Effect of drastic reduction in NOx emissions on particle pollution in the Paris area.

As in most of European countries, a nation-wide lockdown of activities and transport occurred in France (from 17 March to 11 May 2020) due to the Covid-19 pandemia. This lockdown was enforced very strictly in the Paris area, which resulted in a 10-fold reduction in traffic (average 5% traffic congestion against 50% on normal weekdays) during 8 weeks. This situation led to a reduction of NOx emissions of about 70% in the Paris area as estimated by the local air quality network AIRPARIF.
In late winter - early spring, fine aerosols  (PM1/PM2.5) in North Western Europe typically include a significant fraction of ammonium nitrate, a secondary inorganic species which is notably influenced by gaseous precursor emissions from road transport and agricultural activities.
ACSM and aethalometer measurements operating at the ACTRIS SIRTA observatory platform - located in the southwestern suburb of Paris - are currently under investigation to quantify the effect of the strict lockdown on particle pollution. In a preliminary analysis, we compared mass concentrations of several species for the period 17 March - 26 April 2020, to previous year (2011-2019) records in March-April. Then, as a second step, for each day of the “17 March - 26 April 2020” period, we computed the change in concentration compared to 5-20 days in the 2011-2019 period that have analogue atmospheric circulation patterns (using 500-hPa geopotential height). The figure below shows the distribution of daily concentration changes (median, 25-75 percentile and min-max values) for 5 different components and for PM1.

Relative change in mass concentration (%) measured at SIRTA observatory for the period 17 March-26 April 2020 compared to March-April 2011-2019 values in analogue atmospheric circulation conditions.
Figure courtesy of Jean-Eudes Petit (LSCE/CEA-CNRS-UVSQ).

Our preliminary analysis shows that fossil-fuel related Black Carbon mass concentrations were reduced by 70% during the 2020 lockdown period compared to past data in similar meteorological conditions (analogue atmospheric circulation) and during the same period of the year. This result is consistent with the large traffic reduction in the Paris region and all over France. During the same time, a +15% increase in wood-burning related Black Carbon mass concentration was observed, likely related to increased presence of the population at home.
Moreover, a nitrate concentration reduction of about 50% is observed during this 2020 lockdown period, supporting a substantial effect of traffic-related NOx emission abatement on the formation of ammonium nitrate particles. Finally, a -15% overall change in PM1 mass concentration is observed, suggesting significant influences of other/remaining sources (e.g., residential heating) on the overall fine aerosol budget. The influence of other sources was also noticed on Volatile Organic Compound measurements.
Further analyses including the investigation of other compounds and variables measured at the SIRTA observatory are on-going to quantify the temporal evolution of these changes during and after the lockdown.