Trace Gas Monitoring Research Articles

Age of Stratospheric Air: Progress on Processes, Observations, and Long‐Term Trends

AbstractAge of stratospheric air is a well established metric for the stratospheric transport circulation. Rooted in a robust theoretical framework, this approach offers the benefit of being deducible from observations of trace gases. Given potential climate‐induced changes, observational constraints on stratospheric circulation are crucial. In the past two decades, scientific progress has been made in three main areas: (a) Enhanced process understanding and the development of process diagnostics led to better quantification of individual transport processes from observations and to a better understanding of model deficits. (b) The global age of air climatology is now well constrained by observations thanks to improved quality and quantity of data, including global satellite data, and through improved and consistent age calculation methods. (c) It is well established and understood that global models predict a decrease in age, that is, an accelerating stratospheric circulation, in response to forcing by greenhouse gases and ozone depleting substances. Observational records now confirm long‐term forced trends in mean age in the lower stratosphere. However, in the mid‐stratosphere, uncertainties in observational records are too large to confirm or disprove the model predictions. Continuous monitoring of stratospheric trace gases and further improved methods to derive age from those tracers will be crucial to better constrain variability and long‐term trends from observations. Future work on mean age as a metric for stratospheric transport will be important due to its potential to enhance the understanding of stratospheric composition changes, address climate model biases, and assess the impacts of proposed climate geoengineering methods.

Customizing Wettability of Defect-Rich CeO2/TiO2 Nanotube Arrays for Humidity-Resistant, Ultrafast, and Sensitive Ammonia Response.

In all their applications, gas sensors should satisfy several requirements, including low cost, reduced energy consumption, fast response/recovery, high sensitivity, and reliability in a broad humidity range. Unfortunately, the fast response/recovery and sensing reliability under high humidity conditions are often still missing, especially those working at room temperature. In this study, a humidity-resistant gas sensor with an ultrafast response/recovery rate was designed by integrating a defect-rich semiconducting sensing interface and a self-assembled monolayer (SAM) with controllable wettability. As a proof-of-concept application, ammonia (NH3), one of the atmospheric and indoor pollutants, was selected as the target gas. The decoration of interconnected defective CeO2 nanowires on spaced TiO2 nanotube arrays (NTAs) provided superior NH3 sensing performances. Moreover, we showed that manipulating the functional end group of SAMs is an efficient and simple method to adjust the wettability, by which 86% sensitivity retention with an ultrafast response (within 5 s) and a low limit of detection (45 ppb) were achieved even at 75% relative humidity and room temperature. This work provides a new route toward the comprehensive design and application of metal oxide semiconductors for trace gas monitoring under harsh conditions, such as those of agricultural, environmental, and industrial fields.

Zonal variability of methane trends derived from satellite data

Abstract. The Tropospheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor (S5P) satellite is part of the latest generation of trace gas monitoring satellites and provides a new level of spatio-temporal information with daily global coverage, which enables the calculation of daily globally averaged CH4 concentrations. To investigate changes in atmospheric methane, the background CH4 level (i.e. the CH4 concentration without seasonal and short-term variations) has to be determined. CH4 growth rates vary in a complex manner and high-latitude zonal averages may have gaps in the time series, and thus simple fitting methods do not produce reliable results. In this paper we present an approach based on fitting an ensemble of dynamic linear models (DLMs) to TROPOMI data, from which the best model is chosen with the help of cross-validation to prevent overfitting. This method is computationally fast and is not dependent on additional inputs, allowing for fast and continuous analysis of the most recent time series data. We present results of global annual methane increases (AMIs) for the first 4.5 years of S5P/TROPOMI data, which show good agreement with AMIs from other sources. Additionally, we investigated what information can be derived from zonal bands. Due to the fast meridional mixing within hemispheres, we use zonal growth rates instead of AMIs, since they provide a higher temporal resolution. Clear differences can be observed between Northern Hemisphere and Southern Hemisphere growth rates, especially during 2019 and 2022. The growth rates show similar patterns within the hemispheres and show no short-term variations during the years, indicating that air masses within a hemisphere are well-mixed during a year. Additionally, the growth rates derived from S5P/TROPOMI data are largely consistent with growth rates derived from Copernicus Atmospheric Monitoring Service (CAMS) global-inversion-optimized (CAMS/INV) data, which use surface observations. In 2019 a reduction in growth rates can be observed for the Southern Hemisphere, while growth rates in the Northern Hemisphere stay stable or increase. During 2020 a strong increase in Southern Hemisphere growth rates can be observed, which is in accordance with recently reported increases in Southern Hemisphere wetland emissions. In 2022 the reduction in the global AMI can be attributed to decreased growth rates in the Northern Hemisphere, while growth rates in the Southern Hemisphere remain high. Investigations of fluxes from CAMS/INV data support these observations and suggest that the Northern Hemisphere decrease is mainly due to the decrease in anthropogenic fluxes, while in the Southern Hemisphere, wetland fluxes continued to rise. While the continued increase in Southern Hemisphere wetland fluxes agrees with existing studies about the causes of observed methane trends, the difference between Northern Hemisphere and Southern Hemisphere methane increases in 2022 has not been discussed before and calls for further research.

Monitoring of Atmospheric Trace Gases in Urban and Rural Areas of the Baikal Natural Territory during 2019–2021

Monitoring of Atmospheric Trace Gases in Urban and Rural Areas of the Baikal Natural Territory during 2019–2021

Diffusion Height and Order of Sulfur Dioxide and Bromine Monoxide Plumes from the Hunga Tonga–Hunga Ha’apai Volcanic Eruption

A violent volcanic eruption attracting considerable attention occurred on 15 January 2022 near the South Pacific island nation of Tonga. To investigate its environmental impact, we retrieved the sulfur dioxide (SO2) and bromine monoxide (BrO) vertical column densities of environmental trace gas monitoring instrument 2 (EMI-2) based on the differential optical absorption spectroscopy algorithm. The results showed westward and southeastward transport of principal parts of SO2 and BrO plumes, respectively, from the Hunga Tonga–Hunga Ha’apai (HTHH) eruption. On 15 January, most of the released SO2 entered the stratosphere (above 20 km) directly and spread rapidly westward (approximately 30 m/s). In contrast, the principal portion of the BrO spread southeastward slowly (approximately 10 m/s) within the 8–15 km altitude layer on 16 January. Our research results also suggest that during the HTHH eruption, BrO was released from the magmatic melt later than SO2. The total SO2 emissions from this eruption were approximately 0.24 Tg. The majority of SO2 and BrO plumes were transported within the Southern Hemisphere. This study is an important extension to the empirical database of volcanological and magmatic degassing research.

Preliminary inversion of surface UV-B irradiance based on GF-5 environmental trace gas monitoring instrument

Preliminary inversion of surface UV-B irradiance based on GF-5 environmental trace gas monitoring instrument

Principle and Performance Analysis of the Levenberg–Marquardt Algorithm in WMS Spectral Line Fitting

Calibration-free wavelength modulation spectroscopy (WMS) is an efficient technique for trace gas monitoring. It is widely used due to its resistance to light intensity fluctuations, strong suppression of low-frequency noise, fast response time, and excellent environmental adaptability. The calibration-free WMS often employs the Levenberg–Marquardt algorithm for spectral fitting to retrieve gas characteristics. However, to the best of our knowledge, an analysis of the main factors affecting the operational effectiveness of the Levenberg–Marquardt algorithm in calibration-free WMS has merely been reported. In this paper, we have systematically analyzed the Levenberg–Marquardt algorithm’s operating mechanism in WMS-2f/1f. The results show that the number of parameters and the estimation errors of the initial parameters are the main factors limiting the retrieval accuracy of the algorithm, which provides some important guidelines for the subsequent optimization of the spectral fitting scheme.

Understanding the influence of summer biomass burning on air quality in North India: Eight cities field campaign study

Near real-time monitoring of major air pollutants, i.e., particulate matter (PM10, PM2.5, PM1), trace gases (O3, CO, NO, NO2, NOx, NH3, CO2, SO2) and Volatile Organic Compounds (VOCs: benzene, ethylbenzene, m-, p-xylene, o-xylene and toluene) along with climatological parameters was done in eight-cities field campaigns during the rabi (wheat) crop residue burning period in the northwest of Indo-Gangetic Plain (IGP) region. The phase-wise monitoring was done at eight locations representing rural, semi-urban and urban backgrounds. During the whole campaign, the semi-urban site (Sirsa) observed the highest average concentration of PM10 (226 ± 111 μg m−3) and PM2.5 (91 ± 67 μg m−3). The urban site (Chandigarh) reported the minimum concentrations of all the three size fractions of particulate matter with PM10 as 89 ± 54 μg m−3, PM2.5 as 42 ± 22 μg m−3 and PM1 as 20 ± 13 μg m−3 where the monitoring was done in the early phase of the campaign. The highest VOC concentration was recorded at the semi-urban (Sirsa) site, whereas the lowest was at a rural location (Fatehgarh Sahib). NH3 concentration was observed highest in rural sites (31.7 ± 29.8 ppbv), which can be due to the application of fertilizers in agricultural activities. Visible Infrared Imaging Radiometer Suite (VIIRS) based fire and thermal anomalies, along with HYSPLIT back trajectory analysis, show that major air masses over monitoring sites (22 %–70 %) were from the rabi crop residue burning regions. The characteristic ratios and Principal component analysis (PCA) results show that diverse sources, i.e., emissions from crop residue burning, solid biomass fuels, vehicles and industries, majorly degrade the regional air quality. This multi-city study observed that semi-urban regions have the most compromised air quality during the rabi crop residue burning and need attention to address the air quality issues in the IGP region.

Application and algorithm research of TDLAS

Tunable diode laser absorption spectroscopy (TDLAS) is extensively utilized in monitoring of trace gases in the environment. With the relative entropy tomographic reconstruction, simultaneous multiplicative algebraic reconstruction technique algorithm and optimization of existing functions and models, TDLAS has been applied to reconstruction of temperature and humidity field, combustion diagnosis, mass flow monitoring and other domains, this paper will analyze the existing TDLAS application and algorithm research.

Larger-Than-Unity External Optical Field Confinement Enabled by Metamaterial-Assisted Comb Waveguide for Ultrasensitive Long-Wave Infrared Gas Spectroscopy.

Nanophotonic waveguides that implement long optical pathlengths on chips are promising to enable chip-scale gas sensors. Nevertheless, current absorption-based waveguide sensors suffer from weak interactions with analytes, limiting their adoptions in most demanding applications such as exhaled breath analysis and trace-gas monitoring. Here, we propose an all-dielectric metamaterial-assisted comb (ADMAC) waveguide to greatly boost the sensing capability. By leveraging large longitudinal electric field discontinuity at periodic high-index-contrast interfaces in the subwavelength grating metamaterial and its unique features in refractive index engineering, the ADMAC waveguide features strong field delocalization into the air, pushing the external optical field confinement factor up to 113% with low propagation loss. Our sensor operates in the important but underdeveloped long-wave infrared spectral region, where absorption fingerprints of plentiful chemical bonds are located. Acetone absorption spectroscopy is demonstrated using our sensor around 7.33 μm, showing a detection limit of 2.5 ppm with a waveguide length of only 10 mm.

Wideband, high spectral resolution UV convex grating imaging spectrometer based on an Offner structure.

Ultraviolet is an effective electromagnetic spectrum in material detection, which has wide application prospects in aerospace and environmental monitoring. A conventional imaging spectrometer has a narrow UV band and low spectral resolution. To solve this problem, a convex grating imaging spectrometer based on an Offner structure with F#2.5 and a 13 mm long slit was designed and developed. The working wavelength ranges from 200 to 433 nm, and the spectral resolution is greater than 0.5 nm. A hyperspectral data cube with both high spatial and spectral resolutions of external scenes can be obtained by the push-broom imaging mode. Fine Fraunhofer lines can be distinguished in the spectrum. The ultraviolet hyperspectral imager can be used for marine oil spills, trace gas monitoring, and other applications that require high signal-to-noise ratios, wide bands, and high spectral resolutions.

Decomposition-based multiobjective optimization for multipass cell design aided by particle swarm optimization and the K-means algorithm.

We proposed a method to intelligently design two-spherical-mirror-based multipass cells (MPCs) and optimize multiple objectives simultaneously. By integrating the K-means algorithm into the particle swarm optimization (PSO) algorithm, an efficient method is developed to optimize MPC configurations possessing characteristics of both long optical path lengths (OPLs) and circle patterns. We built and tested an MPC with four concentric circle patterns, which achieved an OPL of 54.1 m in a volume of 273.1 cm3. We demonstrated the stability and detection precision of the developed gas sensor. Continuous measurement of methane in ambient laboratory air was realized, with a detection precision of 8 ppb and an averaging time of 13 s. The combination of K-means and PSO algorithms is effective in optimizing MPCs with multiple objectives, which makes it suitable for designing versatile MPCs satisfying various requirements of field applications, including pollution and greenhouse gas emission monitoring and high-sensitivity measurements of other trace gases.

Pre-Launch Radiometric Characterization of EMI-2 on the GaoFen-5 Series of Satellites

The environmental trace gas monitoring instrument (EMI) is a space-borne imaging spectrometer onboard GaoFen-5, which was launched in May 2018, covering wavelengths in the range of 240–710 nm to measure NO2, O3, HCHO, and SO2. An advanced EMI-2 instrument with a higher spatial resolution and sufficient signal-to-noise is currently planned for launch on the GaoFen-5(02) satellite in 2021. The EMI-2 instrument bidirectional scattering distribution function (BSDF) is obtained from the absolute irradiance and radiance calibration on-ground. Based on EMI-2 earth and sun optical paths, the key factors of BSDF parameters are introduced. An NIST-calibrated 1000 W FEL quartz tungsten halogen lamp and a 2D turntable are adopted for the absolute irradiance calibration. A large aperture integrating sphere system is used for the absolute radiance calibration. Based on absolute irradiance and radiance calibration functions, the BSDF parameters are obtained, with accuracy of 4.9% for UV1, 4.3% for UV2, 4.1% for VIS1, and 4.2% for VIS2. The on-ground measurement results show that the reflectance spectrum can be calculated from BSDF parameters. On-orbit application of the EMI-2 instrument BSDF are also discussed.

An indirect-calibration method for non-target quantification of trace gases applied to a time series of fourth-generation synthetic halocarbons at the Taunus Observatory (Germany)

Abstract. Production and use of many synthetic halogenated trace gases are regulated internationally due to their contribution to stratospheric ozone depletion or climate change. In many applications they have been replaced by shorter-lived compounds, which have become measurable in the atmosphere as emissions increased. Non-target monitoring of trace gases rather than targeted measurements of well-known substances is needed to keep up with such changes in the atmospheric composition. We regularly deploy gas chromatography (GC) coupled to time-of-flight mass spectrometry (TOF-MS) for analysis of flask air samples and in situ measurements at the Taunus Observatory, a site in central Germany. TOF-MS acquires data over a continuous mass range that enables a retrospective analysis of the dataset, which can be considered a type of digital air archive. This archive can be used if new substances come into use and their mass spectrometric fingerprint is identified. However, quantifying new replacement halocarbons can be challenging, as mole fractions are generally low, requiring high measurement precision and low detection limits. In addition, calibration can be demanding, as calibration gases may not contain sufficiently high amounts of newly measured substances or the amounts in the calibration gas may have not been quantified. This paper presents an indirect data evaluation approach for TOF-MS data, where the calibration is linked to another compound which could be quantified in the calibration gas. We also present an approach to evaluate the quality of the indirect calibration method, select periods of stable instrument performance and determine well suited reference compounds. The method is applied to three short-lived synthetic halocarbons: HFO-1234yf, HFO-1234ze(E), and HCFO-1233zd(E). They represent replacements for longer-lived hydrofluorocarbons (HFCs) and exhibit increasing mole fractions in the atmosphere. The indirectly calibrated results are compared to directly calibrated measurements using data from TOF-MS canister sample analysis and TOF-MS in situ measurements, which are available for some periods of our dataset. The application of the indirect calibration method on several test cases can result in uncertainties of around 6 % to 11 %. For hydro(chloro-)fluoroolefines (denoted H(C)FOs), uncertainties up to 23 % are achieved. The indirectly calculated mole fractions of the investigated H(C)FOs at Taunus Observatory range between measured mole fractions at urban Dübendorf and Jungfraujoch stations in Switzerland.

High sensitivity open‐path gas sensor based on a triangular multi‐pass cell

AbstractIn this article, a gas sensing technique utilizing a compact multi‐pass absorption cell in a triangular configuration was proposed for open‐path trace gases detection. To demonstrate this technique, an open‐path gas sensor system was developed by combing the innovated triangular multi‐pass cell with a near infrared distributed feedback diode laser (at 1.65 μm) for simultaneous detection of atmospheric CH4 and H2O. A Gaussian‐noise‐limited limit of detection of approximate 52 ppb and 445 ppm were achieved for CH4 and H2O, respectively, with 1 Hz sampling rate and at normal atmospheric pressure. Allan variance analysis indicates that a sensitivity of 5.5 ppb (for CH4) and 75 ppm (for H2O) could be obtained at the optimal averaging time of 300 s, which is comparative with the mid‐infrared laser based gas sensors. This technique will be a critical tool for developing low‐cost open‐path trace gas monitoring networks for environmental, industrial, and public security applications.

First high-resolution tropospheric NO<sub>2</sub> observations from the Ultraviolet Visible Hyperspectral Imaging Spectrometer (UVHIS)

Abstract. We present a novel airborne imaging differential optical absorption spectroscopy (DOAS) instrument: the Ultraviolet Visible Hyperspectral Imaging Spectrometer (UVHIS), which is developed for trace gas monitoring and pollution mapping. Within a broad spectral range of 200 to 500 nm and operating in three channels, the spectral resolution of UVHIS is better than 0.5 nm. The optical design of each channel comprises a fore-optics with a field of view (FOV) of 40∘, an Offner imaging spectrometer and a charge-coupled device (CCD) array detector of 1032×1072 pixels. A first demonstration flight using UVHIS was conducted on 23 June 2018, above an area of approximately 600 km2 in Feicheng, China, with a spatial resolution of about 25 m×22 m. Measurements of nadir backscattered solar radiation of channel 3 are used to retrieve tropospheric vertical column densities (VCDs) of NO2 with a mean total error of 3.0×1015 molec cm−2. The UVHIS instrument clearly detected several emission plumes transporting from south to north, with a peak value of 3×1016 molec cm−2 in the dominant one. The UVHIS NO2 vertical columns are consistent with the ground-based mobile DOAS observations, with a correlation coefficient of 0.65 for all co-located measurements, a correlation coefficient of 0.86 for the co-located measurements that only circled the steel factory and a slight underestimation for the polluted observations. This study demonstrates the capability of UVHIS for NO2 local emission and transmission monitoring.

Approach to enhance trace gas determinations through multi-satellite data fusion

The imaging instruments on the polar-orbiting S-NPP and NOAA-20 satellite platforms [e.g., Visible Infrared Imaging Radiometer Suite (VIIRS)] and on geostationary GOES and Himawari platforms [e.g., Advanced Baseline Imager (ABI) and Advanced Himawari Imager (AHI)] have high horizontal spatial resolution but coarse vertical information about tropospheric gases and temperatures. An approach for fusing a high spatial resolution imager (e.g., VIIRS, ABI, and AHI) and a high information content sounder or trace gas monitor [e.g., Cross-track Infrared Sounder (CrIS) and TROPOspheric Monitoring Instrument (TROPOMI)] offers the opportunity to construct retrieval products (e.g., trace gas concentrations of CO, SO2, and NH3) via product fusion at high spatial resolution. Furthermore, these products can also be provided at high temporal resolution when geostationary imager (ABI and AHI) data are used in the product fusion. Promising results from VIIRS/CrIS, VIIRS/TROPOMI, and AHI/TROPOMI fusion are presented.

First Year On-Orbit Calibration of the Chinese Environmental Trace Gas Monitoring Instrument Onboard GaoFen-5

Environmental trace gas monitoring instrument (EMI) onboard GaoFen-5 was launched in May 2018 and has successfully operated on-orbit for more than a year. EMI contains four grating spectrometers, covering wavelengths in the range 240–710 nm with a spectral resolution of 0.3–0.5 nm, and enables one-day global coverage. For EMI on-orbit calibration, two onboard solar diffusers (SD), one surface reflectance aluminum diffuser, and one quartz volume diffuser (QVD) are used to measure solar spectra. The solar spectra are used to perform accurate spectral and radiometric calibrations. EMI on-orbit spectral calibration contains wavelength calibration and instrument spectral response function (ISRF), both of which are the key quantities in trace gas retrievals based on differential optical absorption spectroscopy (DOAS) analysis. The wavelength calibration is performed using the Fraunhofer lines in the solar spectrum, and the ISRF parameters are obtained by fitting high-resolution and EMI-measured solar spectra. Based on the known solar irradiation and characteristic of SD, SD radiance can be calculated and is used for EMI on-orbit radiometric calibration. The radiometric calibration also determines the absolute Earth reflectance spectra that are used as inputs for atmospheric retrieval algorithms. For EMI on-orbit radiometric monitoring, aluminum diffuser is used as reference SD to monitor QVD degradation. An internal white light source is used to detect pixel performance and monitor radiometric throughput.

Towards an understanding of surface effects: testing of various materials in a small volume measurement chamber and its relevance for atmospheric trace gas analysis

Abstract. A critical issue for the long-term monitoring of atmospheric trace gases is precision and accuracy of the measurement systems employed. Both measuring and preparing reference gas mixtures for trace gases are challenging due to, for example, adsorption and desorption of the substances of interest on surfaces; this is particularly critical at low amount fractions and/or for reactive gases. Therefore, to ensure the best preparation and measurement conditions for trace gases in very low amount fractions, usage of coated materials is in demand in gas metrology and atmospheric measurement communities. This study focuses on testing potential adsorption and desorption effects for different materials or coatings that are currently used or that may be relevant in the future for the measurements of greenhouse gases. For this study we used small volume chambers designed to be used for adsorption studies. Various materials with or without coatings were loaded into the small cylinder to test their adsorption and desorption behavior. We used the aluminum cylinder as the measurement chamber and glass, aluminum, copper, brass, steel and three different commercially available coatings as test materials. Inserting the test materials into the measurement chamber doubles the available geometric area for the surface processes. The presented experiments were designed to investigate the pressure dependency of adsorption up to 15 bar and its temperature dependency up to 80 ∘C for the test materials placed in the measurement chamber. Here, we focused on the species CO2, CH4, CO and H2O measured by a cavity ring-down spectroscopy analyzer. Our results show that the materials currently used in atmospheric measurements are well suited. The investigated coatings were not superior to untreated aluminum or to stainless steel at the tested pressure ranges, whereas under changing temperature aluminum showed better performance for CO2 (<0.05 µmol mol−1 change in measured amount fractions) than stainless steel (>0.1 µmol mol−1). To our knowledge, this study is one of the first attempts to investigate surface effects of different materials in such a setup for the abovementioned gases.

Pollution Trends in China from 2000 to 2017: A Multi-Sensor View from Space

Satellite sensors can provide unique views of global pollution information from space. In particular, a series of aerosol and trace gas monitoring instruments have been operating for more than a decade, providing the opportunity to analyze temporal trends of major pollutants on a large scale. In this study, we integrate aerosol products from MODIS (MODIS Resolution Imaging Spectroradiometer, all abbreviations and their definitions are listed alphabetically in Abbreviations) and MISR (Multi-angle Imaging Spectroradiometer), the AAI (Absorbing Aerosol Index) product from OMI (Ozone Monitoring Instrument), column SO2 and NO2 concentrations from OMI, and tropospheric column ozone concentration from OMI/MLS (Microwave Limb Sounder) to study temporal changes in major pollutants over China. MODIS and MISR consistently revealed that column AOD (Aerosol Optical Depth) increased from 2000, peaked around 2007, and started to decline afterward, except for northwest and northeast China, where a continuous upward trend was found. Extensive negative trends in both SO2 and NO2 have also been found over major pollution source regions since ~2005. On the other hand, the OMI AAI exhibited significant increases over north China, especially the northeast and northwest regions. These places also have a decreased Angstrom exponent as revealed by MISR, indicating an increased fraction of large particles. In general, summer had the largest AOD, SO2, and NO2 trends, whereas AAI trends were strongest for autumn and winter. A multi-regression analysis showed that much of the AOD variance over major pollution source regions could be explained by SO2, NO2, and AAI combined, and that the SO2 and NO2 reduction was likely to be responsible for the negative AOD trends, while the AOD increase over NE and NW China may be associated with an increase of coarse particles revealed by increased AAI and decreased AE. In contrast to aerosols, tropospheric ozone exhibited a steady increase from 2005 throughout China. This indicates that although the recent emission control effectively reduced aerosol pollutants, ozone remains a challenging issue and may dominate future air pollution.