The number of particles and their size distributions were measured in a rural area, during the summer, using a PCASP-X. The aim was to study the influence of wildfires on particle size distributions. The comparative studies carried out reveal an average increase of around ten times in the number of particles in the fine mode, especially in sizes between 0.10 and 0.14
The size of atmospheric aerosols depends greatly on the sources and sinks, as well as on the meteorological processes dominating during their lifetime [
Many studies have found an increased number of aerosols in the fine or accumulation mode during wildfires [
The water-absorption capacity of these particles varies greatly. The hygroscopicity of aerosols originated by biomass burning depends on the internal composition of the organic and inorganic material of the submicrometer particles [
The ability of aerosols generated by biomass burning to form condensation nuclei has been the focus of several studies. Authors such as Warner and Twomey [
In this paper, we will analyze the characteristics of particle size distributions in a rural area in the summer months, and the changes in these distributions caused by the arrival of aerosols from biomass burning, mainly wildfires affecting no-tree woodland, but sometimes also trees, and crops or crop stubble. These wildfires are particularly frequent in the rural study zone. The wildfires considered are all the fires registered in the province of León, Spain. The distance with the probe installed at the sampling site varies between a few kilometres and a maximum of 70 km.
In the case of wildfires, the source of the particles is well defined and located; but in the study of aerosol size distributions, we must also take into account the origin and the type of air mass that carries the particulate matter when it reaches the sampling site [
The measurements have been carried out in the district of Carrizo de la Ribera, in the middle of the province of León, Spain, north-west of the capital (Figure
Location of Carrizo de la Ribera in the province of León. Location of the PCASP-X probe and the river Órbigo, with respect to the town.
The study zone lies on the left bank of the River Órbigo, over a fluvial terrace. The climate is of the continentalized Mediterranean type, with a marked seasonality. Precipitation is scattered irregularly along the year, with minimum values in the summer and the highest values in spring and autumn.
Temperatures are fresh, with an annual average of 10.5°C. The winters are cold, with frequent frosts. The summers are warm, with maximum temperatures that may be over 35°C, although nocturnal temperatures are moderate, as it is an area of irrigated farming systems where land is mainly watered by flooding.
A laser spectrometer (Passive Cavity Aerosol Spectrometer Probe, PMS Model PCASP-X) was installed in a field 2 km from Carrizo de la Ribera (42°35′59′′ N, 5°50′50′′ W) to determine aerosol size spectra in the rural areas. This device measures particles ranging between 0.1 and 10
Here, we are presenting PCASP-X size distributions corrected using Mie theory and implemented with a computer code developed by Bohren and Huffman [
Refractive Index of Atmospheric Aerosol Particle at a He-Ne Laser Wavelength (0.6328
Relative humidity (%) | Rural aerosol |
---|---|
0 | 1.530–6.60 × 10−3 |
50 | 1.520–6.26 × 10−3 |
70 | 1.501–5.60 × 10−3 |
80 | 1.443–3.70 × 10−3 |
90 | 1.399–2.22 × 10−3 |
99 | 1.359–9.16 × 10−4 |
It was also necessary to carry out a number of corrections on the number of counts sampled by the spectrometer in each channel. First, the flow measurement value was set in relation to the altitude of the sampling point; the probe was installed at 834 m introducing a correction factor of 0.905. In addition, each measurement was corrected according to the activity registered. Finally, the last correction was determined by the duration of the samplings, which was 600 seconds. These corrections are described in more detail in Calvo et al. [
The study period covers the 122 days of the months of June, July, August, and September of the year 2000. Eight measurements were carried out daily, measuring the ambient particle size spectrum in the rural study area automatically, during 15-minute intervals every 3 hours.
The probe was installed next to a weather station registering automatically data on precipitation, pressure, temperature, relative humidity, and wind speed and direction. A wind profiler was also used (Sodar SR1000), with a pulse frequency of 5 tones, around 2.150 Hz, pulse power of 300 W, a pulse-repetition period of 8 s, and a maximum range of 1.250 m. This device registered automatically data from the three wind components. The data registered by the weather station and the data registered by the Sodar were all stored on a computer every 30 minutes.
The thermal inversions have been calculated using data from soundings in La Coruña (43.36°N, 8.41°W, altitude 67 m), Madrid (40.50°N, 3.58°W, altitude 633 m), and Santander (43.48°N, 3.80°W, altitude 59 m) provided by the University of Wyoming (
In order to identify the type of weather associated to a particular synoptic situation, a Circulation Weather Type classification (CWTs) was designed based on Jenkinson and Collison [
The Department for the Environment of the regional government of the Junta of Castile and León provided the database with the number of wildfires, the district where each fire occurred, the date of detection and extinction (with exact date and time), the land area affected in hectares (ha), and the type of vegetation burnt during the summer months (June, July, August, and September). This information was used to draw for each day a map of the province of León highlighting all the districts affected by wildfires. One of these maps is shown in Figure
Example of a map representing the districts in the province of León, Spain, with active wildfires on 6th September 2000. The probe was installed in Carrizo.
This material and the data on wind direction at surface level and at a certain altitude, registered by the weather station and the Sodar, were used to identify the measurements carried out by the aerosol probe that could have been affected by the transport of the smoke plumes from any nearby wildfires. The changes in the number of particles revealed the arrival of a plume at the sampling site. In general, a plume affected only 1 out of the 8 measurements carried out daily, as these measurements were carried out every 3 hours. However, in the case of large fires several measurements may be affected in one day or even over several subsequent days, depending on the duration of the fire and the intensity of the wind.
The particle measurements of the 8 daily registers were compared with the time interval of the wildfire, from detection to extinction. The measurements with very large numbers of particles were thus identified. In some cases, there were 20-fold increases with respect to the other measurements registered the same day. After identifying the measurements that were possibly affected by the arrival of a smoke plume to the probe, the data were confirmed with an analysis of the wind direction and speed and the distance between the wildfire and the probe. The increases in the measurements that could not be explained by these variables were excluded from the study.
It must be taken into account that on some days high levels of particles may be explained by the arrival of smoke plumes from wildfires in other provinces close to the province of León, especially Zamora and Orense. However, the data on the wildfires in those provinces during the study period were not made available.
Two different databases were built using the information obtained: one includes the measurements affected by the smoke plumes carried by the wind towards the probe (41 measurements in 28 different days out of the 122 study days); the other one comprises the measurements that were not affected by the smoke plumes from surrounding fires (935 measurements in 121 different days out of the 122 study days). Eight measurements were carried out every single day, so there are both affected and non-affected measurements on most days.
The aerosol size distribution and the daily CWT were analyzed, considering the affected measurements and the non-affected measurements separately.
In the measurements that were not affected by the wildfires the study focuses on the influence of the weather types on the number of particles and the count median diameter of the fine mode. Soundings provided data on thermal inversions, mainly radiative inversions, on the days where the number of particles was higher than average.
The distributions of the measurements affected by wildfires were studied in detail. Measurements contaminated by the fires were compared with those not contaminated, and a detailed analysis was carried out of the geometric diameter of the fine mode and the number of particles in this size range during the 8 daily registers. The evolution of the accumulation mode and of the coarse mode has been analyzed too, as well as the relative humidity at three different stages: in the measurements before the ones affected by the plumes from the fires, in the ones affected by the fires, and in the ones after the fires.
Table
Meteorological study of the months of June, July, August, and September 2000, with data on maximum, minimum and average temperatures, relative humidity, wind intensity, and total precipitation registered.
Months | HR (%) | Wind (m/s) | ||||
---|---|---|---|---|---|---|
June | 31.2 | 2.2 | 16.9 | 62 | 1.6 | 8 |
July | 32.4 | 2.0 | 17.1 | 65 | 1.6 | 12 |
August | 31.7 | 2.3 | 16.7 | 63 | 1.4 | 3 |
September | 30.9 | 3.2 | 14.5 | 71 | 1.3 | 26.8 |
The monthly precipitation accumulated in the study period is low. The highest value was reached in September with 26.8 mm. The average relative humidity is around 65%, with 71% in September. The average wind speed was low, with a monthly average under 2 m/s. The high relative humidity registered, mainly during the night, are due to the irrigation system used in the fields close to the town and also to the fact that the River Órbigo flows close by at a distance of about 2.5 km.
The CWT classification shows that during the months from June to September (Figure
Circulation Weather Type classification in the months of June, July, August, and September 2000.
The province of León is the largest in the region of Castile and León, Spain, with an area of 15,581 km2. The climate is of the Mediterranean type with continental influence and with some areas affected by the Atlantic Ocean too. The result is a wide range of different landscapes, making León one of the provinces in Spain that suffers the highest number of wildfires with the largest areas burnt.
Most of these wildfires are deliberate. According to statistics provided by the Junta of Castile and León, 90% of wildfires are caused by humans, either deliberately or as the result of negligence.
In the summer of the year 2000, a total of 465 wildfires were registered, an average of four per day. The fires are mostly detected during the central hours of the day, between 1000 UTC and 1800 UTC, and only very few fires start during the night.
Most wildfires were classified as of medium size, burning between 1 and 500 ha. In general, it was found that the wildfires that scorch more than 30 ha last longer than the day when they were detected, and some may even be active over several days, in the case of very large fires (those affecting more than 500 ha).
During the study period, it was observed that the number of wildfires increased gradually: August was the month with the highest number of fires, 201, followed by September, with 171, whereas June and July had less than 70 fires each (Table
Number of fires per month and monthly surface burnt according to the type of vegetation affected (trees, nontree woodland, or no-forest land). Total land area affected during the summer of 2000.
Months | No. of Fires | Area Burnt (ha) | |||
Trees | Nontree woodland | No-forest land | Total area burnt | ||
June | 26 | 70 | 389 | 21 | 450 |
July | 67 | 59 | 612 | 57 | 728 |
August | 201 | 703 | 8871 | 275 | 9848 |
September | 171 | 922 | 8075 | 582 | 9580 |
The total balance of hectares burnt was 20,636, affecting forest areas, no-tree woodland, and crops and crop stubble, with 87% of the land area corresponding to low-vegetation areas. The highest number of hectares burnt was registered in August and September, with over 9,500 ha per month.
The surface area affected (S) varies greatly during the summer. In June and July, it is mostly outbreaks of wildfires that are detected (S < 1 ha) or medium-size fires (500 ha > S > 1 ha), with no large fires at all (S > 500 ha). In contrast, in the months of August and September most of the fires are medium-sized with nearly 500 ha burnt, and there are 9 large fires, 5 in August and 4 in September, which together account for 45% of the total area burnt in the summer of the year 2000. The two largest fires took place on the 18th of August 2000 in the districts of Encinedo and Truchas (about 70 km from the study zone), which together burnt nearly 3,000 ha (Table
District where the fire occurred, date of detection and extinction, land area burnt of each type of vegetation (trees, non-tree woodland, and no-forest land) and total land area affected by the large fires (over 500 ha) in the summer of 2000.
Municipality | First detected | Date of extinction | Area burnt (ha) | |||||
Day | Time (UTC) | Day | Time (UTC) | Trees | Nontree woodland | No-forest mass | Total area burnt | |
Encinedo (42°16′15′′N, 6°35′39′′W) | 18/08/2000 | 0311 | 22/08/2000 | 1930 | — | 1034 | 154 | 1188 |
Truchas (42°15′40′′N, 6°26′07′′W) | 18/08/2000 | 1650 | 23/08/2000 | 0800 | 179 | 1539 | — | 1718 |
Castrocalbón (42°11′45′′N, 5°58′43′′W) | 21/08/2000 | 1110 | 22/08/2000 | 1800 | 62 | 773 | — | 835 |
Lucillo (42°24′38′′N, 6°18′16′′W) | 31/08/2000 | 1500 | 03/09/2000 | 1900 | — | 1255 | — | 1255 |
Castrillo de Cabrera (42°20′25′′N, 6°32′39′′W) | 02/09/2000 | 0600 | 02/09/2000 | 1930 | — | 920 | — | 920 |
Villablino (42°55′59′′N, 6°19′00′′W) | 06/09/2000 | 1126 | 13/09/2000 | 0700 | 269 | 761 | — | 1,030 |
Barjas (42°36′40′′N, 6°58′43′′W) | 11/09/2000 | 0830 | 15/09/2000 | 1805 | 33 | 542 | — | 575 |
Bembibre (42°36′54′′N, 6°25′12′′W) | 14/09/2000 | 1605 | 16/09/2000 | 1915 | 104 | 426 | — | 530 |
Villablino (42°55′59′′N, 6°19′00′′W) | 17/09/2000 | 1300 | 18/09/2000 | 1900 | — | 1031 | — | 1031 |
The smoke plumes of these wildfires, medium and large, may be carried away by the wind to far off places, at least at a regional scale, and in this study they are responsible for the huge increases in the number of aerosols registered by the probe in the rural study zone.
In order to study the influence of the weather types onto the number of aerosols in the study zone, we have analyzed the measurements that were not contaminated by aerosols from the wildfires. For each weather type, we have studied the average number of total particles and the standard deviation, as well as the number of days with those weather types (Figure
Number of particles registered and standard deviation for each Circulation Weather Type in 121 days in the months of June, July, August and September, in accordance with the days with each particular weather type.
Next, a comparative analysis was carried out on the relationship between the count median diameter of the fine mode (CMDf) and the weather types (Table
Count median diameter of the fine or accumulation mode (CMDf) for each Circulation Weather Type.
CMDf ( | Circulation Weather Type |
---|---|
<0.13 | ANW-ASW-AW-C-CE-CS-CSE-CSW-E-S-SW |
0.13–0.14 | A-AN-CW-N-NE-NW-SE-W |
0.14–0.15 | ANE-CNW |
>0.15 | AE-CNE |
To sum up, the air masses from the north of Africa carry smaller particles (smaller than 0.13
Figure
Average number of particles and standard deviation in the months of June, July, August, and September 2000.
It must not be forgotten that the province of León is surrounded by two other provinces which are often affected by wildfires too, Orense, in the region of Galicia, to the north-west of León (the distance between both capitals is 274 km), and Zamora, in the region of Castile and León, to the south of León (the distance between the two capitals is 133 km). Both provinces are important emitters of particulate matter to the atmosphere, contributing to the increase in the number of aerosols and to changes in their size distributions.
During the 27 days comprised by these two periods (second half of August and second half of September) there were frequent thermal inversions at altitudes of less than 1000 meters (AGL), both radiative and subsidence inversions. Both types of inversion often occurred the same day (Table
Weather types and radiative and subsidence thermal inversions during the days with an average number of particles exceeding 2000 particles cm−3 (—) means the data are not available.
Day | No. of particles cm−3 | Circulation weather type | Madrid 0000 UTC | A Coruña 0000 UTC | Santander 0000 UTC | |||
Inversions | Inversions | Inversions | ||||||
Radiative | Subsidence (m AGL) | Radiative | Subsidence (m AGL) | Radiative | Subsidence (m AGL) | |||
21/07/00 | 2084 ± 748 | C | 208 | 662 | 252 | |||
01/08/00 | 2501 ± 1041 | NE | 94 | 494 | 837 | 767 | ||
07/08/00 | 2775 ± 3243 | E | 664 | 894 | ||||
09/08/00 | 4723 ± 3233 | NE | 235 | 462 | 277 | |||
10/08/00 | 2376 ± 1671 | N | 361 | 602 | 441 | 634 | ||
11/08/00 | 2380 ± 505 | NW | 113 | — | — | 77 | 798 | |
12/08/00 | 3755 ± 2562 | A | 720 | |||||
13/08/00 | 2431 ± 1092 | A | 102 | |||||
14/08/00 | 3032 ± 1991 | A | 177 | — | — | 111 | ||
15/08/00 | 3014 ± 2090 | ANE | — | — | ||||
16/08/00 | 3101 ± 2911 | NE | 190 | 794 | ||||
17/08/00 | 3000 ± 2766 | N | 132 | 960 | ||||
18/08/00 | 2240 ± 2304 | CNW | 161 | 733 | 85 | |||
20/08/00 | 2330 ± 1155 | N | 65 | 69 | ||||
25/08/00 | 2157 ± 4291 | C | 46 | 324 | ||||
26/08/00 | 2760 ± 3202 | AN | — | — | ||||
09/09/00 | 2028 ± 886 | E | 664 | 68 | 606 | 103 | 436 | |
16/09/00 | 2896 ± 2179 | C | 186 | 945 | ||||
22/09/00 | 2931 ± 2227 | ASW | 292 | 111 | ||||
23/09/00 | 4912 ± 4005 | W | 192 | 295 | 267 | 897 | ||
24/09/00 | 5201 ± 3079 | A | 184 | 278 | ||||
25/09/00 | 2654 ± 3305 | A | 268 | 341 | 16 | |||
26/09/00 | 5405 ± 4341 | AN | 92 | 968 | 372 | |||
27/09/00 | 6972 ± 7035 | A | 109 | |||||
28/09/00 | 6259 ± 4654 | W | — | — | — | — | — | |
29/09/00 | 3625 ± 6597 | NW | — | — | — | — | — | |
30/09/00 | 2736 ± 2870 | NW |
The multi-log-normal function has been used to characterize the size distributions of aerosol particles [
The three parameters that characterize an individual mode
For the count distribution, the geometric mean diameter,
The size distributions analyzed using the PCASP-X were found to be bimodal, that is, they have a fine and a coarse mode. The daily number of particles in the coarse mode was very low, around 5 particles cm−3, so only data for the fine mode are considered in the results.
The influence of wildfires on the particle size distributions was studied taking two separate samples: on the one hand, the measurements not affected by the fires, taken before and after the plume crossed the study zone; and on the other hand, a sample of measurements affected by the smoke plumes from wildfires in a radius of 70 km around the probe. Comparative analyses were then carried out using these data to identify changes in atmospheric aerosols and their evolution during these events.
The comparative analysis between the monthly aerosol size distributions including all measurements and including only data not contaminated by wildfires (Figure
Mean size distributions in the months of June, July, August, and September 2000 in (a) the measurements registered with wildfires and in (b) measurements not contaminated by wildfires. Only the sizes between 0.1 and 1
The increase in this size range (0.1 to 0.2
Aerosol size distributions of two time intervals: (a) between 4th August 2000 at 0700 UTC and 7th August 2000 at 2200 UTC and (b) between 22nd August 2000 at 1300 UTC and 24th August 2000 at 1600 UTC, representing measurements contaminated by wildfires. Only the sizes between 0.1 and 1
To determine the influence of the wildfires on the 8 daily particle measurements, we carried out a comparative analysis of the evolution of the geometric mean diameter of the fine mode (CMDf) and the total number of particles in affected and nonaffected measurements (Figure
Comparative analysis of the evolution of (a) the geometric mean diameter (CMDf) and (b) the total number of particles in the fine mode in the 8 daily registers, for data including contaminated and noncontaminated measurements.
As for the number of particles in the fine mode, we find two completely different situations when comparing affected and non-affected measurements. The data from contaminated measurements show that during the night and in the early hours (from 2200 UTC to 1000 UTC) the number of particles registered is between 2000 and 3500 particles cm−3. From that time on, the number of particles increases, reaching its maximum at 1300 UTC, with nearly 7,000 particles cm−3, coinciding with the hours registering the lowest value in the geometric mean diameter of the fine mode and with the time when the fires are most active. Initial values are back by 1900 UTC. On the other hand, in the data not contaminated by aerosols from wildfires, the number of particles in the fine mode remains stable during the whole day, around 1000 particles cm−3. In conclusion, the increase in the number of aerosols in the smallest fraction of the fine mode is clearly due to the contamination by particulate matter from wildfires, because there is no other anthropogenic source of aerosols nearby, as we are in a rural area.
The aerosol size distributions in the 41 measurements contaminated by particulate matter from the wildfires were compared with the distributions in the 82 measurements taken immediately before and after the ones affected (Figure
Mean size distributions of the measurements carried out immediately before and after those affected by wildfires and of the affected measurements during the months of June, July, August, and September 2000.
The concentration of PM10 particulate matter was estimated considering 1.35 g cm−3 as the density of particles from biomass burning [
The number of particles in the fine mode increases considerably, and to know exactly what happens to the size of these particles when the probe is measuring aerosols from the wildfires, we compared the mean CMD of the fine or accumulation mode before, during, and after the smoke plumes. The influence of humidity on the growth of these particles was also analyzed (Table
Count median diameter of the fine mode (CMDf) and relative humidity (RH) for the measurements before the fire, the ones affected by the fire and the ones after the fire.
Measurements prior to those affected by the fire | Measurements affected by the fire | Measurements after those affected by fire | |||||||||
Day | Time (UTC) | CMDf ( | RH (%) | Day | Time (UTC) | CMDf ( | RH (%) | Day | Time (UTC) | CMDf ( | RH (%) |
19/06/2000 | 1600 | 0.10 | 40 | 20/06/2000 | 2200 | 0.18 | 79 | 20/06/2000 | 1600 | 0.14 | 43 |
19/06/2000 | 1600 | 0.10 | 40 | 20/06/2000 | 0100 | 0.20 | 88 | 20/06/2000 | 1600 | 0.14 | 43 |
19/06/2000 | 1600 | 0.10 | 40 | 20/06/2000 | 0400 | 0.16 | 87 | 20/06/2000 | 1600 | 0.14 | 43 |
19/06/2000 | 1600 | 0.10 | 40 | 20/06/2000 | 0700 | 0.16 | 66 | 20/06/2000 | 1600 | 0.14 | 43 |
08/07/2000 | 1000 | 0.17 | 48 | 08/07/2000 | 1600 | 0.11 | 40 | 09/07/2000 | 2200 | 0.16 | 89 |
21/07/2000 | 1600 | 0.12 | 30 | 22/07/2000 | 2200 | 0.19 | 69 | 22/07/2000 | 0400 | 0.13 | 83 |
04/08/2000 | 0700 | 0.20 | 53 | 04/08/2000 | 1300 | 0.11 | 45 | 05/08/2000 | 2200 | 0.12 | 74 |
05/08/2000 | 1000 | 0.16 | 45 | 05/08/2000 | 1600 | 0.10 | 44 | 06/08/2000 | 2200 | 0.11 | 86 |
06/08/2000 | 1000 | 0.12 | 35 | 06/08/2000 | 1300 | 0.10 | 32 | 06/08/2000 | 1900 | 0.11 | 67 |
09/08/2000 | 0400 | 0.15 | 84 | 09/08/2000 | 1000 | 0.11 | 29 | 09/08/2000 | 1600 | 0.10 | 38 |
11/08/2000 | 0700 | 0.12 | 58 | 11/08/2000 | 1600 | 0.15 | 62 | 11/08/2000 | 1900 | 0.14 | 86 |
13/08/2000 | 2200 | 0.15 | 78 | 13/08/2000 | 0100 | 0.15 | 86 | 13/08/2000 | 0400 | 0.16 | 90 |
14/08/2000 | 0100 | 0.18 | 96 | 14/08/2000 | 0400 | 0.15 | 99 | 14/08/2000 | 0700 | 0.13 | 67 |
15/08/2000 | 1000 | 0.14 | 48 | 15/08/2000 | 1300 | 0.12 | 38 | 15/08/2000 | 1600 | 0.15 | 42 |
16/08/2000 | 1000 | 0.13 | 24 | 16/08/2000 | 1300 | 0.08 | 23 | 16/08/2000 | 1600 | 0.10 | 24 |
17/08/2000 | 0700 | 0.14 | 56 | 17/08/2000 | 1300 | 0.10 | 36 | 17/08/2000 | 1900 | 0.15 | 71 |
18/08/2000 | 1600 | 0.11 | 46 | 18/08/2000 | 1900 | 0.10 | 56 | 19/08/2000 | 0100 | 0.11 | 82 |
19/08/2000 | 0700 | 0.12 | 65 | 19/08/2000 | 1000 | 0.10 | 48 | 19/08/2000 | 1900 | 0.15 | 42 |
22/08/2000 | 1300 | 0.15 | 50 | 22/08/2000 | 1600 | 0.15 | 50 | 22/08/2000 | 1900 | 0.10 | 62 |
23/08/2000 | 1000 | 0.11 | 49 | 23/08/2000 | 1900 | 0.12 | 80 | 24/08/2000 | 2200 | 0.10 | 82 |
24/08/2000 | 1000 | 0.10 | 58 | 24/08/2000 | 1300 | 0.10 | 52 | 24/08/2000 | 1600 | 0.10 | 44 |
28/08/2000 | 1000 | 0.13 | 48 | 28/08/2000 | 1300 | 0.10 | 41 | 28/08/2000 | 1900 | 0.13 | 69 |
08/09/2000 | 1900 | 0.11 | 62 | 09/09/2000 | 2200 | 0.11 | 75 | 09/09/2000 | 0400 | 0.13 | 80 |
09/09/2000 | 0700 | 0.12 | 55 | 09/09/2000 | 1000 | 0.12 | 44 | 09/09/2000 | 1600 | 0.11 | 48 |
09/09/2000 | 1900 | 0.13 | 59 | 10/09/2000 | 0400 | 0.11 | 66 | 12/09/2000 | 2200 | 0.22 | 97 |
09/09/2000 | 1900 | 0.13 | 59 | 10/09/2000 | 0700 | 0.12 | 51 | 12/09/2000 | 2200 | 0.22 | 97 |
09/09/2000 | 1900 | 0.13 | 59 | 10/09/2000 | 1300 | 0.10 | 32 | 12/09/2000 | 2200 | 0.22 | 97 |
09/09/2000 | 1900 | 0.13 | 59 | 10/09/2000 | 1900 | 0.11 | 56 | 12/09/2000 | 2200 | 0.22 | 97 |
09/09/2000 | 1900 | 0.13 | 59 | 11/09/2000 | 2200 | 0.13 | 58 | 12/09/2000 | 2200 | 0.22 | 97 |
09/09/2000 | 1900 | 0.13 | 59 | 11/09/2000 | 1600 | 0.12 | 76 | 12/09/2000 | 2200 | 0.22 | 97 |
18/09/2000 | 1300 | 0.13 | 56 | 18/09/2000 | 1600 | 0.12 | 57 | 18/09/2000 | 1900 | 0.15 | 70 |
21/09/2000 | 1000 | 0.13 | 63 | 21/09/2000 | 1600 | 0.10 | 61 | 21/09/2000 | 1900 | 0.11 | 92 |
23/09/2000 | 1600 | 0.10 | 45 | 24/09/2000 | 2200 | 0.10 | 83 | 24/09/2000 | 0700 | 0.10 | 71 |
23/09/2000 | 1600 | 0.10 | 45 | 24/09/2000 | 0100 | 0.13 | 78 | 24/09/2000 | 0700 | 0.10 | 71 |
25/09/2000 | 0700 | 0.11 | 64 | 25/09/2000 | 1300 | 0.10 | 41 | 26/09/2000 | 0700 | 0.13 | 62 |
25/09/2000 | 0700 | 0.11 | 64 | 25/09/2000 | 1600 | 0.08 | 50 | 26/09/2000 | 0700 | 0.13 | 62 |
25/09/2000 | 0700 | 0.11 | 64 | 25/09/2000 | 1900 | 0.11 | 80 | 26/09/2000 | 0700 | 0.13 | 62 |
25/09/2000 | 0700 | 0.11 | 64 | 26/09/2000 | 2200 | 0.14 | 84 | 26/09/2000 | 0700 | 0.13 | 62 |
25/09/2000 | 0700 | 0.11 | 64 | 26/09/2000 | 0100 | 0.10 | 89 | 26/09/2000 | 0700 | 0.13 | 62 |
27/09/2000 | 0700 | 0.13 | 92 | 27/09/2000 | 0700 | 0.12 | 83 | 28/09/2000 | 1000 | 0.15 | 98 |
27/09/2000 | 0700 | 0.13 | 92 | 27/09/2000 | 1900 | 0.10 | 72 | 28/09/2000 | 1000 | 0.15 | 98 |
It was observed that when the relative humidity in the contaminated measurement is higher than the one in the previous measurement, a humidity of around 80%, the geometric mean diameter of the fine mode increases from 0.10
In the summer season, the analyzed air masses from the north of Africa contributed smaller particles (smaller than 0.13
The average number of particles (excluding the ones generated by biomass burning) registered daily during the study period varies between 2000 and 7000 particles cm−3. However, on certain days as much as 50,000 particles cm−3 were detected in a rural area, like the study zone, with a total lack of anthropogenic sources of pollution this implies that the cause must be the high number of wildfires registered every summer. Part of this increase may be attributed to the stable atmospheric situations typical of the summer season, as well as to the occurrence of deep radiative and/or subsidence thermal inversions. However, in this study, because of the sudden increases observed in the number of particles, the most plausible explanation is the transport of smoke plumes at a regional scale full of particulate matter.
In the province of León, wildfires burnt 20,636 ha between June and September 2000. We claim that wildfires and other large fires in the surrounding provinces are contaminating the air in rural areas during the summer, reaching average values of PM10 concentrations of 86
The wildfires cause not only a considerable increase in the number of particles in the atmosphere but also changes in particle size distributions. Average increases of around 1000% have been found in the number of particles in the fine mode. In particles between 0.10 and 0.14
When the relative humidity increases in the measurements contaminated by the fires when compared with the non-contaminated ones (RH around 80%), the CMD of the fine mode increases by 50%. This may be due to the hygroscopic growth of the aerosol, which absorbs water vapor from ambient air, from the river Órbigo, and from the irrigation system in the study zone.
Summing up, in a rural area, with few anthropogenic sources of aerosols, the considerable increases registered in the number of particles cannot be explained by high pressures leading to atmospheric stability, nor by thermal inversions, but must be attributed to wildfires, large or small, with smoke plumes covering sometimes great distances and causing important ambient pollution of nonanthropogenic origin.
The authors believe this research line must be pursued because the devastating consequences of wildfires cause immediate changes in atmospheric composition at local and regional scales, and the increased number of aerosols produce changes in the Earth’s global radiation balance, causing both direct and indirect radiative forcing. Moreover, exposure to atmospheric particulate matter may have adverse effects on human health.
The authors wish to thank Miguel Angel Olguín, from the Department for the Environment of the Junta de Castilla y León, for his help and cooperation whenever it was needed. The authors are also grateful to Dr. Laura López Campano for her collaboration and to Dr. Noelia Ramón for translating the paper into English. This study was partially supported by the Regional Government of Castilla y León (Grants LE13/00B and LE039A10-2).