The long-term frequency of atmospheric dust observations was investigated for
the southern part of Iceland and interpreted together with earlier results
obtained from northeastern (NE) Iceland (Dagsson-Waldhauserova et al., 2013).
In total, over 34 dust days per year on average occurred in Iceland based on
conventionally used synoptic codes for dust observations. However, frequent
volcanic eruptions, with the re-suspension of volcanic materials and dust
haze, increased the number of dust events fourfold (135 dust days annually).
The position of the Icelandic Low determined whether dust events occurred in the
NE (16.4 dust days annually) or in the southern (S) part of Iceland (about 18
dust days annually). The decade with the most frequent dust days in S Iceland was the
1960s, but the 2000s in NE Iceland. A total of
32 severe dust storms (visibility < 500 m) were observed in Iceland
with the highest frequency of events during the 2000s in S Iceland. The Arctic dust
events (NE Iceland) were typically warm, occurring during summer/autumn
(May–September) and during mild southwesterly winds, while the subarctic dust events
(S Iceland) were mainly cold, occurring during winter/spring (March–May)
and during strong northeasterly winds. About half of the dust events in S Iceland
occurred in winter or at sub-zero temperatures. A good correlation was found
between particulate matter (PM
The frequency of dust episodes is monitored around many of the major desert
areas of the world. Detailed and long-term studies on wind erosion
variability can potentially explain the climatological and environmental
changes in the past. Periodical dust occurrences can affect ecosystem
fertility and spatial and temporal distribution of animal and vegetation
species similarly to climate variations (Fields et al., 2010). Oceanic
ecosystems receive high amounts of nutrient rich dust spread over large
areas where deserts occur near the sea (Arnalds et al., 2014). The long-term
dust variability studies based on meteorological observations present up to
90 year old records from North America, Africa, Asia and Australia
(N'TchayiMbourou et al., 1997; Qian et al., 2002; Natsagdorj
et al., 2003; Ekström et al., 2004; Jamalizadeh et al., 2008; Steenburgh
et al., 2012). Engelstaedter et al. (2003) reported high dust activity at
many weather stations located in high-latitude regions. Cold climate regions
are represented by long-term dust frequency in Northeast (NE) Iceland
(Dagsson-Waldhauserova et al., 2013). Dust emission intensity and deposition
rates in active glacial environments have been found to be very high, in some cases
far exceeding those in lower latitudes (Bullard, 2013). Ganopolski et al. (2009) calculated glaciogenic dust deposition > 50 g m
Dust events in Arctic and subarctic regions have been observed in Alaska (Nickling, 1978; Crusius et al., 2011), Greenland (Bullard, 2013), Svalbard (Dornbrack et al., 2010) and Iceland (Arnalds, 2010; Prospero et al., 2012; Thorarinsdottir and Arnalds, 2012). Arctic coastal zones are considered the windiest regions on Earth (Eldridge, 1980). Strong winds in Iceland cause some of the most extreme wind erosion events recorded on Earth (Arnalds et al., 2013).
The highest dust emissions in Arctic regions occur in summer and early autumn (Nickling, 1978; Bullard, 2013; Dagsson-Waldhauserova et al., 2013). Dust concentrations in subarctic regions peak in spring (April–June, Prospero et al., 2012). In contrast, cold and winter periods have higher glaciogenic dust deposition than warm periods in the past (Ganopolski et al., 2009). Dust events are frequent during dry years (Steenburgh et al., 2012; Dagsson-Waldhauserova et al., 2013), but suspended dust is also observed in high precipitation and low wind conditions (Dagsson-Waldhauserova et al., 2014).
Iceland is an important source of volcanic sediments that are subjected to
intense aeolian activity (Arnalds, 2010; Prospero et al., 2012;
Thorarinsdottir and Arnalds, 2012; Arnalds et al., 2013) and is likely the
largest glaciogenic dust source area in the Arctic–subarctic region. Total
emissions of dust from Icelandic dust sources are of the range 30 to 40
million tons annually, with 5–14 million tons deposited annually over the
Atlantic and Arctic oceans (Arnalds et al., 2014). Seven major dust plume
sources have been identified (Arnalds, 2010). These sources are all in the
vicinity of glaciers. The most active glacial flood plain, Dyngjusandur,
covers an area of about 270 km
Dust suspension is related to reduced visibility. Wang et al. (2008) found a
good correlation between PM
A map showing the locations of weather stations in northeastern and central Iceland (large black circles) and stations in the northwestern and southern part of Iceland (small circles). The red areas depict the major dust sources in Iceland. The base map is drawn from the Agricultural University of Iceland Erosion Database (Soil Erosion in Iceland).
The main objectives of this study were to explore the long-term (63 years) frequency of dust events in Iceland. Emphasis was given to determining the climatology and character of Arctic and subarctic dust events. In addition, the relationship between available dust concentrations and visibility during dust observation was investigated and the frequency of dust events placed in an international perspective.
Weather stations in Iceland reporting synoptic observations. Observation period, number of dust observations, dust days and dust days per year are included. Stations are listed in descending order of number of dust days.
A network of 30 weather stations (15 in S Iceland, 8 in NE Iceland, and 7 in northwest (NW) Iceland) operated by the Icelandic Meteorological Office (IMO) was chosen for the study (Fig. 1). Note the closer distance of the weather stations to the dust sources (red areas) in S Iceland than in NE Iceland. Table 1 shows the duration of station operation: the majority of stations have been in operation since 1949. The data consist of conventional meteorological parameters such as wind velocity, wind direction, temperature and visibility, accompanied by visual observations of present weather. Notice that visibility was not measured but rather estimated by the observer, e.g. on the basis of the visibility of, and known distance to, several landmarks. Present weather refers to atmospheric phenomena occurring at the time of observation, or weather preceding the time of observation (IMO, 1981). The synoptic codes (ww) for present weather that refer to dust observation are 07–09, and 30–35 (WMO, 2009). In addition, codes 04–06 are considered, but only if the codes for primary or secondary past weather (ww1, ww2) are 03 for “blowing soil, dust, sand and dust storm” (IMO, 1981; Dagsson-Waldhauserova et al., 2013). The weather reports were made 3–8 times a day.
Meteorological observations (synoptic codes for dust including 04–06 and
visibility) were evaluated with available particulate matter (PM) mass
concentrations data provided by the Environmental Agency of Iceland (EAI).
The PM
Most of the conventional dust studies do not include the synoptic codes (04–06)
for “visibility reduced by volcanic ashes”, “dust haze” or “widespread
dust in suspension in the air” into the criteria for dust observation
(Dagsson-Waldhauserova et al., 2013). Comparing these codes with available
dust concentration measurements showed that a PM
The initial data set was built from the occurrence of “dust observation” made at one or more weather stations. Long-term dust activity was expressed in dust days. A “dust day” was defined as a day when at least one station recorded at least one dust observation. About 29 % of the observations did not include information on the present weather and they were excluded from the data set. A dust event (DE) refers to the dust observation.
Dust concentration measurements can be compared to the weather observations
at few stations in S Iceland and for a short time period. For the
stations where PM
Dust event classification based on visibility criteria. Frequency of dust events, mean wind velocity, mean temperature, and annual number of dust days of each dust class are included. S is the southern part and NE the northeastern part of Iceland.
A mean of 34.4 dust days per year was observed in Iceland during the period 1949–2011. An annual mean of 16.4 dust days (total of 1033 days) was recorded in NE Iceland (Dagsson-Waldhauserova et al., 2013) and about 17.9 dust days (total of 1153 days) in southern parts of Iceland in 1949–2011. Figure 2 shows that the most dust-active decade in Iceland was the 1960s, while the lowest number of dust days occurred in the 1980s. For S Iceland, the highest frequency of dust events was in the 1950s–1960s, whereas the 2000s had the most frequent dust days in the NE Iceland. The Grímsstaðir station (NE) is the dustiest weather observation location in Iceland with > 12 dust days annually. The following dusty stations with > 3 dust days annually are represented in Table 2: Höfn (S), Vatnsskarðshólar (S), Egilsstaðir (NE), and Hella (S). The stations with highest dust frequency in the S Iceland are displayed in Fig. 2 (NE stations published in Dagsson-Waldhauserova et al., 2013a). The stations Höfn and Vatnsskarðshólar reported the highest number of dust days in the 1950s–1960s; the station Hella observed the highest dust period in the 1960s–1970s; and a new station in Hjarðarland (established in 1990) was the most active in the 2000s. Dust events were less severe in the 2000s than in the 1950s–1990s, as reflected by increased visibility during dust observations. Mean visibility during dust observations in S Iceland was 23.3 km, indicating more severe dust events in S than in NE Iceland (mean DE visibility 26.7 km) or that weather stations in S Iceland are closer to major dust sources. Including codes 04–06 into the criteria for dust observation, the annual mean dust day frequency was 135 dust days with 101 dust days observed in S Iceland and 34 dust days in NE Iceland.
Left panel: total number of dust days, all stations combined (blue bars for southern and northwestern Iceland, brown bars for northeastern Iceland). Lines represent mean visibility (blue for S, brown for NE Iceland). Right panel: individual stations in south Iceland sorted by decade.
Number of dust days (blue bars for the southern and northwestern part of Iceland, brown bars for northeast Iceland) and 3 year moving averages of dust day frequency (lines: red for NE, light blue for S Iceland).
The annual number of dust days in 1949–2011 is depicted in Fig. 3. The dustiest years were 1955, 1966 and 2010, when over 55 dust days occurred annually. The least dusty period was 1987–1990 with 11–15 dust days annually. Dust events occurred more frequently in S Iceland than in NE Iceland in 1949–1954, 1962–1975, 1978–1981, and 2009–2011. The NE dust events were observed more often in 1955–1961, 1976–1977, 1982–1986, and 1992–2008 (except 1994 and 2003). There is a tendency for either the south or the north to be more active at a given time. Dust events observed on the south coast of Iceland and NE Iceland usually do not occur on the same dust day. The years with relatively severe dust events (and annual visibility during dust observations < 15 km) were 1949, 1966, 1975, 1996, and 1998.
The seasonal distribution of dust days in S Iceland showed that about 47 % of dust events occurred in winter (November–March) or during sub-zero temperatures. Dust days, as shown in Fig. 4, were most often in May (18 % of dust days), April (13 %) and March (11 %). The lowest occurrence of dust days (< 6 %) was in January, December, August and September. In contrast, dust events in NE Iceland occurred mainly in summer and early autumn (May–September, Dagsson-Waldhauserova et al., 2013).
The mean DE temperature in S Iceland was 3
Dust observations in S Iceland reported high mean DE wind velocity of 13.6
Number of dust days per month (bars) and monthly means of dust visibility (line) in the southern part of Iceland, 1949–2011.
The most common wind direction during dust events in S Iceland was N–NE, mainly reported from the stations Höfn, Hella, Vatnsskarðshólar, Kirkjubæjarklaustur, Stórhöfði, Eyrarbakki, Vík, Þingvellir, Hjarðarland, Keflavík, and Reykjavík (Fig. 6). Dust events were often observed from the wind direction E–NE (Hæll, Vatnsskarðshólar), E–ESE (Stórhöfði, Vatnsskarðshólar, Þingvellir, Reykjavík, Keflavík), NW–NNW (Höfn), and W–WNW (Vatnsskarðshólar). The DE wind directions in NE Iceland were predominantly SW–S and SSE–SE (Dagsson-Waldhauserova et al., 2013).
Seasonal variability in temperature and wind velocity during dust events in
S Iceland is depicted in Fig. 7. The DE mean temperatures in the October–May
period are several degrees lower than the long-term monthly temperatures
(higher in June–August period). Generally, the DE temperature in S Iceland
was about 1.7
Temperature
The DE wind velocities were significantly higher (5–11
Wind directions during dust events in S
Iceland in 1949–2011. Weather stations that observed mainly wind directions:
0–18
Reported dust events were of different severity. Where no atmospheric dust measurements are available, visibility during dust observation is used to estimate the dust event severity. Table 2 describes the dust event classes based on the visibility ranges. The most frequent were dust observations of “suspended” and “moderate suspended dust” (NE 73 %; S 59 %) with visibility 10–70 km, “severe” and “moderate haze” (NE 24 %; S 32 %) with visibility 1–10 km, and “severe” and “moderate dust storm” (NE 3 %; S 5 %) with visibility < 1 km. There were 32 “severe dust storms” (visibility < 500 m) observed in Iceland (14 in NE mostly in the 1950s, 18 in S mostly in the 2000s).
Monthly mean values (solid lines) and total mean values (dashed lines) of temperature
The DE severity increased with the DE wind velocity, but the DE temperature decreased with the DE severity, except for a “moderate dust storm” recorded mostly at the Vík station in S Iceland (Table 2). The parameters show that dust events in S Iceland were more severe than in NE Iceland.
Most of the dust classes in S Iceland occurred in April and May. Severe dust
storms were most frequent in March and January at Vík, Hella,
Kirkjubæjarklaustur, Hæll, Eyrarbakki and Vatnsskarðshólar stations.
The station Vík, located only about 10 km from the Mýrdalssandur dust source,
reported a mean DE visibility of 2 km indicating very severe dust events.
The following stations had the lowest mean DE visibility: Raufarhöfn (NE,
15 km), Höfn (18.3 km), Kirkjubæjarklaustur (20.1 km), Stórhöfði
(20.4 km), and Hella (21.1 km). The highest mean DE wind velocity was
measured at the most windy station: Stórhöfði (22.6
Monthly wind directions during dust events in the southern part
of Iceland, 1949–2011. Weather stations that observed mainly wind directions of:
0–18
About 18 % of dust days in S Iceland were observed at more than one station at a time (two stations: 12.5 %; three stations: 3.4 %; four or more stations: 1.5 %). Dust co-observations were mostly in Kirkjubæjarklaustur and Höfn, Kirkjubæjarklaustur and Vatnsskarðshólar, and Kirkjubæjarklaustur with Hella. The Reykjavík station observed dust together with Hella or Þingvellir.
Hourly PM
Hourly PM
An annual mean of 34 dust days recorded in Iceland is comparable to dust
studies from the dust-active parts of China (35 dust days yr
Trends in global dust emissions show high dust frequency during the 1950–1960s, and low frequency during 1980s in the USA, Australia and China as well as in Iceland (Steenburgh et al., 2012; Ekström et al., 2004; Qian et al., 2002). The 2000s were reported as the most active decade in Iran and in NE Iceland (Jamalizadeh et al., 2008). Dust periods retrieved from the ice-cores data during the GISP2 project in Greenland correlate with the NE Iceland dust frequency in the 1950–1990 period (Donarummo et al., 2002).
Generally, the period 1950–1965 was warm and dry in Iceland resulting in
frequent dust suspension (Hanna et al., 2004). For NE Iceland, the dustiest
year was 1955 with 37 dust days, and it coincided with one of the warmest
and driest years in NE Iceland (Hanna et al., 2004). For S Iceland, the most productive dust event period was during 1965–1968. It
was a period of below-average precipitation, reported at stations Reykjavík,
Stykkishólmur and Vestmannaeyjar (Hanna et al., 2004), while 1965 was the
driest year in SW Iceland for the past 100 years. The 20th century warm
period in Iceland (1920s–1965) ended very abruptly in 1965 with about a
1
The 1970s were cold with high precipitation, but strong winds were often observed in S Iceland bringing the dust into suspension. The 1980s and 1990s were cold and with high precipitation in S Iceland while the 1990s were warm in the NE (Hanna et al., 2004). A high frequency of dust events in NE Iceland during the 2000s was associated with dry and warm Junes. A high number of dust days in S Iceland in 2010 was often due to the resuspension of volcanic ash from the Eyjafjallajökull eruption during very frequent northerly winds (Petersen et al., 2012). The annual differences in dust event frequency do not correspond to trends of the global climate drivers such as the North Atlantic Oscillation (NAO), the Arctic Oscillation or prevailing ocean currents (Dagsson-Waldhauserova et al., 2013). The main driver is likely an orthogonal pattern to NAO, with the dipole of Sea Level Pressure oscillation oriented east–west (Dagsson-Waldhauserova et al., 2013).
The position of the Icelandic Low determines whether dust plumes travel in a northeasterly or southerly direction. Strong winds in Iceland are almost always associated with extratropical cyclones with strong precipitating systems (fronts). Under such circumstances, there is, in general, only dry weather on the downstream side of the central highlands of Iceland, and this is where the dust is suspended. Higher frequency and severity of DE (low visibility and high wind speeds) in S Iceland than in NE Iceland is likely due to the close proximity of the S stations to the dust sources, a higher number of major dust sources, as well as a higher number of stations in the south (Fig. 1, Table 2). The Grímsstaðir station (NE) is > 100 km from the Dyngjusandur source, while the southerly stations are in the range of tens of km from the sources. Dust deposition rates and DE severity decrease exponentially with distance from the source (Arnalds et al., 2014). This may lead to underestimation of dust events in S Iceland because the stations, located close to the sources, are not able to fully capture the developed dust plume, but only its initiation part, extending several km in width. The dustiest weather station, Grímsstaðir, is located at a great distance downwind of the most active glacial plain in Iceland, Dyngjusandur, N of the Vatnajökull glacier, and it captures a high number of dust events. On the other hand, many dust events are not detected, as dust is often blown directly towards the sea from the sources close to the southern coastline, i.e Mýrdalssandur and Skeiðarársandur. However, the most active stations are equally distributed around the areas with very high dust deposition (Arnalds, 2010) from the central NE, SE, S to SW Iceland. The land reclamation activities from the 1950s and 1970s (Crofts, 2011) resulted in decreased dust activity at Hella and Höfn stations (Fig. 2).
The local dust sources in S Iceland are also affected by milder oceanic climate during the winter, while the NE highland dust sources are covered by snow for much of the winter. The DE temperatures were higher in NE Iceland than S Iceland as the events occurred during summer–autumn, and warm geostrophic southerly winds cause the dust events in NE Iceland. Table 2 shows low DE temperatures in S Iceland, which point to frequent winter–spring dust occurrence and cold strong northerly winds causing dust events in S Iceland. The mean wind speeds are variable each month in S and NE Iceland. In S Iceland, the highest wind speeds were related to the winter months and April, while in NE Iceland, the windiest months were May–June and September. All these months of high winds correlate with high dust frequency. The northerly winds, which caused dust events in S Iceland, were stronger than the winds in NE Iceland, affecting the results in Table 2.
The visibility during dust observations reflects the severity of the dust events. There is an increasing trend in DE visibility through the decades with the maxima in NE as well as S Iceland in the 2000s (Fig. 2). However, most of the severe dust storms with visibility < 500 m occurred in S Iceland in the 2000s. These severe dust storms were related to frequent re-suspension of volcanic ashes at the station Vík, located downwind of the Eyjafjallajökull volcano, in 2010. The increase in dust frequency in the 2000s was coincident with a dust visibility increase. The 2000s were a warmer decade in Iceland compared to the previous decades, 1970s–1990s. This may indicate less availability of fine materials susceptible to dust production determined by changes in flow rates at major glacial rivers in the 2000s, but the reason remains unclear.
The seasonal distribution of dust events in Iceland shows that the high dust period is from March to October. The NE dust events are typically warm, occurring during summer and autumn (May–September) while the S dust events are mainly cold, occurring during winter and spring (March–May). This is related to the sea level pressure pattern which controls the warm southerly winds in NE Iceland and the cold northerly winds in S Iceland (Bjornsson and Jonsson, 2003). The S dust events were, however, more equally distributed throughout the year. The winter season is related to mild temperatures and high winds in S Iceland. Relatively high mean dust concentrations were measured during winter (January–March) at Stórhöfði station (Prospero et al., 2012). Winter cold dust storms were frequently also observed in Mongolia (Natsagdorj et al., 2003). The highest number of dust storms occurred in March–May while the mean March–April temperatures were sub-zero.
The predominant winds
during dust events were NE and NNE winds in March and April, when the mean
wind speeds were about 15 m s
There are several processes responsible for dust events in Iceland. The main drivers were strong winds during periods of low precipitation, enhanced by a limited hygroscopicity of the materials and rapid drying of the dark-coloured surfaces. Dust events in NE Iceland occurred mainly during summer, when the highland dust sources were snow-free and under relatively mild temperatures; while in S Iceland, the dust events occurred also during very low and sub-zero temperatures. Nevertheless, dust events can also be observed during high precipitation seasons < 4 h after rain (Dagsson-Waldhauserova et al., 2014). For instance, even the highest precipitation year (1972) had a relatively high dust frequency. The majority of dust events reported in this long-term study were observed during strong winds.
Visibility during dust observations is an important indicator of dust event
severity. To estimate the empirical relationship between visibility and dust
concentration in Iceland, we compared available PM
The relationship between available PM
This study on long-term dust frequency showed high dust day frequency in the
volcanic and glacial deserts of Iceland. Several dust plumes, captured by
the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra
satellite, exceeded 1000 km travelling towards Europe, North America and the
Arctic. Further, it was calculated that dust is deposited over 370 000 km
This study of long-term dust observations in Iceland showed that dust day
frequency in Iceland can be comparable to the major desert areas in the
world. It was found that dust events often occurred during winter and at
sub-zero temperatures. Observed dust events were more severe in S Iceland than in NE Iceland, most likely because of the closer proximity
of the southerly weather stations to major dust sources. The highest
frequency of dust events was during the 1960s in S Iceland while most of the
dust events in NE Iceland occurred during the 2000s. The highest number of
severe dust storms (visibility < 500 m) was observed in S Iceland during the 2000s. Monitoring dust frequency in active volcanic and glacial deserts requires the inclusion synoptic codes for
“visibility reduced by volcanic ashes” and “dust haze” into the criteria
for dust observation. A moderate correlation was found between
available PM
The work was supported by the Eimskip Fund of The University of Iceland and by the Nordic Centre of Excellence for Cryosphere–Atmosphere Interactions in a Changing Arctic Climate (CRAICC). We would like to thank Joseph Prospero from the University of Miami, USA, and Thorsteinn Johannsson from the Environment Agency of Iceland for providing the PM data for the dust measurements. Edited by: A. Stohl