Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Covid-19 impact on air quality in megacities   Air pollution is among the highest contributors to mortality worldwide,
especially in urban areas. During spring 2020, many countries enacted social
distancing measures in order to slow down the ongoing Covid-19 pandemic. A
particularly drastic measure, the "lockdown", urged people to stay at home and
thereby prevent new Covid-19 infections. In turn, it also reduced traffic and
industrial activities. But how much did these lockdown measures improve air
quality in large cities, and are there differences in how air quality was
affected? Here, we analyse data from two megacities: London as an example for
Europe and Delhi as an example for Asia. We consider data during and before the
lockdown and compare these to a similar time period from 2019. Overall, we find
a reduction in almost all air pollutants with intriguing differences between
the two cities. In London, despite smaller average concentrations, we still
observe high-pollutant states and an increased tendency towards extreme events
(a higher kurtosis during lockdown). For Delhi, we observe a much stronger
decrease of pollution concentrations, including high pollution states. These
results could help to design rules to improve long-term air quality in
megacities.

24
The categories of this paper are: dg-ga
30
The categories of this paper are: q-alg
1
The categories of this paper are: cs
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
25
The categories of this paper are: patt-sol
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
4
The categories of this paper are: cond-mat
21
The categories of this paper are: adap-org
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th