Abstract. Atmospheric information embedded in ice-core nitrate is disturbed by
post-depositional processing. Here we used a layered snow photochemical column
model to explicitly investigate the effects of post-depositional processing on
snow nitrate and its isotopes (δ15N and Δ17O)
at Summit, Greenland, where post-depositional processing was thought to be
minimal due to the high snow accumulation rate. We found significant
redistribution of nitrate in the upper snowpack through photolysis, and up to
21 % of nitrate was lost and/or redistributed after deposition. The model
indicates post-depositional processing can reproduce much of the observed
δ15N seasonality, while seasonal variations in
δ15N of primary nitrate are needed to reconcile the timing of
the lowest seasonal δ15N. In contrast, post-depositional
processing can only induce less than 2.1 ‰ seasonal
Δ17O change, much smaller than the observation
(9 ‰) that is ultimately determined by seasonal differences in
nitrate formation pathway. Despite significant redistribution of snow nitrate
in the photic zone and the associated effects on δ15N
seasonality, the net annual effect of post-depositional processing is
relatively small, suggesting preservation of atmospheric signals at the annual
scale under the present Summit conditions. But at longer timescales when large
changes in snow accumulation rate occur this post-depositional processing
could become a major driver of the δ15N variability in ice-core nitrate.
This study compares recent CO, NOx, NMVOC, SO2, BC, and OC anthropogenic emissions from several state‐of‐the‐art top‐down estimates to global and regional bottom‐up inventories and projections from five Shared Socioeconomic Pathways (SSPs) in several regions. Results show that top‐down emissions derived in several recent studies exhibit similar uncertainty as bottom‐up inventories in some regions for certain species and even less in the case of Chinese CO emissions. In general, the largest discrepancies are found outside of regions such as the United States, Europe, and Japan where the most accurate and detailed information on emissions is available. In some regions such as China, which has recently undergone dynamical economic growth and changes in air quality regulations, the top‐down estimates better capture recent emission trends than global bottom‐up inventories. These results show the potential of top‐down estimates to complement bottom‐up inventories and to aide in the development of emission scenarios, particularly in regions where global inventories lack the necessary up‐to‐date and accurate information regarding regional activity data and emission factors such as Africa and India. Areas of future work aimed at quantifying and reducing uncertainty are also highlighted. A regional comparison of recent CO and NOx trends in the five SSPs indicate that SSP126, a strong pollution control scenario, best represents the trends from the top‐down and regional bottom‐up inventories in the United States, Europe, and China, while SSP460, a low‐pollution control scenario, lies closest to actual trends in West Africa. This analysis can be useful for air quality forecasting and near‐future pollution control/mitigation policy studies.
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