Black Carbon in the Arctic
S. Sharma1, J.A. Ogren2, A. Jefferson2, K. Eleftheriadis3, E. Chan1, P.K. Quinn4, J.F. Burkhart5
1Environment Canada, Atmospheric Science and Technology Directorate, Toronto, ON, Canada
2NOAA ESRL, Boulder, CO, USA
3Institute of Nuclear and Radiological Science & Technology, Energy & Safety N.C.S.R.
"Demokritos" 15310 Ag. Paraskevi, Attiki, Greece
4NOAA PMEL, Seattle, WA, USA
5University of Oslo, Department of Geosciences, 0316 Oslo, Norway
December 2, 2013
- Average equivalent black carbon (soot) concentrations in 2012 at Alert (Nunavut, Canada), Barrow (Alaska, USA) and Ny-Ålesund (Svalbard, Norway) were similar to average concentrations during the decade 2002-2012.
- Annual equivalent black carbon has declined by 55% and 45% since the early 1990s at Alert and Barrow, respectively.
Aerosol black carbon (soot) is released during the incomplete combustion of fossil fuels, biofuels and biomass burning. The largest black carbon emission sources that affect the Arctic are agricultural burning, wildfires and on-road diesel vehicles, followed by residential burning, off-road diesel and industrial combustion, including gas flaring. The burden of atmospheric black carbon north of 70°N in the Arctic is the result of long-range transport from the former Soviet Union, Europe, North America and east Asia (Sharma et al. 2013) (Fig. 15).
Black carbon is a short-lived climate forcer that affects the radiation balance in the Arctic by absorbing solar radiation when suspended in the atmosphere (Charlson et al. 1991; Jacobson 2000, IPCC 2001), by altering cloud properties (IPCC 2007, Liu et al. 2011) and, when deposited on snow and ice, by darkening the surface and enhancing the absorption of solar radiation and melt rates (Flanner et al. 2007, Hegg et al. 2009, Bond et al. 2013). A large proportion of the Arctic climate response to an increase in surface temperature is due to the snow/albedo effect (Fletcher et al. 2009, Serreze and Barry 2011).
Long-term monitoring of black carbon in the Arctic is critical to understanding sources, transport pathways and environmental impacts in the Arctic, and to provide essential information for the development and implementation of mitigation options. Here we report black carbon observations at three high Arctic locations with the longest records: Alert (Nunavut, Canada), Barrow (Alaska, USA) and Ny-Ålesund (Svalbard, Norway) (Fig. 15).
Measuring Black Carbon
In the Arctic, the longest records of black carbon concentration are measured by Aethalometer. This instrument uses a filter-based optical technique changes, which measures light attenuation over time as the amount of black carbon-containing aerosol increases on the filter matrix (Hansen et al. 1984). Black carbon derived from Aethalometer measurements is referred to as Equivalent Black Carbon (EBC) (Petzold et al. 2013). The change in optical transmission is assumed to be due solely to black carbon; no corrections have been applied to account for other aerosols (Weingartner et al. 2003, Collaud Coen et al. 2010, Müller et al. 2013). By not applying the correction, artifacts can affect the EBC measurements by a factor of 2 (Louisse et al. 1993, Sharma et al. 2002, Weingartner et al. 2003). At Barrow, a Particle Soot Absorption Photometer (PSAP) has been used since the Aethalometer broke down in 2002. A good comparison between the two instruments (Sharma, unpublished data) gives us the confidence to continue the Barrow EBC time series with the PSAP data after 2002.
Equivalent Black Carbon (EBC) Observations
EBC in 2012. In 2012, annual average EBC concentrations were 36±36, 32±31 and 23±32 ng m-3 (arithmetic mean ± 1 standard deviation of daily averages in nanograms per cubic meter) at Alert, Barrow and Ny-Ålesund. These values are very similar to the averages (40±45, 30±36 and 27±40 ng m-3) for the decade 2002-2012. This decade is used for the comparison because the continuous Ny-Ålesund record goes back only as far as 2002 (Fig. 16).
Long-Term EBC Trends and Variability. Alert and Barrow have the longest measurement records (1989-present, Fig. 16), which allows meaningful trends to be determined. EBC measurements did not begin at Ny-Ålesund until 1998 and trends there can not be compared directly to the other two sites. However, for the period 2002-2012, daily average EBC concentrations at Ny-Ålesund are similar to those at Barrow and Alert (Fig. 16), in spite of their large geographical separation. This is an indication of the ubiquity of black carbon in the high Arctic.
Overall, there has been a 55% decline in EBC at Alert and a 45% decline in EBC at Barrow between 1990-1993 and 2009-2012 (Fig. 16). The declines are related to decreasing emissions due to the economic collapse in the former Soviet Union during the early 1990s (Sharma et al. 2004, 2006, 2013, Quinn et al. 2008, Hirdman et al. 2010). EBC has not increased since 2000 at Alert, Barrow and Ny-Ålesund, despite rising fossil fuel black carbon emissions in the source regions (Sharma et al. 2013), especially in East Asia. Black carbon from East Asia contributes a small proportion of total deposition in the Arctic (Hegg et al. 2009) because it is transported at higher altitudes than black carbon from Europe and the former Soviet Union (Stohl et al. 2006, Sharma et al. 2013).
Monthly EBC anomalies (Fig. 17) at Alert and Barrow were determined by calculating the difference between monthly EBC concentrations and the monthly mean values for the period 1989-2012. Monthly EBC anomalies at Ny-Ålesund were calculated only for the period of continuous record from 2002 to 2012. At Alert and Barrow, anomalies in the 1990s were significantly higher (P(t<0.01)) than post-2000 anomalies, indicating that prior to 2000 monthly EBC concentrations were significantly higher than the 23 year mean for a given month. These temporal differences at Alert and Barrow are largely due to changes in black carbon source strength and depositional losses rather than changes in transport patterns. EBC anomalies at Ny-Ålesund are significantly different than those at Alert and Barrow for the same period (2002-2012); at Ny-Ålesund, site-specific influences on EBC concentrations, e.g., elevation (500 m above sea level, a.s.l.) are superimposed on larger scale influences such more frequent cyclones and cloud cover that lead to more efficient scavenging.
Seasonal Cycle of EBC. Seasonal EBC variability is evident at Alert, Barrow and Ny-Ålesund (Fig. 16), but is most pronounced at Alert and least pronounced at Ny-Ålesund (Fig. 18). This reflects spatial and inter-annual variability in EBC concentration, which is a function of black carbon source strength, transport pathways from source to receptor, and black carbon deposition (e.g., Stohl 2006, Garrett et al. 2010, Sharma et al. 2013). Also, Barrow (0 m a.s.l.) and Ny-Ålesund (575 m a.s.l.) have Pacific and Atlantic Ocean influences, resulting in higher EBC losses due to deposition than Alert (250 m a.s.l.).
Maximum EBC concentrations occur in winter and spring (Figs. 16 and 18) due to the seasonal influence of Arctic haze, which is transported from mid-latitude source regions (Barrie 1986, Rahn 1981, Sirois and Barrie 1999), with higher transport frequency during winter and spring as the Arctic front extends to lower latitudes. Lower summer values are due to less frequent BC transport into the Arctic and higher wet deposition (Garrett et al. 2011).
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