The characteristics and driving mechanisms of Elevated Stratopause Events (ESEs) are examined in simulations of the ECHAM/MESSy Atmospheric Chemistry (EMAC) chemistry-climate model under present and projected climate conditions. ESEs develop after sudden stratospheric warmings (SSWs) in boreal winter. While the stratopause descends during SSWs, it is reformed at higher altitudes after the SSWs, leading to ESEs in years with a particularly high new stratopause. EMAC reproduces well the frequency and main characteristics of observed ESEs. ESEs occur in 24% of the winters, mostly after major SSWs. They develop in stable polar vortices due to a persistent tropospheric wave forcing leading to a prolonged zonal wind reversal in the lower stratosphere. By wave filtering, this enables a faster re-establishment of the mesospheric westerly jet, polar downwelling and a higher stratopause. We find the presence of a westward-propagating wavenumber-1 planetary wave in the mesosphere following the onset, consistent with in-situ generation by large-scale instability. By the end of the 21st century, the number of ESEs is projected to increase, mainly due to a sinking of the original stratopause after strong tropospheric wave forcing and planetary wave dissipation at lower levels. Future ESEs develop preferably in more intense and cold polar vortices, and tend to be shorter. While in the current climate, planetary wavenumber-2 contributes to the forcing of ESEs, future wave forcing is dominated by wavenumber-1 activity as a result of climate change. Hence, a persistent wave forcing seems to be more relevant for the development of an ESE than the wavenumber decomposition of the forcing.

<p>Air quality monitoring with a high spatial and temporal resolution is essential to understand the sources, processes, and impacts of air quality (AQ) on human health and the environment, especially in densely populated urban areas. Current state-of-the-art instruments don’t allow for such a high spatial resolution AQ monitoring due to costs. Low cost sensors (LCS) provide an opportunity to bridge this divide. There is a large volume of research papers assessing the performance of LCS for particulate matter (PM) against reference PM instrumentation.  Across most studies, a general observation is that LCS PM sensors as offered by the commercial and research markets have an inherent problem with accuracy at high relative humidities and for varying PM composition.</p> <p>This study uses measured and modelled chemical composition of PM<sub>2.5 </sub> at the  Manchester UK urban supersite to addresses the underlying physical chemistry which leads to the mass of the measured PM. Measurements from LCS and reference PM instruments co-located with state-of-the-art research chemical composition and meteorology. To calculate the composition of PM<sub>2.5</sub>, we used XACT for metals, the ACSM for salt ion concentrations, and an Aethalometer for Black Carbon. From this, the LCSs variability in data can be understood.  We present a comparison of LCS PM<sub>2.5</sub> concentrations with measured and calculated total PM<sub>2.5</sub> concentrations from reference instruments for time periods with different Air Quality characteristics and discuss the physico-chemical characteristics leading to varying results of low cost PM sensors. The future research will apply methodology to modelled UK wide data and develop tools for non specialist users to understand LCS in terms of local air pollution and specifically PM<sub>2.5</sub> chemical composition and meteorology.</p>

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