1. Introduction Air pollution, along with its impact on public health, can produce a depressing effect on plant vegetation and photosynthesis processes. The class of phytotoxicants includes such trace gas components of the atmosphere as tropospheric ozone (O 3), sulfur dioxide (SO 2), ammonia (NH 3), ethylene (or ethene, C 2H 4), peroxyacetyl nitrate (PAN), hydrogen fluoride (HF), chlorine (Cl 2), etc. [ 1]. Their high concentrations, caused by powerful anthropogenic (industry, motor transport, agriculture) and natural (wildfires) emissions, can reduce the absorption of CO 2 by vegetation during photosynthesis and, under certain conditions, cause plant death. The importance of this study is due to the fact that NH 3 and C 2H 4 are both phytotoxicants and precursors of secondary atmospheric aerosols (SOAs), which, in turn, have a significant impact on climate, air quality, and public health, and determine atmospheric visibility [ 1, 2]. Ammonia is the most abundant alkaline compound of the Earth’s atmosphere [ 3]. Atmospheric NH 3 is responsible for a significant portion of the transport of reactive nitrogen over distances of up to hundreds of kilometers [ 4]. Changes in ammonia levels alter the biogeochemical nitrogen cycle by increasing the concentration of organic nitrogen and nitrite in the environment, which causes eutrophication and acidification of surface waters and soils. Ammonia can act as a phytotoxicant for some plants, making them less resilient to frost, drought, and other stressful conditions [ 3]. Pogány et al. [ 5] noted that modern global NH 3 emissions have increased by a factor of 2–5 compared to pre-industrial levels. Agriculture, especially livestock and crop production, is the primary source of ammonia emissions, accounting for 70–80% of total atmospheric NH 3. Industrial emissions, transport, solid and liquid waste management systems contribute less than 15% [ 3]. Natural sources of ammonia emissions include soil, vegetation, and wildlife, but these estimates are currently subject to large uncertainties [ 3]. Despite its short atmospheric lifetime (a day or less), NH 3 reacts with sulfuric (H 2SO 4) and nitric (HNO 3) acids to form a fine fraction of atmospheric aerosols (ammonium salts), which remain in the atmosphere for several days to a week [ 2]. Nevertheless, while ammonia does increase the mass of fine particles when it reacts with ambient acidic aerosols, it is also at the same time neutralizing acids and making the particulate matter less toxic. Thus, at atmospheric levels, NH 3 can act as protective of human health by lowering the toxicity of PM2.5: “…an excess of ammonia gave protection from levels of sulfuric acid which, in the absence of ammonia, would have caused 50% mortality” [ 6]. In recent years, due to improvements in measuring equipment and data processing approaches, there has been a growing interest in studying atmospheric NH 3, in particular using ground-based and satellite atmospheric observational systems [ 7, 8, 9, 10]. Ethylene, being one of the most abundant unsaturated hydrocarbons in the atmosphere, is a plant hormone which regulates a growth in response to biotic and abiotic stress [ 11]. C 2H 4 is a highly productive source of tropospheric O 3 in urban air [ 12] and a precursor to SOA formation [ 13]. According to modern concepts, C 2H 4 is not formed in the atmosphere during chemical transformations [ 14]. Total atmospheric emissions of C 2H 4 are approximately 20 Tg/year, half of which come from biomass combustion; the remaining significant sources are industrial and biogenic (including oceanic) emissions [ 14, 15]. Ethylene is widely used as a chemical feedstock, having the highest production level of all industrially produced organic compounds. In agriculture, it is an accelerator of ripening processes [ 11]. The main C 2H 4 removal mechanisms from the troposphere are the reaction with ozone and the hydroxyl radical (OH), and transport to the stratosphere. The lifetime of ethylene in the troposphere varies significantly with time and latitude, it ranges from a few hours in summer to about four days in winter [ 7, 16]. It should be noted that the number of experimental studies of atmospheric ethylene is not numerous due to its low concentration in the atmosphere and, as a consequence, due to the difficulty of its detection, especially for background conditions. For the same reason, published quantitative estimates of sources and sinks are characterized by significant uncertainties [ 11]. The main goal of this study is to investigate the feasibility of using atmospheric monitoring data on NH 3 and C 2H 4 total columns, obtained from ground-based FTIR observations of direct solar radiation, to assess air quality in the context of its impacts on human health and the ecosystem. The specific objectives of this paper are as follows: - to present the strategies that are used to retrieve the NH 3 and C 2H 4 total column (TC) in the atmosphere from high-resolution Fourier Transform Infrared (FTIR) spectra of direct solar radiation recorded at the St. Petersburg State University (SPbU) atmospheric monitoring station; - to analyze the uncertainty budget of the retrieved NH 3 and C 2H 4 TCs; - to analyze the long-term trend, annual cycle and anomalies of NH 3 and C 2H 4 using results of the long-term atmospheric FTIR monitoring of ammonia and ethylene (2009–2025) conducted in the suburbs of St. Petersburg which is the fourth-largest city in Europe with the population ~5.7 million people; - to compare results of NH 3 and C 2H 4 FTIR monitoring at the SPbU site with air quality standards and metrics applied for human health (national) and sensitive ecosystem state (international). 2.1. Description of SPbU Observational Site The SPbU atmospheric monitoring station is a mid-latitude observational station (59.88° N, 29.83° E, 20 m asl), located at a distance of 2.5 km south of the coast of the Gulf of Finland (Baltic Sea) and of about 35 km west of the center of St. Petersburg. Due to the predominant westerly winds, most of the observations at the SPbU site are conducted outside the St. Petersburg pollution plume and mostly reflect the regional atmospheric background. The geographic location of the SPbU station is indicated in Figure 1 (global (a) and local (b) maps) by red circles. The FTIR-observational site is located at the suburban SPbU campus, the territory of which includes the following terrain patterns: pedestrian roads—4%, motor roads and parking lots—12%, commercial buildings of the University campus—18%, and vegetation (grass, bushes, and trees)—66%. The relative distribution (in percents) of land resources in the regional scale (Leningrad Region) by land category is given in Table 1 (according to information presented in [ 17]). The primary urban sources of ammonia in St. Petersburg are wastewater treatment and stationary industrial facilities while in Leningrad Region ammonia emissions are driven by intensive agriculture and mineral fertilizer production. The region is home to major chemical facilities like EuroChem-Northwest (fertilizer manufacturer), which has a production capacity of 1 million tons of ammonia per year. In agriculture, manure management and nitrogen-based fertilizers from livestock operations are main fugitive sources of NH 3. However, official inventories for both St. Petersburg and Leningrad Region are available only for motor vehicles, which contributed ~2300 and ~800 tons of NH 3 in 2024, respectively [ 18, 19]. While there is no official data on ethylene emissions, scientific literature indicates that ethylene typically accounts for less than 5% of total anthropogenic non-methane VOC emissions in urban environments [ 20]. In 2023 total annual emissions of volatile organic compounds (VOCs) from stationary sources and motor vehicles were 12,500 tons in Saint Petersburg [ 18] and 59,500 tons in Leningrad Region [ 19], therefore the approximate values of total annual C 2H 4 emission are less than 625 and 2975 tons respectively. 2.2. FTIR System Since 2009, the FTIR system for atmospheric applications has been used to record spectra of direct solar radiation in the mid-IR range [ 21]. The SPbU FTIR system consists of: - the Bruker IFS125 HR high-resolution Fourier transform spectrometer (FTS) (Bruker, Billerica, MA, USA) installed in a thermostatted room. The highest spectral resolution of the Bruker IFS 125HR is Δν = 0.0019 cm −1; - the original solar tracking system developed at the Dept. of Atmospheric Physics of St. Petersburg State University. The routine monitoring of FTS alignment is being regularly (approximately once a month) carried out by HBr (or N 2O) cell spectra measured for optical path difference OPD = 180 cm (Δν = 0.005 cm −1) using a liquid nitrogen (LN)-cooled Indium Antimonide (InSb) detector and a globar (a standard internal middle IR light source of Bruker IFS 125HR). For the acquisition of HBr cell spectra we set KBr beamsplitter in FTS, while for N 2O cell spectra, we replace it with CaF 2 beamsplitter. Retrievals of modulation efficiency (ME) and phase error (PE) characterizing the quality of FTS alignment are being performed by LINEFIT software [ 22]. Results of ME and PE retrievals for 2012–2025 demonstrating good alignment of FTS are given in Figure 2. These FTIR measurements are carried out in collaboration with the Infrared Working Group (IRWG) of the Network for the Detection of Atmospheric Composition Change (NDACC). The advantage of FTIR observations is the ability to simultaneously determine up to 30 gas components of the atmosphere ( https://www2.acom.ucar.edu/irwg, accessed on 10 April 2026). FTIR monitoring of atmospheric gas composition is carried out under cloudless skies or with small cloud cover. In total, FTIR observations at the SPbU station were conducted over 1045 days from 2009 to 2025 (~70–80 days per year). Gaps in data are due to unsuitable weather conditions or failures of the FTIR system. The typical setup of FTIR system for the NH 3 and C 2H 4 monitoring using the mid-IR spectra of direct solar radiation at the SPbU station is given in Table 2. Currently, the spectra are recorded using the F3 optical filter in combination with the LN-cooled MCT (Mercury–Cadmiun–Telluride) detector. From 2009 to March 2016, another optical filter, F3*, with similar characteristics but providing a lower signal-to-noise ratio (SNR), was used for measurements in this spectral range. To achieve a better SNR, the interferograms are accumulated and then averaged. The number of scans for most atmospheric FTIR measurements at the SPbU station is set as 8–10 (the duration is approximately 10–12 min). For observations conducted under variable cloud cover and/or at large solar zenith angles (SZAs), the number of scans can be set to 6. 2.3. Processing FTIR Spectra The algorithm for solving the inverse problem of atmospheric remote sensing using FTIR observations is aimed at obtaining quantitative information on the total column ( TCGAS) and/or vertical profile of the volume mixing ratio of the target gas in the atmosphere based on measured IR spectra of direct solar radiation. The theoretical fundamentals for solving ill-posed problems of atmospheric remote sensing were developed by Tikhonov [ 23] and Rogers [ 24] in the 1960s and 1970s. Optimal estimation (OE) and Tikhonov–Phillips regularization (T-P) are the two most common approaches implemented in SFIT4 software [ 25], which we used to process the FTIR spectra [ 26]. (1) A priori vertical distributions of NH 3 and the corresponding a priori covariance matrices were formed based on simulations of the GEOS-CHEM model [ 10]. For C 2H 4, as well as interfering components (indicated in column 4 of Table 3), we used the data from the global chemical-transport atmospheric model WACCM v.6 (Whole Atmosphere Community Climate Model) [ 29]. (2) The NOAA/NWS/National Centers for Environmental Prediction (NCEP) data on temperature profiles and geopotential heights at 18 levels from 1000 mb to 0.4 mb were used as input information on atmospheric temperature and pressure ( https://www-air.larc.nasa.gov/missions/ndacc/data.html?NCE12.00UTCP=ncep-list data access up to 10 April 2026 and https://www-air.larc.nasa.gov/missions/ndacc/data.html?NCEP_GFS=gfs-list data access after April 2025; accessed on 10 April 2026). (3) The information on the retrieval strategies used to determine TC NH3 and TC C2H4 is given in Table 3. The second column in Table 3 indicates the spectral ranges. The third column contains information on the spectroscopic database. The fourth column lists the interfering trace gases which were retrieved simultaneously with NH 3 and C 2H 4. And the fifth column indicates the type of regularization. Typical examples of measured and calculated spectra, as well as the differences between them in the spectral ranges used for TC NH3 and TC C2H4 retrievals are shown in Figure 3a,b. The formalism commonly used to solve the ill-posed inverse problems of atmospheric remote sensing involves calculating the so-called averaging kernel matrix [ 34, 35]. This matrix helps to estimate the vertical resolution of the retrieved profiles and to derive some other important characteristics. In particular, the averaging kernels (AVKs) which are the rows of this matrix show how the variations in all individual elements of the true profile will contribute to the value of each element of the retrieved profile. AVKs for the mixing ratio of NH 3 and C 2H 4 are illustrated in Figure 4a,b. The AVKs are functions of altitude (see Figure 4); in an ideal scenario, the retrieval would have an AVK equal to unity in the region of interest (altitude) and zero beyond it. AVKs characterize the degree of smoothing of the true profile in the procedure of solving the inverse problem. From Figure 4, it is evident that our retrieval strategies have the highest sensitivity to the NH 3 and C 2H 4 contents in the troposphere and stratosphere (up to altitudes of ~30–40 km for NH 3 and up to ~15 km for C 2H 4) with maximum sensitivity in the middle and upper troposphere (at altitudes of ~4–10 km). It should be noted that the trace of the AVK matrix characterizes the number of the degrees of freedom for signal (DOFS), which in our case can be interpreted as the number of atmospheric layers where mixing ratios for the target gas are determined independently. The DOFS value shows whether only the TCGAS can be determined (for DOFS < 2) or whether elements of the gas vertical distribution in the atmosphere can be retrieved (for DOFS ≥ 2). Thus, in our case, FTIR measurements can only provide the information on TCs of NH 3 and C 2H 4, since the DOFS values for both target gases are ~1.0 (see Table 4 below). 2.4. Primary Data Analysis and Error Budget The results of the FTIR spectra processing have shown that the C 2H 4 retrieval setup applied to spectra recorded with the F3* optical filter does not allow the SFIT4 iterative procedure to converge due to low SNR value. Therefore, we processed spectra only for the period from April 2016 to 2025 when F3 filter with higher SNR was used. Table 4 presents the most important results including mean values of the total columns of NH 3 and C 2H 4, the column-averaged dry-air mole fraction ( XGAS) of NH 3 and C 2H 4, the root mean square difference between the measured and calculated spectra (RMS), and DOFS. In the case of NH 3, we separately show the results for filters F3 (April 2016–2025) and F3* (2009–March 2016). Column-averaged dry-air mole fraction of the target gas XGAS was calculated as the ratio of the TCGAS and the dry pressure column ( TCdry_air) [ 36]: X GAS = TC GAS/TC dry_air (1) TCdry_air = (Ps/g(φ) − TCH2O·mH2O)/mdry_air (2) where mdry_air—the molecular mass of dry air (28.964 g mol −1); mH2O—molecular mass of H 2O (18.02 g mol −1); Ps—surface pressure (hPa) which is taken from NCEP input data used in the retrievals; g(φ)—the latitude-dependent gravitational acceleration; TCH2O—the H 2O total column obtained as a result of separate retrieval. For convenience, we also provide here the formula for converting concentration q from ppbv to mg/m 3: qmg/m3 = qppbv·P·mGAS/(10 3·R·T) (3) where mGAS—the molecular mass of studied gas (g mol −1); P—pressure (Pa); T—temperature (K); R—ideal gas constant (8.314 J K −1 mol −1). Average values of TCGASXGAS, given in Table 4, are the estimates of the regional background levels of TCGAS_BGXGAS_BG for our station. Since NH 3 and C 2H 4 are most abundant in the troposphere, the XGAS values can be considered as the background tropospheric concentrations of NH 3 and C 2H 4. High values of σ for TCGASXGAS (see Table 4) reaching ~100% for NH 3 and C 2H 4 are caused by high reactivity and deposition rates of these gases. When assessing the uncertainty budget of the retrieved total column of target gas in the atmosphere, we followed the formalism developed by Tikhonov and Rodgers [ 23, 24] which is incorporated into the SFIT4 retrieval tools. A detailed description of the main sources of systematic, random, and smoothing errors which we took into account in our uncertainty analysis can be found in [ 21]. In our case, it is assumed that the smoothing error is random and characterizes the uncertainty caused by the limited vertical resolution of the FTIR technique. The average values of relative random (δ rand) and systematic (δ sys) errors, as well as smoothing (δ sm) errors of NH 3 and C 2H 4 TCs, estimated as a result of processing the FTIR spectra collected at the St. Petersburg State University atmospheric monitoring station, are presented in Table 5. This table shows that using the F3* optical filter in FTIR measurements from 2009 to March 2016 results in higher errors of TC NH3 compared to corresponding errors obtained for F3 filter. The obtained values of the TC NH3 error components presented in Table 5 are consistent with the results of independent studies. The values of random, systematic, and smoothing errors estimated from FTIR measurements at the Hefei station (China) are 8.1 ± 5.8%, 27.8 ± 19.7%, and 0.6 ± 0.4%, respectively [ 37]. For C 2H 4, the δ rand and δ sys values for the SPbU station are generally consistent (taking into account the difference in the geographical location of the stations) with the corresponding errors estimated for the high-latitude Eureka station (Canada): δ rand = 64% and δ sys = 17.9% [ 7]. 2.5. Atmospheric Dispersion Modeling In our study, modeling WF plume evolution was performed by HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) Dispersion Model [ 38, 39] via the online NOAA READY Website Engine ( https://www.ready.noaa.gov/HYSPLIT_disp.php, accessed on 10 April 2026), model setups and details for this run are given in Appendix A ( Figure A1). The HYSPLIT model, which is a comprehensive computer system used to simulate the atmospheric transport, dispersion, and deposition of pollutants and hazardous materials over local to global scales. The HYSPLIT is a hybrid mathematical framework incorporating Lagrangian and Eulerian approaches. The key capabilities and features of the HYSPLIT are as follows: - Trajectory analysis: model computes forward air mass trajectories to determine/forecast the path of pollutant, and backward trajectories to track an air mass back in time to establish its source-receptor relationship; - Atmospheric dispersion modeling: the model simulates pollutant mass distribution by releasing it as point-mass particles, growing 3D cylindrical puffs, or a hybrid combination of both; - Advanced Physics: the model incorporates horizontal and vertical complex wind shear, vertical diffusivity profiles, chemical transformation, radioactive decay, and dry or wet deposition; - Meteorological flexibility: the model utilizes previously gridded, pre-processed regional or global binary meteorological data from agencies like NCEP, utilizing either historical archives or future forecast fields. The model serves as a useful tool for environmental safety, aviation, and regulatory policy and has the following common practical applications: analysis of natural and anthropogenic hazardous release events (tracking the concentration and deposition of radionuclides from nuclear accidents, forecasting the movement of volcanic ash, tracking wildfire smoke plumes, and mapping windblown dust storms); air quality applications (tracking urban pollutants like PM2.5, stationary factory emissions, and identifying transboundary transport of toxic atmospheric components to determine air pollution levels); inverse modeling (utilizing experimental data on atmospheric composition to establish the location of unknown emission source and its emission intensity). For clarity, the methodology used in this study is illustrated in Figure 5. 3. Results Analysis and Discussion The results of long-term FTIR measurements are shown in panels (a) of Figure 6, Figure 7Figure 8 for NH 3 and in panels (b) of Figure 6, Figure 7Figure 8 for C 2H 4. The TC NH3 is characterized by significant spatiotemporal variations; for example, its average level at the SPbU station in 2009–2017 was (3.7 ± 3.2)·10 15 molec/cm 2, which is 2.3 times lower than the corresponding value (8.4 ± 8.6)·10 15 molec/cm 2 observed in Paris over the same period [ 9]. In Bremen, the average total ammonia level between 2004 and 2013 was even higher, at (13.75 ± 4.24)·10 15 mol/cm 2 [ 8]. Comparison of the average TC NH3 observed at SPbU site in 2017–2020 with similar results obtained at the Hefei FTIR station [ 37] showed a more than 6-fold difference in the results: ~2.91·10 15 mol/cm 2 (SPbU station) and ~1.82·10 16 mol/cm 2 (Hefei station). Such differences are explained by the lower intensity of NH 3 sources in the Northwest of the Russian Federation compared to the territories of the European Union and China. The average value of TC C2H4 observed at the SPbU station in 2016–2025 is (1.27 ± 1.25)·10 15 mol/cm 2 which is in good agreement with the ranges of TC C2H4 reported in the literature. Thus, for the background high-latitude Canadian station Eureka [ 7], spectroscopic measurements showed average values at the level of ~6·10 14 mol/cm 2; for the Pasadena station, located in the suburbs of greater Los Angeles, the obtained values are almost two orders of magnitude higher (~2·10 16 mol/cm 2) [ 28]. The TC C2H4 levels less than 1·10 15 mol/cm 2 are considered characteristic of an unpolluted atmosphere [ 28]. The results of NH 3 and C 2H 4 monitoring allowed us to analyze long-term trends, annual cycles, and anomalies of X NH3 and X C2H4 observed in the atmosphere of suburban territory of St. Petersburg. If initially, as a result of solving the inverse problem, we obtain TCGAS values, then switching to the XGAS values minimizes the influence of measurement conditions (by excluding of the atmospheric pressure influence), which is important for FTIR observations with a limited number of measurement days [ 40]. It is for this reason that the further analysis is mainly focused on the X NH3 and X C2H4 values. 3.1. Long-Term Trends To study the long-term trends, we used the original approach proposed in [ 21], which included the following key steps: - preliminary analysis of the XGAS time series including filtering of outliers; - harmonic analysis of the roughly detrended non-even XGAS time series using the Lomb–Scargle technique followed by estimation of the optimal number of harmonics (N) using the cross-validation method. Here we assume that XGAS time series can be approximated as a model function FX = “ linear trend + N harmonics”: F X t = a + b ⋅ t + ∑ i = 1 N c i c o s α i t + φ i (4) where t is time; a, b, ci, αiφi are parameters determined by the least squares method; parameter b is an estimate of the XGAS linear trend; - estimation of the XGAS linear trend and its uncertainty using bootstrapping technique. The results obtained using this approach showed a decrease in the X NH3 and X C2H4 with the rate (−2.3 ± 0.2)%/year for the seventeen-year period 2009–2025 and with the rate (−2.2 ± 0.4)%/year for the ten-year period 2016–2025, respectively. The values of the X NH3 and X C2H4 long-term trends are statistically significant with a confidence level of 99%. According to satellite measurements carried out by the IASI (Infrared Atmospheric Sounding Interferometer) instrument [ 41], the NH 3 trend for the entire territory of Russia in 2008–2018 was negative and amounted to (−4.11 ± 0.80) %/year. For comparison, the X NH3 trend estimated using our ground-based FTIR measurements for the period 2009–2018 amounted to (−3.9 ± 1.5) %/year, which is in good agreement with the above results of the IASI monitoring. The comparison of the X NH3 trends for 2009–2018 and 2009–2025, obtained at the SPbU station, shows that the NH 3 decline rates slowed down by approximately two times from −3.9%/year to −2.3%/year. The number of studies devoted to long-term monitoring of C 2H 4 is small due to its low atmospheric concentration and, consequently, due to the difficulty of detecting this trace gas, especially under background and near-background conditions. Toon et al. [ 28] provided the following estimate of the C 2H 4 total column trend for Pasadena site: a ~3-fold decrease over 25 years (quote: “Despite the increasing population and traffic in southern California, a factor 3 decrease in ethene column density is observed over JPL over the past 25 years…”). This is approximately −2.6%/year, which is consistent with our results (−2.2 ± 0.4%/year) within the error limits. 3.2. Annual Cycle The mean annual cycles of X NH3 and X C2H4 shown in Figure 9 were obtained as follows: at the first stage, the long-term trend was excluded from the corresponding time series of individual measurements, then the daily and monthly means were calculated, and finally monthly means for each month were averaged over all years of FTIR measurements. Averaging periods for X NH3 and for X C2H4 were 2009–2025, and 2016–2025, respectively. Let us characterize the obtained mean annual cycles of X NH3 and X C2H4 shown in Figure 9: (1) The average amplitude of the X NH3 annual cycle is ~95 pptv; the maximum of X NH3 is observed in the warm season with a peak ~207 pptv in May, the minimum—in winter (~16 pptv in December). On average, the relative monthly variability of X NH3 is 30–70% and the most significant variations in X NH3 were observed in March and October. This nature of the X NH3 annual cycle is due to the influence of spring-summer emissions from agricultural production in the nearby region, since livestock and crop production are responsible for ~70–80% of the total ammonia input into the atmosphere, as well as emissions from wildfires [ 3]. (2) The average amplitude of the X C2H4 annual cycle is ~40 pptv; the peak of X C2H4 occurs in January (~116 pptv), while the minimum can be observed in different months of the warm season. In our case, the minimum was usually registered in May (~38 pptv). The highest variability of X C2H4 was observed in June. Such ethylene variability throughout the year is caused by the seasonal dependence of the main mechanism for removing C 2H 4 from the troposphere, which is the reaction of C 2H 4 with the hydroxyl radical OH. Seasonal variations in NH 3 and C 2H 4 concentrations for different geographic locations can vary significantly depending on specific sources and sinks of the target gases [ 9, 28]. For example, for Paris [ 9], the mean annual cycle of TC NH3 based on FTIR measurements in 2009–2017 has two maxima in March and August; the lowest values are detected in winter with a minimum in January, which is generally consistent with the results obtained for the SPbU site. For ethylene, according to ACE-FTS satellite measurements [ 15], average concentrations in the free troposphere are ~50 pptv. Seasonal variations in C 2H 4 for the Northern Hemisphere are characterized by a maximum (~100–200 pptv) in winter—early spring and a summer minimum (~30 pptv in the absence of powerful wildfires), which is in good agreement with the results of our ground-based FTIR observations. 3.3. Anomalies Analysis The long-term time series of NH 3 and C 2H 4 presented in Figure 7 include the dates when anomalously high levels of X NH3 and X C2H4 were observed. When selecting specific cases for further analysis, we used the following criteria: - the XGAS value must exceed the “monthly mean XGAS value + 3σ” level, where σ is the standard deviation (for the corresponding monthly period); - during the measurement day, anomalously high XGAS values must be observed at least twice (we did not consider isolated anomaly of XGAS as it may be an outlier), or high XGAS values must be observed on nearby dates. Table 6 shows the results of this selection: the dates when anomalies of X NH3 and X C2H4 were detected and the corresponding peak values of X NH3 and X C2H4 on these days (highlighted by red diamonds in Figure 7). The observed positive anomalies in X NH3 and X C2H4 are caused by atmospheric emissions from significant natural and anthropogenic pollution sources. At our station, the influence of both types of sources is present, and during certain periods, it can be significant. It should be noted that for both gases, the DOFS value is ≈1. This means that when processing spectra measured in the pollution plume, the inverse problem solving algorithm can determine the total number of gas molecules (which absorb solar IR radiation) along the solar ray trace, but cannot determine the vertical location of atmospheric layers with anomalously high concentrations of NH 3 and C 2H 4. As already mentioned, the majority of NH 3 and C 2H 4 are contained in the troposphere, and most FTIR measurements are conducted under regional background conditions. Therefore, the average values of X NH3 and X C2H4 characterize the mean tropospheric concentrations of these gases. Since NH 3 and C 2H 4 have a short atmospheric lifetime, their most significant anomalies recorded at our station are due to the emissions of powerful nearby ground-based sources (these gases are not formed in the atmosphere). In this case, the bulk of the pollutants will be concentrated in the planetary boundary layer (PBL), where the concentration of NH 3 and C 2H 4 can be estimated using the difference between the recorded anomalies of TC NH3 and TC C2H4 and their monthly mean levels (considered as a background levels). Using the calculated background monthly mean tropospheric levels TCGAS_BGXGAS_BG, and taking ERA5 hourly data on HPBL [ 42] for our region (see Table 6, second column), we can approximately estimate NH 3 and C 2H 4 mean concentrations in PBL for the registered anomalies given in Table 6. For this, we derive the following formula: q BL_GAS = X GAS_BG + (TC GAS_MAX − TC GAS_BG)·k·T/(H PBL·p) (5) where XGAS_BG—background monthly mean tropospheric value of X NH3_BG and X C2H4_BG; TCGAS_MAX—peak values of TC NH3 and TC C2H4 (see Table 6) (molec./m 2); TCGAS_BG—background monthly mean tropospheric value of TC NH3_BG and TC C2H4_BG (molec./m 2); k—the Boltzmann constant (1.38·10 −23 J/K) T—temperature at ground level (K); HPBL—mean PBL height during FTIR observations according to ERA5 data (see Table 6, second column); p—atmospheric pressure at ground level (Pa). The results of calculations of q BL_NH3 and q BL_C2H4 are given in the fifth and eighth columns of Table 6. Over the entire measurement period at SPbU station, the NH 3 and C 2H 4 anomalies were observed for 21 and 8 days, respectively. High concentrations of both gases were recorded simultaneously only once, on 17 October 2017. Based on the information presented in Table 6, we note the following features of the distribution of X NH3 and X C2H4 anomalies throughout the year: - Only one event was recorded during the cold season (November–March), this case is exclusively X C2H4 anomalies; - A total of 20 events, 14 of which were X NH3 anomalies, were recorded during the warm season from April to October; - About 30% (7 events) of the total number of NH 3 anomalies were detected in April–May. The latter is explained by the observed spring peak in NH 3 emissions from agricultural activities (fertilizer application and livestock emissions), as well as the peak of spring wildfires. Since both phytotoxicants are intensely emitted during biomass burning and their local concentrations in pollution plumes can simultaneously reach high levels, this can negatively impact plant growth, especially in the spring. Although our FTIR measurements during the spring did not record simultaneous anomalies in X NH3 and X C2H4, such events can still be observed under certain conditions. For example, on 17 October 2017 we recorded the passage of a pollution plume from forest fires over the SPbU station. The highest TC C2H4 value for the entire period of measurements at our station was recorded on this day (see Table 6). To confirm that the NH 3 and C 2H 4 anomalies on 17 October 2017 were associated specifically with the passage of the wildfire pollution plume, we used the data on the CO, HCN, and C 2H 6 TCs (biomass burning products) in the atmosphere, obtained from FTIR measurements at our station in 2009–2025 [ 27, 43]. Special attention was paid to changes in X HCN (for the fall of 2017), since high concentration of HCN is the most characteristic indicator of the biomass burning products presence in the air [ 27, 43, 44, 45]. Figure 10 shows the daily average XGAS values of NH 3, C 2H 4, and HCN recorded over the period of approximately 20 days before and 20 days after 17 October 2017. It is clearly seen that on 17 October 2017, there was a synchronous significant increase in XGAS concentrations for all three atmospheric gases: by a factor of ~5 for NH 3, by a factor of ~20 for C 2H 4, and by a factor of ~6 for HCN. - In Sweden, 14 fires were recorded, including three of them near 58.35° N, 12.37° E and four of them near 60.13° N, 16.17° E; - In Finland, 10 fires were recorded, localized in the areas of 60.29° N, 25.52° E and 64.64° N, 24.42° E; - In Norway, 2 fires were recorded: at 60.23° N, 10.35° E and 59.12° N, 9.61° E, with the peak of fires occurring on 16–17 October. The power of the most intense forest fires in Finland on 17 October 2017 was estimated at 128 MW (60.29° N 25.52° E), 103 MW (60.30° N 25.52° E) and 88 MW (60.29° N 25.52° E). Simulation of atmospheric dispersion using the HYSPLIT model [ 38, 39] made it possible to establish that the reason of the X NH3, X C2H4 and X HCN anomalies, observed on 17 October 2017 at the SPbU station, was the forest fires in the Helsinki area (located approximately 250 km to the north-west from the observational site; marked in Figure 11 with a purple sign). Figure 12 shows the location of the pollution plume in the atmosphere above the SPbU station (indicated by a red dot) during the period from 9 to 11 UTC on 17 October 2017, when we recorded peak values of TC NH3 and TC C2H4. The results of modeling show that during this time, FTIR measurements were conducted in the central part of the plume with the highest biomass-burning product concentrations (see Figure 12). 3.4. Air Quality Metrics in Comparison with the Results of NH 3 and C 2H 4 FTIR Monitoring If for C 2H 4 the maximum values of X C2H4_MAX = 1243 pptv and q BL_C2H4 ≃ 12 ppbv are obtained on the same date (17 October 2017), then for NH 3 the maxima of X NH3_MAX = 1169 pptv and q BL_NH3 ≃ 55 ppbv were recorded on different measurement days (24 April 2023 and 8 July 2010, respectively). For ammonia and ethylene, classified in the Russian Federation as class 4 and 3 hazard substances, the given values do not exceed the maximum permissible concentration (MPC) levels established in the Russian Federation for the atmospheric air of urban and rural settlements, which are the following [ 47]: (1) For the NH 3 MPC, the maximum one-time value (exposure up to 30 min) is 0.2 mg/m 3 (287 ppbv), and the daily average value (exposure for 24 h) is 0.1 mg/m 3 (143 ppbv); (2) For the C 2H 4 MPC, the maximum one-time value (exposure up to 30 min) is 3.0 mg/m 3 (2.6 ppmv). These values are established taking into account the toxic effects of NH 3 and C 2H 4 on human health. Ethylene, when exceeding the MPC, can have a narcotic effect, causing headaches, dizziness, suffocation, and circulatory problems. Ammonia is a substance with a reflex-resorptive effect, causing severe irritation of the upper respiratory tract. At the same time, the presence of NH 3 in the atmosphere also has a protective effect on human health against population exposures to acidic aerosols. This is particularly important in polluted atmospheres, where if NH 3 levels were controlled, the negative health impacts of air pollutants from fossil fuel combustion would be exacerbated. In other countries, the established standards for NH 3 and C 2H 4 may differ from those in Russia. For example, in European countries, the critical concentration level of NH 3 (CLE NH3), developed for sensitive ecosystems, is also used [ 3, 48]. CLE NH3 is the concentration of atmospheric ammonia above which, according to current understanding, direct adverse effects on an ecosystem may occur [ 48, 49, 50]. These effects include impacts on biodiversity (e.g., on species composition and community composition, such as a decrease in the frequency or number of nitrogen-sensitive species or an increase in the number of nitrogen-tolerant species) and impacts on physiological indicators (e.g., an increase in nitrogen content or nitrogen leaching from the soil). Establishing of CLE NH3 in international studies is more focused on protecting ecosystems and biodiversity, rather than solely on human health. Russian regulations, however, emphasize sanitary and hygienic aspects—protecting public health. As a result, although MPCs in Russia set maximum concentration limits for substances in the air similar to international standards—they are not fully equivalent to CLEs in terms of their objectives and determination methodology. It should be noted that a promising direction for future research based on the obtained results of atmospheric FTIR monitoring is the most detailed study of quantitative indicators of environmental impact and environmental metrics, including the use of additional experimental and model data. Although Critical Level (CLE) criteria are not in use in Russia, we will compare the results of our FTIR NH 3 measurements with established CLE NH3 levels for European countries. It should be noted that in 2009–2010, stricter European standards were introduced for NH 3: CLE NH3 = 1–3 μg/m 3 (1.4–4.3 ppbv), which is significantly lower than the CLE NH3 = 8 μg/m 3 (11 ppbv) previously established in the 1980s [ 3, 48, 51]. The results that we obtained for the mid-tropospheric concentration (X NH3_BG ≃ 160 pptv) are typical for the conditions of the suburbs of St. Petersburg and are more than an order of magnitude lower than CLE NH3, but the situation is changing for observations carried out in the pollution plumes of intense emission sources, when q BL_NH3 can reach 55 ppbv. Since the lifetime of NH 3 is less than 24 h, the anomalies which we recorded demonstrated the presence of both permanent (agricultural enterprises, landfills, etc.) and irregular (wildfires) significant regional sources of ammonia. It should be noted that the number of detected NH 3 anomalies significantly exceeds the number of C 2H 4 anomalies. Thus, despite the generally low levels of NH 3 pollution in the suburb of St. Petersburg, NH 3 concentrations in close vicinities to existing regional sources can permanently exceed CLE NH3, affecting the state of sensitive ecosystems. Ethylene is a key plant hormone produced by plants in response to abiotic stress. It is crucial for plant growth and development under various abiotic stresses, including salinity, hypoxia, and heat. C 2H 4, depending on its concentration in the air, can have both positive and negative effects on plants. Due to its importance in agriculture, the biochemistry of ethylene has been well studied by plant physiologists [ 52]. Ethylene concentrations above 0.01–0.05 ppmv can inhibit flowering, shorten internode length, increase branching, initiate fruit ripening, cause leaf and flower senescence and abscission, chlorosis (yellowing) of leaves, and promote the formation of adventitious roots. Crop sensitivity to C 2H 4 varies, but there are extremely sensitive plant species that react sharply to a short-term (8–24 h) increase in concentration to 0.01 ppmv (e.g., tomatoes and Cuphea hyssopifolia) [ 52]. The mid-tropospheric concentration X C2H4_BG for the suburbs of St. Petersburg is ≃60 ppbv, which is approximately three orders of magnitude lower than the specified thresholds of 0.01–0.05 ppmv. Unlike NH 3, however, a relatively short exposure time to C 2H 4 concentrations at the level of 0.01 ppmv is sufficient to negatively impact certain plant species. The observed plume of wildfires on 17 October 2017 with mean concentrations in PBL of q BL_C2H4 ≃ 12 ppbv could have such an impact. It should be noted that, since the main growing season for open-ground plants in the northwestern region of the Russian Federation usually ends by this time, a similar impact may also affect greenhouse crops. Over the ten-year period of 2016–2025, eight cases with extremely high levels of X C2H4 were registered at the St. Petersburg State University station, only two of them were observed in cold season (17 October 2017 and 20 February 2023) and six events were detected in the summer (the growing season). The levels of X C2H4 observed in the atmosphere of the suburbs of St. Petersburg are low and are close to the background levels. Nevertheless, wildfires, which can lead to significant increases in the C 2H 4 concentrations in the lower troposphere during summer, appear to be an important factor that can negatively impact vegetation on a regional scale. It should be noted that the wildfire plumes usually contain several gaseous products of biomass burning, which are phytotoxicants (for example, C 2H 4, NH 3, SO 2, PAN, etc.), so in this case the inhibitory effect on plants can be mutually reinforcing. 4. Conclusions This work demonstrates the applicability of atmospheric monitoring results of NH 3 and C 2H 4 total columns, obtained from ground-based FTIR observations of direct solar radiation, to assess air quality in the context of its impacts on human health and the ecosystems in urbanized areas. As a result of FTIR measurements, the annual cycle, long-term trends, and positive anomalies of the integral content of NH 3 and C 2H 4 in the atmosphere in a suburb of St. Petersburg agglomeration have been obtained for the period 2009–2025. These parameters have been compared with the maximum permissible concentrations (MPCs) established in the Russian Federation. The potential impact of high pollution levels of NH 3 and C 2H 4 on the region’s sensitive ecosystems has been analyzed. Our results have demonstrated that the retrievals of the NH 3 and C 2H 4 total columns in the atmosphere can be successfully accomplished by processing of FTIR spectra of direct solar radiation in three spectral ranges: 929.40–931.40 cm −1 (NH 3), 962.10–970.00 cm −1 (NH 3), and 948.80–952.40 cm −1 (C 2H 4). The random/systematic errors of the NH 3 and C 2H 4 total column retrievals have been estimated as 6.7%/23% and 26%/15%, respectively. Results of long-term FTIR-monitoring at the SPbU station showed that, despite the proximity of the measurement site to Russia’s second-most populous city (~5.7 million people), the observed levels of X NH3 and X C2H4 can be considered as being close to background levels. Thus, the average long-term levels for the SPbU station are ~163 pptv for NH 3 (2009–2025) and ~59 pptv for C 2H 4 (2016–2025), which are significantly lower than the MPCs established in Russia. The analysis of X NH3 and X C2H4 time series allowed us to evaluate long-term trends, seasonal variability, and identify anomalies in X NH3 and X C2H4. It was shown that for both target gases, a statistically significant decrease in X NH3 and X C2H4 values was observed, amounting to (−2.3 ± 0.2)%/year for the 2009–2025 period and with the rate (−2.2 ± 0.4)%/year for the 2016–2025 period, respectively. The annual cycle of X NH3 with the amplitude of ~95 pptv has a maximum in the warm season with the peak (~207 pptv) in May; the minimum is observed in winter (~16 pptv in December). For C 2H 4, the average amplitude of seasonal fluctuations is ~40 pptv, the peak of X C2H4 occurs in January (~116 pptv), and the minimum is usually registered in May and is ~38 pptv. Over the entire period of FTIR measurements at the St. Petersburg State University station, extremely high X NH3 and X C2H4 values were recorded for 21 and 8 days, respectively. During the warm season from April to October, anomalous X NH3 and X C2H4 values were recorded 20 times, 14 of which were X NH3 anomalies. About 30% (seven events) of the total NH 3 anomalies occurred in April and May. For St. Petersburg and the nearby region, this may negatively impact vegetation, reducing plant resistance to frost, drought, and other stressful situations, as well as reducing CO 2 absorption during photosynthesis. Periodically recorded X NH3 anomalies indicate the presence of intensive emission sources in the region, subjecting ecosystems in adjacent areas to constant exposure to NH 3 concentrations exceeding the critical level (CLE NH3). Anomalously high concentrations (X NH3 and X C2H4) of the target phytotoxicants were recorded simultaneously only once—on 17 October 2017. Using data on HCN total column (as a forest fire indicator) and the results of atmospheric dispersion modeling, it was shown that this pollution event was caused by the influence of biomass burning products emitted from wildfires located approximately 250 km to the north-west from the observational site in the Helsinki area (Finland). Wildfires, which can lead to significant increases in C 2H 4 concentrations in the lower troposphere (PBL), are a factor that can negatively affect vegetation (including greenhouse plants) on a regional scale. This is especially important in light of the observed climate changes, which lead to an increase in the number and area of wildfires. Author Contributions M.V.M.: supervision; conceptualization; methodology; investigation; formal analysis; data processing; writing—original draft. A.A.K.: investigation, formal analysis; methodology. V.S.K.: methodology, writing—review and editing. E.F.M.: funding acquisition. D.V.I.: resources; writing—review and editing. All authors have read and agreed to the published version of the manuscript. Acknowledgments Observational facilities were provided by Geomodel Resource Center of SPbU. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website ( https://www.ready.noaa.gov) used in this publication. The authors would like to express their sincere gratitude to the European Forest Fire Information System ( https://forest-fire.emergency.copernicus.eu/) for providing data on forest fires in Fennoscandia. Figure 1. The geographic location of the atmospheric monitoring site of SPbU is indicated on both global ( a) and local ( b) maps by red circles. Figure 1. The geographic location of the atmospheric monitoring site of SPbU is indicated on both global ( a) and local ( b) maps by red circles. Figure 2. Modulation efficiency ( a) and phase error ( b) as a function of OPD for the 2012–2025. Figure 2. Modulation efficiency ( a) and phase error ( b) as a function of OPD for the 2012–2025. Figure 3. Example of spectral fitting in two spectral intervals used for TC NH3 retrievals ( a) and in a spectral interval used for TC C2H4 retrievals ( b). The date and time when the spectra were recorded are indicated in corresponding plots. Figure 3. Example of spectral fitting in two spectral intervals used for TC NH3 retrievals ( a) and in a spectral interval used for TC C2H4 retrievals ( b). The date and time when the spectra were recorded are indicated in corresponding plots. Figure 4. Typical AVKs (pptv/pptv) for NH 3 ( a) and C 2H 4 ( b). For NH 3 and C 2H 4, the AVKs for altitudes above 4.21 and 26.22 km, respectively, are close to zero and not visible in the figure. Figure 4. Typical AVKs (pptv/pptv) for NH 3 ( a) and C 2H 4 ( b). For NH 3 and C 2H 4, the AVKs for altitudes above 4.21 and 26.22 km, respectively, are close to zero and not visible in the figure. Figure 5. Flowchart diagram illustrating research methodology used in the study. Figure 5. Flowchart diagram illustrating research methodology used in the study. Figure 6. Long-term time series of TC NH3 ( a) and TC C2H4 ( b) obtained at the SPbU FTIR station. Figure 6. Long-term time series of TC NH3 ( a) and TC C2H4 ( b) obtained at the SPbU FTIR station. Figure 7. Long-term time series of X NH3 ( a) and X C2H4 ( b) obtained at the SPbU FTIR station. Red diamonds indicate extremely high levels of X NH3 and X C2H4 in the atmosphere. Figure 7. Long-term time series of X NH3 ( a) and X C2H4 ( b) obtained at the SPbU FTIR station. Red diamonds indicate extremely high levels of X NH3 and X C2H4 in the atmosphere. Figure 8. Sampling histograms for X NH3 ( a) and X C2H4 ( b) measured at the SPbU site in 2009–2025 and 2016–2025, respectively. The last bars with the highest concentrations X C2H4 are not shown in the histogram ( b). Y-axis shows the absolute number of single FTIR measurements; X-axis shows XGAS values for NH 3 ( a) and C 2H 4 ( b). Figure 8. Sampling histograms for X NH3 ( a) and X C2H
