Atmospheric Nitrogen Depositions in a Highly Human-Impacted Area

Abstract

Nutrients that fall on the ground from the atmosphere represent a minor component of the total nitrogen (N) input to soils, especially when compared with agricultural, civil and industrial inputs (i.e. sewage treatment plants or sewage systems, fertilizer and manure applications). However, integrating all nitrogen forms, processes and scales can represent a breakthrough challenge for the understanding and the management of the N cycle. A monitoring experiment was set up to collect wet atmospheric depositions in a human-impacted area with multiple land uses, representing different emission sources. Rainwater collection was executed in the surroundings of Milan, in northern Italy, starting from February 2017 to February 2019. The presence of N compounds and their temporal variations in rainwater are consistent with pollution coming from local anthropogenic emission sources of nitrogen oxides and ammonia, mainly related to the use of the heating systems in the cold seasons and the spreading of fertilizers and manure on agricultural fields. Consequently, the total amount of N wet depositions ranges between 14 and about 30 kg/ha yr in the study area. As leaching of N compounds from soils generally increases at deposition rates higher than about 10 kg(N)/ha yr, this work suggests that the N atmospheric input to soils could not be neglected when evaluating the impacts of N sources to terrestrial and aquatic ecosystems, as well as to groundwater resources. This highlights the need of wisely integrating air, soil and water policies for minimising the risk to deteriorate surficial ecosystems and groundwater.

Appendices

Appendices

1.1 Appendix 1

Microwave plasma atomic emission spectroscopy (MP-AES) and ion chromatography (ICS) methods in measuring major cation concentrations have been compared applying the Bland-Altman graphical technique (Bland and Altman 1999), which allows assessing the relative agreement between two laboratory analytical methods that measure the same chemical substance (Magari 2002).

Figure A1a shows the measurements obtained by ICS plotted against those obtained through MP-AES, to assess visually how well the methods agree: data should lie along a straight line passing through the origin in a 45° angle (i.e. equality line). But, it seems that ICS results are slightly higher than MP-AES results. However, this plot can be misleading: the greater the range of measurements, the better the agreement will appear to be. Instead, measuring the differences between the two methods for each measurement plotted against their means and adding the limits of agreement (Fig. A1b) is a better way of assessing the relationship, as it clearly shows the pattern of the individual differences. The mean difference is 0.07 mg/L, and the standard deviation of the differences is 0.48 mg/L. Thus, the limits of agreement with a 95% confidence level are − 0.86 mg/L and 1.01 mg/L (dashed lines in Fig. A1b). Nonetheless, Fig. A1b shows an increase in variability of the differences as the magnitude of the measurement increases. Thus, a logarithmic (i.e. natural logarithmic) transformation of both measurements has been applied.

Fig. A1

a Measurements of base cations concentrations. b Plot of differences versus average with 95% limits of agreement (SD = standard deviation)

Figure A2 shows the logarithmic transformed data and the difference versus mean plot with superimposed 95% limits of agreement. The range of values has been reduced and data lie along the ‘equality line’ (Fig. A2a). The mean difference (log ICS – log MP-AES) is 0.03 with 95% limits of agreement − 0.071 and 0.076 (Fig. A2b). Thus, it is possible to assure the agreement between ICS and MP-AES methods for measuring major cation concentration and that the two measurement methods can be used interchangeably.

Fig. A2

a Measurements of base cation concentrations after logarithmic transformation (LN = natural logarithm). b Plot of differences versus average after logarithmic transformation with 95% limits of agreement (SD = standard deviation)

It is important to note that instrumental errors are considered within the range of variability of differences between the two methods.

1.2 Appendix 2

(a) Ionic Balance

The completeness of measured parameters and the principle of electroneutrality in precipitation were checked through the ratio of total anions to that of cations. This ratio is expressed as (WMO/GAW 2004; Eq. A1):

$$ \mathrm{BAL}\left(\%\right)=100\times \frac{\mathrm{CE}-\mathrm{AE}}{\mathrm{CE}+\mathrm{AE}} $$

(A1)

with CE and AE being the sum of cation and anion equivalents (meq/L), respectively. CE is the sum of Ca²⁺, Na⁺, Mg²⁺, K⁺, NH₄⁺ and H⁺. AE is the sum of NO₃⁻, NO₂⁻, SO₄²⁻, Cl⁻ and F⁻. CE and AE are calculated as (WMO/GAW 2004; Eqs. A2 and A3):

$$ \mathrm{CE}=\left[\sum \frac{C_{Ci}}{{\left(\mathrm{Eq}.\mathrm{Wt}.\right)}_{Ci}}\right]+\frac{10^{\left(6-\mathrm{pH}\right)}}{1000} $$

(A2)

$$ \mathrm{AE}=\left[\sum \frac{C_{Ai}}{{\left(\mathrm{Eq}.\mathrm{Wt}.\right)}_{Ai}}\right] $$

(A3)

where C_Ci and C_Ai are the concentrations of the ith cation (C) or anion (A) in mg/L, Eq.Wt. is the equivalent weight of the ith cation (C) or anion (A), 10^(6-pH) is the H⁺ concentration in meq/L.

The acceptability criterion was set as BAL ≤ ± 20% to comply with the requirements of the Global Atmosphere Watch Program for precipitation chemistry (WMO/GAW 2004) and according to previous works on precipitation quality (e.g. Balestrini et al. 2000; Yang et al. 2012).

(b) Electric Conductivity Balance

Measured and calculated electric conductivities (EC) are compared according to Eq. A4 (WMO/GAW 2004).

$$ \Delta \mathrm{EC}\left(\%\right)=100\cdotp \frac{{\mathrm{EC}}_{\mathrm{c}}-\mathrm{EC}}{\mathrm{EC}} $$

(A4)

where EC_C is the calculated electric conductivity and EC is the measured electric conductivity, both expressed as μS/cm. For dilute solutions, the total (calculated) conductivity can be calculated in μS/cm from the molar concentrations and molar ionic conductances of the individual ions, as follows (WMO/GAW 2004; Eq. A5):

$$ {\mathrm{EC}}_{\mathrm{c}}=\sum {c}_i\cdotp {\Lambda}_i{}^{\circ} $$

(A5)

where EC_C is the calculated conductivity (μS/cm), c_i is the ionic concentration of the ith ion (mmol/L) and Λ_i° the molaric ionic conductance of the ith ion (Scm²/mol) at infinite solution and 25 °C. Thus (Eq. A6):

$$ {\mathrm{EC}}_{\mathrm{C}}={10}^{\left(3-\mathrm{pH}\right)}\cdotp 349.7+\mathrm{c}\left[{\mathrm{F}}^{-}\right]\cdotp +\mathrm{c}\left[{\mathrm{C}\mathrm{l}}^{-}\right]\cdotp 76.3+\mathrm{c}\left[{\mathrm{NO}}_2^{-}\right]\cdotp 71.8+\mathrm{c}\left[{\mathrm{NO}}_3^{-}\right]\cdotp 71.4+\mathrm{c}\left[{\mathrm{SO}}_4^{2-}\right]\cdotp 160+\mathrm{c}\left[{\mathrm{C}\mathrm{a}}^{2+}\right]\cdotp 119+\mathrm{c}\left[{\mathrm{Mg}}^{2+}\right]\cdotp 106+\mathrm{c}\left[{\mathrm{Na}}^{+}\right]\cdotp 50.1+\mathrm{c}\left[{\mathrm{K}}^{+}\right]\cdotp 73.5+\mathrm{c}\left[{\mathrm{NH}}_4^{+}\right]\cdotp 73.5 $$

(A6)

where 10^(3-pH) expresses the concentration of H⁺.

The acceptability criterion was set as ΔEC ≤ ±30% to comply with the requirements of the Global Atmosphere Watch Program for precipitation chemistry (WMO/GAW 2004) and according to previous works on precipitation quality (e.g. Balestrini et al. 2000; Yang et al. 2012).

(c) Marine Inputs

The sea salt fraction (SSF) and the non-sea salt fraction (NSSF) are calculated according to Keene et al. (1986); WMO/GAW 2004) by comparing ionic ratios in rainwater and seawater using sodium as the reference species (e.g. Moreda-Piñeiro et al. 2014; Deusdará et al. 2017), following Eqs. A7 and A8.

$$ \%{\left(\mathrm{SSF}\right)}_X=\frac{100\cdotp (Na)\left(\frac{X_{\mathrm{sea}}}{{\mathrm{Na}}_{\mathrm{sea}}}\right)}{X} $$

(A7)

$$ \%{\left(\mathrm{NSSF}\right)}_X=100-\%{\left(\mathrm{SSF}\right)}_X $$

(A8)

where X is the concentration of ions as measured in rainwater, X_sea is the concentration of the sea water ions and Na_sea is the concentration of Na⁺ taken as a reference in sea water. All concentrations are expressed in meq/L.

(d) Neutralisation Factors

The acidity of rainwater potentially originates from the dissociation of nitric and sulphuric acid anions (NO₃⁻ and SO₄²⁻) that are neutralised by alkaline species, such as Ca²⁺, Mg²⁺, K⁺ and NH₄⁺ as well as on the neutralisation reactions among them (e.g. Moreda-Piñeiro et al. 2014; Deusdará et al. 2017). The neutralisation factor (NF) is expressed as shown in Eq. A9.

$$ {\mathrm{NF}}_x=\frac{\left[X\right]}{\left[{\mathrm{NO}}_3^{-}+{\mathrm{SO}}_4^{2-}\right]} $$

(A9)

where [X] is the concentration of the species responsible for neutralisation (Ca²⁺, Mg²⁺, K⁺, NH₄⁺). The volume weighted mean for each species, expressed as meq/L, is used for the calculation of the neutralisation factors.

(e) Descriptive Statistics

Descriptive statistics are calculated for ionic concentrations, EC and pH values: arithmetic mean, volume weighted mean, median, minimum and maximum values and standard deviation. Volume weighted mean (VWM) was calculated by using Eq. A10. It allows taking into account the effect of dilution by the rain amount and is useful in comparative studies.

$$ \mathrm{VWM}=\frac{\sum \limits_{i=1}^N{v}_i\left[{X}_i\right]}{\sum \limits_{i=1}^N{v}_i} $$

(A10)

where [X_i] is the concentration of ion X or the value of parameter X (i.e. pH and EC), v_i is the water sample volume and N is the number of samples. Concentration can be expressed as mg/L, meq/L or μeq/L. pH has no unit of measure. EC is expressed as μS/cm.