The present study was carried out between May and June 2012 in the city of Cuernavaca, Mexico. During this time the average ambient temperatures were about 25°C, suggesting the formation of secondary aerosols, consisting mainly of ammonium and sulfate. The average PM2.5 concentration was 37 μg · m-3 for the entire urban area and there were only two days which exceeded the limit established by the official standards for periods of 24 h. The most abundant ionic species associated with PM2.5 were sulfates (3634.82 ng · m-3, average) and ammonium (1709.53 ng · m-3, average). The ratio estimated between total anions and total cations indicated that the concentration of total anions was 1.94 times total cations. The contribution percentage of the ionic species associated with PM2.5 revealed that 76% of the PM2.5 is sulfates and ammonium. The ion balance made for the urban area of Cuernavaca indicated that during the study period, the aerosols showed alkaline characteristics; that is to say the concentration of anions was not sufficient to neutralize the cations, specifically ammonia (m = 0.060). Finally, wind fields showed that during the study the winds came in 50% from the south west, followed by 25% from east and 12.5% of the south east, which in part allowed transport of contaminants into the portion of the city, where the AUSM campus site was located.
The study of atmospheric aerosols has become important in recent years due to their influence on the global climate directly by scattering or absorption of solar radiation and indirectely by altering cloud microphysics and the water cycle. Also, they influence atmospheric chemistry and cause negative effects on human health [
Specifically, particles less than or equal to 2.5 microns (PM2.5) consist largely of water-soluble ionic species (
In México there are standards for air quality for the following air pollutants: sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide (NO2), ozone (O3), total suspended particles (TSP), particulate matter less than ten microns in diameter (PM10) and lead (Pb).
The city of Cuernavaca is not the exception to the current situation of deterioration in air quality; in that in recent years, this urban area has significantly increased its population and the number of human activities, which generate large amounts of air pollutants.
Actually, in order to contribute to environmental improvement in the state, the government has been promoting and supporting environmental studies aimed at the development of diagnostics of the ambient levels of some pollutants considered of high risk to human health and the environment. This was done in order to allow for the design of control programs and/or to mitigate in the short and medium term. Therefore, this study was developed which aims to identify the chemical composition of particulate matter (PM2.5), and to serve as a precedent for further research.
Three sampling sites within the urban area of the city of Cuernavaca were selected. Each one is characterized by different activities, such as vehicular and industrial, in addition to having different geographical and topographical features. The sites selected were: Centro, located in the downtown of the city, the North Campus of the Autonomous University of the State of Morelos (AUSM), located north of the city, and an attached high school Campus of the AUSM (Preparatoria 1), located to the southeast of the metropolitan area. The Centro site is influenced by high vehicular and service activity, while the Preparatoria 1 site is characterized by high vehicular traffic as well as industrial impact, given that there exists an industrial park nearby named CIVAC (Centro Industrial de la Ciudad de Cuernavaca, for its acronym in Spanish).
The North Campus (AUSM) site is characterized by being on the edge of the metropolitan area, on slopes, which suggests frequent movements of air and thus contaminant transport. This area has abundant vegetation; however it is highly influenced by the Mexico-Cuernavaca highway, which passes on one side, and the transport of pollutants, specifically from Mexico City and the south side of the city.
Sampling of PM2.5 was conducted between May 18 and June 30, 2012. For the Preparatoria 1 and Centro sites, low volume equipment (Mini-Vol, Airmetrics), were used, at a flow rate 5 L·min−1, with quartz filters for particle collection. While at the AUSM site, PQ-200 equipment (Waltham, BGI, Inc.) was used at a flow of 16.4 L·min−1, also with filters of 47 mm diameter. In all cases sampling was conducted for periods of 24 h.
The difference in filter weights was obtained under controlled conditions of temperature and relative humidity, before and after sampling, and allowed for the determination of the mass of PM2.5 and estimation of its concentrations in ambient air (corrected to standard conditions of temperature and pressure).
Samples were extracted with Milli-Q water (18.2 MΩ), in an ultrasonic bath (Branson 5510) for 1 hour. Extracts were filtered through nylon membranes (0.45 μm pore diameter) and transferred to vials for chemical analysis by ion chromatography. The ionic composition was determined by an ion chromatograph (Dionex, 1600 with ISC Conductivity Detector).
The anions (
The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT4) model, obtained from the National Oceanic and Atmospheric Administration (NOAA), was used to calculate the wind trajectories. Trajectories tracing back 3 days (72 hr) were calculated. Each trajectory was estimated at 400 m above ground level.
The data obtained from the variables of interest for this study were mainly analyzed with nonparametric statistical tests. The Mann-Whitney U test was applied to compare two sets of data, while the Kruskal-Wallis test was used for comparison of PM2.5 data, anions and cations between the three sampling sites. The Spearman correlation coefficient (r) established the level of the relationship between the variables, whereas the analysis of linear regression by least squares method allowed the determination of the degree of neutralization between anions and cations, as well as determining the degree of acidity or alkalinity of the particles.
To identify the transport pathways of air masses from different potential source regions, backward air trajectory analysis was carried out using the HYSPLIT 4.8 model. Four types of trajectories were observed during sampling period for the city of Cuernavaca. The South West (SW) trajectory prevailed from May 18 to June 30 (50%), it suggests that air masses are transporting some contaminants from the Centro site (SW), to the north of the city where the anthropogenic activities are important; the AUSM site is located in this direction. The East (E) type, accounting for 25% of all trajectories, indicates that a possible source in Cuernavaca is volcanic emissions from the Popocatepetl volcano. For its part, the South East (SE) type, contributes with 12.5 % of all trajectories, passing over the Preparatoria 1 site, where industrial and vehicular activities are a constant feature (
The median of the concentrations of PM2.5, for the entire study period and the three sampling sites, was 28 µg·m−3 (average of 37 µg·m−3).
In Mexico the Official Mexican Standard (NOM-025-SSA1-1993) establishes the average maximum concentration of PM2.5 (65 µg·m−3) for a period of 24 h. The UAEM and Centro sites exceeded this limit, each twice reaching 90 and 75 µg·m−3, respectively, while the Preparatoria 1 site did not exceeded the standard established by the norm on any sampling day. It is likely that at this latter site meteorological and climatic conditions have been more favorable for the dispersion of pollutants, specifically on days with high particulate concentrations in ambient air.
In addition,
N | Average | Median | Minimum | Maximum | SD | Type of area | |
---|---|---|---|---|---|---|---|
AUSM | 6 | 36 | 19 | 4 | 90 | 37 | Urban |
Centro | 6 | 43 | 38 | 7 | 75 | 27 | Urban |
Preparatoria1 | 7 | 34 | 28 | 11 | 56 | 16 | Urban |
Cuernavaca-Morelos* | 19 | 37 | 28 | 4 | 90 | 26 | Urban |
México City1 | 27 | 36 | 10.1 | Urban | |||
Guadalajara, Jal-Mex.2 | 73 | 48 | 20.1 | Urban | |||
Pamplona, España3 | 15.4 | 3.9 | 74.9 | Urban | |||
17.4 | 3.6 | 86.3 | Urban | ||||
Hong Kong4 | 69.3 | 11.8 | Urban | ||||
66.9 | 12.3 | Urban | |||||
49.3 | 17.9 | Urban | |||||
Berna5 | 24.6 | Urban | |||||
Zurich5 | 43.0 | Urban | |||||
Basel5 | 18.9 | Urban | |||||
Payern5 | 13.7 | Suburban | |||||
Chaumont5 | 7.7 | Suburban | |||||
Turín6 | 69.2 | 70.2 | 13.7 | Urban | |||
Verona6 | 51.0 | 60.9 | 17.5 | Urban | |||
Barcelona6 | 30.2 | 31.9 | 5.6 | Urban | |||
París6 | 21.0 | 18.0 | 10.9 | Urban | |||
Reykjavik6 | 4.8 | 4.6 | 2.0 | Suburban |
*This study. SD—standard deviation; N—sampling number. 1Edgerton et al. (2000); 2Hernández et al. (2009), two sampling sites; 3Aldabe et al. (2011), two sampling sites; 4Cheng et al. (2010), three sampling sites; 5Hueglin et al. (2005), three sampling sites; 6Hazenkamp-von Arx et al. (2003).
those observed in the different sites sampled in Cuernavaca. It is important to mention that comparisons should be made with caution because of differences in population size, number of sources and types, topography, etc.
In total five inorganic anions and five cations were analyzed in aqueous extracts obtained from PM2.5 particles collected at three different sites in the city of Cuernavaca. After determining their concentrations in ambient air, it was determined that the concentration of Total Anions (TA) was significantly greater than Total Cations (TC) (Mann-Whitney, p = 0.00001) during the entire period of sampling. These differences allowed for the estimation of the proportion of TA/TC, indicating that the TA concentration was 1.94 times higher than TC at all sampling sites. This ratio was statistically similar between the three study sites (1.84, 1.95 and 1.96 in Preparatoria 1, Centro and AUSM, respectively, p = 0.364). The above ratio only reflects the proportion of concentrations of both types of ions associated with PM2.5, but does not provide information about the chemical properties of the particles (see ion balance section).
Considering all data sets for the entire study, the most abundant ions were
In general, all the cation and anion concentrations showed no significant differences between the three sites (p > 0.05, in all cases); a behavior similiar to that observed with PM2.5. This probably suggests that the source of these chemical species, or atmospheric formation mechanisms, are similar within the urban area of Cuernavaca. In this regard Cheng et al. (2010) [
Taking into account the concentrations of anions and cations associated with PM2.5 and due to the absence of statistically significant differences of ions and particles between sites, the percentage contribution of anions and cations was estimated with respect to the total ion content (TI) for the urban area of Cuernavaca, considering the total data from the three sampling sites. The results indicated that ammonium and sulfate contributed 52 and 24%, respectively (76% between the two species). Other ion species, individually, did not contribute more than 6% to the total. It is important to emphasize that these percentages may vary if the presence of other ion species is taken into account.
In the city of Cuernavaca the precursors (SO2 and NOx) of the most abundant ions, ammonium and sulfate, are subject to oxidation processes once that they are emited to the atmosphere. Thus, it can be suggested that the different combustion processes contribute indirectly to the presence of sulfate and ammonium in the PM2.5, so
Species | N | Average | Median | Minimum | Maximum | SD |
---|---|---|---|---|---|---|
29 | 172.39 | 84.55 | 51.96 | 1598.28 | 297.48 | |
29 | 320.14 | 271.53 | 31.62 | 913.62 | 252.06 | |
29 | 409.65 | 279.21 | 129.19 | 2200.27 | 422.73 | |
21 | 58.26 | 54.91 | 36.33 | 113.11 | 16.08 | |
29 | 3634.82 | 3880.91 | 923.83 | 6373.85 | 1601.92 | |
Na+ | 29 | 177.04 | 123.75 | 54.20 | 1272.39 | 222.61 |
29 | 1709.53 | 1921.32 | 146.74 | 2947.36 | 763.18 | |
K+ | 29 | 245.82 | 231.10 | 72.57 | 619.72 | 126.91 |
Mg2+ | 29 | 37.64 | 24.38 | 10.07 | 285.97 | 50.91 |
Ca2+ | 29 | 283.34 | 162.36 | 77.52 | 2594.05 | 458.32 |
Anions sum | 29 | 4579.19 | 4681.66 | 1308.06 | 10658.20 | 1998.61 |
Cations sum | 29 | 2453.39 | 2475.69 | 368.66 | 7560.88 | 1319.61 |
Total ions | 29 | 7032.58 | 7062.98 | 1676.72 | 18219.08 | 3276.62 |
SD—standard deviation; N—sampling number.
that anthropogenic activities become important as sources of these species [
This situation probably contributed to the higher concentrations of sulfates with respect to the other ion species, including nitrates, which are also of secondary origin. Although
In the background species such as Ca2+ and Mg2+ appear, which are primarily associated with a geological origin (resuspension of soil dust due to wind or vehicular traffic), indicating a contribution of natural sources to the chemical composition of PM2.5 [
To evaluate the degree of association between the anion and cation species, the entire data set was used to calculate the Spearman’s rank correlation, “r”.
Moreover, a linear regression analysis was performed by the least squares method between the levels of anions and cations concentrations, expressed as nM/m3 (nanomolar per m3). This was done with the intention of evaluating the presence of chemical neutralization processes between these species. From this analysis, those results were taken with values of r > 0.82 between anions and cations. The linear regression analysis suggested
Na+ | K+ | Mg2+ | Ca2+ | PM2.5 | TA | TC | TI | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1.00 | ||||||||||||||
0.22 | 1.00 | |||||||||||||
0.38 | 0.60 | 1.00 | ||||||||||||
0.62 | 0.43 | 0.61 | 1.00 | |||||||||||
0.44 | 0.56 | 0.60 | 0.65 | 1.00 | ||||||||||
Na+ | 0.78 | 0.40 | 0.63 | 0.61 | 0.71 | 1.00 | ||||||||
0.46 | 0.49 | 0.55 | 0.63 | 0.99 | 0.69 | 1.00 | ||||||||
K+ | 0.66 | 0.39 | 0.63 | 0.68 | 0.74 | 0.83 | 0.73 | 1.00 | ||||||
Mg2+ | 0.58 | 0.31 | 0.50 | 0.69 | 0.54 | 0.66 | 0.53 | 0.71 | 1.00 | |||||
Ca2+ | 0.32 | 0.42 | 0.56 | 0.70 | 0.61 | 0.55 | 0.57 | 0.63 | 0.70 | 1.00 | ||||
PM2.5 | 0.46 | 0.47 | 0.16 | 0.36 | 0.08 | 0.15 | 0.09 | 0.24 | 0.15 | 0.53 | 1.00 | |||
TA | 0.12 | 0.45 | 0.56 | 0.42 | 0.97 | 0.59 | 0.93 | 0.62 | 0.38 | 0.53 | −0.19 | 1.00 | ||
TC | 0.29 | 0.28 | 0.47 | 0.59 | 0.96 | 0.66 | 0.96 | 0.72 | 0.51 | 0.64 | −0.06 | 0.94 | 1.00 | |
TI | 0.17 | 0.41 | 0.54 | 0.42 | 0.98 | 0.63 | 0.94 | 0.68 | 0.43 | 0.59 | −0.06 | 0.99 | 0.97 | 1.00 |
Bold and italics values (p < 0.05).
that sulfate concentrations are partly neutralized with the ammonia present in the particles (R2 = 0.97, p < 0.00001), and it is highly probable that these species have been present as ammonium sulfate ((NH4)2(SO4)).
In order to establish the acidic or basic properties of the aerosols in the urban area of Cuernavaca, the concentrations of the ionic species tested were transformed to nEq/m3 (nanoequivalents per m3).
After realizing the transformation to nEq/m3 units and applying a linear regression analysis of least squares method, a value for the slope (m) less than unity (m = 0.60, p < 0.00001) was found. This result indicates that during the sampling campaign, the concentration of anions was not enough to neutralize the ammonia and other species of cations present in the PM2.5. Taking into account the above result may suggest that the particles had alkaline characteristics during this period of the year. It is important to note that these results can vary if other ion species and even other particle sizes are taken into account.
A multivariate analysis Cluster type was performed to the data set of the composition of anions and cations associated with PM2.5. Ward’s method for the formation of groups was applied to standardized variables. The dendrogram allowed for distinguishing two basic groups for integration of anion and cation species (
suggest that the geological material is probably an important component in the source of such particles. This is based on the positive correlation, although moderate, of the particles with Ca2+ (
The second group consists mainly of
The trajectories of the winds in the urban area of Cuernavaca indicate that during the study period, the north area (AUSM) may have been influenced by pollutants from the southeast and east of the city.
The results obtained suggest that the PM2.5 concentration is very homogeneous in Cuernavaca (average concentration 37 µg·m−3).
With respect to the ion concentration the results indicate that the most abundant species are sulfate and ammonium. It is also important to note that the concentration of anions and cations in Cuernavaca is very similar, suggesting common sources and/or similar mechanisms.
Meanwhile, the ion balance reveals that in the city of Cuernavaca aerosols presented alkaline characteristics during the study period; this means that the amount of anions was not sufficient to neutralize the ammonium and other cation species.
The authors would like to express their appreciation to Winston Smith of the Peace Corps for the revision of this paper, to the Air Quality Monitoring Network of the State of Morelos (RAMAMOR, for its acronym in English) for allowing the installment of the equipment in their locations Special thanks also to National Council for Science and Technology (CONACyT, for its acronym in English) for the financial support in this project.