Quantifying the impact of global nitrate aerosol on tropospheric composition fields and its production from lightning NO_x

Abstract. Several global modelling studies have previously studied the effects of lightning-generated oxides of nitrogen (LNO_x) on gas-phase chemistry and atmospheric radiative transfer. However, there has been limited attention on quantifying the role of LNO_x production on aerosol, particularly when nitrate aerosol is included. In the present paper, we address two questions: 1) what is the impact of including nitrate aerosol on tropospheric composition fields, aerosol optical depth (AOD) and radiation, and 2) what is the dependency of these impacts on the lightning parameterisation and the amount of LNO_x? For this purpose, we use the Met Office’s Unified Model (UM) – United Kingdom Chemistry and Aerosol (UKCA) global chemistry-climate model, which now includes a nitrate scheme in its modal aerosol component, alongside two empirical lightning flash-rate parameterisations. We find that both nitrate aerosol and changes in LNO_x lead to significant changes in tropospheric composition and aerosol responses. For instance, with the inclusion of nitrate aerosol, the tropospheric ozone burden decreases by approximately 4–5 %, while the tropospheric methane lifetime increases by a similar degree. For an increase of 5.2 Tg N yr^-1 in LNO_x from a baseline of zero, there is a global mean enhancement of 2.8 % in NH₄, 4.7 % in fine NO₃, 12 % in coarse NO₃, and 5.8 % in SO₄ aerosol mass burdens, indicating that LNO_x impacts coarse aerosol the most comparatively. The inclusion of nitrate aerosol affects the aerosol size distribution too, with the most significant changes occurring in the Aitken and accumulation modes. As LNO_x increases, the mean global AOD and top-of-atmosphere net downward radiative flux also increase (the latter being more influenced by tropospheric ozone increases), whereas nitrate aerosol causes a change of –0.4 W m^-2 in this radiative flux in our simulations. The results presented in this paper when considered in the context of the large uncertainty in the global amount of LNO_x suggest that there could be bigger variations in the values of the atmospheric parameters considered.