One-year continuous measurements of aerosol black carbon (BC) at the background site Preila (55°55′N, 21°00′E, 5 m a.s.l., Lithuania) were performed. Temporal and spatial evolution and transport of biomass burning (BB) and volcanic aerosols observed within this period were explained by the air mass backward trajectory analysis in conjunction with the fire detection data produced by the MODIS Rapid Response System and AERONET database. The surface measurements and analysis of the Angström exponent of the absorption coefficient done separately for shorter and longer wavelengths (i.e.,
Atmospheric aerosol particles, which are very small and suspended in the air, affect Earth’s climate directly by scattering and absorbing the atmospheric radiation and indirectly by acting as cloud condensation nuclei and modifying the optical properties of clouds. Furthermore, human exposure to fine particulate matter is associated with adverse impacts on human health [
High concentrations of carbonaceous particles, containing BC or organic compounds (OC) (organic carbon), are found mainly in the submicron size range. The main natural sources of BC are volcanic eruptions, forest fires, and so forth. Wildfires (biomass burning) emit a variety of trace gases and particulates [
The eruption of Grimsvötn in Iceland (May 21, 2011) initiated a great number of research activities to characterize the physical, chemical, and optical properties of airborne volcanic material associated with this eruption. The main challenge in measuring volcanic material in the surface air is that this material is always mixed with other pollutants present in the air. A large fraction of carbonaceous component of the aerosol (in upper troposphere/lower stratosphere) was identified by Murphy et al. [
The long-range transport (LRT) of atmospheric particulate matter (PM) is a transboundary problem that can have significant impacts on PM10 and PM2.5 levels in background European areas [
Combining the variety of terrestrial information operationally derived from satellite data including those from NASA’s MODIS instrument enables linking the different temporal and spatial scales. The use of the Angström exponent
In this work, the pollution events due to biomass burning that occurred in Lithuania on March 21–25, 2011 and pollution episode caused by long-range transported volcanic material from the Grimsvötn eruption that took place at the end of May 2011 in Iceland were investigated. Air mass backward trajectories, fire satellite observations, dispersion modeling results, and emission source data were used to identify the source. The mass and number concentrations of PM2.5 in conjunction with Angström exponent of the absorption coefficient and other optical properties of aerosol were studied.
The aerosol particle concentration measurements were performed at the Preila environmental pollution research station (55°55′N, 21°00′E, 5 m above sea level) in the coastal/marine environment. This station is located on the Curonian Spit, which separates the Curonian Lagoon and the Baltic Sea, and thus can be characterized as a regionally representative background area. The Curonian Lagoon, the largest coastal bay in the Baltic Sea, is a highly eutrophied water body. One of the nearest industrial cities, Klaipeda, is at a distance of about 40 km to the north, and the other major city, Kaliningrad, is 90 km to the south from Preila.
Air mass backward trajectory analysis provides a better understanding of air flow and long-range transport patterns during the events. We analyzed the aerosol characteristics with respect to categorized air mass backward trajectories for the initial estimation of the wildfire and volcanoes potential source locations and quantitative contribution. Backward trajectories were produced using the Flextra model (NILU; Institute of Meteorology and Geophysics, Vienna) [
To identify possible events of regional transport, hotspot/fire the location of a thermal anomaly was detected by MODIS using data from the middle infrared and thermal infrared bands [
The AErosol RObotic NETwork (AERONET) is a ground-based network of Cimel sun photometer that measures the extinction aerosol optical depth at 7 wavelengths
The Navy Aerosol Analysis and Prediction System (NAAPS) model results were used to determine the distribution of aerosols from fires and volcanoes (model description and results are available from the web pages of the Naval Research Laboratory, Monterey, CA, USA;
A Magee Scientific Company Aethalometer, Model AE31 Spectrum, manufactured by Optotek, Slovenia, was deployed at the Vilnius site and provided real-time, continuous measurements of the BC mass concentrations. The optical transmission of carbonaceous aerosol particles was measured sequentially at seven wavelengths
This is the default value set by the manufacturer for a wavelength of 880 nm. The Aethalometer data recorded with a 5-minute time base were compensated for loading effects using an empirical algorithm [
The absorption coefficient
A power law fit is commonly applied to describe the wavelength dependence of aerosol absorption coefficient
The aerosol size distribution was measured by using the Scanning Mobility Particle Sizer (SMPS) built by the Leibniz Institute for Tropospheric Research, Germany. The SMPS is composed of a differential mobility analyzer and CPC UF-02proto. This system had the following general specifications: size range: 8–900 nm; scan time: 5 min; resolution: 71 size channels.
In general, BC particles are mainly produced by anthropogenic activities such as fossil fuel burning (industries and transport) and biomass burning (agriculture or wildfires). However, the natural sources such as wildfires and volcanic eruptions also contribute to the BC mass concentration, but their contributions are relatively much less. We focused on the strongest periods of wildfire and volcanic events. The first event occurred on April 24–30, 2011 and the second on May 25–27, 2011. The concentrations observed during both events are clear outliers in their respective series, both for hourly concentrations and 24 h means.
Box and whisker plots of BC concentrations and mass concentration frequency distribution are shown in Figures
(a) Box and whisker plots for comparison of mass concentrations at Preila; (b) BC concentration frequency distribution.
Active fires detected during April 24–30, 2011 during high BC concentration event: (a) by the MODIS Rapid Response System, ((b), (d)) sulphate, dust, and smoke surface concentration (
The median and outliers of each dataset are also shown. It can be clearly seen that higher concentrations of BC are observed during October to March, when cold weather conditions prevail during any year.
The concentration shows a decrease in June (
Taking 25 ng m−3 value as frequency statistics step, the frequency distribution map of hourly average concentration of BC aerosol is depicted in Figure
Fire Mapper products have provided data about the location and extent of fires during theevent days (Figure
Grass burning products were redistributed over Lithuania by the large-scale and regional atmospheric circulation. The air mass backward trajectory analysis showed that particles from BB with air masses in 3 days were brought to Lithuania and caused high peaks of aerosol number and BC concentrations. BC and aerosol number concentrations rose to higher levels over the whole territory of Lithuania. During this event the 1-hour average aerosol particle number concentration reached 4000 cm−3, BC-3500 ng m−3, while normally average concentration values are about 3100 cm−3 and 580 ng m−3, respectively. It should be noted that during April 24–30, 2011, urban sites belonging to the Lithuanian Automatic Urban Network (EPA) (
Data analysis of the BC combined with TERRA/Moderate Resolution Imaging Spectroradiometer MODIS fire detections (Figure
During the air mass transport to Lithuania, aerosol optical properties changed due to both deposition and mixing with local aerosols. The similar events were detected and studied by lidar networks covering the whole continent [
(a) The spectral variation of instantaneous measurements of AOD for April 2011 showing the gradual buildup and decline of smoke concentrations in this period (April 25–29, blue cycle); (b) values of the Angström exponent of the absorption coefficient (April 29, 2011) showing dynamic variations as a result of both increasing (decreasing) AOD and BC concentration and greater domination of fine mode versus coarse mode optical depth.
The wavelength dependence of the light absorption can be better approximated by separate exponential fits of the shorter (370–520 nm) and longer (660–950 nm) wavelengths obtained by an exponential curve fit over all 7 wavelengths. The Angstrom exponents were calculated by fitting
On the event day (April 29) the mean
Kirchstetter et al. [
These very high values of the absorption exponent are indicative of biomass combustion [
The observed variations in the Angström exponent at 590–950 nm derived from the BC measurements are not consistent with AERONET Angström parameter (Figures
Figure
Backward trajectories of the air masses from the volcano at Grimsvötn to Lithuania on 24–27 ((a)–(d)) May 2011 (12:00 UTC).
The sulphate concentration in the air was used as an indicator of volcanic pollution as sulphur dioxide is the third most abundant gas in volcanic emissions, after water vapor and carbon dioxide [
A carbonaceous aerosol component has been observed in fresh volcanic clouds from several volcanoes [
Time series of the hourly mean Angström exponent of the absorption coefficient on May 24–28, 2011.
Figure
Parallel to the measurement of
The light Angström exponent of the absorption coefficients
It is to be noted that the Angstrom exponent could be a rough indicator of the size distribution of the aerosol particles whereas the Angström exponent of the absorption coefficient tells about the nature of absorbing aerosols. A low
Volcanic particles are assumed to be composed of ash particles and hygroscopic sulfates. Volcanic particles are injected in the atmosphere both as primary particles rapidly deposited due to their large sizes on time scales of minutes to a few weeks in the troposphere and as secondary particles mainly derived from the oxidation of sulphur dioxide [
Contour plot of new particle formation and growth on 24 May 2011; colour represents logarithm of concentration in cm−3.
As shown in Figure
The growth rate of new particles.
Date | ||||
---|---|---|---|---|
24 May | 25 May | 26 May | 27 May | |
Growth rate, nm h−1 | 5.36 | 2.55 | 5.58 | 25.20 |
The growth rates were in the range of 2.55–25.20 nm h−1; the growth rates are in the range of typical observed particle growth rates (1–20 nm h−1) [
A combination of ground-based and satellite observations was used in this study to investigate different sources of aerosol BC loadings over Lithuania during two different time periods, April 24–30 and May 24–2, 2011. The 7-wavelength setup of the Aethalometer has been exploited to obtain information on variation of spectroscopic properties of carbonaceous particles. Although BC mass concentration values do not allow one to a priori determine which process produces BC, the wavelength dependence of the BC absorption gives the option to follow the variation carbonaceous aerosol properties and easily characterize the black carbon/organic component.
Near-source biomass burning smoke is observed routinely in Lithuania, while volcanic effluent plumes provided some new information. During these periods the measured black carbon concentration during BB and volcanic activity events (3500 and 1200 ng m−3, resp.) exceeded previously established average values of 580 ng m−3. Compared to background aerosols, the sampled plumes have higher AOD and contain particles having expected differences in the Angstrom exponent and size. During the wildfire burning event on April 24–30 2011 the highest mean values of the Angström exponent of the absorption coefficients
The observational data and analysis presented here demonstrate that nucleation and subsequent growth can be derived from volcanic eruption gaseous released and that this new secondary particle formation event could occur within the lower troposphere at a large distance from the eruptive activity.
This work was supported by the Lithuanian-Swiss Cooperation Programme “Research and Development” project AEROLIT (Nr. CH-3-ŠMM-01/08). The authors thank Piotr Sobolewski, Brent Holben, and Aleksander Pietruczuk and their staff for establishing and maintaining the Belsk site used in this investigation. The authors declare that they have no competing interests as defined by Advances in Meteorology or other interests that might be perceived to influence the results and discussion reported in this paper.