Comparative aerobiological study between two stations located at different points in a coastal city in southern Spain

doi:10.21203/rs.3.rs-2141383/v1

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Comparative aerobiological study between two stations located at different points in a coastal city in southern Spain

https://doi.org/10.21203/rs.3.rs-2141383/v1

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published 12 Apr, 2023

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Due to the increase in allergies, aerobiological studies carried out in cities are essential to keep the population informed about the pollen atmospheric concentrations detected. However, the high cost and complexity of aerobiological studies often means that the information is generated from a single sampling point what may not be representative of the entire city. So, the precision and functionality of the information and alert systems for allergy sufferers can be reduced. This effect may be increased in coastal cities or in those with an asymmetric growth. In this study, the data obtained by two volumetric pollen traps, located in the coastal city of Malaga (Spain) were analyzed. One of the pollen traps was situated in the city center while the other was in the outskirts of the city, 5 km away from the first. Despite being located within the same city, the data obtained from both collectors showed significant differences in the relative abundance and annual integrals of the main pollen types, as well as in the periods in which elapse their main pollen seasons, probably due to being located in areas with quantitative differences in land uses. These differences were more notable in the case of ornamental plants, weeds that usually appear in brownfields sites, and some taxa that predominate in the agro-forestry areas of the outskirts. For all the above, we consider that it is important to keep operational several sampling points in cities of a certain magnitude to provide more detailed information about atmospheric pollen concentrations.

aerobiology

pollen comparison

land uses

allergy prevention

Pollen is one the main bio-aerosols present in the atmosphere, also being one of the main causes of allergic rhinitis worldwide. It is estimated that between 15 and 40% of the population in Europe (depending on the country) suffer allergy to some pollen type. This percentage has shown an upward trend in the last 30 years and, in general, use to be higher in children (Akdis et al., 2015; Buters et al., 2018; Clot et al., 2020). Additionally, recent studies show that the symptoms of allergic rhinitis increase with air pollution in cities, due to its interaction with the allergens present in the atmosphere (Bédard et al., 2020). This respiratory disorder affects the quality of life and daily activities of the dwellers of the cities, specially work and school activities, which causes great economic losses to society (Akdis and Agache, 2014; Zuberbier et al., 2014).

The incidence caused by the airborne pollen grains to the sensitive population, could be reduced applying appropriated treatments, but also implementing methods for early detection of the pollen concentrations and warning mechanisms, as well as prevention measures (Clot et al., 2020). Due to the long dedication time that aerobiological studies require, the pollen present in the atmosphere of the different cities is usually estimated from data recorded by a single sampling point. However, given the high complexity of urban environments, the concentrations detected at a given sampling point may not be representative of the entire city or extrapolated to the different points of the same. Besides this, it is necessary to take into account, not only where the population lives but also where the inhabitants carry out their daily activities (Charalampopoulos et al. al., 2021). For this reason, different authors recommend sampling the air at several city points in order to increase the precision and usefulness of the released information (Fuentes Antón et al., 2020b; Charalampopoulos et al., 2021; Rojo et al., 2020; Werchan et al., 2017, 2018).

The creation of green urban areas, such as parks and gardens, can help to improve the lifestyle of the dwellers and widely favors physical activities, social relationships and help to reduce stress (Jonker et al., 2014; Lee et al. al., 2015; Maas et al., 2009; Mitchell and Popham, 2008). Such green spaces also reduce the urban heat island effect, noises and air pollutants related to traffic (Dadvand et al., 2015, 2012), but they can also pose a problem for public health, due to the allergenicity of the pollen grains produced by the plants cultivated in such spaces, being cause of diseases in the population sensitized to those pollen types (Cariñanos et al., 2017). On the other hand, rural exodus, land abandonment and industrial development are favoring the growth of the cities, as well as changes in land use and environmental degradation (Caldas et al., 2007), which widely affects the vegetation around the cities, favoring the growth of grasses and weeds.

Malaga (Andalusia, southern Spain), the capital of the “Costal del Sol”, is one of the largest coastal cities in the south of the Iberian Peninsula. National and international tourism has increased notably in Malaga in the last decades, which has led to a fast growth of the city as well as the metropolitan areas (Andrade et al., 2021). Due to be a coastal city surrounded by mountains, this growth has not occurred homogeneously in all directions. The expansion has mainly taken place towards the flat area of the Guadalhorce river valley (west and northwest of the city), with a surrounding vegetation dominated by crops and agroforestry areas. This has led to different land use in different points of the city, what have also resulted in a diversity of urban environments with different vegetation and, therefore, different pollen sources what directly affects the pollen concentrations detected in the different areas. This is why the city of Malaga constitutes an excellent example to study the influence of land uses and urban environments on the atmospheric pollen content registered at different areas of the cities.

In the present work, a comparative study of the pollen concentrations registered in two different sampling points has been carried out in the city of Malaga, one of them located in the old town (city center) and, the other, in a more modern area of the periphery. The aim of this study was to quantify the differences detected between both sampling points, as well as to relate them to differences in the land uses and the influence of nearby vegetation.

2.1. Area of study

The study, as previously mentioned, was carried out in Malaga, a coastal city situated in the south of the Iberian Peninsula. The city is surrounded by mountains except in the west, where the valley of the Guadalhorce river runs, crossing the province from north to south. Malaga presents an oceanic pluviseasonal Mediterranean bioclimate, with a thermomediterranean thermotype and a dry ombrotype (Rivas-Martinez and Rivas-Saenz, 2020). The average annual temperature is 18.4ºC, one of the warmest in Europe (Domínguez-Amarillo et al., 2020). Rainfalls are scarce, around 540 mm per year as average, mainly concentrated in autumn and winter, with periods of heavy drought alternating with episodes of torrential rains.

The city of Malaga presents great urban contrasts between the center and the periphery due to the rapid growth it has suffered in recent years (Andrade et al., 2021). The city center is made up of small and medium-sized buildings, and generally narrow streets. It is common to find some abandoned lots or buildings in ruins in which nitrophilous herbaceous plants are detected, such as species of the Amaranthaceae and Urticaceae families. There are also some parks and gardens with ornamental species. Among them, the Malaga Park stands out, situated just 800 m from the pollen trap. In it abounds a great diversity of ornamental species from all over the world, mainly of tropical and subtropical origin (Cañizo, 1975; Trigo Pérez, 1992, 1996). Among the wind pollinated, shade plane trees, palm trees and some grasses are common. 500 m east from the sampling point we find Mount Gibralfaro, with large areas of repopulation pines (Pinus halepensis Mill.).

On the contrary, on the periphery of the city, large avenues and larger buildings are frequent. There are also lots of building sites and agroforestry areas in transition between the typical Mediterranean scrub and cultivated areas. To the northwest, in the fertile plain of the Guadalhorce river, there are abundant citrus crops, olive groves and orchards (Llamas et. al 2020).

2.2. Pollen data

The airborne pollen samplings were carried out uninterruptedly during the period 2017–2019, by using two volumetric Hirst type pollen traps (Hirst, 1952) installed at different points of the city, separated by a distance of 5 km. One of them was set in the old town (city center), in the plain roof of the multi-services building of the Malaga City Council in Dos Aceras st. (36º43'32.54 ”N 4º25'15.08” W; 29 m.a.s.l.; 12 m.a.g.l.); while the other (Teatinos) was located in the western part of the city, on the roof of the Faculty of Sciences of the University of Malaga, situated in the Campus of Teatinos, a peripheral area, on the edge of the city (36º42'58.09 ”N 4º28'21.86” W; 62 m.a.s.l.; 20 m.a.g.l.) (Fig. 1).

The samples were mounted and counted following the standardized methodology proposed by the Spanish Aerobiology Network (REA) and the European Aeroallergen Society (EAS) (Galán et al., 2007; 2014). The pollen traps were adjusted to aspirate a continuous flow of 10 l/min, silicone fluid being used as adhesive substance and glycerine jelly as mounting mean. Four longitudinal sweeps were analyzed for each sample at a magnification of X400 with the aid of a light microscope. The data used in the figures and tables correspond to the daily mean pollen concentrations, expressed as the number of pollen grains per cubic meter of air (pollen grains/m³). The results were analyzed using the statistical program SPSS v21 and the AeRobiology package, implemented in the R software (Development-Core-Team, 2017; Rojo et al., 2019).

The study is focused on the most abundant pollen types in the atmosphere of Malaga, which were defined as those that exceeded 1% of the annual total pollen detected: Amaranthaceae, Cupressaceae, Olea europaea, Pinus, Plantago, Platanus, Poaceae, Quercus, Urtica membranacea, Parietaria and Casuarina, the last one only in the case of the peripheral station. The pollen type Parietaria corresponds to the whole Urticaceae, excluding U. membranacea which present a different pollen type.

The relative abundance of the main pollen types for the 2017–2019 period was calculated based on the total pollen detected during the three years of the study. The daily mean concentrations from the two sampling points were compared using the Spearman's correlations and the Mann-Whitney-Wilcoxon tests. For these statistical tests, the days included in the main pollen season (MPS) at both sampling points were considered.

For calculating the MPS, the 95% method was used, which defines the beginning as the day in which 2.5% of the annual pollen integral is reached, and the end when 97,5% of the annual total is accumulated (Andersen, 1991). In the case of Casuarina and Cupressaceae, since they are pollen types that present their highest concentrations between the end of one year and the beginning of the next, overlapping years (June to May) were used instead of natural years. To calculate the MPS of Casuarina, Parietaria and Urtica membranacea, as they are pollen types with long tails at the beginning and the end of their pollination period, the 90% method was used (Nilsson & Persson, 1981).

The annual pollen integrals of the main pollen types were also calculated in order to stablish differences between the two sampling stations.

2.3. Land uses

Land uses were obtained from the Corine Land Cover database (version v2020_20u1, data from the year 2018; Copernicus Europe's Eyes on Earth) with a spatial resolution of 100 m. The data was reclassified into 12 categories: Agricultural areas (all crops except olive trees), agroforestry areas (pastures, agro-forestry areas and land mainly occupied by agriculture with significant areas of natural vegetation), coniferous forests, green urban areas (urban green areas, sports and leisure facilities), inland water bodies, mixed forests, olive crops, sclerophyllous forests, sea, shrublands and grasslands (including the categories natural grasslands, sclerophyllous vegetation and transitional woodland-shrub), sparsely natural areas (beaches, bare rocks, sparsely vegetated areas and burned areas) and urban areas (all buildings and urban factories but excluding green urban areas).

Using the R software (Development-Core-Team, 2017), the percentage of surface occupied by each land use was calculated in a radius of 1 km around each pollen trap, as well as in rings of 1 to 5 km, 5 to 10 km and 10 to 25 km.

3.1 Land uses

The types of land uses detected in the surroundings of the two pollen traps were similar, but the percentage of surface occupied by each of them was variable depending on the distance to the sampling point (Fig. 2). Within 1 km radius around the collector, we can find in the city center a clear predominance of built-up areas, together with a small percentage of urban green areas corresponding to the central park (Parque de Malaga) and other gardens distributed throughout the old city. On the contrary, in the periphery, parks and gardens had less prominence, with an increase in the percentage of the agroforestry areas, corresponding to part of the Guadalhorce Valley where agricultural areas are mixed with natural vegetation, in which herbaceous plants and some shrubs predominate.

Within the ring situated between 1 and 5 km away from the sampling sites, remarkable differences were observed in the land uses. The city center of Malaga has a greater proportion of the area occupied by coniferous forests, due to the repopulation of pines carried out in the mountains near the historic center of the city. In addition, due to its proximity to the coast, the percentage of surface occupied by the sea is notably greater in the center than in the periphery. In Teatinos, the percentage occupied by urban areas is greater, as it also covers the areas of industrial estates situated on the outskirts of the city and other nearby coastal towns. The percentage of agroforestry use is also notably higher in the periphery than in the city center, as also is the percentage of surface occupied by olive groves.

Within the ring located between 5 and 10 km away from the pollen traps, we observe how in both sampling points the percentages of land use are similar in terms of urban areas, coniferous forests, scrublands and grasslands, agro-forestry areas, and the influence of the sea. However, in the periphery the use of areas dedicated to agriculture predominates. Finally, in the range between 10 and 25 km, the percentages of land use are similar in both sampling points, with a predominance of the area occupied by the sea, followed by agricultural and scrub and pasture areas.

3.2. Aerobiological study

Regarding the total annual pollen records, no clear pattern was observed for all the sampling years (Fig. 3). In 2017, a greater annual total integral was obtained in the periphery, while in the other years the opposite occurred. A longer time series would be necessary to exclude the particularities of each year and clearly discern if a sampling point generally registers higher integrals than the other. However, the directionality of the interannual changes in these values was the same at both sampling points (i.e. they increased or decreased at the same time at the two sampling points).

The 11 main pollen types constituted, as a whole, the 89.43% of the total pollen collected during the study period. However, despite these pollen types were the same during the 3 years at both sampling points (with the exception of Casuarina), slight differences were detected in the order of abundance from one year to another. Furthermore, the order of abundance of the different pollen types changed depending on the sampling location. In general, almost all pollen types had a higher relative abundance in the city center, compared to the periphery (Fig. 4).

As previous studies highlighted, the abundance of the pollen types detected at certain city area is highly conditioned by the abundance of the emission sources in the surroundings, by ornamental trees and local meteorological conditions (Charalampopoulos et al., 2021; Fuentes Antón et al., 2020a; Pecero-Casimiro et al., 2019; Velasco-Jiménez et al., 2013; Werchan et al., 2018). This may explain the differences observed in the order of abundance of the pollen types detected (Fig. 4). Besides that, the relative abundances are also influenced by the amounts of total pollen detected in each sampling location, so the analysis of these results must be done jointly with those of the annual integrals (Fig. 5).

As can be seen in Fig. 5, the values of the annual pollen integrals showed differences from year to year, depending on the sampling station and the pollen type. Parietaria stands out, always presenting much higher values in the city center, probably due to the greater abundance of ruined buildings, which favors the proliferation of nitrophilous herbaceous plants, as is the case of Parietaria that usually grows in abandoned lots and old walls, which explains not only its higher annual integrals in the center of the city, but also the greater relative abundance (Figs. 4 and 5). On the contrary, Amaranthaceae, Casuarina and Plantago always presented higher values in the periphery. Amaranthaceae and Plantago are abundant plants in agro-forestry and crop transition zones that are more abundant in the outskirts of the city (Fig. 2), which may explain their higher integrals detected at this sampling point. On the other hand, Casuarina is a common ornamental tree on the Teatinos university campus, where the pollen trap is installed, which would have given rise to the highest integrals recorded in the periphery. Previous studies also found similar patterns in the distribution of the airborne pollen concentrations of these pollen types inside cities due to differences in land uses and in the distribution of ornamental trees (Fuentes Antón et al., 2020a; Velasco-Jiménez et al., 2013).

Regarding the remaining pollen types, they reached higher annual pollen integrals in one or another sampling stations depending on the year and did not present patterns as clear as in the case of the mentioned above, despite the fact that some of them presented important differences in terms of relative abundance. This may be explained by a similar abundance of the emission sources of these pollen types at both sampling sites, and wind dynamics during each year being the determinants of the airborne pollen concentrations detected (Fuentes Antón et al., 2020a).

In the case of Cupressaceae, the annual integrals presented scarce differences between the sampling points, being slightly higher in the city center in the years 2017 and 2019, and slightly higher in the periphery in 2018. This similarity is probably due to the fact that cypresses are common ornamental plants in the vicinity of both sampling sites, and they have been previously reported to influence the local airborne pollen concentrations (Charalampopoulos et al., 2021).

The annual integrals of Olea europaea were similar in both sampling points in 2018 and 2019, but much higher in 2017 in the periphery. Given that there are no great differences in terms of the area occupied by olive groves in both sampling sites and that the percentage of olive grove use increases with the distance in a similar way, the observed differences could be caused by punctual variations in the meteorological conditions, especially wind dynamics. It has been observed that when the northwest wind blows, coming from inland, the levels of olive pollen increase, being the peripheral area of the city the most exposed to such winds. On the contrary, winds blowing from the sea (southeast), tent to decrease pollen concentrations, being the pollen trap situated in the city center more exposed to these winds. Something similar happened with Quercus pollen type: its annual integral was higher on the periphery in 2017, similar in 2018 at both sampling points, and higher in the center in 2019. The forests that contribute to increase this pollen type are found in similar abundances around both pollen collectors (Fig. 2), so the differences detected among the years are probably also conditioned by wind dynamics.

In the case of the Pinus, the annual integrals were similar in both locations in 2017, higher in the periphery in 2018 and higher in the center in 2019. Despite their proximity, the repopulation pine forests located in the Montes de Malaga, towards the northwest of the city do not seem to have had a great influence on the quantities detected in the city center pollen trap. This may be due to the fact that the prevailing winds in Malaga are those blowing from SE and NW, and they would not contribute to the transport of pollen from these pine forests to the sampling sites (Recio et al., 2018). In this case, the main source of pollen would be small copses located within the parks and peri-urban areas, frequent in the vicinity of both sampling sites.

The annual integral of Platanus was higher in 2017 in the city center but in 2018 and 2019 it was higher in the periphery. The integral of Poaceae was practically similar in the two sampling points throughout the period studied, and the integral of U. membranacea was higher in the center in 2017, higher in the periphery in 2018 and similar in both sampling sites in 2019. Platanus is represented in the area by ornamental trees whose abundance is similar in both sampling points. Grasses, due to their wide distribution and high number of species, are present in a large number of different land uses, therefore, its pollen concentrations in the air tends to be similar at both locations. Something similar happens in the case of U. membranacea, as it is a nitrophilous ruderal plant, it does not present a special predominance in one or another sampling point, apart from the influence that atmospheric conditions and wind trajectories may exert.

Although several studies have evidenced the influence that different heights above ground level could have on the pollen concentrations detected in the atmosphere, these differences tend to be insignificant when the collectors are located more than 10 m above ground level (Rojo et al., 2019a, 2020). Therefore, we consider that the influence of the scarce difference in the heights of the pollen traps used in this study, has been of little relevance for the results obtained. On the other hand, in other localities, land uses and pollen emission sources located in the vicinity of cities have also been identified as the main variables responsible for the differences detected between nearby sampling points (Fuentes Antón et al., 2020b; Pecero-Casimiro et al., 2019; Picornell et al., 2019; Rojo et al., 2020; Velasco-Jiménez et al., 2013).

At both sampling points, about 85% of the total annual pollen is concentrated from February to June (both inclusive), the period in which the highest daily pollen concentrations of most of the main pollen types took place, with the exception of Casuarina, which is typically autumnal (Figs. 6 and 7). In general, May is the month in which the highest concentrations of the total pollen are detected at both sampling points, a typical behavior in the Mediterranean basin. Although almost all the pollen types presented a single period of maximum concentrations, some of them such as Cupressaceae or Amaranthaceae, presented two peaks of different intensity. In the case of Cupressaceae, two periods of high concentrations were detected, the first, more intense, from January to May and, the second, of lesser intensity, from September to December, due to the presence of different species, some of them with autumn flowering. Something similar occurs in the case of Amaranthaceae, whose maximum concentrations occur between April and May and, after a marked decrease, they slightly increase again at the end of summer (August-September). In any case, sporadic detection of pollen types is frequent throughout the year, due fundamentally to re-buoyancy phenomena.

As previously observed for relative abundances and annual integrals, mean daily concentrations, in general, were also higher in the periphery for the pollen types Amaranthaceae, Casuarina, and Plantago, and higher in the city center for Parietaria (Figs. 6 and 7). This last pollen type showed the greatest differences between the two sampling points, especially during the months of March to July. The rest of the pollen types presented similar daily average concentrations throughout the year at both sampling points, despite the fact that some peaks of greater intensity were sporadically detected at one or another. In general, the timing and intensity of the daily average concentrations detected were similar to those of other nearby cities located in the south of the Iberian Peninsula such as Granada, Ronda or Cordoba (De Linares et al., 2019; Picornell et al., 2019; Velasco-Jiménez et al., 2018).

If we compare the graphs of seasonal behavior of the different pollen types in both sampling points, we observe that there is a great coincidence in terms of their profiles. In fact, the correlation studies carried out between the mean daily concentrations of the different pollen types at both sampling points (center and periphery), resulted in positive and significant correlation coefficients for p ≤ 0.001 in all cases. (Table 1). However, after applying Mann-Whitney-Wilcoxon tests, significant differences (p < 0.05) were observed in 6 of the 11 pollen types studied (i.e. Amaranthaceae, Casuarina, Olea europea, Plantago, Quercus, and Parietaria).

Table 1

Results of the statistical analysis (Spearman correlation and Wilkoxon tests) between the values of the daily mean pollen concentrations of the two sampling sites during the main pollen season (MPS). Significant p-values are marked with an asterisk (p ≤ 0.05).
	Spearman Correlation		Wilcoxon test
Pollen types	Spearman coefficient	P-value	P-value
Amaranthaceae	0.536	< 0.001*	< 0.001*
Casuarina	0.536	< 0.001*	< 0.001*
Cupressaceae	0.767	< 0.001*	0.477
Olea europaea	0.863	< 0.001*	0.006*
Parietaria	0.539	< 0.001*	< 0.001*
Pinus	0.704	< 0.001*	0.652
Plantago	0.619	< 0.001*	< 0.001*
Platanus	0.628	< 0.001*	0.862
Poaceae	0.678	< 0.001*	0.152
Quercus	0.890	< 0.001*	< 0.001*
U. membranacea	0.487	< 0.001*	0.060

Given the proximity of both sampling points, we can expect that the meteorological conditions that influence pollen concentrations in the atmosphere are similar and therefore, the concentrations oscillate in the same direction, even when the absolute values be different due to differences in emission sources or air flows. The significant differences observed according to the Mann-Whitney-Wilcoxon tests were probably due to the fact that, repeatedly, concentration peaks detected in one of the sampling points were not detected in the other, and vice versa.

Regarding the MPS of the pollen types studied (Table 2), differences between both sampling stations have been observed in the dates of start and end, in the duration of the whole MPS and the periods pre- and post-peak, as well as in the dates and intensity of the peak days. Actually, few coincidences in dates have been found, and sometimes these differences are of up to 30 days between one and another sampling sites. It is possible that differences in the beginning, end and duration of the MPS are related to the methodology used, which is based on percentages and, therefore, it is dependent on the value reached by the annual integral of the pollen type in question, but this would not be the case of the peak day or their intensity and they have also been variable.

Some pollen types present their maximum daily peaks earlier in the periphery, as occurred in the case of Amaranthaceae, Casuarina, Olea europaea and Quercus. On the contrary, Parietaria always presented its maximum peaks earlier in the center. The rest of the pollen types did not present a clear behavior regarding the date in which the peak day take place, occurring first in the center or in the periphery depending on the year. For some pollen types, i.e., Casuarina, Plantago and Quercus, the differences are less than 10 days. However, for the rest of the pollen types, the fluctuations were greater. For example, in Platanus the fluctuations oscillated between 1 and 19 days; in U. membranacea between 6–24 days; and in Parietaria, between 10–80 days (Table 2).

Table 2

Phenological parameters related to the main pollen season (MPS) for the main pollen types during the studied years in the city center and the periphery sampling sites.
Year	Station	Start date		Final date	MPS Duration (days)		Pre-peak Duration (days)	Post-peak Duration (days)		Peak date			Peak day concentration (p.g./m³)
Amaranthaceae
2017	Center	16/02		29/11	287		76	211		02/05			14
2017	Outskirts	21/02		09/10	231		54	177		15/04			59
2018	Center	14/02		09/10	238		79	114		03/05			21
2018	Outskirts	03/04		02/10	183		25	158		27/04			61
2019	Center	30/01		30/10	274		100	174		09/05			16
2019	Outskirts	21/02		19/10	241		54	187		15/04			17
Casuarina
2017–2018	Center	15/09/17		20/12/17	97		57	40		10/11/ 17			22
2017–2018	Outskirts	26/10/17		16/01/18	83		11	72		10/11/ 17			43
2018–2019	Center	24/10/18		25/12/18	63		1	62		24/10/ 18			17
2018–2019	Outskirts	27/10/18		02/12/18	37		10	27		05/11/ 18			63
Cupressaceae
2017–2018	Center	14/10/17		24/04/18	193		131	62		21/02/18			427
2017–2018	Outskirts	18/10/17		17/04/18	182		161	21		27/03/18			449
2018–2019	Center	22/09/18		03/05/19	224		173	51		13/03/19			212
2018–2019	Outskirts	08/10/18		23/04/19	198		145	53		01/03/19			273
Olea europaea
2017	Center	11/04		31/08	143		27	116		07/05			282
2017	Outskirts	29/03		11/06	75		29	46		26/04			612
2018	Center	03/05		10/07	69		32	37		03/06			1048
2018	Outskirts	01/05		06/07	67		35	32		04/06			1047
2019	Center	05/04		05/07	92		43	49		17/05			943
2019	Outskirts	03/04		30/06	89		45	44		17/05			893
Parietaria
2017	Center		04/02	01/12		301	46		255		21/03	94
2017	Outskirts		22/01	23/11		306	45		261		07/03	35
2018	Center		07/02	04/10		240	92		148		09/05	212
2018	Outskirts		21/01	17/12		331	28		303		17/02	32
2019	Center		27/01	12/10		196	103		156		09/05	207
2019	Outskirts		08/01	14/10		280	34		246		10/02	29
Pinus
2017	Center	17/02		28/10	254		21	233		09/03			15
2017	Outskirts	18/02		02/09	197		10	187		27/02			60
2018	Center	24/02		05/08	163		22	141		17/03			32
2018	Outskirts	26/02		01/08	157		8	149		05/03			50
2019	Center	08/02		21/07	164		21	143		28/02			74
2019	Outskirts	13/02		19/07	157		17	140		01/03			90

Table 3

Parameters related to the main pollen season for the main pollen types during the period 2017–2019 in the city center and the periphery sampling sites.
Year	Station	Start date	Final date	MPS Duration (days)	Pre-peak Duration (days)	Post-peak Duration (days)	Peak date	Peak day concentration (g.p./m³)
Plantago
2017	Center	06/03	10/08	158	41	117	15/04	21
2017	Outskirts	13/03	27/07	137	34	103	15/04	55
2018	Center	21/02	11/09	203	54	149	15/04	19
2018	Outskirts	28/03	30/06	95	46	49	12/05	41
2019	Center	08/03	24/10	231	59	172	05/05	17
2019	Outskirts	13/03	16/09	188	57	131	08/05	37
Platanus
2017	Center	07/03	12/04	37	4	33	10/03	140
2017	Outskirts	06/03	03/05	59	6	53	11/03	117
2018	Center	07/03	14/04	39	4	35	10/03	85
2018	Outskirts	10/03	12/05	64	20	44	29/03	90
2019	Center	05/03	18/04	45	22	23	26/03	48
2019	Outskirts	04/03	10/04	38	8	30	11/03	102
Poaceae
2017	Center	27/02	12/11	259	81	178	18/05	24
2017	Outskirts	18/02	02/10	227	68	159	26/04	66
2018	Center	03/04	02/08	122	41	81	13/05	71
2018	Outskirts	02/04	14/08	135	58	90	29/05	91
2019	Center	04/03	10/09	191	75	116	17/05	144
2019	Outskirts	11/03	05/09	179	72	107	21/05	143
Quercus
2017	Center	20/03	26/09	191	27	164	15/04	79
2017	Outskirts	13/03	09/08	150	34	116	15/04	330
2018	Center	31/03	28/06	90	29	61	28/04	336
2018	Outskirts	07/04	06/07	91	22	69	28/04	231
2019	Center	02/04	20/07	110	13	97	14/04	1022
2019	Outskirts	02/04	29/08	150	3	147	04/04	448
Urtica membranacea
2017	Center	04/02	25/04	81	24	57	27/02	43
2017	Outskirts	04/02	18/05	104	36	68	11/03	33
2018	Center	24/01	26/07	184	43	141	07/03	29
2018	Outskirts	16/02	02/06	107	47	60	03/04	18
2019	Center	30/01	13/06	135	37	98	07/03	34
2019	Outskirts	27/01	29/05	123	34	89	01/03	28

The peak day values of the pollen types Urtica membranacea and Parietaria were higher during the entire study period in the city center, while Amaranthaceae, Casuarina, Pinus and Plantago obtained higher peak concentrations in the periphery, which seems to be related to the higher abundance of these species in the vicinity of pollen sampler. The remaining pollen types did not show a homogeneous pattern, so the values of their peaks seem to be more conditioned by wind dynamics than by any specific land use pattern.

Previous studies also reported differences in the start, peak and end dates of the MPS when sampling at different locations inside the same city (Charalampopoulos et al., 2021; Fuentes Antón et al., 2020b; Velasco-Jiménez et al., 2013). In accordance with our results, these differences were related to the distribution of the pollen emission sources and wind dynamics.

The study carried out showed differences in the results obtained at both sampling points, not only in timing but also in the values reached by the different pollen types and their relative percentages. The different land uses of the surrounding places in which the two samplers are located, including parks and gardens and proximity to the sea, seem to have influenced the results obtained. In fact, the influence that the vegetation close to the pollen traps has on pollen concentrations already has been highlighted by other authors such as Kasprzyk et al. (2019) or Rojo et al. (2020). However, the influence of the prevailing winds or the structure of the city itself, could explain some of the differences found since the position of the buildings constitutes barriers that can modify the local air flow and affect pollen dynamics (Damialis et al., 2005; Gonzalo-Garijo et al., 2006; Peel et al., 2013; Ciani et al., 2020).

Despite the short distance existing between both sampling points (5 km), the results obtained showed many differences in terms of annual pollen integrals, relative abundance, main pollen seasons and dates on which the maximum peaks occur. The statistical study showed that, although there was a good correlation between the daily mean pollen concentrations of both sampling points for the 11 pollen types studied, there were significant differences in the case of 6 of them.

Some of the differences found could be explained by the different percentages of land use present in the surroundings of the sampling points, since in the area closest to the city center, urban areas, parks and gardens predominate, while in the periphery are the agro-forestry areas. On the other hand, the influence of the sea is greater in the center than in the periphery. However, the direction in which the wind blows, the orography and the structure of the city itself could have contributed to the differences detected.

Based on the results obtained, in cities with a certain size and heterogeneity in terms of land use and characteristics, it would be necessary to install several aerobiological sampling points in order to provide more precise and detailed information to the population, which would contribute to prevent and treat respiratory allergies more effectively.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was partially financed by the Environmental Sustainability Area of the Malaga City Council, by the Ministry of Science and Innovation of Spain and FEDER fundings inside the Operational Plurirregional Program of Spain 2014-2020 and the Operational Program of Smart Growing (Project Environmental and Biodiversity Climate Change Lab, EnBiC2-Lab), and by the University of Malaga under its program for projects leaded by young researchers (I Plan Propio de Investigación y Transferencia; B1-2021_24). A. Picornell was supported by a postdoctoral grant financed by the Ministry of Economic Transformation, Industry, Knowledge and Universities of the Junta de Andalucía (POSTDOC_21_00056).

Author Contributions Statement

R.R-M wrote the main manuscript, prepared the figures, and collected data. M.M.T. reviewed the manuscript, administrated the project and was responsible of funding acquisition and resources. M.R. and E.G-M reviewed the manuscript and collected data. A.P. collaborated in the elaboration of figures, reviewed the manuscript, and coordinated the study.

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Comparative aerobiological study between two stations located at different points in a coastal city in southern Spain

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1. Introduction

2. Material And Methods

2.1. Area of study

2.2. Pollen data

2.3. Land uses

3. Results And Discussion

3.1 Land uses

3.2. Aerobiological study

4. Conclusions

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