1. Introduction
Human health is adversely affected by ozone and particulate matter (PM
10, particulate matter with an aerodynamic diameter of 10 μg or less, and PM
2.5, particulate matter with an aerodynamic diameter of 2.5 μg or less) [
1]. High ozone and particulate matter concentrations affect many major metropolitan cities in the United States, and the El Paso–Juarez airshed is one example. El Paso is a city in the far west corner of Texas, separated only by the Rio Grande River from the Mexican city of Juarez, which is one of the most populous cities in the Mexican state of Chihuahua and is surrounded by the Chihuahua desert. Both cities share the same airshed, the El Paso–Juarez airshed, and in the past, both have violated their air quality standards for ground-level ozone. The El Paso climate is very dry and is characteristic of the urban southwestern US climate. Its air quality problem is known to be partially due to contributions from industrial activities in the region, and to high emissions from automobiles due to prolonged traffic congestion across the international bridges between the two countries [
2,
3,
4]. In addition, the geopolitical region of El Paso–Juarez exhibits exceptional meteorological conditions [
5], such as higher planetary boundary layer heights (PBLHs), than any other Texas city, influenced by the local terrain.
In this work we performed a comprehensive study, using both models and instruments, of the most important factors contributing to the ozone episodes in the El Paso–Juarez region.
In the El Paso–Juarez airshed, higher ozone concentrations occur during the summer, and higher PM10 concentrations occur during the periods of thermal inversions and dust storm events.
The tropospheric formation of ozone and aerosols share much of the same physics and chemistry. Ozone is formed through photochemical reactions of the nitrogen oxides (NO
x = NO + NO
2) and volatile organic compounds (VOCs) [
6]. These reactions produce atmospheric acids, such as nitric acid, sulfuric acid, and organic acids, and these acids are essential aerosol precursors [
7]. Ozone may react with organic compounds, such as isoprene and other alkenes, to produce organic compounds with a low volatility that condense to produce a secondary organic aerosol (SOA) [
8]. There are chemical reaction mechanisms involving ozone that convert nitrogenous compounds to nitric acid and produce SOA precursors during both the day and nighttime [
8,
9]. In turn, there are aerosol particles that affect the gas-phase chemistry of ozone formation. For example, aerosol particles scatter solar radiation, affecting the frequencies of the photolysis reactions [
10]. Aerosol particles are sinks of gas-phase species [
11].
The formation of particulate matter, ozone, and their precursors are affected by their concentrations. Concentrations of air pollutants in the planetary boundary layer (PBL) are affected by the atmospheric mixing height, which is a function of the planetary boundary layer height (PBLH). The PBL is the lowest part of the troposphere, which is directly influenced by the earth’s surface. It is the only part of the atmosphere where frictional forces play an essential role and where the temperature exhibits a diurnal cycle [
12]. The mixing layer height is an essential parameter in modeling air pollution and its transport since it determines the adequate volume in which pollutants are dispersed [
13]. If the surface emissions are consistent, then ozone concentrations will respond to the changes in the PBLH, depending on the volume available for dispersion. Ozone from the previous day can remain in the residual layer, and it can entrain when the convective boundary layer starts rising the following day, depending on the concentration within the PBL. The relationship between the PBLH and ozone is complicated and has not been studied in this region before. Since ozone episodes are frequent during the summer, continuous monitoring of the PBLH, especially in a region like El Paso–Juarez, can provide relevant information for a comprehensive regional air-quality assessment.
Previous studies were conducted in this airshed during the summer of 1996 [
5,
14,
15,
16] in order to quantify the air quality and meteorological parameters contributing to ozone episodes. However, almost no PBL studies have been performed in this region [
4,
5,
14,
17,
18,
19,
20]. Some of the balloon studies observed PBLHs as high as 4 km during the late afternoon, when the average temperature in the region is at its peak. These PBLH observations are unusually high compared to most of the commonly studied regions and need a thorough investigation.
Therefore, the goal of our research was to perform a comprehensive ozone study for this unique region, using both models (e.g., WRF, HYSPLIT) and instruments (e.g., ceilometer, pollutant sensors, available radiosondes), and to analyze how the meteorology (e.g., the winds, temperature, the PBL, the mesoscale and synoptic meteorology), the atmospheric stability, and the ozone precursors impact the ozone concentrations in the El Paso–Juarez airshed during high and low ozone episodes. Given that the El Paso–Juarez airshed experiences high ozone precursor emissions, resulting in high ozone episodes, with unique topographical and meteorological conditions, it is critical to perform a thorough investigation of the significant factors contributing towards the ozone concentrations in this region. In addition, the particulate matter behavior was analyzed during high and low ozone events.
In
Section 2, we describe the methodology, using both models and relevant instruments. In
Section 3, we show our results.
Section 4 is a discussion, and in
Section 5 we provide our conclusions.
4. Discussion
The slow growth in the PBLH in the morning and calm winds during daytime made a significant contribution to the occurrence of high ozone episodes. This observation is in agreement with the recorded field data reported by (Macdonald, C.P. et al; Brown, M.J. et al.) [
5,
14]. In all recorded high ozone events, the wind direction and speed played a significant role. During the high ozone concentrations, the wind speed was low, and the converse held true. This validates the well-known inverse proportionality relationship between the ground-level ozone concentrations and wind speed. In addition, in all high ozone events the wind direction was from the east of ELP. No strong correlation between the modeled (WRF and HYSPLIT) daytime PBLH peak and ozone concentration peak was observed during this study. This result was very much in agreement with the survey conducted previously by (Banta, R.M. et al.) [
46]. A lower MLH implies less volume available for the pollution dispersion and lower vertical mixing. The presence of clouds around 5 km vertical height can be seen more distinctly on the 6th and 7th of June. Conversely, deeper MLH during the three successive low ozone events led to the dilution of precursors and ozone concentration. Strong winds ensured that the existing pollutants, such as ozone and VOCs, are dispersed out from the region (hence a drop in their concentration). The winds, however, brought in dust from the Chihuahua desert.
The contribution of the ozone from the residual layer into the convective boundary layer is beyond the scope of this work, but it deserves future attention. Analysis of the upper air data, the synoptic meteorology, showed a high-pressure ridge present during the high ozone events.
A high peak in the PM10 concentration was recorded on 11 June, in the late afternoon, and can also be noticed in the form of a strong backscattering signal on the ceilometer aerosol backscatter profile.
5. Conclusions
A comprehensive study of the successive high and low ozone episodes in June 2017 was performed using both models and instrumentation, to analyze the major factors, e.g., meteorology (focusing on PBL, synoptic) and ozone precursors, that contributed towards the ozone events.
This work was the first systematic, and rigorous PBLH analysis performed during the ozone episodes in the ELP–CDJ region made using the ceilometer aerosol backscattering and numerical modeling. Ozone events from 4–7 June 2017 were classified as successive high ozone episodes whereas 11–13 June 2017 ozone events were identified as low ozone episodes. This work established the influence of the PBLH and other meteorological parameters on these ozone events. The aerosol backscattered profiles clearly indicate that the boundary layer played a significant role during these successive high and low ozone episodes. Analysis of nearly two decades of data shows that the month of June is a favorable month for ozone events in the ELP–CDJ airshed. The observed aerosol layer height was shallow during the high ozone days and is instrumental in allowing ozone precursors, such as NOx and VOCs, to accumulate and react due to limited availability of volume for dispersion.
The relation between the PBLH and the diurnal pollutant evolution was established using statistical analysis. For ozone diurnal variation, the WRF model performed better than HYSPLIT during the high ozone episodes. The correlation coefficient (R) in
Table 5 and
Table 6 indicated that the PBLHs variation could explain from 48% to 73% of the observed ozone during the high ozone episode period. Whereas, for the low ozone period, only one-day changes in the PBL had an R higher than 50%, indicating that PBL height development was not modulating the diurnal ozone evolution. WRF simulations showed that during the early morning and late evening the PBLHs were lower during the high ozone events compared to the low ozone events. These simulation results were consistent throughout the high and low ozone episodes, respectively. Other than 11 June, all PBLHs showed a constant growth rate between 8 a.m. and 11 a.m. Modeled and observed PBLH growth showed a difference in behavior. This difference was attributed to the way PBLH is defined for each.
The heat deficit calculations provided a greater understanding of the atmospheric stability during the high and low ozone episodes. A high ozone day exhibited strong atmospheric stability together with a high-pressure ridge, whereas during the low ozone episodes the lower atmosphere was less stable. The high atmospheric stability also affected the vertical dispersion of ozone and its precursors, leading to the pollutant build-up within the stable air mass.
High temperatures and clear skies ensured that abundant sunlight was available for the photochemical reactions necessary to produce high ozone concentrations. Lower wind speeds permitted significant accumulation of ozone and its precursors, resulting in lower dispersion rates. The consistent beginning of the ozone growth starting from 6 a.m., during both the high and low ozone events, was a result of the early morning peak traffic hours in the region.
The results of this study demonstrate the importance of continuous boundary layer monitoring in the ELP–CDJ region. This can be achieved using aerosol backscattered profiles from a compact and lightweight remote sensing instrument, such as a ceilometer, and modelling simulations using HYSPLIT (with WRF as input) and WRF. The present study provides a better understanding of the aerosol transport, especially during the synoptic scale frontal passages. The contribution of the PBL, winds, and synoptic scale meteorology on the build-up of ozone and aerosol concentrations in the ELP–CDJ region was successfully demonstrated and constitutes a major achievement of this study.