Abstract. The Paris megacity experiences frequent particulate matter (i.e.PM2.5, particulate matter
with a diameter less than 2.5 µm) pollution episodes in spring
(March–April). At this time of the year, large numbers of the particles
consist of ammonium sulfate and nitrate which are formed from ammonia
(NH3) released during fertilizer spreading practices and transported
from the surrounding areas to Paris. There is still limited knowledge of the
emission sources around Paris, their magnitude, and their seasonality. Using space-borne NH3 observation records of 10 years (2008–2017) and
5 years (2013–2017) provided by the Infrared Atmospheric Sounding
Interferometer (IASI) and the Cross-Track Infrared Sounder (CrIS)
instrument, regional patterns of NH3 variabilities (seasonal and
interannual) are derived. Observations reveal identical high seasonal
variability with three major NH3 hotspots found from March to August.
The high interannual variability is discussed with respect to atmospheric
total precipitation and temperature. A detailed analysis of the seasonal cycle is performed using both IASI and
CrIS instrument data, together with outputs from the CHIMERE atmospheric
model. For 2014 and 2015, the CHIMERE model shows coefficients of
determination of 0.58 and 0.18 when compared to IASI and CrIS,
respectively. With respect to spatial variability, the CHIMERE monthly
NH3 concentrations in spring show a slight underrepresentation over
Belgium and the United Kingdom and an overrepresentation in agricultural areas
in the French Brittany–Pays de la Loire and Plateau du Jura region, as well
as in northern Switzerland. In addition, PM2.5 concentrations
derived from the CHIMERE model have been evaluated against surface
measurements from the Airparif network over Paris, with which agreement was
found (r2 = 0.56) with however an underestimation during spring
pollution events. Using HYSPLIT cluster analysis of back trajectories, we show that NH3
total columns measured in spring over Paris are enhanced when air masses
originate from the north-east (e.g. the Netherlands and Belgium), highlighting
the importance of long-range transport in the NH3 budget over Paris.
Variability in NH3 in the north-east region is likely to impact NH3
concentrations in the Parisian region since the cross-correlation function
is above 0.3 (at lag = 0 and 1 d). Finally, we quantify the key meteorological parameters driving the specific
conditions important for the formation of PM2.5 from NH3 in the
Île-de-France region in spring. Data-driven results based on surface
PM2.5 measurements from the Airparif network and IASI NH3
measurements show that a combination of the factors such as a low boundary
layer of ∼500 m, a relatively low temperature of 5 ∘C, a high relative humidity of 70 %, and wind from the north-east
contributes to a positive PM2.5 and NH3 correlation.
Moisture trajectories during heavy rainfall events using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model
