Non-Volcanic Contributors To Stratospheric Aerosol Load At Northern Midlatitudes: A Multi-Platform Perspective On Asian Pollution And Forest Fires

Abstract:

We use a 24-yr post-Pinatubo observation record from Observatoire de Haute-Provence (OHP) lidars and various satellite aerosol data sets for accurate characterization of background stratospheric aerosol variability and long-term evolution. The background aerosol abundance is found to be modulated remotely by poleward transport of convectively-cleansed air from the deep tropics and aerosol-laden air from the Asian monsoon region. The annual cycle of background aerosol at mid-latitudes, featuring a minimum during late spring and a maximum during late summer, correlates with that of water vapour from Aura MLS instrument.

OHP and satellite observations covering two volcanically-quiescent periods over the last two decades provide indication of a growth in the non-volcanic component of stratospheric aerosol at northern mid-latitudes. We find a statistically-significant factor of two increase of non-volcanic aerosol since 1998, seasonally restricted to late-summer and fall and associated with the influence of the Asian monsoon and growing pollution therein.

Finally, we report an evidence of a thick smoke plume deep in the stratosphere, found traceable to extreme pyro-convection events triggered by severe wildfires in North America during Summer 2017. The plume was observed at OHP for months, and its backscatter and AOD were reaching unprecedentedly high values for a non-volcanic aerosol layer in the 3-decade OHP observation record. We use CALIOP L1B data to track the spatiotemporal evolution of the plume and find a remarkable agreement between ground- and spaced-based lidars sampling the same air mass on a particular date (Fig. 1). CALIOP measurement show that AOD of the two-week old plume above Europe were closely comparable to those recorded at OHP after Pinatubo and El Chichon eruptions. On a monthly time scale, the lidar observations indicate that boreal summer 2017 forest fires had a significant hemisphere-wide impact on stratospheric aerosol load, similar to that of moderate (VEI 4) volcanic eruptions.