A new assessment of global and regional budgets, fluxes, and lifetimes of atmospheric reactive N and S gases and aerosols
Authors: Ge Yao , Vieno Massimo , Stevenson David S. , Wind Peter , Heal Mathew R. 
Published: June 2022

Abstract. We used the EMEP MSC-W (European Monitoring and Evaluation Programme Meteorological Synthesizing Centre – West) model version 4.34 coupled with
WRF (Weather Research and Forecasting) model version 4.2.2 meteorology to undertake a present-day (2015) global
and regional quantification of the concentrations, deposition, budgets, and
lifetimes of atmospheric reactive N (Nr) and S (Sr) species. These
are quantities that cannot be derived from measurements alone. In areas with
high levels of reduced Nr (RDN = NH3+ NH4+),
oxidized Nr (OXN = NOx+ HNO3+ HONO + N2O5 + NO3-+ “Other OXN” species), and oxidized Sr (OXS = SO2+ SO42-), RDN is predominantly in the form of NH3
(NH4+ typically <20 %), OXN has majority gaseous
species composition, and OXS predominantly comprises SO42- except near major SO2 sources. Most continental regions are now
“ammonia rich”, more so than previously, which indicates that, although
reducing NH3 emissions will decrease the RDN concentration, decreasing these emissions will have
little effect on mitigating secondary
inorganic aerosol (SIA). South Asia is the most ammonia-rich region.
Coastal areas around East Asia, northern Europe, and the north-eastern United
States are “nitrate rich” where NH4NO3 formation is limited by
NH3. These locations experience transport of OXN from the adjacent
continent and/or direct shipping emissions of NOx, but NH3
concentrations are lower. The least populated continental areas and most
marine areas are “sulfate rich”. Deposition of OXN (57.9 TgN yr−1,
51 %) and RDN (55.5 TgN yr−1, 49 %) contribute almost equally to
total nitrogen deposition. OXS deposition is 50.5 TgS yr−1. Globally,
wet and dry deposition contribute similarly to RDN deposition; for OXN and
OXS, wet deposition contributes slightly more. Dry deposition of NH3 is
the largest contributor to RDN deposition in most regions except for the Rest of
Asia area and marine sectors where NH3 emissions are small and RDN deposition
is mainly determined by the transport and rainout of NH4+ (rather than
rainout of gaseous NH3). Thus, reductions in NH3 would efficiently
reduce the deposition of RDN in most continental regions. The two largest
contributors to OXN deposition in all regions are HNO3 and coarse
NO3- (via both wet and dry deposition). The deposition of fine
NO3- is only important over East Asia. The tropospheric burden of
RDN is 0.75 TgN, of which NH3 and NH4+ comprise 32 % (0.24 TgN; lifetime of 1.6 d) and 68 % (0.51 TgN; lifetime of 8.9 d) respectively. The lifetime of RDN (4.9–5.2 d) is shorter than that
of OXN (7.6–7.7 d), which is consistent with a total OXN burden (1.20 TgN) almost
double that of RDN. The tropospheric burden of OXS is 0.78 TgS with a
lifetime of 5.6–5.9 d. Total nitrate burden is 0.58 TgN with fine
NO3- only constituting 10 % of this total, although fine
NO3- dominates in eastern China, Europe, and eastern North
America. It is important to account for contributions of coarse nitrate to
global nitrate budgets. Lifetimes of RDN, OXN, and OXS species vary by a
factor of 4 across different continental regions. In East Asia, lifetimes
for RDN (2.9–3.0 d), OXN (3.9–4.5 d), and OXS (3.4–3.7 d) are
short, whereas lifetimes in the Rest of Asia and Africa regions are about twice as long.
South Asia is the largest net exporter of RDN (2.21 TgN yr−1, 29 % of
its annual emission), followed by the Euro_Medi region. Despite
having the largest RDN emissions and deposition, East Asia has only small
net export and is therefore largely responsible for its own RDN pollution.
Africa is the largest net exporter of OXN (1.92 TgN yr−1, 22 %),
followed by Euro_Medi (1.61 TgN yr−1, 26 %).
Considerable marine anthropogenic Nr and Sr pollution is revealed
by the large net import of RDN, OXN, and OXS to these areas. Our work
demonstrates the substantial regional variation in Nr and Sr
budgets and the need for modelling to simulate the chemical and
meteorological linkages underpinning atmospheric responses to precursor
emissions.