Remote sensing of two exceptional winter aerosol pollution events and representativeness of ground-based measurements

Abstract

Two intense winter aerosol pollution events, which took place in winter 2016 2017 in Paris, were monitored using a ground-based N2-Raman lidar, in the framework of WASLIP (Winter Aerosol Survey by Lidar In Paris), a dedicated field campaign that was carried out in this area from 1 November 2016 to 31 January 2017. The data analysis uses the synergy between ground-based and spaceborne lidar observations and data from the air quality monitoring network Airparif. The first severe aerosol pollution event began on 30 November 2016 and ended on 2 December, concerning a circular area of ∼ 250 km in diameter around Paris. The maximum PM10 was 121 ± 63 μgm-3 (regional spatial average ± SD) for the Airparif ground-based PM monitoring stations, and the aerosol extinction coefficient (AEC) ranged from 0.2 to 1 km-1. The second event took place from 20 to 23 January which covered all of the northwestern Europe, with maxima of PM10 around 156±33 μgm-3 and AEC between 0.6 and 1 km-1, within the winter atmospheric boundary layer. Although these two major aerosol pollution events did not occur under identical anticyclonic weather conditions, they share very low planetary boundary layer (PBL) heights, down to 300 m above ground level. Moreover, they are associated with significantly different aerosol lidar ratios: 72 ± 15 and 56 ± 15 sr, respectively in December and January. Such results are consistent with available spaceborne lidar data, 70 ± 25 sr from CALIOP (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations), and values found in the literature. During these two events, the continuous temporal evolution of the aerosol extinction coefficient allows us to investigate the representativeness of optical parameters found in the planetary boundary layer to assess surface aerosol concentration. No one-to-one relationship between the aerosol optical thickness (AOT) and PM2:5 values stands out within our study. In contrast, the maximum aerosol extinction coefficient found within the planetary boundary layer correlates well with PM2:5 at the ground (R2 ∼ 0:75, specific extinction cross section of 9.4 m2 g-1/ for these polluted events. Thus this lidar-derived aerosol extinction coefficient is identified as a consistent variable to monitor the pollution during winter events.