A Study on Three Factors Influencing Uptake Rates of Nitric Acid onto Dust Particles

1. INTRODUCTION

Recently, several studies have reported that although East Asian dust particles have sufficient chemical aging (or contacting) times (say, 1-4 days) with atmospheric air pollutants, they were found to contain only small amounts of nitrate (and/or sulfate) (Song et al., 2007; Song et al., 2005; Maxwell-Meier et al., 2004). This fact possibly indicates that the gas-to-particle nitric acid (HNO₃) uptake rates (R_HNO3) onto dust particles are slower than previously estimated. This could also be an important correction to the results from previous studies (Meskhidze et al., 2003; Song and Carmichael, 2001; Zhang and Carmichael, 1999; Dentener et al., 1996). For example, Meskhidze et al. (2003) inferred from the data of TRACE-P DC-8 Flight #13 that total nitrate (=HNO₃(g)+NO₃^-(p)) distribution between gas phase and East Asian dust particles reaches a near equilibrium (refer to Fig. 4 in Meskhidze et al. (2003)). Their work supported the opposite idea that dust particles could contain a quite large amount of nitrate that could completely neutralize dust-originated cationic components such as Ca²⁺ and Mg²⁺. This study, therefore, intends to discuss possible causes of this discrepancy in order to offer a convincing explanation.

Closely connected with this issue, controversy has continued regarding the magnitude of reaction probability of HNO₃ (γ_HNO3) onto dust particles. Several research groups measured γ_HNO3 onto various types of dust particle, from proxy species of dust particles (e.g., α-Al₂O₃, SiO₂, CaCO₃ etc) to authentic Saharan and Gobi dust. However, a large difference in the magnitude of γ_HNO3 onto dust particles has been reported (Johnson et al., 2005; Umann et al., 2005; Harnisch and Crowley, 2001a, b; Underwood et al., 2001a, b; Fenter et al., 1995), as presented in Table 1. Here, the first group of γ_HNO3 has an order of magnitude from ~10^-1 to ~10^-2 (Umann et al., 2005; Harnisch and Crowley, 2001a, b; Fenter et al., 1995), compared to a range from ~10^-3 to ~10^-5 for the second group (Johnson et al., 2005; Underwood et al., 2001a, b).

Table 1.

Reaction Probability of HNO3 onto dust particles.

The ultimate purpose of estimating (or measuring) γ_HNO3 is to apply the result to the chemistry-transport modeling studies. However, because of the large differences in the magnitude of γ_HNO3 onto dust particles, atmospheric modelers have been confused about which value should be adopted in their modeling studies (Song et al., 2007; Bauer et al., 2004; Zhang and Carmichael, 1999; Dentener et al., 1996; and also the third group of γ_HNO3 as presented in Table 1). In addition, it appears that the parameterizations (or estimation method for γ_HNO3) for describing the heterogeneous interaction between HNO₃ and dust particles are questionable. Therefore, an impartial discussion is urgently required about both the magnitudes of γ_HNO3 onto dust particles and the parameterizations for the heterogeneous interactions between HNO₃ and dust particles. In this study, we discuss all the relevant and yet contentious issues regarding the heterogeneous processes between gasphase HNO₃ and dust particles. In addition, through this study we wish to provide a modeler’s perspective regarding the chemical evolution of dust particles and the magnitude of γ_HNO3 onto dust particles.

2. DISCUSSION

2. 1 Research and Theoretical Background

The HNO₃ uptake into dust particles causes the replacement of carbonate (CO₃^2-) originally associated with crustal Ca²⁺ (and/or Mg²⁺) in dust particles via the following reaction:

$\[ {\text{CaCO}}_{\text{3}}\text{+2HN}{\text{O}}_{\text{3}}\text{→Ca}{\left(\text{N}{\text{O}}_{\text{3}}\right)}_{\text{2}}\text{+}{\text{H}}_{\text{2}}\text{O+C}{\text{O}}_{\text{2}} \]$

(R-1)

Similar carbonate replacement reactions occur with the uptake of other acidic substances such as H₂SO₄ and SO₂. Based on this mechanism, Table 2 presents the “dust chemical aging index”, defined as the ratio of estimated CO₃^2- equivalence ([CO₃^2-] in μeq/m³) to crustal cation equivalence ([Ca²⁺]+[Mg²⁺]) at several different locations in East Asia (Song et al., 2007; 2005). As presented in Table 2, the ratios range from 0.39 to 0.87, indicating that the remaining carbonate fractions in the dust particles range from 39-87%, respectively, even after the chemical aging times of 24-84 hrs. Several single particle chemical analysis studies with East Asian dust particles reached the same conclusions (Ro et al., 2005; Zhang et al., 2003; Zhang and Iwasaka, 1999). The replaced CO₃^2- fractions of 13-61% are believed to be released from dust particles by nitrate (and/or sulfate) formation. The small percentages of the replaced carbonate, even with the long chemical aging times, are obvious evidences of slow R_HNO3 onto dust particles.

Table 2.

Average dust chemical aging indices and maximum HNO3 uptake rates in East Asia.

Assuming pseudo first-order kinetics (eqn. 1), R_HNO3 can be calculated by eqns. (1) and (2). In the equations, the magnitude of R_HNO3 can be affected by three factors: i) γ_HNO3 ii) aerosol surface density (S_a); and iii) gasphase HNO₃ concentration (C_HNO3):

$\[ R_{{HNO}_3}=\frac{d{C}_{HN{O}_3}}{dt}=k{C}_{HN{O}_3} \]$

(1)

$\[ k=\frac{1}{4}{v}_{HN{O}_3}{\gamma }_{HN{O}_3}{S}_a \]$

(2)

where k denotes the mass transfer coefficient (1/s), v_HNO3 the molecular mean velocity of HNO₃ (cm/s), and S_a the aerosol surface density (μm²/cm³). In this study, we discuss these three possibly influential factors to determine which factor (or factors) is really responsible for the observed discrepancy in R_HNO3 onto dust particles.

2. 2 Reaction Probability of HNO₃ (γ_HNO3)

First of all, the slow R_HNO3 could result from low γ_HNO3 onto dust particles. As mentioned in the Introduction, large differences have been reported in the magnitude of γ_HNO3 onto dust particles (Song et al., 2007; Johnson et al., 2005; Umann et al., 2005; Harnisch and Crowley, 2001a, b; Underwood et al., 2001a, b; Fenter et al., 1995). Fig. 1 presents how fast HNO₃ can be transferred from the gas phase into dust particles with different γ_HNO3 values, using eqns. (1) and (2). For example, when γ_HNO3=0.1, the 10 e-folding lifetime (τ₁₀, defined as the time at which C_HNO3/C_HNO3,0=1/10e=0.037; τ₁₀=(1+ln10)/k, where k represent the mass transfer coefficient in eqn. (2)) is only 1.41 hrs. When γ_HNO3=0.01, τ₁₀ becomes 8.03 hrs. In other words, 96.3% of HNO₃ is partitioned into the particulate phase within 8.03 hrs, when γ_HNO3=0.01. The partitioned HNO₃ is then rapidly converted into nitrate, being associated with Ca²⁺ and Mg²⁺ ions inside the dust particles. For example, if the HNO₃ levels of 1-3 ppb are converted into nitrate at STP at a yield of 96.3%, nitrate concentrations of 2.6-7.7 μg/m³ should be found in the dust particles after relatively short chemical aging times (within few hours). Once these amounts of nitrate are formed in the dust particles, the particles (particularly Ca²⁺) can be completely neutralized only by nitrate in most typical dusty situations. HNO₃ levels of 1-3 ppb were frequently observed with high dust concentrations over the downwind areas from the polluted regions in East Asia (this will be discussed in more detail in section 2.4; also refer to Bauer et al. (2004)). However, such large amounts of nitrate have never been found in dust particles, particularly in East Asia (Song et al., 2007; Ro et al., 2005; Song et al., 2005; Zhang et al., 2003). In contrast, when γ_HNO3= 10^-3-10^-5, R_HNO3 become much slower (τ₁₀=3.1-91.8 days). With the initial C_HNO3 of 1-3 ppb and γ_HNO3 of 10^-4, the converted amount of nitrate after the interacting times of 8 hours is 0.095-0.29 μg/m³, which is equivalent to an hourly averaged R_HNO3 of 0.01-0.04 μg/m³∙hr. Such slow R_HNO3 are more consistent with the slow chemical evolution of dust particles observed in the field measurements. For example, in Table 2, we estimated the possibly maximum nitrate concentrations ($$ {\overline{\left[{N{O}_3}^{-}\right]}}^{\text{max}}$$; $$ {\overline{\left[{N{O}_3}^{-}\right]}}^{\text{max}}=\overline{\left[{N{O}_3}^{-}\right]+\left[{{\mathit{S}O}_4}^{2-}\right]}$$). The measurement data used in Table 2 were obtained from the ACE-ASIA C130 flight campaign conducted over the Yellow sea on April 11-13, 2001, and a measurement campaign conducted in Seoul in April, 2005. We selected the data from the dust storm periods of the campaigns. In this calculation, while we cannot estimate the individual amounts of nitrate and sulfate, we can estimate the combined amounts of “nitrate+sulfate” concentration in the dust particles by equating the replaced carbonate equivalences to the “nitrate+sulfate” equivalences. In an extreme case, carbonate in dust particles could be replaced only by NO₃^-. These amounts ($$ {\overline{\left[{N{O}_3}^{-}\right]}}^{\text{max}}=\overline{\left[{N{O}_3}^{-}\right]+\left[{{\mathit{S}O}_4}^{2-}\right]}$$). are then divided by chemical aging times to obtain the maximum possible nitrate uptake rates. As shown in Table 2, the rates range from 0.01 to 0.26 μg/m³∙hr. These values are in general comparable to the estimated rates above (0.01-0.04 μg/m³∙hr), particularly for the cases of ACE-ASIA C-130 Flights #6, #7, and #8 which recorded results in the range 0.05-0.07 μg/m³∙hr (again, it is important to remember that the average uptake rates actually represent the production rates of NO₃^-+SO₄^2-).

Fig. 1.

Ten e-folding lifetimes (τ10) with different reaction probability of HNO3. The parameters for log-normal aerosol distribution used in this figure are presented in the box (Zhang et al., 1994).

Although in this study we do not intend to estimate γ_HNO3 onto dust particles, one of the objectives of this study is to decide which values of “already-measured” γ_HNO3 in Table 1 can be adopted in the dust chemistry modeling studies. Based on the calculations above, the modeling studies with γ_HNO3 of ~10^-3-~10^-5 could produce more consistent results with the field measurement data.

2. 3 Surface Area

There is another possible mechanism that could slow down R_HNO3 into dust particles: the application of a smaller surface area to eqn. (2). For example, in an estimating procedure of γ_HNO3, Umann et al. (2005) introduced an “active surface area (S_A)” (for greater detail, refer to both Umann et al. (2005) and Matter Engineering, Appendix IV of Operating Instructions LQ1-DC, SKM990318-7b). A similar type of active surface area, so-called “Fuchs surface”, was also introduced by Pandis et al. (1991) and Shi et al. (2001). Whichever active surface area is used, S_A has a tendency to become smaller than the “geometric surface area”) (S_G), when the coarse-mode fraction is large, such as in dust and sea-salt aerosol cases. Therefore, if we use S_A instead of S_G for the gas-to-particle mass transfer process, R_HNO3 could become slow. Table 3 presents the typical ratios of S_A to S_G for dust and seasalt particle distributions (Sander and Crutzen, 1996; Zhang et al., 1994; Jaenicke, 1993). The ratios range between 0.11 and 0.41. In particular the ratio is 0.11 for the two typical dust cases, indicating that R_HNO3 become slower by a factor of 0.11, even if the same γ_HNO3 is used. Here, the relevant question is whether S_A can be applied to eqn. (2). Umann et al. (2005) used S_A with the concept of the “actual surface area” that is accessible for impinging gas molecules that can typically be measured by a BET (Brunauer, Emmett and Teller) type of instrument. However, both the active (μm²/cm³) and Fuchs (1/cm³) surface areas are not the “actual/accessible surface area”, but an imaginary aerosol-surface area conveniently adjusted to consider the changing mechanism in the gas-to-particle mass transfer in accordance with the aerosol size changes (Pandis et al., 1991). For the fine particles, the gas-to-particle mass transfer (uptake) rates are proportional to the second moment (i.e., surface area), whereas for the coarse particles, R_HNO3 are proportional to the first moment (i.e., radius). Therefore, the S_A-to-S_G ratios are small when the coarse aerosol fraction is large, whereas the ratios are close to unity when the fine fraction is dominant. The variation in uptake mechanism with the aerosol size can be taken into account by the use of Fuchs and Sutugin kinetics (1971) or other similar kinetics (e.g., see p. 604 in Seinfeld and Pandis, 1998). In Table 3, the Fuchs-Sutugin mass transfer coefficient (k_F) is compared with k_A (k from eqn. 2 with S_A for S_a) and k_G (k from eqn. 2 with S_G for S_a). As shown in Table 3, the values of k_G are consistent with k_F, whereas the values of k_A are smaller than k_F by a factor of ~0.1. Meanwhile, the use of S_A in eqn. (2) also leads to an erroneous estimation of γ_HNO3. One example of the erroneous results is shown in Fig. 2. Umann et al. (2005) estimated γ_HNO3 from the field measurements in Sahara desert, using equations (1) and (2) with S_A. We selected three dust episodes (E3, E4, and E5) from Umann et al.’s work (2005), and then re-estimated γ_HNO3 with S_A and S_G. As shown in Fig. 2, when S_G is used instead of S_A, the values of γ_HNO3 are decreased down to 0.004-0.02, which are smaller than Umann et al.’s values (γ_HNO3=0.03-0.18). These values are also closer to the second group of γ_HNO3 in Table 1 that we recommended to be used in the future dust chemistry modeling studies.

Table 3.

Mass transfer coefficients with different types of surface area (γHNO3=0.033).

Fig. 2.

Estimation of γHNO3 with two different types of surface areas (SA and SG).

2. 4 HNO₃ Concentration in the Gas Phase (C_HNO3)

The third factor that could affect R_HNO3 into dust particles is C_HNO3 (from eqn. 1). If C_HNO3 is low, R_HNO3 becomes slow. Or, if the amounts of HNO₃ are not sufficient, the amounts of nitrate formed in dust particles can be limited by the insufficient the amounts of HNO₃, even with fast R_HNO3. Particularly, extremely low C_HNO3 could occur over the areas where NH₃ concentrations are high. Over such areas, HNO₃ can be depleted by the reaction of HNO₃(g)+NH₃(g) → NH₄NO₃(aerosol). For example, East Asia has large NH₃ emissions (Kim et al., 2006). Fig. 3 presents C_HNO3 and the Ca²⁺ concentrations measured by TRACE-P DC-8 Flights #9 and #13 over the Yellow Sea and East China Sea (see panels (c) and (g)). As presented in panels (d) and (h), the air masses originated from the Gobi desert and the arid areas in Inner Mongolia, and then passed through the highly polluted, high NO_x and NH₃ emission areas in China such as Beijing, Tianjin, Qingdao, Dalian, Nanjing, and Shanghai, as well as various Chinese agricultural areas (Kim et al., 2006). The high levels of Ca²⁺ indicate that the air masses intercepted by DC-8 Flights #9 and #13 contained high levels of dust particles. In the two flights, the observed levels of HNO₃ were increased up to as high as 7 ppb (see panel (g)). The typical HNO₃ levels reported from other TRACE-P DC-8 and P3-B flights in the boundary layer under the continental outflow situations ranged between ~1 ppb and ~3 ppb. Despite the coexistence of the high levels of dust particles and HNO₃, Song et al. (2007; 2005) reported very low nitrate (and/or sulfate) concentrations inside dust particles over the areas close to the flight paths of TRACE-P DC-8 Flights #9 and #13. This is an important correction to the results from Meskhidze et al. (2003). They insisted that the coexistence of high levels of Ca²⁺ and HNO₃ in the TRACE-P DC-8 Flight #13 was firm evidence that HNO₃ had already filled up the dust particles (i.e., complete neutralization of Ca²⁺or 100% carbonate replacement had occurred).

Fig. 3.

The HNO3 and Ca2+ concentrations encountered by TRACE-P DC-8 Flights #9 and #13: (a) and (e) fight paths; (b) and (f) altitudes of the flights; (c) and (g) HNO3 and Ca2+ concentrations; and (d) and (h) five-day backward trajectory analyses, respectively, using the HYSPLIT model and meteorological data available on the NOAA/ARL web site (Draxler and Hess, 1998).

Again, the presence of high HNO₃ levels indicates that the low nitrate concentrations in the dust particles were not a result of the low C_HNO3 levels, but were rather due to the small γ_HNO3 that was reduced even smaller than 10^-3.

3. CONCLUSIONS

The observed R_HNO3 onto dust particles were much slower than previously estimated. In addition, R_HNO3 have been overestimated in several previous dust modeling studies, for which three possibilities were discussed here: (1) the magnitude of γ_HNO3, (2) aerosol surface area, and (3) C_HNO3. Regarding the second factor, we suggested that S_G should be used in eqn. (2) instead of S_A, which had been used, for example, in Umann et al.’s study (2005). With respect to the third factor, we showed that the observed C_HNO3 was relatively high, sometimes increasing up to as high as ~7 ppb in the marine boundary layer in East Asia, for example. C_HNO3 can not limit nitrate formation in dust particles.

The over-predicted R_HNO3 onto the dust particles may have been caused by the first factor-overestimated γ_HNO3 values (γ_HNO3=10^-1-10^-2). In the previous modeling studies of Dentener et al. (1996), Zhang and Carmichael (1999), and Bauer et al. (2004), γ_HNO3 values of 0.1 and 0.01 were used. In contrast to these overestimated γ_HNO3 values, smaller γ_HNO3 values within the range 10^-3-10^-5 have also been reported by Underwood et al. (2001a, b) and Johnson et al. (2005). In this study we concluded that the use of γ_HNO3 values between ~10^-3 and ~10^-5 could produce more realistic results than those within the range of 10^-1-10^-2 could, by more accurately predicting the nitrate formation characteristics in dust particles.

Acknowledgments

This work was financially supported by the Mid-Career Research Program, through a National Research Foundation of Korea (NRF) grant from the Ministry of Education, Science and Technology (MEST) (2010-0014058); and a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. R17-2008-042-01001-0).

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