Nitrate aerosol chemistry in UKESM1.1: impacts on composition and climate&#160;
Authors: Hardacre Catherine , Mulcahy Jane , Jones Anthony , Jones Colin 
Published: March 2022

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&lt;p&gt;&lt;span&gt;UKESM1.1&amp;#160;is the latest generation Earth system model to be developed in the UK. It simulates the core physical and&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;dynamical&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;span&gt;processes of land, atmosphere, ocean and sea ice systems which are extended to incorporate key marine and terrestrial biogeochemical cycles. These include the carbon and nitrogen cycles and interactive stratosphere-troposphere trace gas chemistry. Feedbacks between these components that have an important amplifying or dampening effect on the physical climate, and/or change themselves in response to changes in the physical climate are also included. One focus for the development of UKESM1.1&amp;#160;is&amp;#160;to include a representation of nitrate aerosol chemistry&amp;#160;in the aerosol-chemistry scheme, UKCA-Mode.&amp;#160;Nitrate aerosol&amp;#160;is a major aerosol component and contributes&amp;#160;to&amp;#160;air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing.&amp;#160;In addition, the emission, chemical&amp;#160;transformation&amp;#160;and deposition&amp;#160;processes for&amp;#160;nitrate aerosol&amp;#160;span land-atmosphere boundaries and can impact feedbacks between these systems.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;A new nitrate aerosol scheme has recently been developed for,&amp;#160;and evaluated in,&amp;#160;UKCA-Mode.&amp;#160;We have successfully implemented this new scheme into&amp;#160;the atmosphere only configuration of&amp;#160;UKESM1.1 and explored its impact on aerosol, aerosol-cloud interactions and radiative forcing at the global scale.&amp;#160;UKESM1.1-nitrate&amp;#160;simulates&amp;#160;the largest&amp;#160;nitrate sources&amp;#160;over&amp;#160;China, India, Europe,&amp;#160;eastern USA&amp;#160;and central Africa, and&amp;#160;we see&amp;#160;corresponding&amp;#160;increases in&amp;#160;aerosol&amp;#160;optical&amp;#160;depth (AOD)&amp;#160;of up to 30% over these regions.&amp;#160;The addition of nitrate aerosol also drives increases in cloud droplet number concentration (CDNC)&amp;#160;and&amp;#160;reduces&amp;#160;the net downward radiation at&amp;#160;the&amp;#160;top&amp;#160;of atmosphere&amp;#160;by&amp;#160;0.4 W m&lt;/span&gt;&lt;span&gt;-2&lt;/span&gt;&lt;span&gt;.&amp;#160;The latter is primarily driven by decreases in the outgoing short-wave radiation at the top of atmosphere.&amp;#160;Evaluation&amp;#160;of simulated nitrate aerosol against&amp;#160;observations&amp;#160;surface concentrations&amp;#160;and AOD&amp;#160;from AERONET suggest that&amp;#160;while UKESM1.1 captures the spatial distribution of nitrate aerosol,&amp;#160;the model&amp;#160;may be over-predicting&amp;#160;it&amp;#8217;s atmospheric loading&amp;#160;and&amp;#160;consequently&amp;#160;impacts on&amp;#160;radiative forcing may also&amp;#160;be over-predicted.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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&lt;p&gt;&lt;span&gt;The inclusion of nitrate aerosol chemistry in UKESM1.1 is a&amp;#160;step-change in the model&amp;#8217;s aerosol modelling capability.&amp;#160;We will further investigate the role of&amp;#160;nitrate aerosol&amp;#160;in the Earth system&amp;#160;by calculating&amp;#160;its&amp;#160;contribution to the aerosol&amp;#160;effective radiative forcing&amp;#160;and through development of the&amp;#160;scheme in the fully coupled UKESM1.1 configuration.&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;
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