# PM2.5 Pollution Decrease in Paris, France, for the 2013–2024 Period: An Evaluation of the Local Source Contributions by Subtracting the Effect of Wind Speed

## TL;DR

This study shows that wind speed significantly affects PM2.5 levels in Paris, and local emissions have decreased by about 4% per year from 2013 to 2024.

## Contribution

A novel method to estimate local PM2.5 emissions by subtracting the effect of wind speed is introduced.

## Key findings

- PM2.5 levels decrease with increasing wind speed up to 6 m·s−1, after which they remain stable.
- Local emission sources in Paris contributed to a 4% annual decrease in PM2.5 levels from 2013 to 2024.
- The method isolates background pollution and aligns with WHO recommendations for PM2.5 levels.

## Abstract

What are the main findings:
A direct link is highlighted between PM2.5 mass-concentrations and wind speed in Paris, France.The PM2.5 levels are decreasing with increasing wind speed, up to an inflection point at 6 m·s−1 from which the PM2.5 levels remain almost constant.

A direct link is highlighted between PM2.5 mass-concentrations and wind speed in Paris, France.

The PM2.5 levels are decreasing with increasing wind speed, up to an inflection point at 6 m·s−1 from which the PM2.5 levels remain almost constant.

What is the implication of the main finding:
The trend for PM2.5 mass-concentration decrease can be estimated by subtracting the effect of wind speed.The PM2.5 background contribution due to local emission sources can be estimated to be of around 4% per year in Paris.

The trend for PM2.5 mass-concentration decrease can be estimated by subtracting the effect of wind speed.

The PM2.5 background contribution due to local emission sources can be estimated to be of around 4% per year in Paris.

Measuring the long-term trend of PM2.5 mass-concentration in urban environments is essential as it has a direct impact on human health. PM2.5 levels depend not only on the intensity of local emission sources and on imported pollution, but also on meteorological conditions (e.g., anticyclonic versus windy conditions), which leads to yearly variations in mean PM2.5 values. Two datasets available for Paris, France, are considered: measurements from Airparif air quality agency network and from the Pollutrack network of mobile car-based sensors. Also, meteorological parameters coming from ERA5 analysis (ECMWF) are considered. Annual values are calculated using three different statistical methods, which yield different results. For the 2013–2024 period, a clear relationship between wind speed and PM2.5 mass-concentration levels is established. The results show a linear decrease in both concentration and standard deviation for wind speeds in the 0–6 m·s−1 range, followed by nearly stable values for wind speed above 6 m·s−1. This behavior is explained by the dispersive effect of strong winds on air pollution. Under such conditions, which occur about 10% of the time in Paris, the contribution of persistent background sources can be isolated. Using the 6 m·s−1 threshold, the average annual linear decrease in emissions from local sources is estimated at 4.1 and 4.3% per year for the Airparif and Pollutrack data, respectively. Since 2023, the annual background value attributed to emission has been close to 5 µg·m−3, in agreement with WHO recommendations. This approach could be used to monitor the effects of regulations on traffic and heating emissions and could be applied to other cities for estimating background pollution levels. Finally, future studies should therefore prioritize number concentrations and size distributions, rather than mass-concentrations.