Spatial and Temporal Variations of PM 2 . 5 in North Carolina

Studies have indicated that the adverse effects on human health and the decrease in visibility caused by fine particulate matter (PM2.5) exhibit spatial heterogeneity. Moreover, the environmental effects produced by different chemical compositions of PM2.5 vary on a regional scale. Therefore, understanding the spatiotemporal variations and chemical compositions of PM2.5 is necessary for assessing the regional impacts. Secondary inorganic PM2.5 (iPM2.5) is formed through chemical reactions between the base gas NH3 and acidic gas pollutants (e.g., NO2 or SO2). The major components of iPM2.5 include NH4, SO4, and NO3. To fully comprehend the regional impacts of PM2.5, this research quantifies the spatiotemporal variations of iPM2.5 with the aim of evaluating the contributions from iPM2.5 to PM2.5 in North Carolina (NC). The concentrations (at 34 sites) and chemical components (at 7 sites) of PM2.5 from 2005 to 2014 were extracted from the EPA’s AirData, with the highest concentrations measured in the urban areas of central NC. Notably, PM2.5 concentrations have been significantly reduced over the past 10 years, with a concurrent decreasing trend in iPM2.5. Seasonal variation analysis indicates that PM2.5 concentrations were higher in summer and lower in winter; however, significant variation occurred only between 2005 and 2011. Although iPM2.5 formed the largest mass fraction of PM2.5 for 2005–2011, organic carbon matter (OCM) contributed the dominant share for 2012–2014. Significant seasonal variations in the iPM2.5 mass fractions were also observed, with NO3 and SO4 exhibiting inverse variations. This study links the ambient PM2.5 to various sources by revealing the spatiotemporal variations of PM2.5 and their associated chemical compositions in NC, thereby enabling the development of effective control and mitigation strategies.


INTRODUCTION
By definition, PM 2.5 represents particulate matter (PM) with aerodynamic equivalent diameter ≤ 2.5 µm.As a criteria air pollutant in the United States (U.S.), it has gained intensive attention due to its adverse health effects, special role in visibility degradation, and the potential impact on climate (Boucher, 2000;Dominici et al., 2006;Haywood and Pope et al., 2009;Pui et al., 2014).Franklin et al. (2007) studied the relationship between PM 2.5 and mortality across the U.S. with a conclusion that elevated PM 2.5 mass concentration can lead to the increase in mortality; the responses to PM 2.5 exposures may differ for people of different ages and genders in different areas of the U.S. Dominici et al. (2006) estimated the health risk of PM 2.5 exposures and discovered that cardiovascular risks were increased by PM 2.5 short-term exposures with higher risk in the eastern U.S.Moreover, research has indicated that standalone PM 2.5 mass concentration is inadequate to vigorously explain the health effects of PM 2.5 ; detailed information of PM 2.5 chemical compositions is needed to establish a relationship between human health and PM 2.5 exposures (Franklin et al., 2008).In addition to spatially varied adverse effects on human health, a visibility research conducted by Malm et al. (1994) indicated that sulfate (SO 4 2-) and organic materials were more effective in reducing the visibility across the U.S. than the other components of PM 2.5 .In the eastern U.S., SO 4 2-is the dominant component to contribute to light extinction.Similar investigations performed by Brewer and Adlhoch (2005) and Brewer and Moore (2009) suggested that ammonium sulfate ((NH 4 ) 2 SO 4 ) and organic carbon matter (OCM) were the dominant contributors to light extinction in the southeastern U.S.; however, wildfires and windblown dust dominated in light extinction in the western U.S.
In ambient air, large spatiotemporal variations exist in mass concentration and chemical compositions of PM 2.5 , caused by spatiotemporal variations of emission sources, formation and deposition processes, meteorological conditions, and atmospheric fate and transport (Martuzevicius et al., 2004;Seinfeld and Pandis, 2006;Bell et al., 2007;Wang-Li, 2015).The regional impacts of PM 2.5 may vary due to such spatiotemporal variations.Hu et al. (2014) used satellite-retrieved aerosol optical depth (AOD) data to estimate the spatiotemporal variations of PM 2.5 mass concentrations in the southeastern U.S. in 2001-2010; the results indicated that urban areas and major highways can display higher PM 2.5 concentrations than rural or mountain areas.In addition, around 20% reduction in PM 2.5 concentrations has been achieved over the past 10 years in the southeastern U.S. Previous research on the spatiotemporal variations of PM 2.5 mass and chemical compositions in the U.S. has indicated that PM 2.5 mass concentrations had higher values in the eastern U.S. and California and lower values in the central and northwestern regions of the nation.Moreover, PM 2.5 chemical components such as SO 4 2-and nitrate (NO 3 -) exhibited inverse seasonal variation patterns in eastern and western coast areas of U.S. (Bell et al., 2007).In addition, Hasheminassab et al. (2014) identified the main contributors to the PM 2.5 to be secondary inorganic PM 2.5 (iPM 2.5 ) in California.Another study performed in Cincinnati metropolitan area found that major components of PM 2.5 can be identified as organic carbon (OC) followed by SO 4 2-, elemental carbon (EC), crustal elements and trace metals (Martuzevicius et al., 2004).Similar research conducted by Saunders et al. (2015) in the northeastern U.S. identified two main PM 2.5 pollutants: OC and (NH 4 ) 2 SO 4 .Goetz et al. (2008) analyzed the major compositions of PM 2.5 in eastern NC from 2001 to 2004 at sites representing urban and rural sites; the results indicated that OC, SO 4 2-, ammonium (NH 4 + ), NO 3 -and EC were the major contributors to PM 2.5 mass; SO 4 2-and NH 4 + tended to be slightly higher in the rural site.Previous research has discovered that different areas exhibited different PM 2.5 chemical characteristics such that the regional impact of PM 2.5 may differ correspondingly.
The chemical compositions of PM 2.5 include ions (e.g., NH 4 + , SO 4 2-, and NO 3 -), OC, EC, elements (e.g., crustal materials) and other unknown components (Malm et al., 1994;Frank, 2006).The iPM 2.5 is defined as the ion components, including NH 4 + , SO 4 2- , and NO 3 -, in this paper.Different compositions of PM 2.5 can be linked to different emission sources and formation processes (Abdeen et al., 2014).As a subset of total PM 2.5 , iPM 2.5 is formed through the partitioning of gas-phase NH 3 and particlephase NH 4 + .The formation of secondary iPM 2.5 can be characterized by the reactions between precursor gases such as ammonia (NH 3 ), nitric acid (HNO 3 ) and sulfuric acid (H 2 SO 4 ).While H 2 SO 4 and HNO 3 are mainly transformed from the primary pollutants, SO 2 and NO x (NO x = NO + NO 2 ), through photochemical reactions, NH 3 is directly emitted from emission sources such as agricultural operations (Seinfeld and Pandis, 2006).Because secondary iPM 2.5 constitutes a significant part of total PM 2.5 (Sawant et al., 2004;Katzman et al., 2010;Philip et al., 2014), it is important to establish a holistic understanding of the spatiotemporal variations of secondary iPM 2.5 such that the regional impact of PM 2.5 may be fully understood.Aneja et al. (2003) investigated the relationship between NH 3 emissions from agricultural sources and ambient NH 4 + concentrations in the southeastern U.S. with spatiotemporally varied NH 3 emissions and NH 4 + concentrations data.This research indicated that higher NH 3 emissions led to higher NH 4 + at the given location.Other research discovered that in the rural areas of North Carolina (NC), ambient NH 3 concentration was high, sometime reaching up to 45.87 ppb (34.80 µg m -3 ); ambient air was dominated by NH 3 -rich conditions in those rural areas due to high NH 3 emissions from agricultural operations (Robarge et al., 2002;Walker et al., 2000b).Walker et al. (2004) compared the secondary iPM 2.5 concentrations at three sites located in NC Coastal Plain region with different NH 3 emission densities, and reported that NH 3 emitted from agricultural sources posed large influences on iPM 2.5 concentrations; higher total NH 3 emissions led to higher iPM 2.5 concentrations in rural areas.Furthermore, Walker et al. (2006) performed another study and reported a twoyear average concentration of secondary iPM 2.5 from 1999 to 2000 to be 8.0 ± 5.84 µg m -3 at an agricultural site in the southeastern area of NC; among the measured various iPM 2.5 chemical components, SO 4 2-, NO 3 -, and NH 4 + were the most dominant components.
As a complex mixture, various chemical components of PM 2.5 contribute to total PM 2.5 mass differently.The reduction of any specific chemical component may have different effects on the reduction of PM 2.5 mass.The in-depth understanding of spatial and temporal variations of PM 2.5 characteristics is the foundation to study regional impact of PM 2.5 and to develop effective control strategies of PM 2.5 ; more research is necessary to quantitatively characterize PM 2.5 mass and chemical compositions in spatial and temporal scales.This research was to quantitatively assess the variations of PM 2.5 and its associated chemical compositions in spatial and temporal scales in NC to advance our understanding of spatial and temporal variations of PM 2.5 in the region.

PM 2.5 Monitoring Stations in North Carolina
Investigation of PM 2.5 spatial and temporal variations started with identification of the EPA's PM 2.5 monitoring stations in NC and obtaining PM 2.5 data for each given station.Under the State and Local Air Monitoring Stations (SLAMS) for criteria pollutants (EPA, 2016a), PM 2.5 mass concentrations were routinely monitored at 34 stations in NC from 2005 to 2014.In addition, under the EPA's Chemical Speciation Network (CSN) (EPA, 2016b), PM 2.5 chemical speciation samples were taken and analyzed at 7 stations, where PM 2.5 mass measurements were concurrently taken.Fig. 1 shows the spatial distribution of these monitoring stations in the state.While the PM 2.5 mass concentration monitoring stations were evenly distributed across the whole state, most of the PM 2.5 chemical speciation monitoring stations were located in central areas of the state.Based on the topography of NC, the whole state can be divided into three geographic areas: mountains in the west, the Piedmont in the center, and the Coastal Plain in the east (NC SOS, 2018).
Under the SLAMS, PM 2.5 concentration measurements were taken once every day using either Federal Reference

Method (FRM) or Federal Equivalent Method (FEM).
Under the CSN, PM 2.5 chemical speciation samples were taken once every six days using the chemical speciation samplers that had three filter types with Teflon for mass concentration and elemental analyses, nylon for ion analyses, and quartz for OC and EC analyses.

Data Acquisition
To investigate the temporal and spatial variations, the 10 years (2005-2014) of PM 2.5 concentrations and chemical speciation data for all the monitoring sites in Fig. 1 were extracted from EPA's AirData (EPA, 2016c).For analysis of seasonal variations, dataset was grouped into four seasons with spring in March, April and May; summer in June, July and August; fall in September, October and November; and winter in December, January, and February.

Data Adjustment
Organic carbon measurements reported in EPA's AirData database should be adjusted to derive total mass of organic compounds.According to Turpin and Lim, (2001), Weber et al. (2003), Frank (2006), El-Zanan et al. (2009), and Dillner et al. (2012), the adjusted organic carbon matter (OCM) may be calculated using Eq. ( 1): where OCM = organic carbon matter; OC m = measured organic carbon; OC b = field blank organic carbon.In this research, the adjusted OCM concentrations were used to check the mass closure of PM 2.5 chemical speciation.
In an effort to understand the contribution of each chemical component to the total PM 2.5 mass concentration, chemical speciation data were used to construct PM 2.5 chemical speciation distribution pie chart, also named the mass closure.In this PM 2.5 mass closure analysis, percentage of each chemical component was calculated using Eq. ( 2).For valid data selection, only those days having simultaneous measurements of PM 2.5 mass and chemical components were used to construct PM 2.5 mass closure.In addition, it was discovered in many cases, the sum of individual chemical component masses exceeded the measured PM 2.5 mass.This may be due to some artifacts such as adsorption of organics, inappropriate estimation of OCM, the loss of ammonium nitrate, water existing in the samples, and measurement errors (Andrews et al., 2000;Frank, 2006;Watson et al., 2008;Chow et al., 2015).To conduct mass closure analysis, when measured PM 2.5 mass concentration was less than the sum of individual chemical compositions masses, those data were excluded.
where P i = percentage of the chemical component i to the total PM 2.5 mass concentration, C i = measured or adjusted concentration of the chemical composition i, C m = measured PM 2.5 mass concentration from Teflon filter.
In the period of 2004-2009, the PM 2.5 chemical speciation dataset included SO 4 2-, NO 3 -, NH 4 + , Na + and K + , OC/EC, and 48 elements, whereas for the years of 2010-2014 the PM 2.5 chemical speciation dataset included SO 4 2-, NO 3 -, NH 4 + , Na + and K + , OC/EC, and 33 elements.Since each element only accounted for very small portion of total PM 2.5 mass concentration, the individual elements were grouped into one category to construct the mass closure.In addition, since S has been taken into account in SO 4 2-measurements, and Na + and K + have been taken into account in Na and K elemental measurements, S element, Na + , and K + were excluded to avoid double counting when analyzing the mass closure.

Statistical Analysis and Map Development
The relationship between PM 2.5 mass concentration and PM 2.5 chemical compositions were characterized by Spearman correlation coefficient (R).In addition, Tukey test and Mann-Kendall trend test were performed to characterize the general change trend of the dataset in spatial and temporal scales under 0.05 significance level.The dataset in this research may not follow normal distribution; therefore, parametric test such as Tukey test may lack power to interpret the change trend of the data.The distribution of the data was examined as well.All the statistical tests were conducted using R software.To visually illustrate spatial variations, maps reporting PM 2.5 monitoring stations and associated PM 2.5 concentrations over time were developed using ArcMap 10.4.

Spatial and Temporal Variations of PM 2.5 Concentration in North Carolina
The PM 2.5 concentrations measured by the FRM samplers from 2005 to 2014 were used to detect the spatiotemporal variations.For illustration purpose, Fig. 2 shows the annual average PM 2.5 concentrations at each site for 2005, 2008, 2012 and 2014.Based on continued research and updated scientific evidence on the adverse effects of PM 2.5 , the PM 2.5 National Ambient Air Quality Standards (NAAQS) have been revised two times in the past 10 years.The PM 2.5 24-hour average concentration was strengthened from 65 µg m -3 to 35 µg m -3 in 2006; in addition, the primary PM 2.5 annual average concentration threshold was strengthened from 15 µg m -3 to 12 µg m -3 in 2012.Selection of these years was to reflect the PM 2.5 regulation changes through the time (EPA, 2018).
In general, the annual average PM 2.5 concentrations were higher in central urban NC, the Piedmont areas, while the other areas exhibited lower annual average PM 2.5 concentrations.Specifically, large cities in central NC such as Charlotte and Lexington exhibited higher annual average PM 2.5 concentrations for the past 10 years, while lower PM 2.5 concentrations always occurred in western mountain areas and eastern Coastal Plain areas.The observation of such spatial variations in annual average PM 2.5 concentrations in NC was consistent with the finding of Hu et al. (2014).
The spatial pattern of PM 2.5 concentrations may be attributed to the spatial heterogeneity of emission sources because major emission sources of primary PM 2.5 and precursor gases of secondary iPM 2.5 such as NO x and SO 2 were mainly located in urban areas.More specifically, airport operations, foundries, food processing plants and steel mills were located in Charlotte (see Fig. 1), while glass plants and furniture manufacturing plants were located in Lexington.Among these emission sources, airport operations, steel mills, and glass plants can emit primary PM 2.5 , NO x Fig. 2. Spatial and temporal variations of annual average PM 2.5 concentrations (µg m -3 ) in NC.
and SO 2 at the same time (NEI, 2008).In addition, the topography might also affect atmospheric airflow dynamics and transport of PM 2.5 .As reported by Chow et al. (2006), high elevation may prevent the transport of PM 2.5 toward mountain areas.The air mass blowing from coastal areas is cleaner than the inland.Various air pollutions may be picked up during the movement of air mass toward Piedmont area (central NC).If the air mass was trapped and circulated within the central NC, this may result in higher PM 2.5 mass concentrations.Whichever direction the wind may come from, the monitoring sites in the mountain areas may not be exposed to the air mass laden with higher PM 2.5 concentrations.
As for temporal variation, Mann-Kendall trend test indicates that the annual average PM 2.5 concentrations were significantly decreased from 2005 to 2014 at all the monitoring sites; annual average reduction of PM 2.5 concentrations ranged from 0.51 µg m -3 in the city of Raleigh to 0.86 µg m -3 in the city of Hickory.In 2005, there were 9 sites having annual average PM 2.5 mass concentration greater than 15 µg m -3 , the annual concentration threshold under the NAAQS back then; on the other hand, in 2014, all the sites had the annual concentrations below the latest NAAQS threshold of 12 µg m -3 .The temporal reduction of PM 2.5 mass concentration from 2005 to 2014 was reflective of the implementation of NAAQS.Under the Clean Air Act (CAA) and Clean Air Act Amendment (CAAA), each state has to develop effective reduction plans of six criteria pollutants known as the SIPs for nonattainment areas to meet NAAQS (EPA, 2016d).In 2002, NC Clean Smokestacks Act was passed to regulate the NO x and SO 2 emissions from coal-burning power plant facilities and set the reduction plan of 77% in NO x by 2009 and 73% in SO 2 by 2013.The utilization of control devices such as scrubbers and low-NO x burners have guaranteed the compliance of the act (NCDAQ, 2014).The annual average PM 2.5 concentrations were significantly decreased to be below 14 µg m -3 in 2008 and 11 µg m -3 in 2009 across the whole state.The significant reduction of annual average PM 2.5 concentrations was also reflective of EPA's Clean Air Interstate Rule (CAIR), which was implemented in 2009.As requested by the rule, NC had to control both ozone and PM 2.5 pollutants, and electric generating units (EGUs) were required to control both NO x and SO 2 emissions (EPA, 2016e).Moreover, as the replacement of CAIR, the implementation of Cross-State Air Pollution Rule (CSAPR) in 2012 further guaranteed the reduction of PM 2.5 concentrations, measures such as the lower S coal and improved pollution control equipment in power plants helped reduce the emissions of both SO 2 and NO x (EPA, 2016f).Consequently, the annual average PM 2.5 concentrations became below 10 µg m -3 in 2014 across the state.
To further investigate the reduction trend of PM 2.5 concentrations in NC, higher temporal resolution datasets (monthly average PM 2.5 concentrations) were analyzed.Results at 4 representative sites are shown in Fig. 3.
The monthly average PM 2.5 concentrations at these 7 sites exhibited reduction trend from 2005 to 2014 as well.In addition, a seasonal variation of PM 2.5 concentrations can also be observed from the above plots.Furthermore, the 10 years can be divided into 3 consecutive periods based upon the reduction trend of PM 2.5 concentrations: 2005-2007, 2008-2011, and 2012-2014.The seasonal variations of PM 2.5 mass concentrations in these 3 periods are summarized in Table 1.
As can be seen from Table 1, in general, PM 2.5 concentrations were significantly higher in summer and lower in winter at 7 monitoring sites; however, the Tukey test indicated that this seasonal trend was only significant in 2005-2007 and 2008-2011, while no significant seasonal variation can be observed in 2012-2014 at 6 out of 7 sites.The seasonal variations were only significant at sites in the city of Winston-Salem in 2012-2014 period.This may be due to the aforementioned implementation of CSAPR in 2012 and effective emission control on NO x and SO 2 continued in 2012 in NC.Thus, NO x and SO 2 were less available in all four seasons compared with the emission conditions before 2012; less seasonal variations of various chemical components of PM 2.5 combined with opposite seasonal variations of OCM and SO 4 2-salts led to insignificant variations of PM 2.5 in 2012-2014.

Spatial and Temporal Variations of the iPM 2.5 Chemical Compositions
The variations of PM 2.5 concentrations may result from the variations in various PM 2.5 chemical compositions; therefore, spatiotemporal variations of PM 2.5 chemical compositions were analyzed and compared with the PM 2.5 concentration change trend at each location.Fig. 4 shows the analyses results of major iPM 2.5 composition changes over time at 7 urban sites located in the cities of Asheville, Hickory, Lexington, Winston-Salem, Rockwell, Charlotte, and Raleigh (see the map in Fig. 1).All the PM 2.5 mass concentrations were measured by the non-FRM samplers.
In general, there is no significant difference among 7 PM 2.5 chemical speciation monitoring sites for each of the 3 major iPM 2.5 chemical components, SO 4 2-, NH 4 + and NO 3 -.The exceptions were that SO 4 2-concentrations in Lexington were significantly higher than in Raleigh in 2006, and NH 4 + concentrations in Lexington were significantly higher than in Asheville in 2008 and 2009.Asheville was located in western mountain areas while Raleigh was located on the border of Piedmont and Coastal Plain areas (see Fig. 1); therefore, the impact of topography may explain the spatial variations of SO 4 2-and NH 4 + in these years.Similar spatial distribution of the three major iPM 2.5 chemical compositions at 7 urban sites may indicate the similarity of the atmospheric chemistry background at the 7 monitoring sites, as all of them are located in urban areas.In rural areas, large NH 3 emissions from agricultural operations have resulted in NH 3 -rich conditions in ambient air (Saylor et al., 2010;Walker et al., 2000a, b); therefore, atmospheric chemistry background in rural areas was different from those in urban areas.Some spatial variations of iPM 2.5 chemical compositions may be expected between urban and rural areas; further research is needed to establish holistic understanding of the spatial variations in the future.concentrations also decreased significantly at this location from 6.42 ± 3.71 µg m -3 to 1.71 ± 0.76 µg m -3 , from 2.16 ± 1.18 µg m -3 to 0.50 ± 0.34 µg m -3 , and from 0.88 ± 0.96 µg m -3 to 0.62 ± 0.75 µg m -3 , respectively.Quantitatively, at Charlotte site, a 9.09 µg m -3 annual average PM 2.5 mass   concentration reduction was achieved from 2005 to 2014; at the same time, 4.71 µg m -3 , 1.66 µg m -3 , and 0.26 µg m -3 annual average concentration reduction were achieved for SO 4 2-, NH 4 + , and NO 3 -, respectively.Together, iPM 2.5 contributed to around 73% of the PM 2.5 mass concentration reduction.The similar reduction trend of iPM 2.5 was observed at the other monitoring sites as well.Annual average OCM concentration was reduced from 5.63 ± 2.57 µg m -3 in 2005 to 3.63 ± 1.89 µg m -3 in 2014 at Charlotte site.The OCM contributed to 22% of the reduction in PM 2.5 mass concentration.The exceptions were Asheville, Hickory, and Rockwell; the OCM concentrations did not exhibit a significant reduction trend over the past 10 years.Concurrent variations of PM 2.5 mass and iPM 2.5 chemical compositions provided evidence of important contribution of iPM 2.5 to the reduction of total PM 2.5 mass in NC.

Trends in Extrema of PM 2.5 and its Chemical Component Concentrations
The trends in extrema of PM 2.5 mass and its chemical component concentrations at one of the seven sites (Charlotte) are shown in Fig. 5.
As can be seen from Fig. 5, the temporal variation in the distribution of PM 2.5 mass and its chemical component concentrations occurred in the three consecutive periods.Fewer high concentrations can be observed in the period of 2012 to 2014 in Charlotte.The 98 th percentile of PM 2.5 mass concentration in 2012-2014 was about 18.6 µg m -3 ,

Correlations between PM 2.5 and Secondary iPM 2.5 Component Concentrations
To better understand the contribution of iPM 2.5 components to the total PM 2.5 mass, the correlations between PM 2.5 total mass and iPM 2.5 chemical components were analyzed and are shown in Table 2.The analyses include yearly and seasonal correlations to account for the impact of ambient conditions in four seasons.
As can be seen from Table 2, at 7 monitoring sites of NC, NH 4 + and SO 4 2-were strongly correlated with PM 2.5 mass concentration (R > 0.58); however, the correlation between NO 3 -and PM 2.5 mass concentration varied in 4 seasons with stronger correlation in winter than in summer, spring and fall.The observation was consistent with the research performed by Bell et al. (2007), in which NH 4 + , SO 4 2-, and NO 3 -were determined to be 3 significant components of PM 2.5 , exhibiting stronger correlation with total PM 2.5 mass.Stronger correlation of PM 2.5 mass concentration with NH 4 + and SO 4 2-indicated the day-today covariation, while the fluctuated correlation between PM 2.5 and NO 3 -can be attributed to the semi-volatile characteristic of NH 4 NO 3 (Olszyna et al., 2005).

Evaluation of PM 2.5 Mass Closure
Analysis of the mass fraction of each chemical component may help to understand the individual chemical components' contributions to the total PM 2.5 concentrations.Fig. 6 shows the resultant mass distribution charts of the analysis at 4 selected monitoring locations, separated by the three periods.
The OCM was the dominant component of PM 2.5 in the 3 periods at 7 monitoring sites, especially in 2012-2014 accounting for 34.4-37.0% of total PM 2.5 .Of the total PM 2.5 at seven monitoring sites, the three major iPM 2.5 components together accounted for 34.1-45.9% from 2005 to 2007, 34.8-39.1% from 2008 to 2011, and 28.3-30.9%from 2012 to 2014.Among the three major iPM 2.5 chemical components, SO 4 2-was the dominant contributor to the total PM 2.5 mass.The mass fraction of SO 4 2-exhibited a significant reduction trend in the 3 periods.Contribution of NH 4 + to total PM 2.5 mass was also significantly reduced, while no significant reduction trend can be observed for the mass fraction of NO 3 -.The contribution of unidentified components ("other") to total PM 2.5 mass was actually increased throughout the whole periods.The increase of unidentified component mass fraction may be caused by the inappropriate estimation of OCM through the OCM/OC ratio of 1.4; this conversion ratio may be higher in the later period (Blanchard et al., 2013).The relative contributions of various chemical components to the total PM 2.5 mass resulted from different emission sources and atmospheric physiochemical processes.As mentioned above, in the past 10 years, several regulations such as CAIR and CSAPR have been promulgated to reduce the emissions of SO 2 and NO x in eastern U.S. to control the pollution of PM 2.5 ; the reduction of the mass fraction of the secondary iPM 2.5 proved the effectiveness of all these regulation and control policies.
In addition, seasonal variation analysis indicates that NO 3 -contribution to the total PM 2.5 had a significant seasonal pattern with higher fraction in winter and lower fraction in summer, while the mass fraction of SO 4 2exhibited an inverse seasonal pattern with higher fraction in summer and lower fraction in winter.No significant seasonal variations can be observed for the mass fraction of NH 4 + .This can be justified by the semi-volatile characteristic of NH 4 NO 3 .Low temperature (T) and high relative humidity (RH) in winter favored the formation of NH 4 NO 3 (Olszyna et al., 2005).However, in contrast to NO 3 -salts, SO 4 2-salts had very low vapor pressure; therefore, most of the (NH 4 ) 2 SO 4 stayed in particle phase even in summer.H 2 SO 4 was transformed from SO 2 through photochemical reactions; more intense solar radiation in summer facilitated the formation of H 2 SO 4 leading to more formation of (NH 4 ) 2 SO 4 (Seinfeld and Pandis, 2006).
No significant spatial variations of 3 major iPM 2.5 chemical components were observed at the 7 monitoring sites.This may be due to similar chemistry background of atmosphere at these 7 sites.However, high NH 3 emissions from agricultural sources in rural areas may pose greater influence on the chemical compositions of PM 2.5 ; the relative contributions of iPM 2.5 to PM 2.5 mass may vary correspondingly.The monitoring data in rural areas are scarce in NC; future research is needed to fill this data gap so that analysis may be conducted for agricultural areas, where the highest base precursor gas (NH 3 ) concentrations occurred.At the 7 sites in NC, the 3 major iPM 2.5 components accounted for the largest portion of PM 2.5 mass in 2005-2011. However, in 2012-2014, OCM accounted for the largest portion of PM 2.5 mass.The reduction trend of PM 2.5 chemical components was consistent with finding of the research performed by Saylor et al. (2015) in other locations in southeastern U.S.

CONCLUSIONS
The variations of PM 2.5 mass concentrations in North Carolina from 2005 to 2014, measured at the EPA's monitoring sites, were found to be statistically significant on spatial and temporal scales, with the highest values recorded in the urban areas of central NC.A significantly decreasing trend in PM 2.5 levels was also observed across the state during this period.Temporal variations in the iPM 2.5 were consistent with this trend, exhibiting a concurrent reduction in concentration and thereby indicating the critical contribution of secondary iPM 2.5 to the total PM 2.5 mass.Although the PM 2.5 concentrations displayed a significant seasonal variation, with higher 2-exhibiting inverse variations.The results of this study improve our holistic understanding of PM 2.5 and iPM 2.5 on a regional scale.

Fig. 1 .
Fig. 1.PM 2.5 mass and chemical component measurement stations across North Carolina.

Table 1 .
Seasonal variations of PM 2.5 concentrations in 3 monitoring periods.

Table 2 .
Correlation of secondary iPM 2.5 chemical compositions with PM 2.5 total mass at the 7 sites of NC.All the values were calculated as Spearman correlation coefficient (R).R ≤ 0.5 were marked bold.