response concentrations of BC. PM pollution builds rapidly at high levels under static weather situations and then experiences cross-border transportation processes, resulting in complex health and environmental consequences [11][12][13][14]. In addition, air pollution has dynamic relationships with widespread climate and weather [12][15][16]. Li et al. [17] found that during the COVID-19 in China’s Yangtze River Delta (YRD), human activities—industrial operations, travel vehicles, operating buildings, etc.—were substantially reduced, resulting in lower PM2.5 emissions of up to 27–46%.
A study described a higher concentration of organic carbon (OC) in PM1.8 and PM2.5 in winter compared to summer [18]. The researchers further explained that a colder and stable environment always favors newly formed organic substances condensing from vehicular emission [19][20]. In addition, an apparent seasonal change in PAHs has also been reported [18], with a higher and lower level in the winter and summer seasons, respectively. According to this research, more biomass burning occurs in winter, and the lower temperature favors less volatility and increases the gas conversion rate into PM-bound particles of PAHs.

2. Inference of Lockdown on the Primary and Secondary Formation of PM

Overall, there have been major reductions in PM formation in some cities of China [21] but no evidence of a decrease in PM concentration in European countries and the US  [3][7]. This is because non-transportation sources, which include domestic heating, biomass burning, and food cooking, contribute significantly to aerosol concentration in some contexts [3][7]. The concentrations of PM10 and PM2.5 were 36.5 and 35.9 μg/m3 in Suzhou during lockdown, lower than the pre-COVID concentrations of 37.2% and 38.3%, respectively, although the daily variance of PM during lockdown corresponded to its pre-COVID variance, irrespective of the substantial drop [22]. During the lockdown in many major cities worldwide, air pollutants decreased dramatically. Studies have shown that the lockdown syndrome attributed to COVID-19 has influenced the mechanism of primary and secondary particulate matter formation [22]. In addition, the findings show that travel restrictions have, in most cases, significantly decreased NO2 and CO contaminants directly connected with the transport sector [23][9][24].
On average, NO2 concentrations in Barcelona and Madrid exhibited a 50% and 62% decline in March 2020, respectively, compared to the 2019 results. However, these reductions have not been recorded in American cities like New York and Memphis [25][26]. It could also show that the pollution caused by traffic in these cities is small [25]. In comparison, many investigations of Brazilian, Chinese, and South Asian cities show that greening the transport sector will offer significant advantages in terms of air quality excellence [9][27][28]. Compared to changes in SO2, NO2,, and CO during the lockdown, O3 concentrations were significantly enhanced due to the sharp decrease in NOx. An increased concentration of O3 as an atmospheric oxidant can increase the formation rate of secondary organic and inorganic PM. Significant declines in transport NOx emissions during the lockdown were also reported by Huang et al. [29], which encouraged the production of secondary particulates caused by elevated ozone levels and night NO3 radical production during night lockdown. Officials should also be mindful that steps to reduce such contaminants, such as NO2 and PM, may raise the concentration of secondary pollutants, such as ground-level ozone, and trigger other health problems. However, more research is required to better identify the primary reaction mechanisms and the implications of other atmospheric influences [30].
These regulatory steps have significantly reduced primary emissions of PM, while secondary pollutant like ozone (O3) is still prominent [31]. In addition, many investigations have revealed that complex air pollution has come from primary industrial pollutants, traffic, heating processes, and power plants, while secondary pollutants are produced by complex chemical, biological, and physical processes [14][32][33][34][35][36].

3. Influence of Lockdown on the Composition of PM

Ambient PM consists of various biological and chemical components [37]. The chemical constituents of PM include minerals (metal oxides), secondary inorganic PM, rare earth metals, elemental carbon (EC), sea salt and organic matter, water-soluble ions, rare earth metals, organic constitutes (e.g., PAHs, OC, organic matter, and volatile organic compounds (VOCs)), inorganic constituents, inorganic secondary aerosol, marine salt, and trace elements [19]. From these PAHs, secondary inorganic species described as the main components of PM, such as nitrate, sulfate, ammonia, and carbonaceous species (OC, EC), are of great concern due to their toxicity and carcinogenicity [38]. Figure 1 shows different chemical and biological constituents of PM. The PM components with a biological origin are termed bioaerosols OC, and are included in a similar category in some studies [39][40]. These bacteria, pollens, and plant-related fragments are usually found in coarse PM [41]. However, some bacterial and fungal spores were also reported in fine PM[42]. These tend to attach to coarser particulate fractions.
Figure 1. Composition of the different chemical and biological components of PM.
Ambient PM contains diverse chemical elements such as carbonaceous, elemental, and organic substances. The individual concentration of these components forms 10 to 30% of the total mass of PM [43][44]. They are highly variable in concentration, depending upon source emission, meteorological conditions, and other factors [45]. The following are the main chemical species present in PM:
During the lockdown in several major cities, air pollutants decreased significantly. For example, PM2.5, PM10, and BC concentration in Suzhou was recorded at 37%, 38%, and 53% less during lockdown than in the pre-COVID period, respectively [22]; while in Wuhan, PM2.5 level decreased by 41% and PM10 by 33% [46]. In Delhi, during the lockdown phases, PM10, PM2.5, and BC decreased by about 52%, 53%, and 78%, respectively, compared to the pre-lockdown period [47][48]. In Washington, PM2.5 and BC concentrations decreased by 33% and 25% during lockdown implementations [49].
Concentrations of water-soluble ions (WSI) were 58.6% less than in the pre-COVID period. In addition, the PM2.5 and ion ratios showed the lowest lockdown values, up to 27.4%. This disparity demonstrated the significant changes during the lockdown in the chemical composition of PM2.5. Specifically, during the lockdown actions, as per Zheng et al. [50], primary emissions declined while secondary production of PM2.5 increased, resulting in less total mass concentration of PM2.5 and different chemical composition. According to Sun et al. [51], 25–46% of all gaseous species (NO2, SO2, and CO) were decreased, with a 30% to 50% reduction in aerosol form (fossil-fuel related PM, predominantly from coal combustion emissions, cooking-related organic PM, and biomass-burning organic aerosol) due to Chinese New Year. Through the lockdown period in Suzhou, the ionic arrangement, in order of concentration, was NO3 > NH4 > SO42− > Cl> Ca2+ > K > Mg2 > Na+; while during the pre-COVID phase, they were rearranged into NO3 > SO42− > NH4 > Cl > Ca2+ > K > Na+ > Mg2+. Compared with the pre-COVID ion levels, it was reported that ions NO3−, NH4+-, SO42−, Cl, Ca2+, K+, and Na+ dropped by 66.3, 48.8, 52.9, 57.9, and 76.3 in terms of percentage concentrations, respectively, in the lockdown period. At the same time, Mg2+ exhibited an increase of 30.2% [22]. Overall, compared to the pre-COVID time, during the lockdown in Suzhou, the PM10, PM2.5, BC, and WSIs decreased by 38.3, 37.2, 53.3, and 58.6%, respectively [22].
Furthermore, most research on air pollution “lockdown” focuses on “classic” contaminants such as NO2, CO, SO2, PM2.5, and PM10 [52][53][54][55]. Ivana et al. [56] reported a 35% decrease in NO2 and PM1, alongside a 26% decrease in total PAHs, near road traffic measuring sites. Only the concentration of NO2 decreased marginally at the residential measurement site; PM1 and PAHs levels were comparable to the previous year. Zhang et al. [57] found that PAH concentrations decreased 52.6%, 36.6%, and 36.7% from February to April of 2020 relative to the same time in the previous year. The changes in northern China are consistent with a decrease in SO2 and NO2 that grew during COVID-19 control and moderated a bit after the lockdown was lifted. In addition, the composition of PAHs in Kanazawa University Wajima Air Monitoring Station (KUWAMS) changed little before, during, and compared to previous years in the COVID-19 outbreak, indicating a stable source composition. These findings emphasize the importance of reducing the emission intensity in China for reducing PAH transport over long distances and pollution levels in downwind areas.

4. Influence of Lockdown on PM2.5− and PM10− Based Air Quality Index

Sarmadi and his colleagues [58]studied variations in the AQI during the first four months of each year (from 2018–2021), evaluating the AQI from 87 industrialized, polluted, and highly populated metropolises in 57 countries. Noticeably, of these 87 metropolises, 58 were capital cities, while the remainder were among the world’s top 100 heavily polluted and industrialized cities. The cities with the lowest PM2.5 and PM10 AQI values were Edmonton, Washington, Zurich, and Tallinn, with corresponding AQI of 0.10, 0.18, 2.31, and 3.98. Meanwhile, in 2020, the highest AQI levels were in Dhaka, Delhi, Ulaanbaatar, Seoul, and Jerusalem, with AQIs of 182.18, 106.36, 11.19, 26.86, and 36.62, respectively.
According to AQI, during the first quarter of each year, the AQI in 2020 improved significantly in most cities compared to pre-COVID (2019) time; however, most of the metropolises regained poor AQI scores in 2021. Similar trends were observed in other lockdown-impacted AQI assessment studies[59][60]. The greatest percentage decrease in PM2.5 and PM10 in 2020 compared to 2019 was seen in places such as Stockholm and Abu Dhabi (−40.05% and −40.13%), while the greatest increases were seen in Ankara and Buenos Aires (+37.97% and +16.95%, respectively). In countries with a +ve variance percentage, the AQI increased as well as dropped over time. Only 13% (7 of 55) and 25% (17 of 67) of cities with smaller AQI–PM10 and AQI–PM2.5 values in 2020 than in 2019 showed a declining trend in 2021, respectively, while AQI values rose in some other cities in 2021.
The mean AQI–PM2.5 in 2020 declined by 7% and 15%, respectively, when compared with 2019 and 2018, and the mean AQI–PM10 decreased by 18% and 24%, showing a better AQI, attributed to a decline in PM. It’s worth noting that those same stations measuring AQI may be situated near major roadways and airports, where PM levels are likely to be elevated [61].


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