Mexican agricultural soil dust as a source of ice nucleating particles

. Agricultural soil erosion, both mechanical and eolic, may impact cloud processes as some aerosol particles are able to facilitate ice crystals formation. Given the large agricultural sector in Mexico, this study investigates the ice nucleating abilities of agricultural dust collected at different sites and generated in the laboratory. The immersion freezing mechanism of ice nucleation was simulated in the laboratory via the Universidad Nacional Autónoma de México (UNAM)- Micro Orifice Uniform Deposit Impactor (MOUDI)- 25 Droplet freezing technique (DFT) (UNAM-MOUDI-DFT). The results show that agricultural dust from the Mexican territory promote ice formation in a temperature range from -11.8ºC to -34.5ºC, with ice nucleating particle (INP) concentrations between 0.11 L -1 and 41.8 L -1 . Furthermore, aerosol samples generated in the laboratory are more efficient than those collected in the field, with T 50 values (i.e., the temperature at which 50% of the droplets freeze) higher by more than 2.9ºC. The mineralogical analysis indicated a high 30 concentration of feldspars i.e., K-feldspar and plagioclase (> 40%) in most of the aerosol and soil samples, with K-feldspar significantly correlated with the T 50 of particles with sizes between 1.8 µm and 3.2 µm. Similarly, the organic carbon (OC) was correlated with the efficiency of aerosol samples from 3.2 µm to 5.6 µm and 1.0 µm to 1.8 µm. Finally, a decrease in the efficiency as INPs, after heating the samples at 300ºC for 2 h, evidenced that the organic matter from agricultural soils can influence the role of INPs in mixed-phase clouds. as a consequence of higher particle concentrations of larger particles. The XRD analysis allowed the identification of the different mineral phases present in the aerosol and soil samples, where high concentrations of plagioclase, K-feldspar, quartz, kaolinite, smectite, and mica-illite were 375 detected. These minerals have been previously identified in dust samples. In particular, feldspars were found in higher concentrations (> 40%) for most of the samples. Additionally, the significant correlation between the T 50 and the K-feldspar for particles with sizes from 1.8 to 3.2 µm shows the influence of K-feldspar in the INPs efficiency on mineral particles. The concentrations of OC indicate that despite the low percentage observed in most of the samples (<17%) in comparison to the mineral concentration, the organic components increase the 380 efficiency as INPs to promote ice crystals formation. This is evidenced by the decrease in the efficiency of the ice nucleating abilities after the removal of the organic matter, and the statistically significant correlations between the OC concentration and the T 50 , for particle size from 3.2 to 5.6 µm and 1.0 to 1.8 µm. The present results improve the current gap in knowledge of field measurements of aerosol particles at tropical 385 latitudes, focusing on agricultural emissions and highlights the importance of both the chemical composition and the particle size in their efficiency as INPs. However, more analysis and especial attention to the organic compounds of agricultural dust are needed to improve the current understanding of soil components and the development of new parametrizations.


Laboratory generated samples
All soil samples were air dried, crushed, and sieved to a pore particle size of 425 µm. Aerosol particles were then generated using a dry system (Fig. 2)

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Measuring Systems) was used to obtain the particle size distribution (PSD) of the aerosol samples. The OPC was operated at a flow rate of 28.3 L min -1 , and the aerosol concentrations were for particles ranging between 0.3 µm and 10 µm.
With the MOUDI, siliconized glass substrates (HR3-215; Hampton Research) and 47 mm aluminum foil filters 130 (0100-47-AF, TSI) were used for the INPs and mineral analysis, respectively. Aerosol particles collected over aluminum filters on the eight stages of the MOUDI (0.18 to 10 μm) were grouped in a single sample for the mineral analysis. Particulate matter with diameter less than 10 µm (PM10) was collected over 47 mm quartz filters (2500QAO-UP, Pall Life Science) using the MiniVol at a flow of 5 L min -1 for the OC analysis. The quartz filters were previously conditioned at 500°C for 4 h to remove trace pollutants, especially volatile organic 135 compounds.

Analysis of INPs
The ice nucleating abilities of the agricultural dust particles collected in the field and generated in the laboratory were analyzed through the immersion freezing mode using the UNAM-MOUDI- DFT (Córdoba et al., 2021).
where ( ) is the number of unfrozen droplets (dimensionless) at a temperature (°C), is the total number of droplets (dimensionless), is a correction factor that accounts for the uncertainty associated with the number of nucleation events (dimensionless), is the total area of the aerosol deposit on the glass 160 substrates (mm 2 ), is the area of the glass substrates analyzed by the DFT (mm 2 ), is the volume of air sampled with the MOUDI (L), and are correction factors to account for the aerosol deposit inhomogeneity (dimensionless).

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X-ray diffraction (XRD) has been widely used for the characterization of crystalline materials (Kohli and Mittal, 2019). Therefore, the mineralogical composition of the aerosol and soil samples were determined using a Xray diffractometer Empyrean (Malvern Panalytical, with CuKα radiation) operated with a PIXcel 3D detector.
The mineral phases were identified and quantified by the Rietveld method (Rietveld, 1969) using the HIGHScore v4.5 software and ICDD (International Center for Diffraction Data) and ICSD (Inorganic Crystal
The OC content was derived using a thermal-optical technique (Sunset Lab) based on Birch and Cary (1996) procedure. Briefly, the quartz filters were introduced in an oven, where the samples were volatilized and oxidized to CO2. Finally, the CO2 was reduced and quantified by a flame ionization detector. To verify the 175 influence of the organic matter in the ice nucleating abilities of the agricultural dust, the soil samples were heated at 300ºC for 2 h to remove the organic components, following Tobo et al. (2014).

Microbiological Analysis
To determine culturable microorganisms present on the soil samples collected in ZAC, 500 mg of each sample

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The warmest freezing temperatures shown by L samples suggest that the aerosol particle generation during soil tillage is not fully simulated by the process used in the laboratory. The discrepancies in the INP abilities can be attributed to different environmental conditions as the F samples are exposed to a variety of physicochemical processes while in the atmosphere (Boose et al., 2016;Cziczo et al., 2013), which is unlikely the case in the L samples. The differences between laboratory and field environments are also reflected in different PSD 205 observed during the aerosolization process (Fig. S1). As this figure shows, mean particle concentrations between 5.0x10 -4 and 0.4 particles cm -3 characterized L samples, while lower values were observed for the F samples. Furthermore, the highest particle concentration for the L samples was found for particles between 1.0 µm and 5.0 µm (Fig. S1a), while the F samples are enriched in smaller particles, i.e., 0.3 µm (Fig. S2b).
Therefore, the larger particles present in the L samples likely promoted ice nucleation at warmer temperatures.

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It was also found that the ice nucleating abilities of the different soils seem to be influenced by the specific crop grown previously and the type of soil, with particles from the nopal, corn 1, and corn 2 crops showing the warmest freezing temperatures and beans and wheat showing the coldest freezing temperatures (Fig. 3). As Table 1 shows, the ZAC samples were collected in calcisols; however, the concentrations of the mineral phases 215 identified on bean, chili, wheat, and onion samples differ, suggesting that additional parameters to the soil type may influence the samples properties and their abilities to act as INPs. Kalev and Toor (2018) found that soil composition determines their properties. This fact may influence the ice nucleating abilities of the soils, as shown in Fig. 3. Further details of the mineral composition and the organic content of each sample are discussed below.
220 Tegen and Fung (1995) and Tegen et al. (2004) also proposed that the PSD of the aerosol particles can vary according to soil type. The size of the aerosol particles is well known to influence their behavior as INPs (Diehl and Wurzler, 2004;DeMott et al., 2010;Mason et al., 2015b;Córdoba et al., 2021). This fact is evidenced in the different PSD distributions observed for each sample in Fig. S1. Figure S2 shows a clear trend for the F 225 samples, where the larger the particle size, the higher the T50. However, this behavior was not observed for the L samples, corroborating differences in the PSD between L and F samples.
Although the laboratory generated aerosol particles do not fully reproduce the characteristics of the ambient agricultural particles, the ice nucleation experiments of the L samples highlight the importance of agricultural soils in ice formation in Mexico. The ice nucleation temperatures observed in the present study are on the same order as those reported for agricultural dust in Wyoming (USA), from -18ºC to -36ºC for dp=0.6 µm (Tobo et al., 2014), and Argentina, China, and Germany from -11ºC to -26ºC for dp <5 µm (Steinke et al., 2016). This suggests that they are able to influence ice formation in clouds regardless of the origin or location of the agricultural soils. The comparison of the present results with literature data is further discussed in Sect. 3.4. Figure 4 shows that the OC concentration represents a small fraction of the agricultural dust samples (dp <10 µm), with values between 5% and 17%, while the mineral components predominate (i.e., from 33% to 95%).

The influence of organic matter
Conen et al. (2011) found that the OC fraction for non-agricultural soils with dp <15 µm collected in Mongolia, concentrations of organic compounds (37%) for agricultural dust dp <0.6 µm in the US. The aforementioned studies reported that the organic components of soil dust (with different particle sizes and concentrations) can enhance the ice nucleating abilities of dust particles. This behavior is also observed for the agricultural dust 245 analyzed here, as summarized in Fig. 5. Figure 5 shows that for the four particle sizes ranges analyzed here (i.e., 0.56-1.0 µm, 1.0-1.8 µm, 1.8-3.2 µm, and 3.5-5.6 µm), there is a significant reduction in the freezing temperature, referred as ΔT50, after the organic matter was degraded through a heating treatment. The T50 of the heated samples got reduced between 0.7ºC and 250 14.0ºC, with the largest mean ΔT50 reported by particles in the size range between 1.8 and 3.2 µm, as shown in Fig. S3. The highest reduction in the ice nucleating abilities as a consequence of the heat treatment, was observed on the corn 2 sample (ΔT50=-14ºC, Table S1) with 17% of OC, while the other samples have OC values <9% (Fig. 4). The highest reduction was observed for particles with sizes ranging between 1.8 µm and 3.2 µm, which were also reported as the most efficient INPs (Fig. 3). Even though the OC fraction is small 275 As soils contain more than organic compounds, the effects of heat treatments on minerals have been questioned.

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Note that the decrease observed in freezing temperatures varies as a function of particle size (Figs. 5 and S3), suggesting a relationship between the organic content and the size of the particles. These observations agree with Chen and Chiu (2003) and Lin et al. (2010), who reported that the composition of the organic matter contained in soils from Taiwan and the HULIS fraction from China soils varies with particle size. In addition, the positive significant correlations found between the T50 and the OC concentration for particles ranging 290 between 1.0 and 1.8 µm (r=0.79, p-value<0.05) and between 3.5 and 5.6 µm (r=0.86, p-value<0.05) shown in Fig. S5, support the importance of particle size and chemical composition in the ice nucleating abilities of agricultural dust particles.

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The presence of plagioclase, K-feldspar, quartz, kaolinite, smectite, mica-illite, and minor constituents were identified through XRD analysis. Figure 6 shows a high content of feldspars (i.e., K-feldspar, plagioclase) in both soil and aerosol samples, except in the corn 2 sample collected in Hunucmá, where the presence of feldspars was not identified. In particular, the plagioclase fraction (i.e., Na/Ca feldspar) together with the Kfeldspar reaches more than 50% of the total concentration for the corn, nopal, and bean samples. The absence . Furthermore, the differences in the mineralogical composition between the soil and aerosol samples indicate that not all the soil particles are aerosolized. These differences can also be a consequence of the particle sizes analyzed, as aerosol samples varied between 0.18 and 10 μm, while the soils particles were smaller than 425 μm.

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The presence of feldspars, quartz, and clays (e.

INP concentrations and atmospheric implications
The INP concentration emitted during soil tillage of the agricultural soils in ZAC was found to vary between  ice nucleating abilities after the removal of the organic matter, and the statistically significant correlations between the OC concentration and the T50, for particle size from 3.2 to 5.6 µm and 1.0 to 1.8 µm.
The present results improve the current gap in knowledge of field measurements of aerosol particles at tropical

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Competing interests. The authors declare that they have no conflict of interest.