Taiyuan is a major industrial city in northern China, often experiencing elevated concentrations and enrichment factors (EFs) of PM_2.5-bound heavy metals, influenced by local emissions, regional transport, and complex terrain (Li et al., 2021). A winter observation campaign was conducted from 3 December 2023 to 14 January 2024, coinciding with the centralized heating season characterized by intensive coal combustion. The sampling site (37.88° N, 112.55° E; approximately 800 m a.s.l.) is located on the rooftop (∼20 m above ground level) of a four-story building at the Taiyuan Municipal Ecology and Environment Bureau. Situated near Taoyuan 3rd Alley, this site is representative of a typical central urban environment. Although it is removed from direct major industrial point sources, it remains suitable for monitoring urban fine particulate pollution driven by mixed anthropogenic activities.

Key atmospheric pollutants were continuously monitored at the station, including concentrations of water-soluble ions, heavy metals, elemental carbon (EC) and organic carbon (OC), gas concentrations (SO2, NOx, O3, CO), particulate matter concentrations (PM_2.5 and PM₁₀), together with meteorological parameters (wind speed, wind direction, temperature, relative humidity, and atmospheric pressure). In addition, INP filter samples were collected using a two-channel sampler equipped with two parallel filter channels from 4 December 2023 to 5 January 2024. Polycarbonate filters (47 mm, Nuclepore Track-Etch Membrane, 0.2 µm pore size, Whatman) were used for sampling. The sampler was operated without a size-selective inlet. The sampling flow rate was maintained at 6 Lmin-1 by a mass flow controller (MFC), with a 24 h sampling duration. Considering flow fluctuations and filter switching time, the total sampled air volume for each filter was approximately 8000 L. Except for the filter samples, all observation data were recorded as hourly averages.

Figure 1 presents the temporal evolution of atmospheric INP concentrations and relevant aerosol parameters during the observation period in Taiyuan, including size-segregated particle number concentrations, carbonaceous components (OC and EC), as well as mass concentrations of PM_2.5 and PM₁₀, and water-soluble ions. Daily NINP at different temperatures are shown in Fig. 1a (data for 15, 16, and 19 December are missing). At and above -15 °C, the cumulative NINP(T), representing the total INP concentration active at and above this temperature, ranged from 10-2 to 101 L-1 with a campaign mean of 1.75 L-1. The corresponding 95 % confidence intervals for NINP at selected temperatures are provided in Table S3 in the Supplement. Correspondingly, the ns values at and above -15 °C (see Fig. 3) spanned 105 to 107 m-2, with a campaign mean of 3.42×106 m-2.

Several distinct events can be identified throughout the campaign, as highlighted in Fig. 1 by dashed boundary lines and colored arrows. The first event (5–9 December, marked by the orange arrow in Fig. 1a) was characterized by a sharp increase in coarse-mode particle concentrations, represented here by N>1µm derived from the merged SMPS–OPC size distribution (Figs. 1b, S1b, and S2 in the Supplement). During this episode, the mean N>1µm increased to 5.94 cm-3, which was approximately 2.7 times higher than the average concentration during the remaining observation periods (2.21 cm-3). A peak daily concentration of 12.08 cm-3 was recorded on 6 December, accompanied by a steep rise in PM₁₀ that reached nearly 800 µgm-3 (Fig. 1b and d).

72 h back trajectory analysis by HYSPLIT model confirmed that air masses during this period predominantly originated from the Taklamakan, Gurbantünggüt, or Badain Jaran Deserts in northwestern China, with trajectory paths highly overlapping the desert regions (Fig. S3 in the Supplement). This pattern is consistent with desert dust transport events previously observed in northern China (Chen et al., 2021), further supporting the identification of this period as a desert dust intrusion event. Wind rose analysis (Fig. S6 in the Supplement) shows that both the dust-event and non-desert-dust periods were dominated by northerly flow, whereas the dust-event period exhibited a more concentrated northerly flow pattern. This serves as supplementary meteorological support for the event classification. Given the well-established high ice-nucleating activity of mineral dust, this episode is identified as a desert dust-intrusion event responsible for the pronounced enhancement in NINP, especially at temperatures >-12.5 °C. The remaining observation periods are not assumed to be free of mineral dust influence; rather, no similarly clear desert dust intrusion signature was identified outside this event. Further discussion of its characteristics and implications is provided in Sect. 3.2.

Other episodes (13–14, 18, and 28–30 December, marked by the gray arrows in Fig. 1a) were characterized by simultaneous increases in sulfate, nitrate, ammonium, and organic carbon, accompanied by elevated PM_2.5 and PM₁₀ levels (Fig. 1c and d). This indicates the influence of air pollution driven by regional secondary aerosol accumulation under stagnant winter meteorology. Moderate NINP increases were observed during the earlier two episodes (13–14 and 18 December), suggesting that secondary aerosol formation may indirectly modulate ice-nucleating activity through coating or mixing with pre-existing ice-active particles. The late-December event showed no corresponding NINP enhancement despite high PM levels, indicating that aerosol mass concentration alone may not determine INP abundance. This contrast emphasizes that particle composition and surface properties, rather than overall pollution intensity, govern INP activity during pollution episodes characterized by enhanced secondary inorganic and organic aerosol accumulation (Chen et al., 2018; Wagh et al., 2021; Zhang et al., 2022).

Around 2 January 2024 (marked by the red arrow in Fig. 1a), a short-lived pollution event occurred, accompanied by sharp increases in PM mass and concurrent peaks in K, OC, EC (Fig. 1c and d), which are consistent with firework emissions during New Year celebrations. Despite elevated particle concentrations and combustion-related aerosol components, NINP remained low, suggesting that freshly emitted anthropogenic particles, including black carbon and other combustion products, are relatively inefficient as INPs, consistent with previous observations that even locally high concentrations of combustion aerosols contribute little to atmospheric ice-nucleating activity (Adams et al., 2020; Zhang et al., 2022).

Figure 2 compares the measured NINP in this study with values reported previously. All samples in Taiyuan activated before -15 °C, with the most active samples initiating freezing near -5 °C. The NINP varied by more than two orders of magnitude among different samples, particularly in the -10 to -15 °C range, where values ranged from below 0.01 L-1 to above 10 L-1. Such variability primarily reflects the influence of the desert dust intrusion identified earlier (highlighted in darker orange in Fig. 2), during which enhanced NINP values were observed due to long-range transport of natural mineral particles. However, the enhanced warm-temperature INP activity during this episode may not be fully attributable to mineral dust alone. During East Asian dust events, Chen et al. (2021) showed that heat-sensitive INPs made substantial contributions at relatively warm temperatures. Similarly, Hu et al. (2023) reported enhanced warm-temperature INP activity during spring dust transport in Beijing and suggested, based on heat-treatment analysis, that transported dust particles may be mixed with heat-sensitive, potentially biological components. Together, these studies support the possibility of similar heat-sensitive contributions during the Taiyuan dust episode. In the present study, however, no heat-treatment analysis was performed; therefore, such a contribution can only be inferred and cannot be directly assessed.

For the remaining non-desert-dust periods, NINP in Taiyuan was generally higher than wintertime urban observations in Beijing measured by INDA (Chen et al., 2018) and in Tokyo (Tobo et al., 2020) over much of the overlapping temperature range. This difference may be related to the semi-arid setting of Taiyuan and its proximity to the Loess Plateau and northwestern dust source regions, where mineral and resuspended dust particles can provide a more persistent background of coarse-mode aerosol than in the Beijing and Tokyo observations. Consistently, particles larger than 1 µm remained at appreciable concentrations even outside the identified desert dust event, although no clear desert-dust intrusion signature comparable to the 5–9 December episode was identified. Differences in sampling size range should also be considered, because the Taiyuan INP filters were collected without a size-selective inlet and can be regarded as approximately total suspended particles (TSP), whereas Chen et al. (2018) collected PM_2.5 samples in Beijing. The anthropogenic-dust samples collected in Beijing in summer (Chen et al., 2024) overlap mainly with the lower part of the Taiyuan data. These comparisons suggest the higher Taiyuan NINP levels likely reflect a combination of regional dust influence, local dust-associated coarse particles, seasonal differences, and sampling-size-range differences, rather than a single controlling factor.

The linear NINP parameterizations derived from field measurements in Beijing (Bi et al., 2018) and Huangshan (Su et al., 2014) also capture the overall magnitude of the present measurements. Additionally, the gray shaded area denotes the NINP range derived from precipitation samples summarized by Petters and Wright (2015), which is used here as a broad reference for background atmospheric INP abundance. At temperatures warmer than -15 °C, the Taiyuan NINP values fall mostly within this range during the non-desert-dust periods. At colder temperatures, however, the measured NINP values exceed this range substantially, suggesting contributions from additional ice-active components active at lower temperatures. Taken together, these features indicate that episodic desert dust transport was associated with the most prominent INP enhancements during this campaign, whereas the remaining variability during the non-desert-dust periods likely reflects a more complex mixture of sources and particle properties that cannot be uniquely resolved by the present dataset.

During the desert dust event (5–9 December; marked by the orange arrow and dashed boundary lines in Fig. 1a), both NINP and ns increased markedly. At and above -15 °C, NINP reached 13.37 L-1 and ns peaked at 3.11×107 m-2, while their mean values rose from 0.61 to 7.47 L-1 and from 9.52×105 to 1.57×107 m-2, respectively. These coherent increases in concentration and surface-active-site density indicate that the episode was characterized by both an increased abundance of INPs and substantially enhanced intrinsic ice-nucleating activity. The ns values discussed here were calculated from the standard-condition merged SMPS–OPC size distribution, thereby accounting for contributions from coarse particles in the surface-area estimate. Figure 1a also compares the measurements with the DeMott et al. (2015) parameterization (D15), which links INP concentrations to the number concentration of particles larger than 0.5 µm and was originally developed for desert dust aerosol. In this study, the D15 values were calculated using the observed particle number concentrations larger than 0.5 µm and are therefore shown mainly as a dust-based reference. Accordingly, the comparison is most meaningful for the identified dust episode, whereas for the remaining periods it should be interpreted with caution. While D15 captures the correct order of magnitude, it tends to underestimate the INP peak during the desert dust event (5–9 December) and overestimate INP concentrations during the pollution episode (28–30 December). This discrepancy suggests that coarse-particle abundance alone may not fully constrain INP variability in Taiyuan. Particles larger than 0.5 µm in Taiyuan outside the identified desert dust event may differ in source characteristics and ice-nucleating efficiency from those of the highly active desert dust represented by the D15 parameterization. The apparent overestimation by D15 during pollution episodes implies that these coarse particles may possess lower ice-nucleating efficiency than the highly active desert dusts represented by the parameterization. Therefore, while size-based parameterizations provide a useful approximation, incorporating source-specific or compositional factors could further improve INP predictability in such complex industrial-urban environments.

Figure 3 compares ns as a function of temperature for desert dust-affected samples (red) with non-desert-dust samples (blue). Across the full temperature range, dust-affected samples exhibit systematically higher ns values than non-desert-dust samples. Between -10 and -15 °C, desert dust-affected samples exhibited ns values approximately one order of magnitude higher than the most active non-desert-dust samples. In the colder range (-20 to -15 °C), most dust-affected samples fall within about one order of magnitude of the desert-dust parameterizations proposed by Niemand et al. (2012) and Chen et al. (2021), indicating ice-nucleating behavior consistent with long-range transported mineral dust. Considering that Taiyuan is located on the western edge of the North China Plain while Beijing lies farther downwind, dust arriving in Beijing may have experienced a longer transport history and more atmospheric processing. As shown in Fig. 3, the Beijing dust parameterization broadly overlaps with the ns values of the dust samples in this study across the measured temperature range. This comparison is consistent with Chen et al. (2023), who reported observational evidence that atmospheric chemical modification did not suppress the ice nucleation activity of East Asian dust. However, because the present study did not directly characterize the aging state of the dust particles or the specific aging processes involved, this comparison is used only to suggest that strong ice-nucleating activity was retained during the Taiyuan dust episode, rather than to demonstrate or quantify the effect of atmospheric aging.

It is also noteworthy that at relatively warm temperatures (approximately T>-12 °C), ns values of the ambient desert dust samples substantially exceed the parameterizations for pure K-feldspar (Atkinson et al., 2013; Harrison et al., 2019), suggesting that mineralogy alone cannot account for the elevated warm-temperature activity. Previous laboratory and field studies have shown that mineral dust particles can act synergistically with biological macromolecules or cellular fragments, leading to pronounced enhancement of heterogeneous freezing at warmer temperatures (Augustin-Bauditz et al., 2016; O'Sullivan et al., 2016; Yahya et al., 2019). The enhanced ns observed at T>-12 °C in this study may therefore reflect the combined influence of mineral dust and potentially co-transported biological INP components, rather than mineral dust alone. The TSP-like sampling configuration may have facilitated retention of such coarse ice-active components, but it cannot by itself explain the selective enhancement observed during the dust event.

The Pearson correlation coefficients between NINP at different temperatures and various pollutants were calculated for non-desert-dust periods. As shown in Table S1 in the Supplement, only Ca, Co, Ni, and Au exhibited statistically significant correlations (p<0.05; n=24). The mean concentrations of Co, Ni, and Au were all below 10 ngm-3, suggesting that their correlations are of limited physical relevance. By contrast, Ca averaged about 405 ngm-3 but was notably correlated only with NINP at -10 °C. As a typical crustal element, Ca is closely linked to particles larger than 1 µm (Chen et al., 2024). Excluding the influence of long-range dust transport, this observation likely points to contributions from local or regional dust-associated particles, such as resuspended soil and fugitive dust.

The weak correlations for all pollutants highlight that individual chemical species provide limited explanatory power for INP variability during non-desert-dust periods. This reflects the fact that INPs constitute only a minor and compositionally distinct subset of the total aerosol population, and thus bulk pollutant concentrations are not expected to correlate strongly with NINP. To further examine whether the dominant PM_2.5 source factors were associated with NINP variability during the non-desert-dust periods, PMF analysis was applied to the PM_2.5 chemical composition dataset, resolving five major factors (industry, dust-related particles, secondary aerosols, coal combustion and traffic, and fireworks). The corresponding factor profiles (Fig. S4 in the Supplement) are consistent with the assigned source categories, including crustal tracers in the dust-related factor and Cu–Ba–K enrichment in the fireworks factor. The inter-species correlation patterns shown in Fig. S5 in the Supplement also support these factor assignments. Because the PMF analysis was based on PM_2.5 whereas the INP filter samples can be regarded as approximately TSP, the PMF results are used here only to assess covariation with NINP, rather than to provide direct source apportionment for INPs. Data from the identified desert dust event (5–9 December) are not shown in Fig. 4, because the PMF–NINP correlation analysis focused on non-desert-dust periods. The dust-related factor exhibits characteristic signatures of dust-associated particles, including high loadings of Ca and Fe. During the non-desert-dust periods, this factor is interpreted as mainly reflecting local dust-associated particles, such as road dust, fugitive dust from industrial and construction activities, and resuspended soil. This dust-related PM_2.5 factor showed no significant correlation with NINP during the non-desert-dust periods (Table 1), indicating that local dust-associated PM_2.5 variability did not explain NINP variability outside the identified long-range desert dust event. The reconstructed daily PM_2.5 contributions (Fig. 4) provide the temporal context for the subsequent PMF–NINP correlation analysis, showing that secondary aerosols and coal/traffic emissions dominated the PM_2.5 mass during much of the non-desert-dust period, whereas fireworks emissions became prominent during the New Year period.

To evaluate whether any of these major PM_2.5 sources covaried with INP, Pearson correlations were calculated between NINP at -20 °C and the time series of the PMF factors during non-desert-dust periods (Table 1). All correlations were weak (|r|<0.3) and statistically insignificant (p>0.05), indicating that the dominant PM_2.5 source factors did not show clear covariation with INP abundance. Importantly, even during periods with strong increases in secondary inorganic and organic aerosols, such as 13–14 December, 18 December, and 28–30 December, NINP did not rise proportionally (Fig. 4). This behavior suggests that the observed pollution episodes, which were largely associated with fine-mode secondary aerosol accumulation, were not accompanied by corresponding INP enhancements, consistent with previous urban observations showing weak relationships between INPs and bulk pollution indicators (Chen et al., 2018; Zhang et al., 2022).

These results suggest that, during non-desert-dust periods of this campaign, NINP variability was not clearly explained by any of the major PMF-resolved source categories. This is consistent with previous observations at urban sites in different regions, where INPs remain scarce and episodic relative to total aerosol loading, and where their variability is often not well explained by the major anthropogenic aerosol components. Similar analyses were also conducted at -10, -12.5, -15, and -17.5 °C (Table S2 in the Supplement). No statistically significant correlations were found between NINP and any PMF-resolved factor at the examined temperatures (p>0.05), indicating that this result was not specific to the choice of -20 °C.