Responses of Secondary Inorganic PM2.5 to Precursor Gases in an Ammonia Abundant Area In North Carolina

Received: October 28, 2018
Revised: January 18, 2019
Accepted: January 22, 2019

Download Citation: RIS|BibTeX|https://doi.org/10.4209/aaqr.2018.10.0384  

Cite this article:

Cheng, B. and Wang-Li, L. (2019). Responses of Secondary Inorganic PM2.5 to Precursor Gases in an Ammonia Abundant Area In North Carolina. Aerosol Air Qual. Res. 19: 1126-1138. https://doi.org/10.4209/aaqr.2018.10.0384

HIGHLIGHTS

Keywords: Ammonia; Inorganic PM2.5; Thermodynamic equilibrium modeling; ISORROPIA II; Animal feeding operations.

INTRODUCTION

Particulate matter (PM) with an aerodynamic equivalent diameter less than or equal to 2.5 µm (i.e., PM_2.5) is one of the six criteria air pollutants regulated under National Ambient Air Quality Standards (NAAQS) (U.S. EPA, 2015a). Due to its adverse impacts on environment and human health, PM_2.5 has been an intensive research topic since 1987 (Donham et al., 1995; Heederik et al., 2007; Pope et al., 2009; Pui et al., 2014). Various chemical components contribute to PM_2.5 in different proportions, and the major chemical components of PM_2.5 include ammonium (NH₄⁺), sulfate (SO₄^2–), nitrate (NO₃^–), organic carbon (OC), elemental carbon (EC), elements and other unknown components (Bell et al., 2007). The secondary inorganic PM_2.5 (iPM_2.5) is formed through chemical reactions between basic and acidic gases (e.g., ammonia [NH₃], nitric acid [HNO₃] and sulfuric acid [H₂SO₄]) (Hinds, 1998; Seinfeld and Pandis, 2006). The iPM_2.5 mainly consists of NH₄⁺ salts including ammonium nitrate (NH₄NO₃), ammonium sulfate ((NH₄)₂SO₄), ammonium bisulfate (NH₄HSO₄) and ammonium chloride (NH₄Cl) (Tanner et al., 1979; Tolocka et al., 2001; Walker et al., 2004; Li et al., 2012, 2014a).

Ammonia is the major alkaline gas that may react with acidic gases to form iPM_2.5 in ambient air, and this process is also called gas-particle partitioning of NH₃-NH₄⁺. The neutralization degree of NH₃ can be characterized by gas ratio (GR), which is in Eq. (1) (Ansari and Pandis, 1998): 

where TA is total available ammonia, including NH₃ and NH₄⁺ (in the unit of µmole m^–3). TS is total sulfate including SO₄^2–, bisulfate (HSO₄^–) and H₂SO₄ (in the unit of µmole m^–3). TN is total available nitrate, including NO₃^– and HNO₃ (in the unit of µmole m^–3). When GR > 1, the total available ammonia exceeds the amount needed to fully neutralize both total sulfate and total available nitrate, and this is defined as NH₃-rich condition; under this condition, the changes of total available ammonia may not be a key factor to affect the concentration of iPM_2.5. When 0 < GR < 1, the amount of total available ammonia is adequate to fully neutralize total sulfate but not total available nitrate, and only NH₄NO₃ formation is limited by NH₃; under this condition, the decrease of NH₃ may lead to corresponding decrease of NH₄NO₃. When GR < 0, the amount of total available ammonia is not enough to fully neutralize either total sulfate or total available nitrate, and both (NH₄)₂SO₄ and NH₄HSO₄ are limited by NH₃ (Wang-Li, 2015).

Based on USEPA’s National Emission Inventory (NEI), animal feeding operation (AFO) contributed to more than 70% of the total NH₃emissions in the United States (U.S.) (U.S. EPA, 2015b). While the AFO NH₃ emissions present a great potential to the formation of secondary iPM_2.5 in some regions where a significant amount of AFO facilities are located, the dynamic contribution of such emissions to the ambient iPM_2.5is not well understood spatially and temporally. To gain holistic understanding of atmospheric PM_2.5, it is essential to understand the dynamic responses of atmospheric iPM_2.5 to the AFO NH₃ emissions under different atmospheric conditions and geographical locations (Wang-Li, 2015).

To study the thermodynamic equilibrium processes of iPM_2.5 and its precursor gases, thermodynamic equilibrium model such as ISORROPIA was developed to simulate the gas-particle partitioning of NH₃-NH₄⁺ (Nenes et al., 1998, 1999). In ISORROPIA, the phase changes (e.g., gas, liquid, and solid) and interaction of different chemical species (NH₄⁺, NO₃^–, SO₄^2–, Cl^–, and Na⁺) as well as the impacts of temperature (T) and relative humidity (RH) on partitioning of NH₃-NH₄⁺ are simulated (Fountoukis and Nenes, 2007; Fountoukis et al., 2009). In an application of ISORROPIA to assess the formation of iPM_2.5 at an agricultural site located in eastern North Carolina (NC), Walker et al. (2006) examined the change of iPM_2.5 concentration in response to the 50% reduction of total NH₃ (gas + aerosol), total HNO₃ (gas + aerosol) and total H₂SO₄(aerosol) in winter and summer of 1999–2000. It was discovered that the 50% reduction of total NH₃ had the least impact on iPM_2.5 concentration as compared to the 50% reductions in total HNO₃ and H₂SO₄. This research suggested that at the agricultural sites with elevated atmospheric NH₃concentration, the iPM_2.5 is more sensitive to acidic gases rather than NH₃. In another iPM_2.5 study, Goetz et al. (2012) investigated the effect of NH₃ emissions from swine production facilities on iPM_2.5 concentration at three locations in eastern NC. The iPM_2.5 chemical composition data obtained from air quality monitoring stations of NC Division of Air Quality and gaseous pollutant concentrations measured or cited from literature were used to conduct iPM_2.5 simulation using ISORROPIA under three T and RH conditions. The results indicated that the simulation results of iPM_2.5 by ISORROPIA agreed well with the observation. Furthermore, this research revealed that high precursor gas concentrations, low T, and high RH led to higher chance for secondary iPM_2.5 formation.

In order to gain advanced understanding of the formation of iPM_2.5 as impacted by AFO NH₃ emissions, Li et al. (2014b) investigated the formation of secondary iPM_2.5 in response to total NH₃ inside a production house and in the vicinity of an egg farm in the southeastern U.S. Onsite measurements of NH₃ concentrations and PM_2.5 chemical components at in-house and ambient locations were used to conduct ISORROPIA IIsimulation to predict gas-particle partitioning of NH₃-NH₄⁺. Li et al. (2014b) confirmed that the most significant reduction of iPM_2.5 can be caused by the reduction of total H₂SO₄ instead of NH₃ and this is because the formation of iPM_2.5 is limited by the availability of acidic gases when NH₃ exceeds the amount needed to fully neutralize acid gases.

In addition to the local-scale simulation, ISORROPIA II has been embedded into chemical transport model (CTM) to study more complicated atmospheric gas-particle partitioning on regional and/or global scales. Paulot and Jacob (2014) estimated the contribution of agricultural NH₃emissions to the ambient PM_2.5 in the U.S. using GEOS-Chem global CTM coupled with ISORROPIA II. This modeling practice reported that there is a 0.36 µg m^–3 increase of ambient PM_2.5 concentration caused by NH₃ emissions associated with the U.S. food export activities.

As the research gap exists in quantifying the formation of secondary iPM_2.5 experimentally and/or through model simulation in AFO region, the objectives of this research were as follows: (1) investigation of the neutralization degree of NH₃; (2) examination of seasonal variation of PM_2.5mass closure; and (3) study of the responses of secondary iPM_2.5 to the changes of total NH₃, total HNO₃, and total H₂SO₄ in an animal production area of NC under different meteorological conditions. 

METHODS

Since NH₄⁺, SO₄^2–, and NO₃^– account for the majority of atmospheric iPM_2.5 (Bell et al., 2007), this research focuses on the responses of NH₄⁺, SO₄^2–, and NO₃^– to the changes of total NH₃, total HNO₃ and total H₂SO₄ at a site where atmospheric NH₃ is abundant due to NH₃emissions from AFO.

 
Research Site Selection and Data Collection

To achieve the research objective, the research site has to be in an area where a significant amount of AFO facilities are present. In addition, simultaneous measurements of gas-phase pollutants (e.g., NH₃ and HNO₃) and particle-phase ions (e.g., NH₄⁺, SO₄^2–, and NO₃^–) are required input data to conduct ISORROPIA II simulation. Thus, the availability of the required measurement data is another criterion for the site selection. Based upon these two site selection criteria, a monitoring site (35.23146 N, 77.568792 W) in Lenoir County of NC was selected for this research. Fig. 1 shows this site marked on the AFO distribution map developed by Zhao and Wang-Li (2015). The majority of the swine and poultry farms are in the south-central area of NC where the research site was also located.

Fig. 1. Spatial distribution of poultry and swine farms across the entire NC (the black star: iPM_2.5 and NH₃ monitoring site; the red triangle: swine farms; the green solid circle: poultry farms).

At the selected site, 24-hr average measurements of PM_2.5 chemical components (e.g., NH₄⁺, SO₄^2–, NO₃^–) and hourly measurements of NH₃, reactive oxides of nitrogen (NO_y), T and RH were taken in 2002–2004 and reported at the EPA’s Air Quality System (AQS) website (https://aqs.epa.gov/api). The 24-hr PM_2.5 chemical composition data were measured every sixth day while the hourly NO_y concentrations, T, and RH were measured continuously for three years. To keep these data on the same time scale, hourly data on specific days when the PM_2.5 chemical composition data were taken were converted into 24-hr average data to match daily measurements of PM_2.5 chemical components. While the HNO₃gas concentration is a required input for ISORROPIA II model, the HNO₃ gas measurements were not available. Thus, the NO_y concentration may be used to indirectly determine the HNO₃ gas concentration. In general, NO_y includes nitric oxide (NO), nitrous oxide (N₂O), HNO₃, peroxyacetylnitrate (PAN), nitrous acid (HONO), nitrate radical (NO₃), dinitrogen pentoxide (N₂O₅), organic nitrates etc. In addition, NO, NO₂, HNO₃, and PAN are the major components of NO_y (Finlayson-Pitts and Pitts, 2000; Seinfeld and Pandis, 2006). In atmosphere, HNO₃ is mainly formed through the oxidation of NO₂ following Eq. (2) in the daytime (Pun and Seigneur, 2001; Jacobson, 2005):

where reaction (2) mainly happens in the daytime in which the photochemical reaction can provide adequate hydroxyl radical (OH).

To estimate the HNO₃ concentration through NO_y concentration, NO₂ concentration should be first determined. Luke et al. (2010) discovered that the fractional contribution of NO₂ in NO_y varied with the time of day. While no NO₂ measurement data were available in Lenoir County, there were simultaneous measurements of hourly NO_y and NO₂ concentrations in 2014 in Wake County, which is another county in southeastern NC (Fig. 1). The median NO₂/NO_y ratios for each of 24 hours in Wake County were used to estimate NO₂/NO_y ratios in Lenoir County. The fractional contributions of NO₂ to the total NO_y are set at different values in each of 24 hours based on the measurement data in Wake County, and this trend is consistent with the measurements performed by Luke et al. (2010).

In estimation of the HNO₃ concentrations, it was assumed that different percentages (0%, 0.5%, 1%, 5%, 10%, 25%, 50%, and 100%) of NO₂might be converted to HNO₃ through Eq. (1). As for H₂SO₄, since it has very low vapor pressure, nearly all of the H₂SO₄ partition into the particlephase (Seinfeld and Pandis, 2006; Makar et al., 2009). 

Mass Closure Profile

The contribution of inorganic PM_2.5 to total PM_2.5 mass contribution was analyzed using mass closure profile of PM_2.5 chemical components. Major ions including iPM_2.5 anions/cations, up to 47 crustal elements, EC and OC as well as PM_2.5 mass concentration were simultaneously measured by MetOne SASS sampler. To develop a mass closure profile, PM_2.5 mass concentration measured by MetOne SASS Teflon was used to analyze the mass closure of PM_2.5 chemical components. When the sum of all the chemical component concentrations were greater than the measured PM_2.5mass concentration, those data were excluded from mass closure analysis.

According to Dillner et al. (2012) and Weber et al. (2003), organic carbon matter (OCM) can be calculated using Eq. (3) to account for elements other than carbon:

where OCM = organic carbon matter, OC_m = organic carbon measurement, and OC_b = field blank.

The contributions of various chemical components to PM_2.5 were calculated using Eq. (4):

where P_i = percentage of the chemical component i in PM_2.5 mass concentration, C_i = concentration of chemical component i, and C_m = measured PM_2.5 mass concentration from MetOne SASS Teflon filter.

ISORROPIA II Settings

For this study, all the iPM_2.5 chemical components are assumed to be internally mixed, and the thermodynamic equilibrium is also assumed to be established very rapidly. The ISORROPIA II allows user to specify the problem type (forward or reverse) and thermodynamic state (stable or metastable); in this study, ISORROPIA II is set as forward + stable. As NH₃-NH₄⁺-SO₄^2–-HNO₃-NO₃^– system is determined as the research focus, all the other species concentrations, including total sodium (Na⁺), total hydrochloric acid (HCl), total calcium (Ca²⁺), total potassium (K⁺), and total magnesium (Mg²⁺) are set as 0.

The examination of responses of iPM_2.5 to the total NH₃, total HNO₃, and total H₂SO₄ was based on the minimum, median and maximum concentrations of the input parameters measured in 2002–2004 using ISORROPIA II (Table 1).

Five T and RH conditions were used to test the responses of secondary iPM_2.5 to the changes of total NH₃, total H₂SO₄, and total HNO₃. These combinations include maximum T + minimum RH, minimum T + maximum RH, median T + median RH, maximum T + maximum RH, minimum T + median RH, of which, maximum T + maximum RH and minimum T + median RH represent the summer and winter conditions, respectively.

Statistical Analysis

All the data analyses, analysis of variance (ANOVA) test, paired t-test, and Tukey’s Honest Significant Difference (HSD) test were performed using R.

RESULTS AND DISCUSSION

 
Statistical Characterization of the Field Measurements

Table 2 lists the summary of three-year measurements. Median NH₃ gas concentrations in winter and summer were 0.74 and 3.17 µg m^–3, respectively, and median NH₄⁺ concentrations in winter and summer were 1.15 and 1.64 µg m^–3, respectively. Research performed by Walker et al.(2006) at a site in the neighboring county, Sampson County (Fig. 1), reported the median NH₃ gas concentrations of 2.60 and 6.18 µg m^–3 in winter and summer, respectively, and median NH₄⁺ aerosol concentrations of 1.90 and 1.69 µg m^–3 in winter and summer, respectively. Comparatively, the NH₃ gas and NH₄⁺ aerosol concentrations at the site in Sampson County were greater than the concentrations measured in this research. This can be justified by the difference of AFO distribution and density in these two counties shown in Fig. 1. The AFO density is much higher in Sampson County than in Lenoir County. In addition, minimum, median and maximum total H₂SO₄ concentrations of this site (0.59, 3.53 and 14.60) are comparable with those (0.58, 3.43 and 14.30) in Sampson County reported by Goetz et al. (2012).

Responses of Total iPM_2.5 to Different Conversion Percentages of NO₂ to HNO₃

As it has been stated, 0%, 0.5%, 1%, 5%, 10%, 25%, 50% and 100% of NO₂ were assumed to be converted through Eq. (1) to HNO₃; in each case scenario, gas-phase HNO₃ and total HNO₃ concentrations were calculated based upon the different conversion ratios. The calculated total HNO₃ along with other model inputs (median values) were then used to simulate the formation of iPM_2.5 using ISORROPIA II. The predicted iPM_2.5 was compared with the measured iPM_2.5 concentrations under different NO₂-to-HNO₃ conversion percentages (Fig. 2).

Fig. 2. The ratio of predicted iPM_2.5 concentration over measured iPM_2.5 concentration against conversion percentage of NO₂ to HNO₃.

Fig. 2 shows that as the percentage of NO₂ to HNO₃ conversion was increased from 0% to 100%, the ratio of predicted iPM_2.5 over measured iPM_2.5 was increased from 0.95 to 1.80, correspondingly. This can be explained that in Lenoir County, ambient NH₃ is abundant (GR > 1); while NH₃preferentially reacted with H₂SO₄ to form (NH₄)₂SO₄ and ammonium bisulfate (NH₄HSO₄), excessive NH₃ could react with HNO₃ to form NH₄NO₃ (Tanner et al., 1981; Yu et al., 2005). When NH₃ concentration in the atmosphere exceeds the amount needed to fully neutralize both total HNO₃ and total H₂SO₄, more HNO₃ can favor the formation of NH₄NO₃, then increase the concentration of iPM_2.5.

When the conversion percentage of NO₂ to HNO₃ was set at 3.63%, the ratio of predicted iPM_2.5 over measured iPM_2.5 was close to 1, which may be indicative of a reasonable estimate of the conversion ratio. Moreover, Goetz et al. (2012) reported the minimum, median, and maximumconcentrations of total HNO₃ were 0.26, 1.07, and 5.24 µg m^–3, respectively, in eastern NC in 2001–2004. The estimated minimum, median, and maximum total HNO₃ in this research were 0.24, 1.17, and 4.24 µg m^–3, respectively in 2002–2004. Thus, the estimation of the HNO₃concentration generally agreed with the previous research.

 
Neutralization Degree of NH₃: GR

The neutralization degree of NH₃ was characterized by GR. The ranges of GR in four seasons are listed in Table 3. This area was classified as the NH3-rich area since the GR values were greater than 1 in most of the time. Specifically, in summer, 29 out of 30 data points were characterized as GR ≥ 1; even in winter, 23 out of 28 data points were GR > 1. In addition, the median GRs in four seasons were all larger than 1 with the highest GR-12.69 in summer. This observation indicated that the research site in Lenoir County was dominated by NH3-rich condition such that there was a great potential for neutralizing acidic gases with excessive NH3 in atmosphere.

Seasonal Variations of PM_2.5 Chemical Speciation

In Lenoir County, PM_2.5 mass concentration was measured in 2002–2004 using two methods, Federal Reference Method (FRM) and MetOne SASS chemical speciation sampler; both datasets came from gravimetric analysis. To check the data quality, the comparison between these two datasets were performed and the results are listed in Table 4.

As indicated by Table 4, the PM_2.5 mass concentration measurements from MetOne SASS sampler are significantly greater than the measurements from FRM method. The seasonal mass closure profile is shown in Fig. 3.

 Fig. 3. PM_2.5 chemical speciation mass fractions in Lenoir County in four seasons.

As can be seen from Fig. 3, iPM_2.5 accounted for a large proportion of the total PM_2.5, and this fraction was highest in spring (51%) and lowest in summer (40%). For iPM_2.5 alone, three chemical species, SO₄^2–, NO₃^– and NH₄⁺, also accounted for different fractions in four seasons. The results of Tukey’s test were shown in Table 5.

As indicated by Table 5, the PM_2.5 concentration in summer was significantly higher than in winter; this can be explained by the seasonal variations of the major chemical components. The OCM and SO₄^2– together accounted for 41.4–54.2% of PM_2.5 mass concentration in four seasons; both species’ concentrations were higher in summer than in winter. In addition, SO₄^2– was also the major component of iPM_2.5 and it accounted for 47.9–67.0% in the whole iPM_2.5. The NO₃^– concentration was significantly higher in winter than in summer. This may be due tosemi-volatile characteristic of NH₄NO₃, according to Olszyna et al. (2005), SO₄^2– salts and NO₃^– salts own different levels of thermal stability, the NH₄NO₃ is not thermally stable so that it may decompose to gaseous HNO₃ and NH₃ when environmental conditions (high T and low RH) do notfavor the particle phase. On the other hand, SO₄^2– salts are thermally stable compared with NO₃^– salts. In summer, the ambient conditions do not favor the formation of NH₄NO₃. However, the (NH₄)₂SO₄ can still form in summer; thus, SO₄^2– was the year-round major component of iPM_2.5and this is consistent with the finding from Holt et al. (2015). However, as for NH₄⁺, there is no significant difference between four seasons. The NH₃-rich conditions dominated in four seasons; NH₃ exceeded the amount needed to fully neutralize both HNO₃ and H₂SO₄. The NH₄⁺ in the form of (NH₄)₂SO₄ was higher in summer than in winter and NH₄⁺ in the form of NH₄NO₃ was higher in winter than in summer; thus, both seasonal variations offset the change of the NH₄⁺ concentration in winter and summer.

Response of Secondary iPM_2.5 to Total NH₃

In this analysis, median total H₂SO₄ concentration and median total HNO₃ concentration were used under five T and RH case scenarios (Table 1) to simulate the iPM_2.5 formation with total NH₃ concentration ranging from 0.54 to 37.94 µg m^–3. The responses of iPM_2.5 to the change of total NH₃ are shown in Fig. 4. Under different conditions, the responses of iPM_2.5 to total NH₃ were different. The concentration of iPM_2.5responded to the total NH₃ nonlinearly.

Fig. 4. Responses of different iPM_2.5 chemical components to total NH₃ concentration under five T and RH conditions.

Under maximum T + minimum RH (305.95 K + 39%), the change of iPM_2.5 concentration is caused by the change of SO₄^2–, HSO₄^–, and NH₄⁺. When the total NH₃ concentration is changed from 0.54 µg m^–3 to 37.94 µg m^–3, the GR is also changed from –2.17 to 116.29; during this process, the response of iPM_2.5 to total NH₃ is changed from sensitive to insensitive. When GR ≤ 0, increasing total NH₃ concentration can increase the concentration of particle-phase SO₄^2– and NH₄⁺ while at the same time decrease the concentration of HSO₄^–. This is because that when NH₃ gas is insufficient for fully neutralizing total H₂SO₄, both (NH₄)₂SO₄ and NH₄HSO₄ exist in the system and adding more NH₃ can react with the available H₂SO₄ and at the same time convert NH₄HSO₄ to (NH₄)₂SO₄. When GR ˃ 0, NH₃ is adequate to fully neutralize total H₂SO₄, and there is excessive NH₃ in the system. In the whole process, NO₃^– aerosol is close to 0, and HNO₃ concentration remains the constant. This is because that high T and low RH do not favor the formation of semi-volatile compound NH₄NO₃; even when there is excessive NH₃ existing in the system, HNO₃ will not react with NH₃ to be converted to NO₃^–. Thus, in the end, increasing NH₃ will not change the concentration of any component in the system, and the response of iPM_2.5to total NH₃ is insensitive.

Under minimum T + maximum RH (271.95 K + 95%), the change of iPM_2.5 concentration is caused by the changes of SO₄^2–, HSO₄^–, NH₄⁺and NO₃^–. When GR ≤ 1.63, increasing total NH₃ concentration can increase the concentration of SO₄^2–, NH₄⁺ and NO₃^– while at the same time decrease the concentration of HSO₄^–. This is owing to the fact that low T and high RH can favor the formation of NH₄NO₃; when GR ≤ 0, adding more NH₃ can react with H₂SO₄ and at the same time convert NH₄HSO₄ to (NH₄)₂SO₄. When 0 < GR ≤ 1.63, the excessive NH₃ can react with HNO₃ to form NH₄NO₃ until all the HNO₃ is depleted. After GR ˃ 1.63, adding more NH₃ will not change the concentration of iPM_2.5significantly; this is due to lack of available acidic gases, which have been fully neutralized by NH₃ gas.

Under median T + median RH (292.05 K + 77%), changes in the iPM_2.5 chemical composition are similar to those under the condition of minimum T + maximum RH (271.95 K + 95%); the difference is the peak iPM_2.5 concentration value and the GR at which the system reaches the peak iPM_2.5concentration. Fig. 4 shows that the difference in peak iPM_2.5 concentration is caused by the NO₃^– concentration. Lower T and higher RH can favor the formation of NH₄NO₃; thus, the peak iPM_2.5 under median T + median RH is about 6.13 µg m^–3, which is less than the peak iPM_2.5 concentration(6.24 µg m^–3) under minimum T + maximum RH (271.95 K + 95%) and greater than the peak iPM_2.5 concentration (4.76 µg m^–3) under maximum T + minimum RH.

According to Table 2, at summertime, the ambient meteorology was characterized by high T + high RH; at wintertime, the ambient meteorology was characterized by low T + median RH. In order to capture the seasonal variation of iPM_2.5, these two combinations of T and RH are added to the analysis. As shown in Fig. 4, under maximum T + maximum RH (305.95 K + 95%), the change of iPM_2.5 concentration is similar to the condition of median T + median RH. The difference is the peak iPM_2.5 concentration; the peak iPM_2.5 concentration under maximum T + maximum RH is close to 6.00 µg m^–3, which is lower than the peak iPM_2.5 concentration, 6.13 µg m^–3, under median T + median RH. This is owing to the fact that higher T does not favor the formation of NH₄NO₃ and this case scenario can represent ambient condition in the summertime of this region. Under minimum T + median RH (271.95 K + 77%), the change of iPM_2.5 concentration is caused by the change of SO₄^2–, NH₄⁺, NO₃^–, and HSO₄^–, and this trend is similar to the condition of minimum T + maximum RH; the difference is the threshold value of total NH₃ concentration that indicates the transition from sensitive to the insensitive region.

Upon the above analysis, the threshold values of total NH₃ concentration at which transition from sensitive to the insensitive happens are identified for all the case scenarios (Table 6). Greater than threshold values, the iPM_2.5 becomes insensitive to the change of total NH₃ concentration.

Table 6 shows that under condition of median T + median RH, the threshold values of total NH₃ concentration and GR are the largest. Under condition of minimum T + maximum RH, the threshold values of total NH₃ concentration and GR are the smallest. According to Table 3, the ambient condition is dominated by NH₃-rich condition; thus, under most of the cases, the response of iPM_2.5 to total NH₃ is insensitive. Thus, reducing total NH₃ concentration will not lead to significant decrease of iPM_2.5 concentration.

Response of Secondary iPM_2.5 to Total H₂SO₄

In this analysis, median total NH₃ concentration and median total HNO₃ concentration were used under five T and RH case scenarios (Table 1) to simulate the iPM_2.5 formation with total H₂SO₄ concentration changing from 0.59 to 14.60 µg m^–3. The results are shown in Fig. 5.

Fig. 5. Response of different iPM_2.5 chemical components to total H₂SO₄ concentration under five T and RH conditions.

Fig. 5 shows that the response of iPM_2.5 to total H₂SO₄ is linear. Increasing total H₂SO₄ concentration can lead to the increase of iPM_2.5concentration. Under maximum T + minimum RH (305.95 K + 39%), the change of iPM_2.5 concentration is caused by the change of SO₄^2–, HSO₄^–, and NH₄⁺. As the formation of NH₄NO₃ is not favored under this condition, the concentration of NO₃^– is close to 0, and HNO₃ gas concentration remains the constant. While under the conditions of minimum T + maximum RH (271.95 K + 95%), median T + median RH (292.05 K + 77%), maximum T + maximum RH (305.95 K + 95%), and minimum T + median RH (271.95 K + 77%), the change of iPM_2.5 concentration is caused by the change of SO₄^2–, HSO₄^–, NH₄⁺ and NO₃^–. With the increase of total H₂SO₄, NH₃ gas concentration is decreased due to the reaction with H₂SO₄, and during this process, H₂SO₄ also competes with HNO₃ to react with NH₃ to form (NH₄)₂SO₄ and NH₄HSO₄. When total H₂SO₄concentration is greater than 9.0 µg m^–3, HSO₄^– begins to increase; this is because that if total NH₃ is not adequate to fully neutralize H₂SO₄, adding more H₂SO₄ can convert (NH₄)₂SO₄ to NH₄HSO₄.

In summary, the response of secondary iPM_2.5 to total H₂SO₄ concentration is linear. Adding more H₂SO₄ can increase iPM_2.5 concentration significantly; this is because of the excessive NH₃ existing in the system. When all the NH₃ gas is neutralized by H₂SO₄, SO₄^2– is converted to HSO₄^–; thus concentration of iPM_2.5 is increased. Response of secondary iPM_2.5 to total H₂SO₄ is not determined by NH₃-rich or NH₃-poor conditions in this research. This is because that in NH₃-NH₄⁺-SO₄^2–-HNO₃-NO₃^– system, NH₃ gas preferentially reacts with H₂SO₄, and if total NH₃ is in excess of fully neutralizing H₂SO₄, excessive NH₃ can react with HNO₃. In this research, when there is excessive NH₃ existing in the system, adding more H₂SO₄ can directly increase the concentration of (NH₄)₂SO₄. After all the NH₃ is neutralized, (NH₄)₂SO₄ is then converted to NH₄HSO₄. In total, the iPM_2.5 concentration keeps increasing but with lower increasing rate (there are two stages in the Fig. 5, higher increase rate in the first stage and lower increase rate in the second stage).

 
Response of Secondary iPM_2.5 to Total HNO₃

In this analysis, median total NH₃ concentration and median total H₂SO₄ concentration were used under the five T and RH case scenarios (Table 1) to simulate the iPM_2.5 formation with total HNO₃ concentration changing from 0.24 to 4.24 µg m^–3. The results are shown in Fig. 6.

Fig. 6. Response of different iPM_2.5 chemical components to total HNO₃ concentration under five T and RH conditions.

Under maximum T + minimum RH (305.95 K + 39%), iPM_2.5 is insensitive to the change of total HNO₃. This is owing to the fact hat high T and low RH do not favor the formation of NH₄NO₃; even if there is excessive NH₃ gas in the system, it will not react with the added HNO₃ to form NH₄NO₃. Thus, all the other chemical components remain the constant and NO₃^– concentration is close to 0.

Under the conditions of minimum T + maximum RH (271.95 K + 95%), median T + median RH (292.05 K + 77%), maximum T + maximum RH (305.95 K + 95%), and minimum T + median RH (271.95 K + 77%), the changes of iPM_2.5 concentration is caused by the change of NH₄⁺ and NO₃^–. This is because that under these four conditions, the formation of NH₄NO₃ is not limited by the ambient meteorology; thus, adding more HNO₃ can react with the available NH₃ to form NH₄NO₃, which leads to the increase of NO₃^– and NH₄⁺ simultaneously. The SO₄^2–concentration remains the constant; this is due to the full neutralization of H₂SO₄ by NH₃.

In summary, the response of secondary iPM_2.5 to total HNO₃ is linear; adding more HNO₃ to the system can increase the iPM_2.5 concentration linearly due to the formation of NH₄NO₃. Under different ambient conditions, the increase rate of iPM_2.5 is different; lower T and higher RH favor the formation of NH₄NO₃ and lead to higher iPM_2.5 concentration.

Possible Mitigation Strategies for Ambient iPM_2.5 Reduction

Analysis of the iPM_2.5 responses to the precursor gases may lead to the development of the iPM_2.5 mitigation strategy. Under different ambient conditions, the responses of iPM_2.5 to total NH₃, total HNO₃, and total H₂SO₄ are different. In general, iPM_2.5 is more sensitive to the change of total H₂SO₄ concentration. The change of total NH₃ and total HNO₃ may also lead to the change of iPM_2.5 but may be limited by the neutralization degree of NH₃ and ambient conditions. In NH₃-rich condition, the change of total NH₃ concentration has the least impact on the iPM_2.5concentration. Reducing total H₂SO₄ is more effective to decrease iPM_2.5 as compared to the reduction of total NH₃ and total HNO₃.

It needs to be noted that the two acidic gases used in ISORROPIA II, H₂SO₄ and HNO₃, are not directly emitted, but are transformed from the oxidation of nitrogen oxides (NO_x) and sulfuric dioxide (SO₂) in the atmosphere. Reducing the two acidic gases would require the reduction of their associated primary pollutants, NO_x and SO₂. Thus, in development of the total H₂SO₄ and HNO₄ concentration reduction strategies, strategies for SO₂ and nitrogen oxide (NO_x) emissions reduction should be taken.

 
CONCLUSIONS

In this research, the responses of iPM_2.5 to changes in the total NH₃, the total HNO₃, and the total H₂SO₄ were simulated by ISORROPIA II based upon three-year measurements of the chemical components in iPM_2.5 and gaseous pollutants as well as meteorological conditions. It was found that iPM_2.5 responds to precursor gases differently under different T and RH conditions. In general, the response of iPM_2.5 to the total NH₃is nonlinear, whereas its response to the total H₂SO₄ and the total HNO₃ is linear. In NH₃-rich regions, iPM_2.5 is insensitive to the total NH₃ but highly sensitive to the total H₂SO₄ and/or the total HNO₃. This research determined that the threshold values for the total NH₃ concentrations, below which the iPM_2.5 was insensitive to changes in the total NH₃, varied under different ambient conditions. Although dry and wet deposition and the nonlinear conversion of NO_x and SO₂ to HNO₃ and H₂SO₄ were not simulated with the thermodynamic model, inferences can still be drawn from our results about the dynamic changes in iPM_2.5 in response to changes in precursor gases, offering insight into controlling and regulating iPM_2.5 in NH₃-rich regions. These findings also illuminate the impact of AFO NH₃ emissions on the formation of secondary iPM_2.5, which may help to develop strategies for reducing ambient PM_2.5. Future studies may examine the quantitative contribution of AFO NH₃ emissions to forming iPM_2.5 using the Chemical Transport Model, thus further exploring the dynamic contribution of these emissions to the gas-particle partitioning of NH₃-NH₄⁺.

 
ACKNOWLEDGMENTS

The AFO farm distribution map was provided by Yijia Zhao. Help from Yijia and individuals from EPA and NCDENR is greatly appreciated. This project was supported by the NSF Award No. CBET-1804720.