Vertically resolved optical and microphysical properties of biomass burning aerosols, measured in 2011 with a multiwavelength Raman lidar, are presented. The transportation time, within 1-2 days (or less), pointed towards the presence of relatively fresh smoke particles over the site. Some strong layers aloft were observed with particle backscatter and extinction coefficients (at 355 nm) greater than 5 Mm−1 sr−1 and close to 300 Mm−1, respectively. The particle intensive optical properties showed features different from the ones reported for aged smoke, but rather consistent with fresh smoke. The Ångström exponents were generally high, mainly above 1.4, indicating a dominating accumulation mode. Weak depolarization values, as shown by the small depolarization ratio of 5% or lower, were measured. Furthermore, the lidar ratio presented no clear wavelength dependency. The inversion of the lidar signals provided a set of microphysical properties including particle effective radius below 0.2
Forest fires, agricultural burns, and the widespread use of wood as fuel for heating and cooking are major sources of pollution related to biomass since they dominate the production of primary carbonaceous particles (see, e.g., [
Despite the recognition of the importance of free tropospheric aerosol layers in the climate forcing, the determination of their temporally and vertically resolved properties still poses numerous problems. The spatial (3-D) and temporal distribution of aerosols are a crucial input in the global atmospheric models to access the influence of aerosols on climate. The limitations of the observational techniques for characterizing free tropospheric aerosols are one of the difficulties to overcome. On the one hand, satellite and ground-based passive remote sensing devices detect the whole atmospheric column and thus cannot separate particles within the boundary layer from layers aloft, that is, in the free troposphere or stratosphere. On the other hand, using in situ techniques on board aircrafts is costly, and therefore not practical for long term studies. One approach consists in using the space-borne lidar observations (with the Cloud-Aerosol Lidar with Orthogonal Polarization, CALIOP), on board of the satellite CALIPSO (Cloud-Aerosol Lidar and Pathfinder Satellite Observations) [
In this work, fresh smoke aerosol from forest fires, detected as several plumes in the free troposphere over Portugal, are studied in terms of optical and microphysical properties, based on multiwavelength Raman lidar measurements. The lidar measurements were carried out between 17 and 19 October 2011, while numerous forest fires were occurring in several regions of the western Iberian Peninsula, both in Portugal and in Spain.
In the following sections, the site is briefly presented as well as the multiwavelength lidar system PAOLI (Portable Aerosol and Cloud Lidar), the inversion algorithm for the retrieval of microphysical properties, and other used methodologies. The results are shown and discussed in Section
Évora (38.5° N, 7.9° W, 300 m above sea level (a.s.l)) is located inland in the south-western region of the Iberian Peninsula. The city (<60000 inhabitants) is the capital of Alentejo, a rural region in Southern Portugal, which covers about one-third of the area of Portugal but has a low population density. The distance to the capital, Lisbon, is about 130 km. The regional landscape consists primarily of soft rolling hills and wide plains. No polluting industries exist in the vicinity of Évora; thus local anthropogenic pollution is caused by traffic and, in winter, domestic fuel burning as well [
The Portable Aerosol and Cloud Lidar (PAOLI) of the Évora Geophysics Center (CGE) is a multiwavelength Raman lidar of the type
The profiles of particle extinction coefficients,
A set of lidar measurements which were performed during nighttime between 17 and 19 October 2011 were used in the analysis. Therefore both the particle backscatter and extinction coefficients profiles, at 355 and 532 nm, were obtained by applying the so-called Raman method [
The optical data retrieved with the lidar
Additional information was provided by an automatic sun tracking photometer (CIMEL CE-318-2) also operating at CGE facilities in the framework of the Aerosol Robotic Network (AERONET) [
Moreover, three-day backward trajectories arriving at different heights in the free troposphere were computed with the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) [
The vertically resolved characterization of the smoke particles that follows, in terms of their optical and microphysical properties, is based on a set of nighttime measurements available from 17 October 2011 to 19 October 2011 at dawn.
The daytime maximum columnar atmospheric turbidity during this event was recorded by the AERONET sun photometer during the afternoon of 18 October 2011. For that reason, focus was given to lidar measurements performed around this time period, that is, during the previous and following evenings. A large number of forest fires occurred in the preceding days, as shown in Figure
Fire hot spots detected by MODIS on board the Terra and Aqua satellites in the period from 15 to 18 October 2011 (
The HYSPLIT trajectories drawn in the MODIS fire map were computed for the height range between 2 and 4 km for times centered during the different periods of the lidar measurements. They suggest the transport of particles from the areas with large number of fire hot spots. In the first period of the measurements (evening of 17 October 2011) air masses were transported towards Évora after crossing southern and central regions of Spain. In the subsequent measurement periods (evening of 18 October and 19 October at night) the airflow changed and the trajectories indicated air masses that transported smoke from the forest fires occurring in the northwest regions of the Iberian Peninsula. The backward trajectories suggested a relatively short transport time, between about 1 and 2 days, or even less.
On 18 October 2011 the smoke plumes over Portugal and Spain and the atmospheric turbidity caused by those fires were particularly evident in the MODIS data. The respective Terra-MODIS products for that day, depicted in Figure
(a) Terra-Modis aerosol optical depth at 550 nm and (b) Terra-Modis Ångström exponent (470/660 nm) on 18 October 2011.
Figures
Profiles of (a) backscatter coefficients (355-blue, 532-green, and 1064 nm-red), (b) extinction coefficients (355-blue and 532 nm-green), (c) backscatter and extinction-related Ångström exponent (wavelength pair 355/532 nm), (d) lidar ratios (355 and 532 nm), and (e) particle linear depolarization ratio (532 nm) measured on (a) 17 October 2010 20:20–22:10 UTC.
Profiles of (a) backscatter coefficients (355-blue, 532-green, and 1064 nm-red), (b) extinction coefficients (355-blue and 532 nm-green), (c) backscatter and extinction-related Ångström exponent (wavelength pair 355/532 nm), (d) lidar ratios (355 and 532 nm), and (e) particle linear depolarization ratio (532 nm) measured on 18 October 2011 at 19:00–20:45 UTC.
Profiles of (a) backscatter coefficients (355-blue, 532-green, and 1064 nm-red), (b) extinction coefficients (355-blue and 532 nm-green), (c) backscatter and extinction-related Ångström exponent (wavelength pair 355/532 nm), (d) lidar ratios (355 and 532 nm), and (e) particle linear depolarization ratio (532 nm) measured on 18 October 2010 21:45–22:15 UTC.
Profiles of (a) backscatter coefficients (355-blue, 532-green, and 1064 nm-red), (b) extinction coefficients (355-blue and 532 nm-green), (c) backscatter and extinction-related Ångström exponent (wavelength pair 355/532 nm), (d) lidar ratios (355 and 532 nm), and (e) particle linear depolarization ratio (532 nm) measured on 19 October 2010 00:00–02:00 UTC.
Multilayered structures were visible during the different periods, which can be noticed by the variability within the backscatter and extinction profiles. Maximum values of
The Ångström exponent was usually high, showing a strong wavelength dependency of the backscatter and extinction as indicated in Figures
Mean values of the optical and microphysical properties of the aerosol layers observed between 17 and 19 October 2011.
Day | ||||||
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17 October 2011 | 18 Oct 2011 | 19 October 2011 | ||||
Time (UTC) | 20:20–22:10 | 19:00–20:45 | 21:45–22:15 | 00:07–02:05 | ||
Layer center, m | 2500 | 3600 | 2800 | 2500 | 3400 | 3250 |
Thickness (m) | 500 | 600 | 600 | 400 | 600 | 500 |
AOD355 | 0.09 | 0.13 | 0.12 | 0.11 | 0.16 | 0.10 |
AOD532 | 0.05 | 0.07 | 0.06 | 0.07 | 0.09 | 0.06 |
LR355, sr |
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Reff ( |
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CRIreal |
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CRIimag |
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SSA355 |
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SSA532 |
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SSA1064 |
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Relative frequency distributions of (a) backscatter and extinction related Ångström exponents (355/532 nm) and (b) lidar ratios at 355 and 532 nm, considering the four different periods of measurements on 17, 18, and 19 October 2011.
Analysis of the backward trajectories suggested travel times in the order of one to two days, or less, for the smoke plumes observed at Évora, although affected by some uncertainties due to the different source regions contributing to the aerosol load over Évora. In any case, the estimated travel times, in conjunction with the range of observed å, are fairly consistent with the conclusions of Müller et al. [
Extinction-related Ångström exponent and effective radius of forest fires smoke, based on Raman lidar measurements versus transport time. The results from the different cases measured at Évora are shown (crosses) as well as the average values (open squares). The average values from Alados-Arboledas et al. [
The lidar ratio profiles at 355 and 532 nm, shown in Figures
The average value of the particle linear depolarization ratio was 5.0 ± 0.6%. In all investigated periods, consistently small particle depolarization ratios were found, mainly in the range of 4–6%. Our results are in agreement with what should be expected for small size particles which were close to sphericity. Comparable values were also reported by other authors [
The particle backscatter and extinction coefficients of the most prominent smoke layers for each of the different periods (a total of six cases) were used as input of the inversion algorithm for the retrieval of the microphysical properties. For doing so, the optical data were averaged over height ranges of 400–600 m centered at the altitudes of maximum intensity (i.e., centers of the plumes). The results of the inversion algorithm are discussed in Section
Table
In this study, effective radii were small, in the range of 0.14–0.19
Volume concentrations in the centers of the plumes varied from 18 to 34
Real and imaginary parts of the refractive index were derived from the inversion of the lidar data for each case. The real part of the refractive index varied between 1.49 and 1.61, while the imaginary part was in the range 0.01–0.024. Given the comparably large uncertainties, the refractive indexes are not so different between the different cases. The retrievals reported by Alados-Arboledas et al. [
The single scattering albedo (SSA) varied between 0.82 and 0.96 with no clear wavelength dependence between 355 and 532 nm. At 1064 nm somewhat smaller values of the single scattering albedo were found, which was also observed by Dubovik et al. [
Free tropospheric smoke plumes were detected over Évora, Portugal, with Raman lidar during October 2011. Additional ground-based sunphotometer measurements, during daytime, were also used for a basic characterization of the aerosol population present in the column over the site. The main findings of our study can be formulated as follows.
The smoke particles were transported towards Évora from various regions in the Iberian Peninsula, mainly from the north-western areas facing the Atlantic Ocean, as well as from southern Spanish areas, where numerous forest fires were active.
The transportation paths suggested on the presence of relatively fresh smoke, with a lifetime of about 1-2 days, or less, and the optical and physical properties derived from the measurements also indicated that the observed smoke was not aged but relatively fresh.
These aerosol layers were observed up to about 4 km. Particle backscatter coefficients greater than 5 Mm−1 sr−1 and particle extinction coefficients close to 300 Mm−1, at 355 nm, were measured in the center of the layers aloft.
The wavelength dependence of the backscatter and extinction coefficients was usually high, indicating the presence of small particles, which is in agreement with the results obtained from the sunphotometer and MODIS satellite data. Furthermore, in general the lidar ratio presented no clear wavelength dependence and the particle depolarization ratio was consistently low, about 5%.
The microphysical properties retrieved by inversion of the lidar data provided effective radius below 20
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by FCT (Fundação para a Ciência e a Tecnologia) through the National Re-equipment Program under REDE/1527/RNG/2007, through the project PTDC/CTEATM/65307/2006 and through the projects PTDC/AAC-CLI/104925/2008 and PTDC/GEO-MET/4222/2012. The authors also acknowledge the funding provided by the Évora Geophysics Centre, Portugal, under the contract with FCT (the Portuguese Science and Technology Foundation), PEst-OE/CTE/UI0078/2011. Sérgio Nepomuceno Pereira and Jana Preißler were funded by FCT with Grants SFRH/BPD/81132/2011 and SFRH/BD/47521/2008, respectively. CGE benefits from the membership in SPALINET, EARLINET, and ACTRIS. ACTRIS Research Infrastructure Project is supported by the European Union Seventh Framework Programme (FP7/2007–2013) under Grant agreement (no. 262254). This work was also supported by the Andalusia Regional Government through the project P10-RNM-6299. The authors also thank the lidar team of the Leibniz-Institute for Tropospheric Research in Leipzig, Germany, for their support with PAOLI. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (