A new ammonium nitrate scheme in UKCA-mode: Initial sensitivity tests - UKESM
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A new ammonium nitrate scheme in UKCA-mode: Initial sensitivity tests Anthony Jones Adrian Hill1, Samuel Remy2,3, Luke Abraham4, Mohit Dalvi1, Catherine Hardacre1, Alan Hewitt1, Ben Johnson1, Jane Mulcahy1, and Steve Turnock1 1 Met Office 2 Institut Pierre-Simon Laplace 3 HYGEOS 4University of Cambridge www.metoffice.gov.uk © Crown Copyright 2020, Met Office
Why is nitrate important? • Sulphate (SO4) concentrations have declined since ‘clean air’ regulation introduced • Ammonium (NH4) concentrations have held constant or increased • SO4 and nitrate (NO3) compete for NH4 in the atmosphere – thus SO4 reduction → NO3 increase • NH4 and NO3 precursors are mostly anthropogenic (agriculture, land-use change, fossil-fuel combustion) • NO3 contributes to urban air pollution, radiative forcing, cloud properties, and biosphere impacts such as soil acidification and lake eutrophication
Why is ammonium nitrate hard to model? • Precursor gases NH3 and HNO3 condense at different speeds depending on acidity and partial pressures • Long timesteps → numerical instability • Modelling approaches: equilibrium, dynamical, and hybrid equilibrium- dynamical • HyDiS-1.0 developed in Leeds for GLOMAP-mode is a hybrid Benduhn et al., 2016, GMD, doi:10.5194/gmd-9-3875-2016
Equilibrium schemes • The equilibrium constant of the ammonia-nitrate system related temperature and relative humidity • Complex equilibrium schemes like ISORROPIA account for many chemical reactions simultaneously • Mozurkewich model compares well with ISORROPIA NH4 NO3 ՞ NH3 + HNO3 Simple Mozurkewich (1993) scheme compares well with ISORROPIA Hauglustaine et al., 2014, ACP doi:10.5194/acp-14-11031-2014
An equilibrium nitrate scheme in UKCA • Changes to Rose – two UKCA flags added to control emission of NH4.NO3 and coarse NO3 (hetNO3) separately • NH4 and NO3 emitted into the Aitken and accumulation soluble modes • hetNO3 associated with sea-salt and dust emitted into the accumulation and coarse soluble modes Mode Species On the trunk since 11.8 NUC SOL SO4, OM AIT SOL SO4, BC, OM, NH4, NO3 MODE SETUP 10 ACC SOL SO4, BC, OM, SS, NH4, NO3, hetNO3 COA SOL SO4, BC, OM, SS, NH4, NO3, hetNO3 AIT INS BC, OM
GA7.1+Strattrop simulations UM11.7 • Rate at which NH4·NO3 reaches equilibrium is limited by first order uptake theory, i.e. Initial Equilibrium conditions 2 = 4 0.47 1+ × 1− 3 1 + • The uptake coefficient (γ) is altered between Simulation Rose Description Jobid FAST (0.193) and SLOW (0.001) in sensitivity simulations CNTL u-bz552 No nitrate • UKCA testing branch at UM11.7 with Strattrop, INSTANT u-ca284 Nitrate – instant equilibrium GA7.1, N96L85, and perpetual year 2000 FAST u-bz424 Nitrate – fast uptake ( = 0.193) conditions SLOW u-bz615 Nitrate – slow uptake ( = 0.001) • Run for 25 years – 5 year spin up followed by 20 year validation period NB To change γ in a branch, change the following in ukca_prod_no3_mod.F90 REAL, PARAMETER :: zghno3 = 0.193 ! HNO3 uptake coefficient
Column burdens • NH4 and NO3 primarily over US, Europe, India, China, and Eq. Africa • hetNO3 more evenly spread than NO3 • Twice as much NO3 in FAST than in SLOW (0.2 vs 0.1 mg[N] m-2) and 20% more NH4 in FAST • Burdens compare well with other models Xu & Hauglu- Burden FAS SLOW Penner staine AeroCom Tg[N] T 2012 2014 HNO3 0.48 0.48 0.3 0.3 0.56 [0.15, 1.27] NH3 0.04 0.06 0.07 0.09 0.16 [0.04, 0.7] NO3+hetNO3 0.2 0.15 0.17 0.18 0.14 [0.03, 0.42] NH4 0.42 0.36 0.26 0.22 0.25 [0.13, 0.58]
Surface concentrations • Twice as much NO3 near surface in FAST than SLOW (0.11 vs 0.05 μg[N] m-3) • NO3 and hetNO3 confined to lowest 1km of atmosphere
Surface concentrations • Peak NH4.NO3 concentrations in US and Europe in summer • In SLOW, hetNO3 concentrations are of similar magnitude to NO3 • Regional NH4.NO3 concentrations mostly correlated with NH3 emissions suggesting NH3 is limiting factor Surface concentrations NH3 emissions
Surface concentrations vs observations • HNO3 concentrations biased high in east US, particularly in SLOW, compared to CASTNET • NH4 concentrations too high in FAST and acceptable in SLOW • NO3 concentrations too high in FAST and too low in SLOW • Spatial distribution of NO3 better in FAST in east → Too much HNO3 in baseline Credit: Catherine Hardacre
Surface concentrations vs observations • Worse spatial correlations over Europe for HNO3, NO3 and NH4 concentrations compared to EMEP observations • Better magnitudes in SLOW, e.g. bias of 3 μg m-3 for NO3 in FAST • Poor spatial correlation may be due to perpetual year 2000 conditions in simulation Credit: Catherine Hardacre
Surface concentrations vs observations • Diurnal NO3 cycles are similar to observations at Auchencorth Moss (SE Scotland) • Peak at night and early morning (00Z and 06Z) • Too much NO3 and NH4 in summer – JJA NH3 emissions too high? • Too much NH3 at night – apply diurnal cycle to emissions • Spurious HNO3 peak in the day also observed in AQUM
Radiation and AOD • ∆ AOD between -0.003 in SLOW and 0.005 in FAST • Effective radiative forcing between -0.07 Wm-2 in SLOW and -0.19 Wm-2 in FAST Credit: Ben Johnson • Significant negative forcing over India, Europe and the DRC = (-10, -4, -4) Wm-2 in FAST
Next steps • Introducing nitrate to UKESM • Catherine Hardacre is repeating simulations in UKESM1.1 (UKESM ticket #781) • Coupling with JULES • Coupling with MEDUSA (probably after UKESM2) • High resolution simulations over London – Hamish Gordon and Anthony Jones • Already works over Colorado (see right) • AQUM ?
Jones, A. C., Hill, A., Remy, S., Abraham, N. L., Dalvi, M., Hardacre, C., Hewitt, A. J., Johnson, B., Mulcahy, J. P., and Turnock, S. T. Exploring the sensitivity of atmospheric nitrate concentrations to nitric acid uptake rate using the Met Office's Unified Model Atmos. Chem. Phys. Discuss. [preprint], https://doi.org/10.5194/acp-2021- 400, in review, 2021.
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