Nitrogen dioxide (NO2) is one of the most important reactive nitrogen species in the atmosphere and is emitted from combustion processes and atmospheric oxidation of nitric oxide 1. In addition to its critical role in tropospheric photochemistry, NO2 is involved in the multiphase or heterogeneous chemical reactions within cloud droplets and aerosol particles, affecting climate, air quality, human health, and ecosystems 2. NO2 hydrolysis in aqueous solution (R1) has long been known to produce nitrate (NO3−) and nitrite (NO2−) ions 3–8. Nitrite undergoes the reversible acid-base reaction (R2). The produced nitrous acid is partitioned between the gas phase (designated as HONO) and aqueous phase (HNO2) 9.
Nitrate is an increasingly important component in atmospheric aerosol particles with the trend of dominating over sulfate in places such as the North China Plain and the eastern United States 10,11. HONO is a critical product as a primary source of hydroxyl radical (OH), the atmospheric “detergent” 12,13.
Traditionally, the reaction pathway R1 is considered to proceed far too slowly under ambient conditions. Its second-order reaction rate constant is reported to be 107–108 M–1 s–1 in pure water or dilute solution 3,4. The uptake coefficient of NO2 on pure water has been measured as small as 10–7 3. Hence, the reaction R1 is thought to contribute a negligible amount to reactive nitrogen cycle in the atmosphere. However, it is highly uncertain whether the reaction kinetics obtained from dilute solutions with low ionic strengths are applicable to atmospheric aerosol particles which are often with much higher ionic strengths 14,15. Ionic strength can greatly affect reaction kinetics occurring within deliquesced aerosol particles through its integrated effects on the activity coefficients of reactants and products 16,17. In deliquesced aerosol particles, it is several orders of magnitude higher than that in cloud droplets (dilute conditions) and thermodynamic model calculations predict I values up to ~ 40 mol kg–1 14. To the best of our knowledge, reaction kinetics of NO2 hydrolysis have not been measured under high ionic strength conditions, hampering our understanding of the significance of NO2 hydrolysis in deliquesced aerosol particles.
In this study, we investigate the effect of ionic strength on the reaction kinetics of NO2 hydrolysis in deliquesced aerosol particles. The experiments of NO2 uptake by deliquesced particles were performed using a custom-made aerosol flow cell coupled with Raman spectroscope 18; see the Methods for details on the uptake experiments, particle types to study, and data analysis. Seven types of particles were studied at various relative humidities (RHs) to achieve a wide range of I from 0.1 up to 44 mol kg–1: 1) sodium chloride (NaCl); 2) ammonium chloride (NH4Cl); 3) sodium sulfate (Na2SO4); 4) ammonium sulfate ((NH4)2SO4); 5) ammonium bisulfate (NH4HSO4)/(NH4)2SO4; 6) oxalic acid; and 7) malonic acid. Furthermore, we use a state-of-the-art chemical transport model, the Weather Research and Forecasting‒Community Multiscale Air Quality (WRF-CMAQ) and incorporate in this model the enhancement effect of R1 due to the ionic strength of aerosols. The modeling results are then compared with field observations over the North China Plain to quantitatively examine its contribution in rectifying the model-observation gaps for nitrogen-containing species (NO2, HONO, and particulate nitrate). Last, we evaluate the potential effect of this chemical mechanism on atmospheric oxidation capacity.
Ionic Strength Effect On Reaction Rate Constants
Our laboratory experiments show that nitrate formation rates are sensitive to gaseous NO2 concentrations, and no nitrate is formed in the absence of NO2 (Supplementary Fig. 1). The results ascertain that nitrate formation is driven by the heterogeneous hydrolysis reaction of NO2 in aerosol particles. Figure 1A presents nitrate formation rates as a function of I for all particle types. All the experimental results are summarized in Supplementary Table 1. Nitrate formation rates are found to increase with decreasing RH (Supplementary Fig. 2) or increasing I values. Overall, we find that ionic strength has a positive influence on the reaction rates of R1. Notably, at high ionic strengths (I > 40 mol kg− 1), the formation rates become two orders of magnitude higher than those at low ionic strength (I < 1 mol kg− 1). Although the ionic strength influences the physical solubility or the Henry’s law constant of gases in liquid water 19, NO2 has a weak salting-out effect 20, and the high I does not enhance the solubility of NO2 into liquid water. Hence, the observed increase of nitrate formation rates at high I are attributed to the enhanced reaction rate constants of R1.
Figure 1B summarizes the logarithm of the measured reaction rate constants at a given I, kI, normalized by that at infinite dilution (i.e., kI=0 = 108 mol–1 kg s–1) as a function of I for all particle types examined. In non-ideal solutions, a reaction rate is proportional to reactant activities in place of the concentrations. The reactant activity is the product of its concentration and activity coefficient 21. The activity coefficient is related to the ionic strength I 22, and hence a reaction rate is strongly affected by I (see details in Text S1 in Supplementary Information). For neutral species, its activity coefficient may be expressed as 10bI according to the Debye and McAulay approach 23, where b is the kinetic salting coefficient 16. Therefore, this ionic strength effect on reaction rate constants can be quantified as 16,24:
Equation (1) predicts that the logarithm of the ratio between kI and kI=0, log10(kI/kI=0), reaches zero as I decreases down to dilute conditions. Indeed, log10(kI/kI=0) has a strong linear dependence on I (Fig. 1B). Based on the Debye and McAulay framework, the positive dependence on I is attributable to an increase in the activity coefficient of reactant (i.e., NO2) and/or the stabilization of the activated complex that is approximated by the bI term 16. When I is less than 1 mol kg–1 (e.g., oxalic acid), kI measured is 1.2 × 108 mol–1 kg s–1, which is consistent with the rate constant obtained in dilute solutions 3. Supplementary Table 2 shows that the rate constants determined in this study at high ionic strengths are several orders of magnitude higher than those in the literature under dilute conditions. These results highlight that the high ionic strengths of deliquesced aerosol particles significantly enhance NO2 hydrolysis.
In contrast, kI does not appear to be particularly sensitive to pH (Fig. 1B and Supplementary Fig. 3) and the chemical composition of the particles. For instance, Na2SO4 and NH4HSO4/(NH4)2SO4 particles at 80% RH show comparable I and kI values of 14.8 mol kg–1 and 1.8 × 109 mol–1 kg s–1, and 15.3 mol kg–1 and 2.3 × 109 mol–1 kg s–1, respectively, whereas they have different initial pH values of 5.6 and 0.9. Several studies report the effect of anions on NO2 accommodation at air-aqueous interfaces. The presence of Cl− in aqueous microdroplets significantly promotes the accommodation of gaseous NO2 by Cl− as Cl-NO2– at air-aqueous interfaces, followed by the reaction of Cl-NO2– with another NO2 25. Carboxylic acids also catalyze the reaction of R1 at the air-particle interface. Colussi et al. 25 found an increased production of nitrate during the reaction by around 3 times as malonic acid concentrations increased from 0.01 to 0.1 mmol kg–1. However, further increase in the acid concentration did not promote nitrate production. This suggests that the anion catalysis effect may be limited to the low ionic strength conditions (i.e., I < < 1 mol kg–1). Our results (Fig. 1B) also show that the rate constant of R1 in malonic acid particles is about 4 times higher than in dilute solutions despite its low ionic strength of 0.1 mol kg–1. Hence, the estimated rate constants may reflect the overall effects of ionic strength and catalysis by anions, particularly at low ionic strengths. Nonetheless, the ionic strength effect enhances the R1 reaction much more than the catalysis effect at high ionic strengths. We conclude that the ionic strength effect can be a more important factor affecting the reaction kinetics at high ionic strengths of deliquesced aerosol particles. Thus, we have decided to simplify the parameterization of the kinetics using the ionic strength alone for model implementation. A linear fit (R2 = 0.83, p < 0.05) to all the data points in Fig. 1B yields b = 0.058 in Eq. (1), which primarily represents the ionic strength effect. This quantitative relationship is used in the model analysis described in the next section.
Model Analysis And Evaluation With Field Observations
To examine the significance of the ionic strength effect on heterogeneous NO2 hydrolysis, we incorporate the effect of ionic strength into the WRF-CMAQ air quality modeling system and evaluate the model performance against field observations over the North China Plain (NCP). This region is chosen because intensive anthropogenic emissions lead to high NO2 levels and aerosol loadings, which are expected to facilitate the heterogeneous hydrolysis of NO2 26,27. The field observations were taken during 2015‒2018 at five stations (Supplementary Fig. 4) and the sampling periods have been summarized in Supplementary Table 3 28–32. For all the stations, we compare the observed and modeled concentrations of NO2 and HONO (particulate nitrate data were not available at some stations). Two WRF-CMAQ modeling scenarios, namely, Base and IS_Enh, are analyzed in this section. The Base scenario generally follows the default WRF-CMAQ settings, whereas the IS_Enh scenario differs in considering the enhancement effect of ionic strength on the reaction rates of heterogeneous NO2 hydrolysis using the exponential relationship obtained from our laboratory experiments (see the Methods).
The model‒observation comparisons of HONO concentrations are shown in Fig. 2 and Supplementary Table 4. The observed HONO concentrations exhibit both seasonal and spatial variations. The highest mean value 2.8 parts per billion (ppb) was obtained during the wintertime at Gucheng, while the lowest mean (0.2 ppb) was found during the summertime at Dongying, Shandong Province. The linear regression analysis suggests that the Base and IS_Enh modeling scenarios can explain 69% and 75% of the variance in HONO observations, respectively. The Base scenario greatly underestimates the concentrations of HONO by more than a half, although it has considered various HONO sources including direct emissions from biomass burning and motor vehicles, homogeneous reaction of OH and nitric oxide (NO), and heterogeneous reactions of NO2 on ground surfaces and deliquesced aerosols (without ionic strength effect). The IS_Enh scenario reduces the low bias by 27% on average, demonstrating the significant enhancement effect of ionic strength on heterogeneous NO2 hydrolysis. Both scenarios predict NO2 concentrations which agree well with the field observations over the NCP, and heterogeneous NO2 hydrolysis has a very minor effect on the levels of NO2 (Supplementary Fig. 5). The stoichiometric factor of HONO/NO2 is 1:2 in R1, while the level of HONO is commonly 1‒2 orders of magnitude lower than that of NO2; therefore, this reaction pathway is more important to HONO.
A more detailed comparison is made for the McFAN campaign (Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain) conducted at Gucheng, Hebei Province during winter 2018 since all the relevant meteorological variables and chemical species are available 28. Time series of the observed and simulated concentrations of NO2, HONO, and particulate nitrate during the McFAN field campaign are shown in Supplementary Fig. 6 and the model performance is summarized in Table 1. Generally, the simulated NO2 concentrations agree well with the McFAN observation in the Base scenario with a normalized mean bias (NMB) of 8%. However, HONO concentrations are significantly underestimated in the Base scenario with a NMB of − 49% and this bias is evident during both daytime and nighttime. Similarly, nitrate concentrations are underestimated with a NMB of − 34%. In the IS_Enh scenario, the ionic strength enhancement effect significantly improves the simulated NO2, HONO, and nitrate concentrations and reduces their NMB values (Table 1).
Table 1
Statistics of model performance during the McFAN campaign.
Species
|
Time
|
Observation
|
Base Scenario
|
IS_Enh Scenario
|
Conc.
|
Conc.
|
NMB* (%)
|
Conc.
|
NMB (%)
|
NO2
(ppb)
|
Daytime
|
30.0
|
31.7
|
4.0
|
30.3
|
−0.6
|
Nighttime
|
39.7
|
43.7
|
9.4
|
40.7
|
1.9
|
Average
|
36.1
|
39.2
|
7.7
|
36.8
|
1.1
|
HONO
(ppb)
|
Daytime
|
1.6
|
0.4
|
−74.7
|
0.6
|
−60.6
|
Nighttime
|
3.5
|
1.9
|
−41.8
|
3.1
|
−8.9
|
Average
|
2.8
|
1.3
|
−48.8
|
2.1
|
−19.9
|
Nitrate
(μg m− 3)
|
Daytime
|
14.6
|
7.7
|
−47.1
|
10.7
|
−26.5
|
Nighttime
|
12.8
|
9.3
|
−26.5
|
14.7
|
16.5
|
Average
|
13.4
|
8.7
|
−34.4
|
13.2
|
0.1
|
*NMB refers to normalized mean bias.
Figure 3 shows the diurnal patterns of two meteorological parameters (RH and downward shortwave radiation) and three nitrogen-containing chemical species in the McFAN campaign. The simulated nighttime HONO in the IS_Enh scenario is close to the observations with a NMB of − 9%. However, the simulated HONO during the daytime only shows a slight improvement (from − 75% to − 61%). The diurnal difference can be attributed to two factors. First, elevated shortwave radiation during the daytime enhances the photolysis of HONO, making the extra HONO formed in this pathway (R1) photolyze quickly, which can be observed from the integrated reaction rate analysis (IRR) results in Supplementary Fig. 7. Second, the relatively low RH during the daytime (Fig. 3A) may result in a hygroscopic growth factor (HGF) smaller than the criterion for the aerosol solid‒liquid phase state transition, thus hindering the enhancement effect (Supplementary Fig. 8). Overall, the enhanced NO2 hydrolysis rate due to the ionic strength effect leads to significantly better model performance of HONO and nitrate, particularly during the nighttime, while it hardly affects the satisfying NO2 results.
Impact On Nitrogen-containing Species And Atmospheric Oxidation Capacity
The WRF-CMAQ simulated difference between the IS_Enh and Base scenarios is used to examine the influence of rapid NO2 hydrolysis on nitrogen-containing species and atmospheric oxidation capacity. Figure 4 shows the modeled surface concentrations of HONO and nitrate over East Asia during winter. The most significant areas are the NCP (marked in square in Fig. 4) and North India, as well as parts of Northeast China and Republic of Korea. It is shown that surface concentrations of HONO and nitrate over the NCP and North India increase by 71% and 10%, and 96% and 9%, respectively, in the IS_Enh model scenario when compared to the Base. Field measurements of HONO are still rare in India, and a very recent study33 observed high levels of HONO of about 3 ppb during winter in Delhi, similar to its value found in the NCP. During summer, the difference between the IS_Enh and Base scenarios is minor (Supplementary Fig. 9). Two factors are considered to contribute to this seasonal variation. First, frequent temperature inversion events in winter result in high levels of NOx, aerosol, and water vapor being accumulated in the shallow planetary boundary layer, providing abundant precursor and reaction space for heterogeneous NO2 hydrolysis 29,34. Second, the stronger and longer solar radiation during summer enhances the photolysis of HONO and thus hampers its accumulation.
The formation of HONO can further enhance the atmospheric oxidation capacity through its photolysis and the subsequent free radical reactions (Supplementary Fig. 10). Over the NCP during winter, the modeled OH and O3 concentrations in the IS_Enh scenario are 19% and 6% higher, respectively, compared to those in the Base scenario. Nevertheless, the ionic strength effect of heterogeneous NO2 hydrolysis has little influence during summer (< 0.1% increase in OH and O3).