Absorbing aerosols over Asia – an inter-model and model-observation comparison study using CAM5.3-Oslo

Figures & data

Table 1. Summary and short description of the used AERONET data.

Station name	Location	Variable	Site classification	Meas. type	Meas. period
Kanpur	Northern India	AOD	Urban	Remote sensing	2001–2018
	(26.45 N, 80.33E)	AAOD	Polluted	(AERONET)
Beijing	Eastern China	AOD	Urban	Remote sensing	2001–2018
	(39.98 N, 16.38E)	AAOD	Polluted	(AERONET)
Karachi	Pakistan	AOD	Urban	Remote sensing	2006–2014
	(24.95 N, 67.14E)	AAOD	Coast	(AERONET)
Solar Village	Saudi Arabia	AOD	Continental	Remote sensing	1999–2013
	(24.9 N, 46.39E)	AAOD	Desert	(AERONET)
Pokhara	Nepal	AOD	Continental	Remote sensing	2010–2018
	(28.19 N, 83.98E)	AAOD	Mountain	(AERONET)
Chiang Mai	Thailand	AOD	Urban	Remote sensing	2007–2017
	(18.77 N, 98.97E)	AAOD	Elevated	(AERONET)

Table 2. Summary and short description of MERRA-2 and AeroCom phase III models.

Model/Reanalysis	MERRA-2	CAM5.3-Oslo	GEOS5-assimilate	GEOS5-freegcm	HadGEM3-GA7.1
Short name	MERRA-2	CAM5.3-Oslo	GEOS-A	GEOS-F	HadGEM
Spatial resolution	0.5 × 0.5${}^{°},$ L72	1 × 1${}^{°},$ L30	0.5 × 0.5${}^{°},$ L72	0.5 × 0.5${}^{°},$ L72	N216 (60 km), L85
BC refractive index	1.75 − 0.44i^a	1.95 − 0.79i	1.75 − 0.44i^a	1.75 − 0.44i^a	1.85 − 0.71i^b
OA refractve index	1.53 − 0.009i^a	1.53 − 0.006i	1.53 − 0.009i^a	1.53 − 0.009i^a	1.5–0i^b
Dust refractive index	1.53 − 0.0026i^a	1.53 − 0.0055i	1.53 − 0.0026i^a	1.53 − 0.0026i^a	1.52 − 0.0015i^b
References	Molod et al. (2015)	Kirkevåg et al.	Molod et al. (2015)	Molod et al. (2015)	Williams et al. (2018)
	Randles et al. (2017)	(2018)

The refractive index is given for 550 nm.

^a Veselovskii et al. (2018).

^b Mollard (2018).

Table 3. Summary and short description of the control and sensitivity experiments.

	Experiment name	Experiment description
Control	Control	Control nudged simulation from 2006 to 2012
Emissions	CMIP6_PD	CMIP6 aerosol emissions from 2000
	CMIP6_transient	CMIP6 aerosol emissions are synchronous with the simulated year
	OM-OC_1.7	Factor for conversion of OC emissions to organic matter set to 1.7 for all emission sources
	DUemissions_tuned	Tuning of dust emissions, halved emission fluxes
Meteorology	ERA5	Nudged with ERA5 instead of ERA-interim
	AMIP	AMIP-type simulation
Deposition	BCscav_lower	Below-cloud scavenging coefficient of BC decreased by a factor of 2
	DUscav_higher	Below-cloud scavenging coefficient of dust increased by a factor of 2
	DUdrydep_increased	fall velocity for dust increased by 10%
Optics	BCrefrac_1.0	Imaginary part of the BC refractive index at 550 nm increased from 0.79 to 1.0
	OArefrac_MERRA	OA refractive index at 550 nm same as in MERRA-2
	DUrefrac_MERRA	Dust refractive index at 550 nm same as in MERRA-2

The sensitivity experiments are performed only for the year 2010 and compared to the year 2010 of the control simulation.

Fig. 1. Left: Temporal mean of the years 2006 to 2012 of aerosol optical depth (AOD) for MODIS (a), MERRA-2 reanalysis (c) and the model control simulation (e) in the focus region. AERONET observations are illustrated as coloured circles. Right: Scatter plots of MODIS (b), MERRA-2 (d) and modelled AOD (f) against AERONET retrievals (monthly average). Note, the AOD of MODIS, MERRA-2 and AERONET is clear-sky while the modelled AOD is for all-sky.

Fig. 2. Left: Temporal mean of the years 2006 to 2012 of absorption aerosol optical depth (AAOD) for MERRA-2 reanalysis (a) and the model control simulation (c) in the focus region. AERONET observations are illustrated as coloured circles. Right: Scatter plots of MERRA-2 (b) and modelled AAOD (d) against AERONET retrievals (monthly average). Note, the AAOD of MERRA-2 and AERONET is clear-sky while the modelled AAOD is for all-sky.

Fig. 3. AAOD timeseries from the nudged control (blue) and AMIP (black, dashed) simulations compared to AERONET (grey bars) and MERRA-2 (grey line) AAOD monthly means (Level 1.5) of selected AERONET stations. The AAOD contribution of BC (red), dust (yellow) and OA (green) to the nudged control simulation is shown as well. Note, the AAOD of BC, OA and dust does not add up to their sum due to the technical realisation of internal mixing in the model. The map in the bottom left corner shows the respective locations of the AERONET stations.

Fig. 4. Left: Annual mean of the year 2010 of aerosol optical depth (AOD) for four AeroCom phase III models in the focus region (a, c, e, g). Note, HadGEM uses a clear-sky AOD while GEOS and CAM5.3-Oslo use all-sky. AERONET clear-sky AOD observations as annual mean of the year 2010 are illustrated as coloured circles. Right: Scatter plots of modelled AOD against AERONET (monthly average) observations (b, d, f, h).

Fig. 5. Left: Annual mean of the year 2010 of absorption aerosol optical depth (AAOD) for four AeroCom phase III models in the focus region (a, c, e, g). Note, HadGEM uses a clear-sky AOD while GEOS and CAM5.3-Oslo use all-sky. AERONET clear-sky AAOD observations as annual mean of the year 2010 are illustrated as coloured circles. Right (b, d, f, h): Scatter plots of modelled AAOD against AERONET observations (monthly average).

Table 4. Mean AAOD, normalised mean bias (NMB) and correlation (R) compared to AERONET, aerosol emissions, column burden and residence times for absorbing aerosols for MERRA-2 and AeroCom phase III models on the regional scale for the year 2010.

Variable	MERRA-2	CAM5.3-Oslo	GEOS-A	GEOS-F	HadGEM
AAOD	0.014	0.012	0.015	0.012	0.018
NMB (%)	6	–9	8	–18	46
R	0.68	0.18	0.59	0.43	0.44
BC emissions (kg m^–2 s^–1)	2.2$·{10}^{-12}$	1.7$·{10}^{-12}$	2.2$·{10}^{-12}$	2.2$·{10}^{-12}$	2.7$·{10}^{-12}$
OA emissions (kg m^–2 s^–1)	1.2$·{10}^{-11}$	9.8$·{10}^{-12}$	1.2$·{10}^{-11}$	1.2$·{10}^{-11}$	1.3$·{10}^{-11}$
Dust emissions (kg m^–2 s^–1)	3.0$·{10}^{-10}$	6.8$·{10}^{-10}$	2.4$·{10}^{-10}$	3.1$·{10}^{-10}$	2.8$·{10}^{-10}$
BC burden (kg m^–2)	1.1$·{10}^{-6}$	5.3$·{10}^{-7}$	1.1$·{10}^{-6}$	7.8$·{10}^{-7}$	8.7$·{10}^{-7}$
OA burden (kg m^–2)	5.5$·{10}^{-6}$	5.9$·{10}^{-6}$	6.1$·{10}^{-6}$	4.4$·{10}^{-6}$	4.8$·{10}^{-6}$
Dust burden (kg m^–2)	1.1$·{10}^{-4}$	9.7$·{10}^{-5}$	1.3$·{10}^{-4}$	1.2$·{10}^{-4}$	2.9$·{10}^{-5}$
BC residence time (days)	8.1	5.0	7.5	7.9	5.7
OA residence time (days)	6.3	5.0	8.1	7.6	6.2
Dust residence time (days)	13.1	1.9	17.8	18.3	1.3

Fig. 6. Absolute differences in AAOD between the nudged model control simulation and the model sensitivity experiments in the categories ’Emissions’ (a, b), ’Meteorology’ (c, d), ’Optics’ (e, f, g) as well as one combined sensitivity experiment (h) for the year 2010 in the focus region.

Fig. 7. AAOD seasonal variability for the year 2010 from the AMIP and nudged control simulations compared to AeroCom phase III models and AERONET AAOD monthly means (Level 1.5). The blue shaded area indicates the range of AAOD changes for all sensitivity experiments. The map in the bottom left corner shows the respective locations of the AERONET stations.

Fig. 8. Regionally averaged AAOD for the control simulation, sensitivity experiments as well as MERRA-2 and the AeroCom phase III models. The error bars show the inter-quartile range. The regions are (a) the focus region Asia, (b) India, (c) East China and (d) Arabia.

Table 5. Absolute regional mean in AAOD for the year 2010, the normalised mean bias (NMB, in %) and correlation (R) compared to AERONET data.

Simulation	AAOD	NMB	R	BC	OA	Dust	BC	OA	Dust	BC	OA	Dust	Radiative
				emissions	emissions	emissions	burden	burden	burden	residence	residence	residence	effect
										time	time	time
Control	0.0121	–13	0.18	1.66$·{10}^{-12}$	1.02$·{10}^{-11}$	6.57$·{10}^{-10}$	5.78$·{10}^{-7}$	5.97$·{10}^{-6}$	9.41$·{10}^{-5}$	5.5	5.08	1.88
	Δ AAOD				Δ emissions			Δ burden
	(%)			(%)	(%)	(%)	(%)	(%)	(%)
CMIP6_PD	1.3	–9	0.19	7	–23	–0.5	12	–11	–0.4	5.70	5.14	1.88	–0.003
CMIP6_transient	20.8	8	0.26	51	–4	–0.6	58	–3	–0.7	5.90	5.20	1.87	–0.002
DU_emissions_tuned	–2.8	–13	0.18	0	0	–6	0	–0.3	–6	5.49	5.07	1.87	–0.06
OM-OC_1.7	–1.6	–14	0.18	0	–16	–0.3	–0.2	–11.6	–0.1	5.46	5.02	1.88	+0.104
BCscav_lower	0.8	–13	0.18	0	0	0	1.2	0	0	5.60	5.09	1.88	+0.002
DUscav_higher	–0.9	–14	0.18	0	0	–0.1	–0.1	–0.3	–1.2	5.48	5.06	1.86	+0.02
DUdrydep_increased	0	–13	0.18	0	0	–0.1	0	0	0	5.49	5.08	1.88	+0.003
ERA5	–1	–13	0.18	0	0	1.3	–0.6	–1	1.5	5.40	4.97	1.91	+0.017
AMIP	–18.5	–23	0.15	0	–0.2	–17.7	–6.3	–8.3	–23.9	5.41	4.82	1.85	+ 1.525
BCrefrac_1.0	11.2	–7	0.21	0	0	–0.5	0.1	0.1	–0.4	5.52	5.10	1.88	+0.05
OArefrac_MERRA	4.6	–12	0.19	0	0	–0.3	0	0	–0.2	5.50	5.09	1.88	+0.01
DUrefrac_MERRA	–16.7	–32	0.30	0	0	0.3	0	0	0	5.50	5.09	1.88	–0.021
Combined	3	–12	0.38	51	–4	1.4	57	–3	1.6	5.81	5.10	1.9	–0.01

The aerosol emissions (in kg m^–2 s^–1) and column burden (in kg m^–2) for the control simulation and the relative regional mean changes between the control and sensitivity experiments for each property, as well as the global residence time in days and radiative effect (in W m^–2) for each simulation.

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Data availability statement

Data produced with model simulations using the model CAM5.3-Oslo is available from the corresponding author upon request. Observational data from AERONET and MODIS as well as the MERRA-2 reanalysis is openly available. AeroCom model output is available via the AeroCom archive.