Mist cannon trucks can exacerbate the formation of water-soluble organic aerosol and PM 2 . 5 pollution in the road environment

Abstract. Mist cannon trucks have been widely applied in megacities in China
to reduce the road dust, since they are considered to be more water saving
and efficient than the traditional sprinkling trucks. However, their effect
on the formation of water-soluble organic compounds and the pollution
control of fine particles (PM2.5) remains unknown. We characterized the
variations of chemical compositions in PM2.5 collected on the road
sides during the simulated operations of mist cannon truck and traditional
sprinkling truck via Fourier transform ion cyclotron resonance mass
spectrometry and ion chromatography. The mass concentrations of
water-soluble organic carbon in PM2.5 showed a significant increase
(62 %–70 %) after air spraying. Furthermore, we found that water-soluble
organic compounds, particularly organic nitrates, increased significantly
via the interactions of reactive gas-phase organics, atmospheric oxidants
and aerosol liquid water after air spraying, although the air spraying had a
better effect on suppressing road dust than the ground aspersion. Moreover,
the formation of PM2.5 on the road segment where the mist cannon truck
passed by was promoted, with an increase of up to 13 % in mass
concentration after 25–35 min, on average. Thus, the application of
mist cannon trucks potentially worsens the road atmospheric environment
through the increase in PM2.5 levels and the production of a large
number of water-soluble organic compounds in PM2.5. The overall results
provide not only valuable insights to the formation processes of
water-soluble organic compounds associated with aerosol liquid water in the
road environment but also management strategies to regulate the operation of
mist cannon trucks in China.



S1. Parameter Calculation and Compound Categorization
The value of double-bond equivalent (DBE) was calculated to reflect the sum of π-bonds and rings in a neutral molecule (Lechtenfeld et al., 2014;Qiao et al., 2020).
The equation was shown below.DBE = 1 + NC -NH/2 + NN/2 (1) where the NC, NH, and NN denote the number of carbon, hydrogen, and nitrogen atoms in a molecular formula, respectively.
The modified aromaticity index (AImod) can be used to reflect the aromaticity of organic molecules, which was calculated according to the following equation (Schmidt et al., 2017;Koch and Dittmar, 2006).
The carbon oxidation state (OSc) is an indicator to describe the evolving composition of aerosol organics undergoing oxidation processes (Kroll et al., 2011).
For assignable molecular formulas, OSC was calculated with following equation.
where the NC, NH, NO, and NN denote the number of carbon, hydrogen, oxygen, and nitrogen atoms in a molecular formula, respectively.Although the heteroatoms (N, S, and P) can introduce some uncertainties to the OSC value of a given molecule in the measurement of ultrahigh resolution ESI-MS, the influence of these heteroatoms on the OSC value of orgainc aerosols is generally small (Kroll et al., 2011).
Considering the presence of isomers for identified formula, the divided categories only represent the compounds cantaining the most likely functional structure (Butturini et al., 2020;Xie et al., 2021).

S2. Aerosol Liquid Water (ALW) Prediction
The model ISORROPIA-II was used to estimate the mass concentration of ALW with particle-phase concentrations of Na + , NH4 + , K + , Ca 2+ , Mg 2+ , SO4 2− , NO3 − , and Cl − , as well as meteorological data (ambient temperature and relative humidity) as inputs (Guo et al., 2015;Nguyen et al., 2016;Tan et al., 2017).In this study, the model was run in the "reverse mode" without inputs of gas-phase parameters (Nguyen et al., 2015;Xu et al., 2020).In addition, the thermodynamically metastable state was set in the subsequent calculation (Guo et al., 2015;Nguyen et al., 2015;Nguyen et al., 2016).The "forward mode" was also run with inputs of only particle-phase ion concentration data, temperature, and relative humidity.The calculation results of water concentrations showed little difference irrespective of the mode used, which is consistent with the previous measurements (Guo et al., 2015;Hennigan et al., 2015).
Aerosol organics typically have complex compositions.It is difficult to directly quantify the mass concentration of water associated with organic fraction (Nguyen et al., 2016;Sareen et al., 2013;Cruz and Pandis, 2000).Accordingly, the mass concentration of water derived from organic compounds was predicted using a simplified model with the Zdanovskii−Stokes−Robinson (ZSR) mixing rule, as suggested by previous studies (Nguyen et al., 2016;Nguyen et al., 2015).Briefly, the hygroscopic growth of aerosol mixtures can be estimated using weighted hygroscopicity of each component according to their dry volume fractions (Bian et al., 2014;Nguyen et al., 2016;Nguyen et al., 2014).The detailed calculation was shown below (Petters and Kreidenweis, 2007;Kreidenweis et al., 2008).
where Vw, o and Vo are the volumes of water and organics, respectively.κorg is dimensionless and represents the hygroscopicity parameter of the organics.aw is dimensionless and indicates water activity.The typical value of 1.4 g cm −3 for organic density was used to calculate the Vo value (Davidson et al., 2005;Turpin and Lim, 2001).
A κorg value of 0.08 was used in this study, which has been considered as a representative κorg value for urban aerosols (Cerully et al., 2015;Dusek et al., 2010;Gunthe et al., 2009;Nguyen et al., 2016).The aw value can be treated as relative humidity to simplify the calculation (Nguyen et al., 2015).This consideration was based on the following assumptions.The effect of aerosol curvature is insignificant.

Figure S1 .
Figure S1.Classification of CHO, CHON, CHOS, and CHONS species into subgroups according to the number of O atoms in their molecules in WSOM in PM2.5 collected from different cases: (a, b, c) air spray vs ground aspersion and (d) no water spray (Ⅰ) vs no water spray (Ⅱ).

Figure S2 .
Figure S2.Van Krevelen diagrams of unique CHO compounds in WSOM in PM2.5 collected from different cases: air spray vs ground aspersion on (a) March 23, (b) March 24, and (c) March 25 and two road segments without water spray (Ⅰ vs Ⅱ) on (d) March 26.For the above comparative cases, the unique CHO compounds indicate the CHO molecules identified in PM2.5 collected from the air spray (/no water spray-Ⅰ) road segments.The classifications of compounds include unsaturated aliphatic-like (UA), highly unsaturated-like (HU), highly aromatic-like (HA), polycyclic aromatic-like (PA), and saturated-like (Sa) molecules.

Figure S5 .
Figure S5.Van Krevelen diagrams of unique CHON compounds in WSOM in PM2.5 collected from different cases: air spray vs aspersion on (a) March 23, (b) March 24, and (c) March 25 and two road segments without water spray (Ⅰ vs Ⅱ) on (d) March 26.For the above comparative cases, the unique CHON compounds indicate the CHON molecules identified in PM2.5 collected from the air spray (/no water spray-Ⅰ) road segments.The classifications of compounds include unsaturated aliphatic-like (UA), highly unsaturated-like (HU), highly aromatic-like (HA), polycyclic aromatic-like (PA), and saturated-like (Sa) molecules.

Figure S6 .
Figure S6.OSc of unique CHON2 molecules in WSOM in PM2.5 collected from different cases: air spray vs ground aspersion on (a) March 23, (b) March 24, and (c) March 25 and two road segments without water spray (Ⅰ vs Ⅱ) on (d) March 26.For the above comparative cases, the unique CHON2 compounds indicate the CHON2molecules identified in PM2.5 collected from the air spray (/no water spray-Ⅰ) road segments.The light orange background indicates areas of HOA (hydrocarbon-like organic aerosol), BBOA and VEOA (biomass burning and vehicle emission organic aerosols)(Kroll et al., 2011;Tong et al., 2016), SV-OOA (semivolatile oxidized organic

Figure S7 .
Figure S7.OSc of each CHON molecule in WSOM in PM2.5 collected from different cases: air spray vs ground aspersion on (a) March 23, (b) March 24, and (c) March 25 and no water spray (Ⅰ) vs no water spray (Ⅱ) on (d) March 26.

Table S1 .
The arithmetic and peak-intensity-weighted averages of the elemental ratios and DBE values for different compound subgroups in