Global and regional radiative forcing from 20 % reductions in BC, OC and SO<sub>4</sub> – an HTAP2 multi-model study

Abstract

Abstract. In the Hemispheric Transport of Air Pollution Phase 2 (HTAP2) exercise, a range of global atmospheric general circulation and chemical transport models performed coordinated perturbation experiments with 20 % reductions in emissions of anthropogenic aerosols, or aerosol precursors, in a number of source regions. Here, we compare the resulting changes in the atmospheric load and vertically resolved profiles of black carbon (BC), organic aerosols (OA) and sulfate (SO4) from 10 models that include treatment of aerosols. We use a set of temporally, horizontally and vertically resolved profiles of aerosol forcing efficiency (AFE) to estimate the impact of emission changes in six major source regions on global radiative forcing (RF) pertaining to the direct aerosol effect, finding values between. 51.9 and 210.8 mW m−2 Tg−1 for BC, between −2.4 and −17.9 mW m−2 Tg−1 for OA and between −3.6 and −10.3 W m−2 Tg−1 for SO4. In most cases, the local influence dominates, but results show that mitigations in south and east Asia have substantial impacts on the radiative budget in all investigated receptor regions, especially for BC. In Russia and the Middle East, more than 80 % of the forcing for BC and OA is due to extra-regional emission reductions. Similarly, for North America, BC emissions control in east Asia is found to be more important than domestic mitigations, which is consistent with previous findings. Comparing fully resolved RF calculations to RF estimates based on vertically averaged AFE profiles allows us to quantify the importance of vertical resolution to RF estimates. We find that locally in the source regions, a 20 % emission reduction strengthens the radiative forcing associated with SO4 by 25 % when including the vertical dimension, as the AFE for SO4 is strongest near the surface. Conversely, the local RF from BC weakens by 37 % since BC AFE is low close to the ground. The fraction of BC direct effect forcing attributable to intercontinental transport, on the other hand, is enhanced by one-third when accounting for the vertical aspect, because long-range transport primarily leads to aerosol changes at high altitudes, where the BC AFE is strong. While the surface temperature response may vary with the altitude of aerosol change, the analysis in the present study is not extended to estimates of temperature or precipitation changes.

Global source attribution of sulfate concentration and direct and indirect radiative forcing

“…Note that the receptor region of ROW is used to calculate efficiencies of the VOL and DMS source sectors, which leads to low biases in efficiencies. The efficiencies over the Middle East show high values in almost all seasons due to dry atmospheric conditions favoring long aerosol lifetime, especially in DJF and SON (e.g., Stjern et al, 2016). The efficiencies are also high over South Asia in DJF and SON, but low in MAM and JJA due to strong wet removal during the South Asian summer monsoon season.…”

“…The underestimation of sulfate over China could lead to the underestimation of contributions from East Asia to remote sulfate concentrations, global DRF and incremental IRF, as well as their efficiencies. Table S9 compares the annual sulfate radiative forcing efficiencies simulated in this study to those in previous multimodel studies Bellouin et al, 2016;Stjern et al, 2016). As in the previous studies, the DRF efficiency is calculated as the response of global DRF to a 20 % reduction in local emissions divided by the 20 % of sulfur emissions based on two separate simulations rather than 100 % of local emissions in a single simulation (Table S6).…”

“…Some previous studies examined the impact of emission reductions on global and regional DRF and the influence of long-range transport Bellouin et al, 2016;Stjern et al, 2016). Yu et al (2013) examined changes in aerosol DRF resulting from a 20 % reduction in anthropogenic emissions from four major polluted regions (namely North America, Europe, East Asia and South Asia) in the Northern Hemisphere, using simulations by nine models from the first phase of the Hemispheric Transport of Air Pollution (HTAP1).…”

“…They found that 31 % of South Asia sulfate aerosol optical depth over South Asia was contributed by non-local sources. Based on the HTAP2, Stjern et al (2016), using results from 10 models, further assessed global and regional DRF from a 20 % reduction in emissions over seven regions including North America, Europe, South Asia, East Asia, Russia, the Middle East and the Arctic. They found that the 20 % reduction in emissions in South Asia and East Asia largely perturbed the radiative balance for other regions.…”

Similar spatial patterns of global climate response to aerosols from different regions

“…2). Previous studies by the Hemispheric Transport of Air Pollution (HTAP) project 14,15 have also investigated the importance of transport for regional aerosol forcing, and found similarly that for sulphate, the dominant forcing contribution generally remains close to the source region. Sulphate aerosol strongly scatters incoming solar radiation, and so its removal in our experiments results here in a positive radiative heating mainly over the perturbed region.…”