Seasonal Composition of PM_2.5 and Secondary Inorganic Aerosols at a Rural Site near Livestock Facilities in Ulsan, South Korea

Abstract

Seasonal observations of particulate composition are needed to characterize rural air quality near livestock facilities without assuming that nearby emissions determine ambient aerosol chemistry. This study examined PM_2.5 and eight water-soluble inorganic ions in weekly 24 h filter samples collected in Duseo-myeon, Ulsan, South Korea, from June 2023 to November 2025. Seasonal concentrations and nitrate mass fractions were evaluated alongside particulate ammonium-to-sulfate ratios and a stoichiometric neutralization index. PM_2.5 was highest in spring, whereas nitrate was highest in winter and lowest in summer, with seasonal means of 3.00 and 0.26 µg m⁻³, respectively. Spring and winter both frequently exhibited elevated ammonium-to-sulfate ratios, but nitrate exceeded sulfate by mass much more often in winter. The ratio intervals summarize observed composition and do not define thermodynamic thresholds or precursor-sensitivity regimes. The study provides a multi-year composition baseline for this rural site and shows that the seasonal PM_2.5 maximum and the strongest nitrate contribution did not coincide. Weekly sampling, incomplete gas-particle time matching, and the absence of concurrent thermodynamic constraints limit attribution of the observed nitrate abundance to specific sources or formation processes.

1. Introduction

A substantial fraction of fine particulate matter (PM_2.5) consists of secondary particulate material formed through atmospheric reactions of precursor gases [1]. Secondary inorganic aerosol (SIA), a major component of PM_2.5, is composed primarily of sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺) [2]. Ammonia (NH₃) is the principal basic precursor that neutralizes sulfuric acid (H₂SO₄) and nitric acid (HNO₃), thereby forming ammonium sulfate ((NH₄)₂SO₄) and ammonium nitrate (NH₄NO₃) [2,3]. Agricultural and livestock activities are major atmospheric NH₃ sources; one European assessment attributed approximately 95% of NH₃ emissions in the EU-28 to agricultural activities and identified manure storage and management as important emission processes [4]. Nearby livestock activities can therefore provide a plausible source of NH₃, but their presence alone does not establish their contribution to ambient NH₃ or SIA at a monitoring site [4,5].

Sulfate and nitrate differ in their volatility and atmospheric processing. Sulfuric acid has very low volatility, whereas ammonium nitrate is semivolatile and its gas-particle partitioning depends on temperature, humidity, and the gas and particle composition [2,3]. Lower temperature generally favors particulate ammonium nitrate, but no single temperature defines its stability under all atmospheric conditions. Field observations near a swine facility also illustrate the coupling between NH₃, acidic gases, and fine particles [5]. Consequently, the presence of NH₃ alone does not establish the amount of particulate nitrate or its response to precursor changes.

Recent changes in East Asian emissions have altered SIA composition. Reductions in SO₂ emissions have been accompanied by decreasing particulate sulfate and increasing relative contributions of nitrate [6,7]. An analysis of PM_2.5 changes in China and South Korea during 2015–2018 reported annual decreases in surface PM_2.5 of 9.1% yr⁻¹ in China and 4.3% yr⁻¹ in South Korea [8]. Over the same period, SO₂ concentrations in China decreased to approximately half of their 2015 level. At Baengnyeong Island, located along a long-range transport pathway from China to South Korea, sulfate decreased whereas nitrate increased, a pattern attributed to nonlinear interactions among SIA components [8]. These changes indicate that nitrate has become increasingly important in SIA assessment and management.

The response of particulate nitrate to precursor reductions depends on the chemical environment. In an agricultural NH₃-reduction scenario for the Hai River Basin, a 60% reduction in NH₃ emissions was estimated to decrease SIA by 22–43% and PM_2.5 by 9–24% in high-emission areas [9]. In the San Joaquin Valley, long-term NO_x reductions were associated with lower wintertime ammonium nitrate, although daytime and nighttime production responded differently [10]. These results concern different atmospheric settings and do not identify a universally preferable control precursor. In particular, sensitivity to HNO₃ availability cannot be equated directly with sensitivity to NO_x emissions, because the conversion of NO_x to HNO₃ depends on oxidant chemistry [10].

The nonlinear response of nitrate to NH₃ reduction is already established in the literature [11]. Thermodynamic assessments show that aerosol pH and liquid water affect sensitivity to NH₃ and HNO₃ availability [12], while particulate ion ratios alone are not reliable proxies for aerosol pH [13]. The need to consider these factors is therefore an established basis for interpreting composition measurements, rather than a new mechanism proposed here.

The particulate [NH₄⁺]/[SO₄²⁻] molar ratio has been used to describe ammonium relative to sulfate, and a value near 1.5 has been discussed as an empirical transition in particular observational datasets [14,15]. Neither this value nor a higher ratio is a universal criterion for nitrate exceeding sulfate by mass. Furthermore, nitrate can be associated with non-ammonium cations, including those in sea-salt and crustal particles [16]. Relative nitrate abundance and thermodynamic precursor sensitivity therefore require different evidence.

Korean agricultural research has characterized paddy-field and upland-field sites [17], while other studies describe urban, industrial, and suburban environments [18]. A complementary observational need is to document seasonal particulate composition near livestock facilities, where potential NH₃ emissions should not be confused with demonstrated control of ambient SIA. The present study adds observations from Duseo-myeon, Ulsan, spanning June 2023 to November 2025. Its contribution is a multi-year, site-specific description of PM_2.5 and eight inorganic ions, with emphasis on the contrast between the spring PM_2.5 maximum and the winter nitrate maximum. We compare seasonal nitrate concentrations and mass fractions with particulate ammonium-to-sulfate ratios to characterize their co-occurrence. The ratio-bin analysis is exploratory; the study does not seek to establish a new equilibrium threshold or quantify livestock source contributions.

2. Materials and Methods

2.1. Study Site and Measurement Period

Atmospheric measurements were conducted at an air-monitoring station within the Duseo-myeon Administrative Welfare Center, 285 Inbo-ri, Duseo-myeon, Ulju-gun, Ulsan, Republic of Korea (35.64795° N, 129.16180° E; Figure 1). The site is located in a basin-like setting surrounded by forest, and livestock facilities, including cattle barns, are distributed around the monitoring station. The instruments were installed in an open area to minimize airflow obstruction by nearby structures. The measurement period extended from 12 June 2023 to 10 November 2025.

Figure 1. Location of the study site in Duseo-myeon, Ulju-gun, Ulsan, South Korea. The white star indicates the sampling site, and different shades of gray represent administrative divisions.

The nearby livestock facilities characterize the site setting. Their contribution to measured NH₃ and particulate ions was not quantified, and the present observations do not separate local livestock emissions from other local sources or regional transport.

2.2. Sample Collection and Chemical Analysis

PM_2.5 mass and water-soluble inorganic ions (WSIIs) were determined from 24 h samples collected once per week, generally on Mondays. Gaseous pollutants and NH₃ were measured continuously and managed as 1 min data. The principal sampling and continuous-monitoring instruments are shown in Figure 2.

Figure 2. Instruments used for atmospheric measurements: (a) low-volume PM_2.5 sampler; (b) outdoor air-quality and meteorological monitoring system; and (c) cavity ring-down spectroscopy NH₃ analyzer.

2.2.1. PM_2.5 Sampling and Gravimetric Determination

PM_2.5 samples were collected using the low-volume air-sampling method specified in the Korean Standard Method for the Examination of Air Pollution, ES 01115 [19]. A low-volume PM_2.5 sampler designed to operate under conditions consistent with the U.S. Environmental Protection Agency Federal Reference Method (PMS-204, APM Co., Bucheon, Republic of Korea) was operated at 16.7 L min⁻¹. Instantaneous, mean, and cumulative flow values recorded by the mass-flow controller were checked for each sample. Particles were collected on 47 mm polytetrafluoroethylene filters for subsequent gravimetric and ionic analyses.

Before sampling, filters were conditioned for at least 24 h at 20.0 ± 0.2 °C and 40 ± 2% relative humidity and weighed using an automated weighing system equipped with a precision balance with a readability of 0.1 µg. Exposed filters were reconditioned for at least 24 h under the same conditions and reweighed. Laboratory blanks were used to confirm the stability of the weighing process, and field blanks were used to correct mass changes associated with transport, handling, and field exposure. PM_2.5 mass concentration was calculated by subtracting the field-blank mass change from the pre- and post-sampling filter mass difference and dividing the result by the actual sampled air volume.

2.2.2. Analysis of Water-Soluble Inorganic Ions

Three anions (Cl⁻, NO₃⁻, and SO₄²⁻) and five cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) were quantified. Each filter was placed in a beaker with the sampled surface facing downward, wetted with 200 µL of reagent-grade or higher-purity ethanol, and then extracted with 20 mL of ultrapure water by shaking at 120 rpm for 120 min.

The extract was filtered and analyzed by ion chromatography (IC). Calibration standards ranged from 0.1 to 20.0 μg mL⁻¹, and the reported coefficients of determination (R²) were 0.999 for all eight ions. Ion-specific detection limits ranged from 0.002 to 0.041 μg mL⁻¹, and relative standard deviations (RSDs) ranged from 0.1% to 1.0% (Table 1). Ambient ion concentrations were calculated by multiplying the blank-corrected extract concentration by the extraction volume and dividing by the actual sampled air volume; concentrations are reported in µg m⁻³.

Table 1. Quality-control results for the ion chromatographic analysis of water-soluble inorganic ions.

Ion	Calibration R2	Detection Limit (μg mL−1)	RSD (%)
Cl−	0.999	0.004	0.2
NO3−	0.999	0.003	0.4
SO42−	0.999	0.020	1.0
Na+	0.999	0.002	0.1
NH4+	0.999	0.025	0.1
K+	0.999	0.003	0.1
Ca2+	0.999	0.010	0.1
Mg2+	0.999	0.041	0.1

2.2.3. Continuous Measurement of NH₃ and Gaseous Pollutants

Ambient NH₃ was measured using a cavity ring-down spectroscopy analyzer (G2103, Picarro Inc., Santa Clara, CA, USA). The manufacturer specifies a guaranteed measurement range of 0–500 ppb and a lower detection limit of <0.09 ppb at 3σ with 300 s averaging [20]. Raw data generated at 1 s intervals were aggregated to 1 min means; the manufacturer’s 300 s detection-limit specification is therefore not a field-derived detection limit for these 1 min data. The upstream filter was replaced approximately every three months. Instrument operation, optical-cavity temperature and pressure, data output, and remote communication status were checked routinely. These operational checks and manufacturer specifications do not independently establish field accuracy or long-term calibration stability during the study.

NO₂, SO₂, and O₃, together with temperature, relative humidity, atmospheric pressure, wind direction, and wind speed, were measured continuously using a sensor-based outdoor air-quality and micrometeorological monitoring system (OAQ-C300, Kweather Co., Seoul, Republic of Korea). Gaseous pollutants were measured with electrochemical sensors at a sample flow rate of 1.0 L min⁻¹, and data were generated at 1 min intervals. Measurements were stored internally and transmitted through LTE to the Air365 platform for monitoring instrument status and data acquisition. Optical-scattering and electrochemical sensor modules were inspected periodically; after module replacement, data were used only after a one-week calibration and stabilization period.

Data collected during the stabilization period following sensor replacement were excluded from the analysis. The NO₂, SO₂, and O₃ time series were used as supporting observations to describe temporal variations in air quality and were not used to quantify oxidation rates or precursor sensitivity.

2.3. Data Processing and Ion-Composition Indicators

Filter-based PM_2.5 and WSII data were integrated by sampling date. Samples were excluded from composition-indicator calculations when required analytes were missing or when field records and instrument-operation information indicated abnormal sampling. Continuous data were organized at 1 min resolution after removal of instrument-malfunction and missing-data periods. Long-term gaseous-pollutant variations were summarized as monthly means and standard deviations. Seasons were defined as spring (March–May), summer (June–August), autumn (September–November), and winter (December–February). Seasonal PM_2.5 and ion concentrations are presented as mean ± standard deviation.

The overall and seasonal filter-based means represent the sampled 24 h periods rather than continuous calendar-period averages. The analyses presented here use monthly gas summaries, without paired NH₃–ion averages for individual filter-sampling windows. Meteorological measurements were not used to stratify the filter samples or to estimate source contributions. Consequently, neither sample-matched gas-particle relationships nor the independent effects of temperature, relative humidity, wind speed, or wind direction were quantified.

For ion-composition calculations, the mass concentrations of NH₄⁺, SO₄²⁻, and NO₃⁻ were converted to molar concentrations by division by their respective molecular weights. The neutralization ratio (NR), representing the extent to which particulate NH₄⁺ could stoichiometrically neutralize the acidic equivalents of SO₄²⁻ and NO₃⁻, was calculated as follows [2]:

$$\mathrm{N}\mathrm{R}={\displaystyle \frac{\left[{{\mathrm{N}\mathrm{H}}_4}^{+}\right]}{2\left[{{\mathrm{S}\mathrm{O}}_4}^{2-}\right]+\left[{{\mathrm{N}\mathrm{O}}_3}^{-}\right]}}$$

(1)

Here, brackets denote molar concentrations, whereas c(X) denotes the mass concentration of species X. NR compares ammonium with the charge equivalents of measured sulfate and nitrate. An NR of at least 1 indicates sufficient ammonium equivalents for those two anions within this restricted calculation; it does not establish neutral aerosol pH. The calculation excludes other ions and is not a complete ionic charge balance. The [NH₄⁺]/[SO₄²⁻] molar ratio describes particulate ammonium relative to sulfate and is not a direct measure of gaseous NH₃ availability or precursor sensitivity [12,13].

For descriptive comparisons, samples were grouped into [NH₄⁺]/[SO₄²⁻] intervals of <1.5, 1.5–<4.0, and ≥4.0. The value 1.5 provides a link to empirical ion relationships discussed in previous studies [14,15]. A ratio of 2.0 corresponds to the stoichiometry of pure ammonium sulfate but does not identify the composition or acidity of an ambient particle mixture. The 1.5–<4.0 interval was further subdivided at 2.0 for subsequent exploratory comparisons. The 4.0 boundary is a dataset-specific descriptive cut point, not a fitted or independently validated transition. The interval labels refer only to measured particulate ratios and do not classify the atmosphere as NH₃-deficient or NH₃-rich.

SIA mass concentration was defined as c(NO₃⁻) + c(SO₄²⁻) + c(NH₄⁺). The NO₃⁻/SIA mass fraction was calculated for each sample before averaging within a season or ratio interval. The operational term “nitrate-dominant” denotes c(NO₃⁻) > c(SO₄²⁻); it does not require nitrate to exceed 50% of SIA or PM_2.5, and it does not establish nitrate speciation as NH₄NO₃. For each season, the proportions of samples with NR ≥ 1, [NH₄⁺]/[SO₄²⁻] ≥ 1.5, [NH₄⁺]/[SO₄²⁻] ≥ 4.0, and nitrate dominance were summarized. Frequencies and nitrate mass fractions were also described across the <1.5, 1.5–<2.0, 2.0–<4.0, and ≥4.0 ratio intervals. These comparisons summarize composition and are not tests of nitrate formation rates or responses to emission reductions.

The seasonal joint distribution of the [NH₄⁺]/[SO₄²⁻] molar ratio and the NO₃⁻/SIA mass fraction was visualized using a scatter plot and bivariate kernel density estimation (KDE). To reduce right-skewness, KDE was performed after logarithmic transformation of the molar-ratio axis. The 50% density region for each season was displayed as a contour and projected back onto the original ratio scale for visualization. Vertical reference lines were placed at ratios of 1.5 and 4.0.

2.4. Statistical Analysis

Seasonal differences in PM_2.5 and WSII concentrations were evaluated using one-way analysis of variance (ANOVA), with season as the independent variable and concentration as the dependent variable. Normality within each seasonal group was assessed using the Shapiro–Wilk test [21], and homogeneity of variance was evaluated using Levene’s test [22].

When both assumptions were satisfied, one-way ANOVA was applied, and the F statistic and p-value were calculated. For variables showing an overall seasonal difference at p < 0.05, Tukey’s honestly significant difference test was used for pairwise comparisons while controlling the family-wise error rate [23]. When either normality or homogeneity of variance was violated, the Kruskal–Wallis test was used as a rank-based nonparametric alternative [24]. Significant Kruskal–Wallis results were followed by Dunn’s test with Bonferroni adjustment [25].

The significance level was set at 0.05. Analyses were performed using R version 4.4.2 [26]. ANOVA and Tukey tests were conducted using aov() and TukeyHSD() in the stats package; the Shapiro–Wilk and Kruskal–Wallis tests used shapiro.test() and kruskal.test(), respectively; Levene’s test used leveneTest() in the car package; and Dunn’s test used the dunn.test package.

3. Results

3.1. Seasonal Concentrations of PM_2.5 and Water-Soluble Inorganic Ions

Monthly mean concentrations (±1 standard deviation) of NO₂, SO₂, O₃, and NH₃ are shown in Figure 3. The NH₃ record shows its presence during both warm and cold portions of the study period, including the warm-season periods in which filter-based nitrate concentrations were low. This is a qualitative comparison of different temporal summaries, rather than a sample-matched relationship between NH₃ and particulate nitrate.

Figure 3. Monthly mean concentrations (±1 standard deviation) of NO₂, SO₂, O₃, and NH₃ during the study period.

The mean PM_2.5 concentration across the filter samples was 12.20 ± 6.62 µg m⁻³ (Table 2). Seasonal means were highest in spring (14.77 ± 7.19 µg m⁻³) and winter (13.99 ± 7.73 µg m⁻³) and lowest in autumn (9.47 ± 4.24 µg m⁻³). NO₃⁻ was highest in winter (3.00 ± 2.78 µg m⁻³) and lowest in summer (0.26 ± 0.28 µg m⁻³). In contrast, SO₄²⁻ was higher in spring (1.97 ± 1.22 µg m⁻³) and summer (1.89 ± 1.65 µg m⁻³) and lower in winter (1.32 ± 1.10 µg m⁻³). NH₄⁺ was highest in winter (1.39 ± 0.99 µg m⁻³).

Table 2. Seasonal mean concentrations (mean ± standard deviation) of PM_2.5 and water-soluble inorganic ions (µg m⁻³).

Class	Species	Annual	Spring (n = 23)	Summer (n = 28)	Autumn (n = 33)	Winter (n = 21)
	PM2.5	12.20 ± 6.62	14.77 ± 7.19	11.77 ± 6.44	9.47 ± 4.24	13.99 ± 7.73
Anions	Cl−	0.20 ± 0.24	0.15 ± 0.17	0.26 ± 0.22	0.29 ± 0.32	0.05 ± 0.05
Anions	NO3−	1.24 ± 2.15	1.80 ± 2.86	0.26 ± 0.28	0.58 ± 1.04	3.00 ± 2.78
Anions	SO42−	1.68 ± 1.26	1.97 ± 1.22	1.89 ± 1.65	1.52 ± 0.80	1.32 ± 1.10
Cations	Na+	0.57 ± 0.29	0.62 ± 0.36	0.59 ± 0.24	0.64 ± 0.32	0.39 ± 0.17
Cations	NH4+	0.94 ± 0.84	1.12 ± 0.87	0.78 ± 0.86	0.66 ± 0.50	1.39 ± 0.99
Cations	K+	0.06 ± 0.07	0.02 ± 0.02	0.07 ± 0.08	0.07 ± 0.07	0.08 ± 0.06
Cations	Mg2+	0.00 ± 0.01	0.00 ± 0.00	0.01 ± 0.01	0.01 ± 0.01	0.00 ± 0.00
Cations	Ca2+	0.01 ± 0.02	0.00 ± 0.01	0.02 ± 0.02	0.02 ± 0.03	0.00 ± 0.00

The overall seasonal differences in PM_2.5 and SIA concentrations were statistically significant (p < 0.05). The clearest seasonal contrast was observed for NO₃⁻: the winter mean was approximately 11.5 times the summer mean, and the winter–summer difference was significant in the post hoc comparison. PM_2.5 differed significantly between spring and autumn, and NH₄⁺ differed significantly between winter and summer. By contrast, the lower-concentration ions Cl⁻, Na⁺, K⁺, Mg²⁺, and Ca²⁺ did not show significant seasonal differences (p ≥ 0.05).

Among the remaining ions, Na⁺ and Cl⁻ had overall mean concentrations of 0.57 ± 0.29 and 0.20 ± 0.24 µg m⁻³, respectively (Table 2). The overall mean K⁺ concentration was 0.06 ± 0.07 µg m⁻³, while Mg²⁺ and Ca²⁺ were present at still lower reported concentrations. Values displayed as 0.00 µg m⁻³ reflect the reporting precision of Table 1 and should not be interpreted as proof of absence.

3.2. Seasonal Ion Ratios and Nitrate Dominance

Seasonal NR, [NH₄⁺]/[SO₄²⁻] molar ratios, nitrate-dominance frequencies, and NO₃⁻/SIA mass fractions are summarized in Table 3 and Figure 4. The proportion of samples with NR ≥ 1 was highest in winter (47.6%), followed by spring (34.8%), autumn (33.3%), and summer (10.7%). Mean NR values followed the same order: winter, 1.26; spring, 1.08; autumn, 0.83; and summer, 0.59. Thus, even in winter, fewer than half of the samples had NR ≥ 1.

Table 3. Seasonal distribution of ion-composition and nitrate-dominance indicators.

Season	n	NR ≥ 1 (%)	Mean NR	[NH4+]/[SO42−] ≥ 1.5 (%)	[NH4+]/[SO42−] ≥ 4.0 (%)	Mean [NH4+]/ [SO42−]	c(NO3−) > c(SO42−) (%)	Mean NO3−/SIA (%)
Spring	23	34.8	1.08	95.7	30.4	3.19	17.4	25.0
Summer	28	10.7	0.59	60.7	0.0	1.29	0.0	7.8
Autumn	33	33.3	0.83	81.8	6.1	2.14	15.2	14.7
Winter	21	47.6	1.26	100.0	66.7	5.72	66.7	40.4

NR, neutralization ratio; SIA, secondary inorganic aerosol. Brackets denote molar concentrations, and c(X) denotes the mass concentration of species X. Nitrate dominance denotes c(NO₃⁻) > c(SO₄²⁻). Mean NO₃⁻/SIA is the mean of the sample-specific mass fractions.

Figure 4. Relationship between the [NH₄⁺]/[SO₄²⁻] molar ratio and the NO₃⁻/SIA mass fraction. Points represent individual samples. Colored shaded areas represent the 50% kernel-density regions for each season, and the colored outlines indicate their corresponding boundaries. These regions were estimated in log-transformed [NH₄⁺]/[SO₄²⁻] space and then projected onto the original scale. Dashed vertical lines denote molar ratios of 1.5 and 4.0. The vertical lines are descriptive reference values, not validated transition thresholds.

The proportion of samples with [NH₄⁺]/[SO₄²⁻] ≥ 1.5 was 100.0% in winter, 95.7% in spring, 81.8% in autumn, and 60.7% in summer. Mean molar ratios were 5.72 in winter, 3.19 in spring, 2.14 in autumn, and 1.29 in summer. The proportion with [NH₄⁺]/[SO₄²⁻] ≥ 4.0 was 66.7% in winter, 30.4% in spring, 6.1% in autumn, and 0.0% in summer. Nitrate-dominant samples ([NO₃⁻] > [SO₄²⁻]) showed the same seasonal maximum in winter (66.7%) but occurred less frequently in spring (17.4%) and autumn (15.2%) and were absent in summer. Mean NO₃⁻/SIA mass fractions were 40.4% in winter, 25.0% in spring, 14.7% in autumn, and 7.8% in summer.

3.3. Seasonal Distribution of Molar-Ratio Intervals

Figure 5 shows the seasonal distribution of the particulate [NH₄⁺]/[SO₄²⁻] ratio intervals. The 1.5–<4.0 interval accounted for 65.2%, 60.7%, and 75.8% of spring, summer, and autumn samples, respectively. The <1.5 interval accounted for 4.3%, 39.3%, and 18.2% of spring, summer, and autumn samples, respectively. The ≥4.0 interval accounted for 30.4% of spring samples and 6.1% of autumn samples and was absent in summer. In winter, 66.7% of samples were in the ≥4.0 interval and 33.3% in the 1.5–<4.0 interval; no samples were in the <1.5 interval.

Figure 5. Seasonal frequencies of the particulate [NH₄⁺]/[SO₄²⁻] molar-ratio intervals. The intervals describe observed composition and do not represent thermodynamic or precursor-sensitivity regimes.

3.4. Nitrate Dominance Across [NH₄⁺]/[SO₄²⁻] Ratio Intervals

Nitrate dominance varied among the [NH₄⁺]/[SO₄²⁻] molar-ratio intervals (Table 4 and Figure 6). In the <1.5 interval, 1 of 18 samples (5.6%) was nitrate-dominant. No nitrate-dominant sample was observed among the 23 samples in the 1.5–<2.0 interval. The frequency increased to 19.5% (8 of 41 samples) in the 2.0–<4.0 interval and to 60.9% (14 of 23 samples) in the ≥4.0 interval. Mean NO₃⁻/SIA mass fractions were 11.4%, 7.9%, 21.2%, and 38.0% in the <1.5, 1.5–<2.0, 2.0–<4.0, and ≥4.0 intervals, respectively. The highest nitrate-dominance frequency and mean nitrate mass fraction therefore occurred in the ≥4.0 interval. These frequencies are descriptive comparisons without adjustment for season.

Table 4. Nitrate-dominance frequency and NO₃⁻/SIA distribution across [NH₄⁺]/[SO₄²⁻] molar-ratio intervals.

[NH4+]/[SO42−] Interval	n	c(NO3−) > c(SO42−) (%)	Mean NO3−/SIA (%)
<1.5	18	5.6	11.4
1.5–<2.0	23	0.0	7.9
2.0–<4.0	41	19.5	21.2
≥4.0	23	60.9	38.0

Figure 6. Nitrate dominance across [NH₄⁺]/[SO₄²⁻] molar-ratio intervals: (A) frequency of nitrate-dominant samples, defined as c(NO₃⁻) > c(SO₄²⁻); and (B) distribution of the NO₃⁻/SIA mass fraction. Black diamonds indicate means. In panel (B), gray dots represent individual samples, and black diamonds indicate the mean NO₃⁻/SIA mass fraction within each interval.

4. Discussion

4.1. Seasonal Shift Toward Nitrate-Rich SIA

The measured seasonal contrast was most apparent in the major SIA ions: nitrate and ammonium had their highest mean concentrations in winter, whereas sulfate was higher in spring and summer. This establishes a seasonal composition pattern at the rural study site. It does not, by itself, determine whether temperature, moisture, transport, or local emissions caused that pattern.

This pattern is consistent with observations from agricultural and urban environments in South Korea. At an agricultural site in Jeonbuk, ionic species accounted for more than 40% of PM_2.5, with higher sulfate in summer and higher nitrate in winter; wintertime nitrate and sulfate contributions were 26% and 12%, respectively [17]. At the present site, wintertime nitrate also exceeded sulfate, accounting for 48.15% and 21.19% of the total measured WSII mass, respectively. In Seoul, PM_2.5 increased from 13.3 ± 6.7 µg m⁻³ in summer to 28.3 ± 14.5 µg m⁻³ in winter, and nitrate was reported as the largest component of both PM_1.0 and PM_2.5 during winter [27].

The annual PM_2.5 concentration at the present site (12.20 ± 6.62 µg m⁻³) was lower than the range of 21.8–26.44 µg m⁻³ reported for Korean urban, industrial, and suburban environments [18]. Nevertheless, the direction of the seasonal compositional change was similar: nitrate increased in winter, while sulfate was relatively enhanced during warmer seasons. This indicates that a comparatively low annual PM_2.5 level does not preclude a pronounced seasonal shift in SIA composition.

The winter nitrate maximum did not coincide with the spring maximum in PM_2.5 mass, demonstrating a change in composition within the sampled periods. Winter nitrate enhancement has also been reported in Seoul [28], where isotope-based observations have helped distinguish oxidation pathways [29]. In the present study, higher particulate nitrate is interpreted as greater observed abundance, not a measured increase in nitrate production rate. It can reflect changes in chemical production of total nitrate, redistribution of existing nitrate toward the particle phase, transport, atmospheric dilution, or removal [2,10,12]. Likewise, a higher nitrate-to-SIA fraction can result from increased nitrate, decreased sulfate or ammonium, or both. The filter measurements alone cannot distinguish these contributions.

Comparable winter nitrate behavior has been observed in several North American valleys, although the controlling conditions differ. In the San Joaquin Valley, observations linked changes in winter ammonium nitrate to NO_x chemistry and showed different daytime and nighttime responses [10]. In Idaho’s Treasure Valley, nitrate variability was associated with time of day, NO_x, and relative humidity, with cold, humid conditions favoring particulate nitrate [30]. In Cache Valley, Utah, measurements of both NH₃ and HNO₃, combined with thermodynamic calculations, supported nitric-acid limitation during the investigated pollution episodes [31]. In Yakima Valley, the onset of persistent low cloud reduced several primary pollutants without a corresponding decline in particulate ammonium nitrate, illustrating the combined relevance of mixing and secondary aerosol processes [32].

These studies place the Ulsan winter nitrate maximum in a wider observational context; they do not establish the same precursor sensitivity or boundary-layer conditions at Ulsan. The low sulfate contribution reported during the northern Utah episodes also differs from the appreciable sulfate observed here, particularly in spring and summer [31]. Differences in particle-size range, sampling frequency, and the denominator used for mass fractions further preclude direct equivalence of the reported percentages.

The continuous NH₃ record provides complementary context: NH₃ was observed during periods when particulate nitrate was low, so low particulate nitrate should not be equated with the absence of gas-phase NH₃. However, monthly NH₃ means cannot establish the NH₃ concentration or gas-particle partitioning during an individual filter sample. Lower temperature generally favors particulate ammonium nitrate, while the effects of relative humidity depend on aerosol composition, liquid water, and acidity [2,12]. Relative humidity is not itself a measurement of aerosol liquid water or pH. Wind direction and speed can affect exposure to nearby sources and regional air masses, but no filter-matched wind analysis was performed. Thus, meteorology and nearby livestock emissions remain plausible influences rather than quantitatively demonstrated controls at this site.

4.2. Particulate Ammonium Ratios and Relative Nitrate Abundance

The spring–winter comparison illustrates the distinction between particulate composition indices. In spring, 95.7% of samples had [NH₄⁺]/[SO₄²⁻] ≥ 1.5, but nitrate exceeded sulfate by mass in only 17.4%; the corresponding winter frequencies were 100.0% and 66.7%. These observations describe seasonal co-occurrence and do not establish an independent predictive effect of the molar ratio. Seoul PM₁ studies likewise provide examples of increased nitrate contributions under winter or stagnant conditions [33,34], although their particle-size ranges and mass-fraction denominators differ from those used here.

NR includes sulfate and nitrate in its denominator, whereas [NH₄⁺]/[SO₄²⁻] includes sulfate alone. The two indices can therefore give different impressions of ammonium relative to the measured anions. Neither index identifies aerosol pH, and NR is not a complete charge balance because non-ammonium cations and other anions are excluded [13]. The Na⁺, Cl⁻, K⁺, Mg²⁺, and Ca²⁺ measurements in Table 1 show that the measured inorganic mixture extends beyond the three SIA ions. Their presence does not uniquely identify marine, crustal, combustion, or livestock contributions, nor does bulk ion analysis determine which salts were present [16].

Prior observations already demonstrate why ratio-only interpretations require caution. Guangzhou measurements found substantial acidity despite abundant NH₃ and high particulate ammonium-to-sulfate ratios [15], while Beijing and Shanghai observations included nitrate-rich, ammonium-poor samples [14]. These findings and thermodynamic work on aerosol pH and liquid water [12] support a limited interpretation of the Ulsan results: seasonal composition is documented, but gas-phase ammonia excess, particle acidity, and limiting precursors are not diagnosed.

4.3. Exploratory Interpretation of Molar-Ratio Intervals

Nitrate dominance was most frequent in the ≥4.0 interval, but it occurred in only 14 of its 23 samples (60.9%). This interval also contained a large winter contribution, so the pooled comparison does not separate the ratio association from season. No independent threshold validation or season-adjusted predictive analysis was performed. The value 4.0 is therefore retained only as a descriptive grouping used in Table 2 and Table 3 and Figure 4, Figure 5 and Figure 6, and no site-transferable cut point is inferred.

The indices are also chemically and mathematically coupled. Ammonium associated with particulate nitrate can increase the ammonium-to-sulfate ratio, while lower sulfate can increase both that ratio and the nitrate mass fraction without an increase in nitrate concentration. Their association is consequently not independent evidence that additional NH₃ caused nitrate formation. Studies from Chengdu, the Po Valley, and the eastern United States illustrate that ammonium–sulfate relationships vary with chemical setting and cannot be interpreted independently of other aerosol components and gas-phase conditions [35,36,37].

Modeling near animal feeding operations and observations in Nanjing provide examples of nonlinear precursor responses and heterogeneous nitrate processing [38,39]. These studies motivate possible explanations, but cannot assign those mechanisms to the Ulsan samples. The transferable point is the need to distinguish descriptive particle ratios from independently constrained gas-particle chemistry; the numerical boundary itself is not generalized.

4.4. Implications and Limitations

The main implication is observational: the seasonal maximum in PM_2.5 mass and the strongest nitrate contribution were different at this site, and particulate molar ratios do not independently identify a precursor-control strategy. This composition record can support comparisons with other rural observations, but does not quantify livestock emissions or a thermodynamic sensitivity regime.

Weekly 24 h sampling smooths within-day variability and can miss short pollution episodes between sampling days. Because samples were generally collected on Mondays, the seasonal summaries may also reflect day-of-week selection. Weekday–weekend changes in NO_x and nitrate chemistry have been observed elsewhere [10], but neither the direction nor the magnitude of a Monday-related bias can be determined from this dataset. The reported means and frequencies therefore apply to the sampled periods and are not assumed to represent all days equally.

Semivolatile nitrate can be lost from filter samples during sampling and subsequent handling, with temperature-dependent losses documented in previous work [40]. No site-specific volatilization correction is reported here. Greater warm-period losses could exaggerate the observed winter–summer nitrate contrast, but their magnitude cannot be quantified from the available measurements. This uncertainty also affects the interpretation of nitrate mass fractions and ion ratios.

The reported IC calibration fit and operational gas-monitor checks do not fully characterize analytical uncertainty or long-term field calibration stability. In particular, the sensor gas records are contextual and cannot validate nitrate oxidation pathways. Monthly NH₃ summaries are not paired with individual filter samples, and no filter-matched meteorological attribution was performed. NO₂ measurements are not measurements of total NO_x, oxidation rates, or HNO₃ production. Concurrent HNO₃, aerosol liquid water, and particle pH constraints are also lacking.

Finally, one monitoring site cannot resolve spatial variability or separate local livestock influence from regional transport. Identifying limiting processes would require temporally aligned gas and particle measurements, appropriate thermodynamic constraints, and spatial or source-specific evidence. The present study therefore supports a site-specific seasonal composition interpretation, rather than causal source attribution or an emission-control prescription.

5. Conclusions

This study documented seasonal variations in PM_2.5 and secondary inorganic aerosol composition at a rural site near livestock facilities in Ulsan, South Korea, from June 2023 to November 2025. The seasonal mean PM_2.5 concentration was highest in spring, whereas nitrate concentration and its mass fraction within SIA were highest in winter. These observations show that the seasonal maximum in total particle mass and the greatest relative contribution of nitrate occurred in different seasons.

The spring–winter comparison further highlighted differences in inorganic aerosol composition. Although elevated particulate ammonium-to-sulfate molar ratios occurred in both seasons, nitrate exceeded sulfate by mass in 17.4% of spring samples and 66.7% of winter samples. This contrast illustrates the distinction between particulate ammonium abundance relative to sulfate and nitrate predominance over sulfate at this site. The findings support interpreting particulate molar ratios together with seasonal concentrations and mass fractions when characterizing inorganic aerosol composition.

Weekly filter sampling and the absence of concurrent HNO₃, aerosol liquid water, and particle-pH measurements limit the attribution of nitrate formation processes and precursor sensitivity. The study provides a multi-year observational reference for a Korean rural environment near livestock facilities, documenting the seasonal contrast between total PM_2.5 mass and nitrate contribution. This record provides a basis for comparing seasonal inorganic aerosol composition across rural monitoring sites and for designing subsequent observations of wintertime nitrate.