Remote sensing and in-situ measurements of tropospheric aerosol, a PAMARCMiP case study
Highlights
► Arctic haze has been studied by airborne and ground based remote sensing and in-situ instruments. ► Extinction has been derived using two different methods. ► In-situ number concentrations compare well with Raman lidar based microphysical inversions. ► Particles with diameter 0.18 μm and decreasing number concentrations with altitude were found. ► BC concentration and aerosol absorption was very low.
Introduction
The Arctic is a remote region with generally very low concentrations of aerosols. However, the concentration of especially accumulation mode aerosols and associated aerosol optical depth (AOD) annually peaks in the spring months March through April as a result of poleward aerosol transport. This increase in tropospheric aerosols is commonly referred to as Arctic haze (Shaw, 1995; Sirois and Barrie, 1999 and Quinn et al., 2007). It was first observed as a visible layer of unknown origin in the 50's by pilots crossing the American Arctic. Its anthropogenic origin was shown by Rahn et al., (1977) and Rahn (1981) and others about 35 years ago. A strong annual increase in particulate sulfates, with maximum values in March and April, has been monitored at different sites throughout the Arctic for almost 30 years (Barrie et al., 1981, Quinn et al., 2000). Arctic haze consists of well-aged aerosol of 0.2 μm or less in diameter, i.e. dominated by the accumulation mode. Sulfate is the most abundant compound, but also nitrates, chlorides and carbonaceous compounds are present, e.g. a haze event in 2000 was characterized by a high mixing ratio of externally mixed soot particles (Yamanouchi et al., 2005). Arctic haze is assumed to mainly originate from long-range transported anthropogenic pollution from Europe and western Asia. Depending on the location of the polar front, effective meridional air mass exchange between the polar and mid-latitudes is possible. It has been shown that Arctic haze can get trapped for up to 15–30 days in late winter (Quinn et al., 2007). Additionally, biomass burning events are considered to contribute to Arctic haze (Law and Stohl, 2007, Warneke et al., 2010). Vertically and spatially highly inhomogeneous haze layers have been observed frequently above Spitsbergen, as reported by Gerding et al. (2004), Yamanouchi et al. (2005), Stohl (2006) and Hoffmann et al. (2009). Due to the large abundance of accumulation mode sized particles, Arctic haze layers are very efficient in scattering solar radiation. The presence of light absorbing material such as soot also enhances the absorption during periods of Arctic haze.
The airborne research project PAMARCMiP (Polar Airborne Measurements and Arctic Regional Climate Model Simulation Project) studied the meteorology, air quality and sea ice thickness in the Arctic (Stone et al., 2010). Over the Arctic, little information is available on the spatial distribution of atmospheric aerosols, e.g. soot, and clouds and on their radiative impact (Liu et al., 2010). Data obtained with several instruments on board the Polar 5 aircraft are compared to ground based remote sensing data from the French German Arctic Research Base, AWIPEV (11.92°E, 78.92°N) as well as continuous in-situ data collected at the Zeppelin Mountain Research Station (11.89°E, 78.91°N), located in and close to Ny-Ålesund.
The AWIPEV Research Base is operated by the Alfred Wegener Institute for Polar and Marine Research (AWI) and the Institut polaire français Paul-Emile Victor (IPEV). It facilitates a uniquely well equipped atmospheric laboratory, which provides long term remote sensing technologies including lidar and sun photometer as well as atmospheric monitoring using balloon soundings. These technologies can be combined to characterize the Arctic atmosphere. In the first week of April 2009, the AOD measured by ground based sun photometer was relatively high and, as confirmed by lidar data, concentrated within the lowest few kilometers of the atmosphere. Within this period, the largest AOD of up to 0.12 at 532 nm was measured on 4 April 2009. Coincidentally, this day was one of the two days of the PAMARCMiP campaign, where instrument comparison flights in the Ny-Ålesund vicinity were performed. These data are used for a remote sensing and in-situ intercomparison, which is important to better understand the aerosol radiative forcing. Closure studies between experiments measuring optical and in-situ aerosol properties are sparse so far; Wandinger et al., (2002) describes a case from central Europe. However, we are not aware of such intercomparisons for Arctic aerosols so far.
Section snippets
Methods
Ground based data obtained at the AWIPEV Research Base in Ny-Ålesund and the Zeppelin Mountain Research Station are compared to airborne data collected during a comparison flight of the research aircraft Polar 5. From 10:00 to 12:00 UTC, Polar 5 circled the Ny-Ålesund vicinity at altitudes from 100 m to 3600 m above ground. The detailed flight pattern can be seen in Fig. 1 with the flight sections, in which the different instruments on board were active, being color coded. The average speed of
Analyses
The enhanced aerosol loading on April 4th is illustrated in Fig. 2, where the mean backscatter ratio (ratio of the backscattering by particles to total backscattering) obtained in two cloud-free periods in March 2007 and April 2009 is compared to the backscatter ratio as measured on 4 April 2009.
Discussion
AOD data obtained with airborne and ground based sun photometer show very stable enhanced values over the course of the day, which is also confirmed by the KARL lidar (Fig. 5). The airborne photometer measured slightly larger AODs although it did not include the first 100 m above ground. This might be due to the fact, that the Spitsbergen orography could have influenced, e.g. partly blocked, the constant northerly air flow to Ny-Ålesund, while the airplane's descent took place above the open
Summary
For the first time, according to our knowledge, a comparison of microphysical aerosol properties derived from in-situ and remote sensing data was performed in the European high Arctic was given. With 4 April 2009, a day with increased aerosol load and quite stable meteorological conditions during the 2009 PAMARCMiP campaign was chosen.
It has been shown, that by combining the presented data sets, an overview on the meteorological situation as well as the optical and microphysical properties of
Acknowledgments
We would like to thank the PAMARCMiP crew as well as the AWIPEV Base personnel, whose effort provided the present comprehensive data set. We would like to thank NILU for the access to the meteorological data at the Zeppelin Mountain Research Station. Furthermore, we acknowledge the support from Holger Deckelmann and Alexander Schulz in data preparation.
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Now at: Institute of Aerospace Systems, Technische Universität Carolo-Wilhelmina zu Braunschweig, Hermann-Blenk-Str. 23, 38108 Braunschweig, Germany.