Bin Cheng, Lingjuan Wang-Li 


Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC 27695, USA


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

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

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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

  • The Southeastern North Carolina was dominated by NH3-rich condition.
  • The AFOs NH3 emissions contributed to NH3-rich condition.
  • Responses of secondary iPM2.5 to total NH3, total HNO3 and total H2SO4 varied.
  • Reduction of total H2SO4 is more effective to reduce secondary iPM2.5.
 

ABSTRACT


Secondary inorganic fine particulate matter (iPM2.5) constitutes a significant amount of the atmospheric PM2.5. The formation of secondary iPM2.5 is characterized by thermodynamic equilibrium gas-particle partitioning of gaseous ammonia (NH3) and aerosol ammonium (NH4+). To develop effective strategies for controlling atmospheric PM2.5, it is essential to understand the responses of secondary iPM2.5 to different precursor gases. In southeastern North Carolina, the amount of NH3 is in excess to fully neutralize acidic gases (i.e., NH3-rich conditions). NH3-rich conditions are mainly attributed to the significant NH3 emissions in the region, especially from the large amounts of animal feeding operation (AFO). To gain a better understanding of the impact of NH3 on the formation of secondary iPM2.5 in this area, the responses of iPM2.5 to precursor gases under different ambient conditions were investigated based upon three-year monitoring data of the chemical components in iPM2.5, gaseous pollutants, and meteorological conditions. The gas ratio (GR) was used to assess the degree of neutralization via NH3, and ISORROPIA II model simulation was used to examine the responses of iPM2.5 to changes in the total NH3, the total sulfuric acid (H2SO4), and the total nitric acid (HNO3). It was discovered that under different ambient temperature and humidity conditions, the responses of iPM2.5 to precursor gases vary. In general, iPM2.5 responds nonlinearly to the total NH3 but linearly to the total H2SO4 and the total HNO3. In NH3-rich regions, iPM2.5 is not sensitive to changes in the total NH3, but it is very sensitive to changes in the total H2SO4 and/or the total HNO3. Reducing the total H2SO4, as opposed to the total HNO3 or the total NH3, leads to a significant reduction in iPM2.5 and is thus a more effective strategy for decreasing the concentration of iPM2.5. This research provides insight into controlling and regulating PM2.5 in NH3-rich regions.


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., PM2.5) 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, PM2.5 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 PM2.5 in different proportions, and the major chemical components of PM2.5 include ammonium (NH4+), sulfate (SO42–), nitrate (NO3), organic carbon (OC), elemental carbon (EC), elements and other unknown components (Bell et al., 2007). The secondary inorganic PM2.5 (iPM2.5) 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 iPM2.5 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 iPM2.5 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 SO42–, 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 iPM2.5. 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 iPM2.5 in some regions where a significant amount of AFO facilities are located, the dynamic contribution of such emissions to the ambient iPM2.5is not well understood spatially and temporally. To gain holistic understanding of atmospheric PM2.5, it is essential to understand the dynamic responses of atmospheric iPM2.5 to the AFO NH3 emissions under different atmospheric conditions and geographical locations (Wang-Li, 2015).

To study the thermodynamic equilibrium processes of iPM2.5 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, SO42, 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 iPM2.5 at an agricultural site located in eastern North Carolina (NC), Walker et al. (2006) examined the change of iPM2.5 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 iPM2.5 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 iPM2.5 is more sensitive to acidic gases rather than NH3. In another iPM2.5 study, Goetz et al. (2012) investigated the effect of NH3 emissions from swine production facilities on iPM2.5 concentration at three locations in eastern NC. The iPM2.5 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 iPM2.5 simulation using ISORROPIA under three T and RH conditions. The results indicated that the simulation results of iPM2.5 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 iPM2.5 formation.

In order to gain advanced understanding of the formation of iPM2.5 as impacted by AFO NH3 emissions, Li et al. (2014b) investigated the formation of secondary iPM2.5 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 PM2.5 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 iPM2.5 can be caused by the reduction of total H2SO4 instead of NH3 and this is because the formation of iPM2.5 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 PM2.5 in the U.S. using GEOS-Chem global CTM coupled with ISORROPIA II. This modeling practice reported that there is a 0.36 µg m3 increase of ambient PM2.5 concentration caused by NH3 emissions associated with the U.S. food export activities.

As the research gap exists in quantifying the formation of secondary iPM2.5 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 PM2.5mass closure; and (3) study of the responses of secondary iPM2.5 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+, SO42, and NO3 account for the majority of atmospheric iPM2.5 (Bell et al., 2007), this research focuses on the responses of NH4+, SO42, 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+, SO42, 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: iPM2.5 and NH3 monitoring site; the red triangle: swine farms; the green solid circle: poultry farms).Fig. 1.
 Spatial distribution of poultry and swine farms across the entire NC (the black star: iPM2.5 and NH3 monitoring site; the red triangle: swine farms; the green solid circle: poultry farms).

At the selected site, 24-hr average measurements of PM2.5 chemical components (e.g., NH4+, SO42–, NO3) and hourly measurements of NH3, reactive oxides of nitrogen (NOy), 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 PM2.5 chemical composition data were measured every sixth day while the hourly NOy 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 PM2.5 chemical composition data were taken were converted into 24-hr average data to match daily measurements of PM2.5 chemical components. While the HNO3gas concentration is a required input for ISORROPIA II model, the HNO3 gas measurements were not available. Thus, the NOy concentration may be used to indirectly determine the HNO3 gas concentration. In general, NOy 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 NOy (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 NOy concentration, NO2 concentration should be first determined. Luke et al. (2010) discovered that the fractional contribution of NO2 in NOy varied with the time of day. While no NO2 measurement data were available in Lenoir County, there were simultaneous measurements of hourly NOy and NO2 concentrations in 2014 in Wake County, which is another county in southeastern NC (Fig. 1). The median NO2/NOy ratios for each of 24 hours in Wake County were used to estimate NO2/NOy ratios in Lenoir County. The fractional contributions of NO2 to the total NOy 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 PM2.5 to total PM2.5 mass contribution was analyzed using mass closure profile of PM2.5 chemical components. Major ions including iPM2.5 anions/cations, up to 47 crustal elements, EC and OC as well as PM2.5 mass concentration were simultaneously measured by MetOne SASS sampler. To develop a mass closure profile, PM2.5 mass concentration measured by MetOne SASS Teflon was used to analyze the mass closure of PM2.5 chemical components. When the sum of all the chemical component concentrations were greater than the measured PM2.5mass 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, OCm = organic carbon measurement, and OCb = field blank.

The contributions of various chemical components to PM2.5 were calculated using Eq. (4):

where Pi = percentage of the chemical component i in PM2.5 mass concentration, Ci = concentration of chemical component i, and Cm = measured PM2.5 mass concentration from MetOne SASS Teflon filter.


ISORROPIA II Settings

For this study, all the iPM2.5 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+-SO42–-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 iPM2.5 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).


Table 1. Inputs of the ISORROPIA II simulation.

Five T and RH conditions were used to test the responses of secondary iPM2.5 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.


R
ESULTS 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).


Table 2. The statistics of concentrations of different gases and PM2.5 chemical components by season. All the concentration values are expressed in µg m–3.


Responses of Total iPM2.5 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 iPM2.5 using ISORROPIA II. The predicted iPM2.5 was compared with the measured iPM2.5 concentrations under different NO2-to-HNO3 conversion percentages (Fig. 2).


Fig. 2. The ratio of predicted iPM2.5 concentration over measured iPM2.5 concentration against conversion percentage of NO2 to HNO3.Fig
. 2. The ratio of predicted iPM2.5 concentration over measured iPM2.5 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 iPM2.5 over measured iPM2.5 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 iPM2.5.

When the conversion percentage of NO2 to HNO3 was set at 3.63%, the ratio of predicted iPM2.5 over measured iPM2.5 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 m3, 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 m3, 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.


Table 3. Numbers in different GR ranges in four seasons of Lenoir County.


Seasonal Variations of PM2.5 Chemical
 Speciation

In Lenoir County, PM2.5 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.


Table 4. Comparison of FRM vs. MetOne SASS measured PM2.5 mass concentration.

As indicated by Table 4, the PM2.5 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. PM2.5 chemical speciation mass fractions in Lenoir County in four seasons. Fig. 3. PM2.5 chemical speciation mass fractions in Lenoir County in four seasons.

As can be seen from Fig. 3, iPM2.5 accounted for a large proportion of the total PM2.5, and this fraction was highest in spring (51%) and lowest in summer (40%). For iPM2.5 alone, three chemical species, SO42, NO3 and NH4+, also accounted for different fractions in four seasons. The results of Tukey’s test were shown in Table 5.


Table 5. Comparisons of different PM2.5 chemical component concentrations by season.

As indicated by Table 5, the PM2.5 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 SO42 together accounted for 41.4–54.2% of PM2.5 mass concentration in four seasons; both species’ concentrations were higher in summer than in winter. In addition, SO42 was also the major component of iPM2.5 and it accounted for 47.9–67.0% in the whole iPM2.5. 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), SO42 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, SO42 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, SO42 was the year-round major component of iPM2.5and 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 iPM2.5 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 iPM2.5 formation with total NH3 concentration ranging from 0.54 to 37.94 µg m3. The responses of iPM2.5 to the change of total NH3 are shown in Fig. 4. Under different conditions, the responses of iPM2.5 to total NH3 were different. The concentration of iPM2.5responded to the total NH3 nonlinearly.


Fig. 4. Responses of different iPM2.5 chemical components to total NH3 concentration under five T and RH conditions.Fig. 4.
 Responses of different iPM2.5 chemical components to total NH3 concentration under five T and RH conditions.

Under maximum T + minimum RH (305.95 K + 39%), the change of iPM2.5 concentration is caused by the change of SO42, HSO4, and NH4+. When the total NH3 concentration is changed from 0.54 µg m3 to 37.94 µg m3, the GR is also changed from –2.17 to 116.29; during this process, the response of iPM2.5 to total NH3 is changed from sensitive to insensitive. When GR ≤ 0, increasing total NH3 concentration can increase the concentration of particle-phase SO42 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 iPM2.5to total NH3 is insensitive.

Under minimum T + maximum RH (271.95 K + 95%), the change of iPM2.5 concentration is caused by the changes of SO42, HSO4, NH4+and NO3. When GR ≤ 1.63, increasing total NH3 concentration can increase the concentration of SO42, 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 iPM2.5significantly; 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 iPM2.5 chemical composition are similar to those under the condition of minimum T + maximum RH (271.95 K + 95%); the difference is the peak iPM2.5 concentration value and the GR at which the system reaches the peak iPM2.5concentration. Fig. 4 shows that the difference in peak iPM2.5 concentration is caused by the NO3 concentration. Lower T and higher RH can favor the formation of NH4NO3; thus, the peak iPM2.5 under median T + median RH is about 6.13 µg m3, which is less than the peak iPM2.5 concentration(6.24 µg m3) under minimum T + maximum RH (271.95 K + 95%) and greater than the peak iPM2.5 concentration (4.76 µg m3) 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 iPM2.5, 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 iPM2.5 concentration is similar to the condition of median T + median RH. The difference is the peak iPM2.5 concentration; the peak iPM2.5 concentration under maximum T + maximum RH is close to 6.00 µg m3, which is lower than the peak iPM2.5 concentration, 6.13 µg m3, 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 iPM2.5 concentration is caused by the change of SO42, 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 iPM2.5 becomes insensitive to the change of total NH3 concentration.


Table 6. Threshold values of insensitive response of iPM2.5 to total NH3.

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 iPM2.5 to total NH3 is insensitive. Thus, reducing total NH3 concentration will not lead to significant decrease of iPM2.5 concentration.


Response of Secondary iPM2.5 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 iPM2.5 formation with total H2SO4 concentration changing from 0.59 to 14.60 µg m3. The results are shown in Fig. 5.


Fig. 5. Response of different iPM2.5 chemical components to total H2SO4 concentration under five T and RH conditions.Fig. 5.
 Response of different iPM2.5 chemical components to total H2SO4 concentration under five T and RH conditions.

Fig. 5 shows that the response of iPM2.5 to total H2SO4 is linear. Increasing total H2SO4 concentration can lead to the increase of iPM2.5concentration. Under maximum T + minimum RH (305.95 K + 39%), the change of iPM2.5 concentration is caused by the change of SO42, 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 iPM2.5 concentration is caused by the change of SO42, 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 m3, 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 iPM2.5 to total H2SO4 concentration is linear. Adding more H2SO4 can increase iPM2.5 concentration significantly; this is because of the excessive NH3 existing in the system. When all the NH3 gas is neutralized by H2SO4, SO42 is converted to HSO4; thus concentration of iPM2.5 is increased. Response of secondary iPM2.5 to total H2SO4 is not determined by NH3-rich or NH3-poor conditions in this research. This is because that in NH3-NH4+-SO42–-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 iPM2.5 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 iPM2.5 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 iPM2.5 formation with total HNO3 concentration changing from 0.24 to 4.24 µg m3. The results are shown in Fig. 6.


 Fig. 6. Response of different iPM2.5 chemical components to total HNO3 concentration under five T and RH conditions.Fig. 6.
 Response of different iPM2.5 chemical components to total HNO3 concentration under five T and RH conditions.

Under maximum T + minimum RH (305.95 K + 39%), iPM2.5 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 iPM2.5 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 SO42concentration remains the constant; this is due to the full neutralization of H2SO4 by NH3.

In summary, the response of secondary iPM2.5 to total HNO3 is linear; adding more HNO3 to the system can increase the iPM2.5 concentration linearly due to the formation of NH4NO3. Under different ambient conditions, the increase rate of iPM2.5 is different; lower T and higher RH favor the formation of NH4NO3 and lead to higher iPM2.5 concentration.


Possible Mitigation Strategies for Ambient iPM2.5 Reduction

Analysis of the iPM2.5 responses to the precursor gases may lead to the development of the iPM2.5 mitigation strategy. Under different ambient conditions, the responses of iPM2.5 to total NH3, total HNO3, and total H2SO4 are different. In general, iPM2.5 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 iPM2.5 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 iPM2.5concentration. Reducing total H2SO4 is more effective to decrease iPM2.5 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 (NOx) and sulfuric dioxide (SO2) in the atmosphere. Reducing the two acidic gases would require the reduction of their associated primary pollutants, NOx and SO2. Thus, in development of the total H2SO4 and HNO4 concentration reduction strategies, strategies for SO2 and nitrogen oxide (NOx) emissions reduction should be taken.

 
CONCLUSIONS


In this research, the responses of iPM2.5 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 iPM2.5 and gaseous pollutants as well as meteorological conditions. It was found that iPM2.5 responds to precursor gases differently under different T and RH conditions. In general, the response of iPM2.5 to the total NH3is nonlinear, whereas its response to the total H2SO4 and the total HNO3 is linear. In NH3-rich regions, iPM2.5 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 iPM2.5 was insensitive to changes in the total NH3, varied under different ambient conditions. Although dry and wet deposition and the nonlinear conversion of NOx 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 iPM2.5 in response to changes in precursor gases, offering insight into controlling and regulating iPM2.5 in NH3-rich regions. These findings also illuminate the impact of AFO NH3 emissions on the formation of secondary iPM2.5, which may help to develop strategies for reducing ambient PM2.5. Future studies may examine the quantitative contribution of AFO NH3 emissions to forming iPM2.5 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.



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