Factors controlling BC deposition in the Arctic

Ling Qi1, Qinbin Li1,2,3, Yinrui Li4, Cenlin He1,3

1 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095, USA

2 Jet Propulsion Laboratory, California Institute of Technology, CA 91109, USA

3 Joint Institute For Regional Earth System Science and Engineering, University of California, Los Angeles, CA 90095, USA

4 School of Physics, Peking University, Beijing, China

BC deposition and BC in snow in the Arctic: upto 5X discrepancy

• We examined the sensitivity of BC deposition and BC concentration in snow in the Arctic to wet scavenging, dry deposition velocity and emissions.

(Jiao et al., 2014)

AeroCom Phase II

BC deposition BC in snow

(Jiao et al., 2014)

Washout ratio

• Washout ratio (Hegg et al., 2011)

• The median washout ratio observed at Svalbard (79.0°N,12.0°E): ~770 (Hegg et al., 2011).

• The median washout ratio observed at Abisko (68.9°N,18.8°E): ~100 (Noone and Clarke, 1988)

• The large difference is because of riming vs. WBF (Hegg et al., 2011).– The falling snow at Svalbard was commonly rimed– The snow at Abisko shows no evidence of riming, consisting almost exclusively

of pristine crystals formed by Wegener-Bergeron-Findeisen (WBF) process.

Mass mixing ratio of BC in fresh snow

Mass mixing ratio of BC in surface air

BC wet scavenging: Riming vs. WBF

• Riming: Snow crystals nucleated high in the cloud and falling through a liquid water zone and collect the water drops. All BC particles in the condensed phase are removed.

• WBF process can significantly reduce the effective scavenging by essentially evaporating off cloud drops containing previously scavenged BC particles and returning them to the gas phase.

ew > e > ei

Wet scavenging: Riming vs. WBF

• Differentiating riming vs. WBF– Riming dominates when -12°C ≦ T -8°C and Liquid water content ≦

(LWC) > 1.0 g m-3 (Fukuta et al., 1999)Incorporation efficiency F = 100%

– WBF dominates when -40°C < T < -12°C or -8°C < T < 0°C Incorporation efficiency F = 0.031 + 0.93/{1 + exp[-(T+3.64)/3.42]}

(Verheggen et al., 2007; Cozic et al., 2007)

• Standard GEOS-Chem: All riming, no WBF• All hydrophilic BC incorporated into condensed phase and removed

by coalescence and riming.• NO hydrophobic BC is removed.

Site Observations All riming

Svalbard (riming dominates) ~770 785

Abisko (WBF dominates) ~100 535

Riming or WBF

250

245

WBF decreases wet dep. by 6–33% yet total dep. increases

• Wet dep. as a fraction of total dep. decreases by 5–17%.

WBF increases BC in snow (median 11.8 ng/g) by ~17%

• In Canadian Arctic and Alaska, WBF increases BC in snow by ~3 ng/g and reduces model discrepancy.

• High riming rates are rare in these two regions (~12%, Fan et al., 2011). • WBF increases BC lifetime in the Arctic from 9 to 16 days.

11.8

Probability distribution function Canadian Arctic Alaska

OBS. WBF ALLRiming

OBS. WBF ALLRiming

6

12

18

5

10

15

BC c

once

ntra

tion

in s

now

(ng/

g)

Observed dry deposition velocity over snow/ice varies dramatically (from 0.01 cm s-1 to 1.52 cm s-1 for PM2.5)

• Standard GEOS-Chem: vd = 0.03 cm s-1 over snow and ice based on observations over sea ice in Arctic Ocean (Nilsson and Rannik, 2001).

• We compute vd using a standard resistance-in-series scheme (Wesely, 1989)

Fraction of BC dry deposition increases with increasing dry deposition velocityBC in snow NOT sensitive to dry deposition velocity

11.8

Flaring emissions in Russia increase BC in snow in the Arctic by ~25%

Flaring emissions (Stohl et al., 2013) almost double the total BC emissions in the Arctic (from 67 to 115 Gg yr-1).

Without flaring emissions With flaring emissions

11.8

Summary

• Wet scavenging: WBF process improves the simulation of BC washout ratio

(at low riming rate site) and BC concentration in snow. We are currently working on improving the parameterization

of WBF and riming using ACAPEX observations.

• Dry deposition: non-uniform, varying dry deposition velocity increases the fraction of dry deposition flux by up to 12%. BC in snow is rather insensitive to such changes in dry deposition velocity.

• Emission: Gas flaring emissions (predominantly in Russia) contribute ~25% (3 ng/g out of 11.8 ng/g, median) to BC in snow. Better constraint on the flaring emission factor is needed.