Understanding of the Fate of Atmospheric Pollutants Using a Process Analysis Tool in a 3-D Regional Air Quality Model at a Fine Grid Scale

The process analysis is performed for August and December, 2002 using the process analysis tool embedded in the Community Multiscale Air Quality (CMAQ) modeling system at a fine horizontal grid resolution of 4-km over an area in the southeastern US that is centered at North Carolina. The objectives are to qunatify the contributions of major atmospheric processes to the formation of major air pollutants and provide the insights into photochemistry that governs the fate of these pollutants at a fine grid scale. The results show that emissions provide a dominant source for gases including ammonia (NH3), nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2) and Particulate Matter (PM) species including fine PM (PM2.5) and its composition such as sulfate, elemental carbon, primary organic aerosol, and other inorganic fine PM in both months. While transport acts as a major sink for NH3, NO, and SO2 at most sites and PM2.5 and most of PM2.5 composition at urban sites, it provides a major source for nitric acid (HNO3) and ozone (O3) at most sites in both months, and secondary PM species in August and most PM species in December at rural and remote sites. Gas-phase chemistry serves as a source for NO2 and HNO3 but a sink for O3 at urban and suburban sites and for NO and SO2 at all sites. PM processes contribute to the formation of PM2.5 and nitrate ( ) at the urban and suburban sites and secondary organic aerosol (SOA) at most sites in December and ammonium ( ) in both months. They reduce formation at most sites in August and at rural and remote sites in December and the formation of PM2.5 and SOA at most sites in August. Dry deposition is an important sink for all these species in both months. The total odd oxygen (Ox) production and the total hydroxyl radical (OH) reacted are much higher at urban and suburban sites than at rural sites. Significant amounts of OH are consumed by biogenic volatile organic compounds (BVOCs) in the rural and remote areas and a combination of anthropogenic VOCs (AVOCs) and BVOCs in urban and subareas areas in August and mainly by AVOCs in December. The amount of NO2 produced by the reactions of hydroperoxy radical (HO2) is similar to that of organic peroxy radical (RO2) at all sites in August but higher than that by the reactions of RO2 in December. The production rate of HNO3 due to the reaction of OH with NO2 dominates in both months. The ratio of the production rates of hydrogen peroxide (H2O2) and HNO3 (PH2O2/PHNO3) is a more robust photochemical indicator than the ratios of their mixing ratios (H2O2/HNO3) and the afternoon mixing ratios of NOy in both months, and it is highly sensitive to the horizontal grid resolution in August. The use of PH2O2/PHNO3 simulated at 4-km indicates a VOC-limited O3 chemistry in urban and suburban areas in August that was not captured in previous model simulations at a coarser grid resolution. 3 NO 


Introduction
Process Analysis (PA) is a useful tool embedded in a 3-D air quality model that calculates the Integrated Process Rates (IPR) for major atmospheric processes such as emissions, chemical reactions, horizontal and vertical transport, and removal processes and the Integrated Re-action Rates (IRR) for all gas-phase chemical reactions in all model grid cells.The results from IPR provide the relative contributions of individual physical and chemical processes to the formation of gas and Particulate Matters (PM) species.These processes include emissions, vertical and horizontal transport, gas-phase chemistry, PM processes, aqueous-phase processes (or cloud processes), and dry deposition.The results from IRR provide individual gas-phase reaction rates that can be used to identify key chemical pathways for ozone (O 3 ) and its precursors, the chemical regimes of O 3 , as well as gaseous precursors of secondary PM with aerodynamic diameter less than and equal to 2.5 m (PM 2.5 ) [1-3].For example, the net production and loss of total odd oxygen (O x ) represent the total oxidation capacity that affects the formation efficiency of O 3 and secondary PM.The list of typical IRR products can be found in Zhang et al. [3].PA has been conducted in several studies to quantify the contributions of atmospheric processes and chemical reactions to the formation of O 3 and PM 2.5 [e.g., [3][4][5][6][7][8][9].All those studies focused only criteria pollutants such as O 3 and PM 2.5 and used a horizontal grid resolution of 36-km or coarser.Very few studies include PA for agriculturally-emitted pollutants such as ammonia (NH 3 ) and ammonium ( 4 NH  ) and are performed at a horizontal grid spacing of 4 -12 km.
In this study, 3-D model simulations and PA are conducted at a horizontal grid spacing of 4-km to simulate O 3 , PM, and their precursors.The objective of this study is to identify the governing atmospheric processes of major air pollutants including both creteria and noncreteria air pollutants and associated seasonalities at a fine grid resolution.An area in the southeastern US that centers over North Carolina (NC) is selected for this study.This area feasures with very high emissions of NH 3 from agricultural livestock, which account for about 91% (i.e., 482.9 tons•day −1 ) in August and 81% (i.e., 253.4 tons•day −1 ) in December of total NH 3 emissions [10].PA over this area allows an understanding of the fate of non-creteria pollutants such as NH 3 , 4 NH  , nitric acid (HNO 3 ), and reduced nitrogen (NH x = NH 3 + 4 NH  ), in addition to that of creteria air pollutants such as nitric oxide (NO), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), O 3 , and PM 2.5 .

Modeling Domain and Simulation Setup
The modeling system consists of the Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR) Mesoscale Modeling System Generation 5 (MM5) version 3.7 [11], the Sparse Matrix Operator Kernel Emissions (SMOKE) Modeling System version 2.1 [12], and the Community Multiscale Air Quality (CMAQ) modeling system version 4.4 [13].The PA tool embedded in CMAQ is used to quantify the contributions of major atmospheric processes of major air pollutants.[10], the model evaluation showed that MM5/CMAQ gave an overall good performance for meteorological variables and O 3 mixing ratios and a reasonably good performance for PM 2.5 .A more detailed description of the model configurations, ICs and BCs, the databases used for the operational evaluation for meteorological and chemical predictions, and the model performance evaluation for both MM5 and CMAQ can be found in Wu et al. [10].
A detailed PA analysis is performed at 17 sites from three surface networks: seven sites from the Speciation Trends Network (STN): Kinston, Asheville, Hickory, Fayetteville, Winston-Salem, Charlotte, and Raleigh; four sites from the Interagency Monitoring of Protected Visual Environments (IMPROVE): GRSM1, LIGO1, SHRO1, and SWAN1; and six sites from the Clean Air Status Trends Network (CASTNET): BFT142, CND125, COW-137, PNF126, SPD111, and VPI120.Their locations are shown in Figure 1.Among the STN sites, Kinston and Fayetteville are located in the Coastal Plain region, Ashville is located in the Mountains, and Hickory, Winston-Salem, Charlotte, and Raleigh are located in the Pidemont region.Among the IMPROVE sites, GRSM, LI-GO1, and SHRO1 are located in the Mountains and SWAN1 is in the Coastal Plain region.Among the CASTNET sites, BFT142 is a Coastal Plain site, CND125 is a Pidemont site, and COW137, PNF126, SPD111, and VPI120 are all located in the Mountains.

Integrated Process Rates
Figure 2 shows the monthly-mean contributions of individual processes to the mixing ratios of gaseous species in the surface layer at the 17 locations in August and December 2002.Emissions provide a dominant source of NH 3 at all STN sites except for Asheville in August and at all STN sites in December.Among the 17 sites, the largest emissions occur at Kinston and Fayetteville where NH 3 emissions from agricultural livestock are high.Transport reduces the mixing ratios of NH 3 at most sites (except for CND125 in August), particularly at Kinston in August and at Kinstron, Fayetteville, and Charlotte in December.Dry deposition also acts as a sink for NH 3 at all sites, particularly at Kinstron, Fayetteville, and CND125 in August and at Kinston in December.PM processes such as gas-to-particle mass transfer convert NH 3 to 4 at most sites in both months.Emissions of NO are high in both months at all STN sites expect for Kinston, providing the main source of nitrogen oxides (NO x = NO + NO 2 ) at these sites.Major loss processes of NO in both months include gas-phase chemistry (i.e., its titration reactions with O 3 ) and horizontal and vertical transport.The same titration reaction of NO with O 3 produces NO 2 , which is the most important source of NO 2 at the STN sites in both months.Emissions of NO 2 provide additional sources at several STN sites including Hickory, Fayetteville, Winston-Salem, and Charlotte.The major loss processes of NO 2 in both months include transport at most sites, particularly at the STN sites, and dry deposition at all sites.Comparing to the STN sites, the process contributions to NH 3 and NO x at the IMPROVE and CASTNET sites are relatively small, due to a lack of pollutant sources in the Costal Plain and mountain regions.While transport contributes to the accumulation of HNO 3 at all 17 sites in August and at all sites except for Kinston, Hickory, and CND125 in December, dry deposition is a major sink of HNO 3 at these sites.Cloud and PM processes also contribute to its sink in December.In August, emissions at all STN sites except for Kinston provide a major source of SO 2 , and transport is the main process to accumulate SO 2 at the IMPROVE and CAST-NET sites except for SWAN1 and COW137.While both dry deposition and transport are important sinks at the STN sites, dry deposition dominates the loss of SO 2 at the IMPROVE and CASTNET sites.In December, emissions provide a main source of SO 2 at all STN sites except for Kinston, Asheville, and Raleigh where transport is either a dominant source or equally important to its emissions.Transport also helps accumulation of SO 2 at all IMPROVE and CASTNET sites.Dry deposition, gasphase chemistry, and cloud processes including aqueousphase chemistry and wet scavenging contribute to the loss of SO 2 at all these sites.In August, O 3 comes primarily from transport and it is lost due mainly to gasphase chemistry at Hickory, Fayetteville, and Winston-Salem, both gas-phase chemistry and dry deposition at Charlotte, and dry deposition at all remaining sites.In December, transport accumulates O 3 at all STN sites except for Kinston, two IMRPOVE sites (LIGO1 and SHRO1) and one CASTNET site (PNF126).Gas-phase chemistry provides a major sink at all sites, in particular at all STN sites except for Kinston.

NH 
Figure 3 shows the monthly-mean contributions of individual processes to the mass concentrations of PM 2.5 and its major composition including sulafte ( 2 , elemental carbon (EC), primary organic aerosol (POA), secondary organic aerosol (SOA), and other inorganic fine PM (OIN) in the surface layer at the 17 sites.For PM 2.5 in August, emissions provide a dominant source at all STN sites except for Kinston and transport helps its accumulation at three IMPROVE sites (i.e., GRSM, LIGO1, and SHRO1) and two CASTNET sites (i.e., PNF126 and VPI120).Transport is a major sink process at most STN sites and dry deposition contributes the most to the loss of PM 2.5 at LIGO1, SHRO1, and PNF126.In December, both emissions and PM processes are important sources of PM 2.5 at most STN sites, and transport helps its accumulation at GRSM, LIGO1, SHRO1, and PNF126.Transport plays a similar role to that in August, depleting PM 2.5 at the STN sites.Dry deposition is a major sink of PM 2.5 at several sites including Ashville, LIGO1, SHRO1, and PNF126.Cloud processes also contribute to the loss of PM 2.5 at all sites in both months.For    NO  at all sites comes primarily from transport in August, and it is removed mainly through PM processes such evaporation back to the gas-phase, aqueous processes such as aqueous-phase chemistry and wet scanvenging, and dry deposition.For comparison, in December, 3 NO  at all STN sites is produced by PM processes such as the condensation of HNO 3 but by transport at all o her sites.O 3 under the favorable weather and chemical conditions at these sites.For EC, POA, and OIN, the main production and loss are emissions and transport, respectively, at most STN sites.OIN may also be produced by emissions at other sites such as SWAN1 and BFT142 in both months or LIGO1, SHRO1, and PNF126 in December.Different from POA, transport is a dominant source for SOA at most sites in August and December.Gas/particle mass transport is a major contributor to SOA formation at Hickory in August and SWAN1 and BFT142 in December.
Figure 4 shows the monthly-mean contributions of individual processes to the mass concentrations of reduced h a major gain from emissions and major loss by transport and deposition at most STN sites in both months.The fate of TNO 3 is dominated by that of HNO 3 , with a major gain from transport and a major loss by deposition at all sites in both months.Figure 6 shows the production and loss rates of O x , OH reacted with AVOCs and BVOCs (referred to as OH-A August VOCs and OH-BVOCs hereafter), the production of NO 2 from the reactions involving hydroperoxy radicals (HO 2 ) and organic peroxy radicals (RO 2 ), and the productions of HNO 3 due to the reaction of OH with NO 2 and that of VOCs with nitrate radical (NO 3 ) at the 17 sites in both months.At all locations, O x production rates are higher than its loss rates by factors of 2.7 -14.1 in August and 4.9 -10.7 in December.The production rate of O x is much higher at most STN sites (except for Kinston, which is an agricultural site with very high NH 3 emissions located in Coastal Plain region) in August than other sites.In December, the production rates of O x at all sites are overall similar, with higher values at several ppb by Sillman [14].The values of NO y larger than these threshold values indicate a VOC-limited O 3 chemistry, otherwise a NO x -limited O 3 chemistry.Among these three photochemical indicators, PH O /PHNO has been mountain sites such as COW137, Asheville, and Hickory.The rates of OH-AVOCs and OH-BVOCs are much higher at all sites in August than in December.In August, the rates of OH-BVOCs are higher than those of OH-AVOCs at all rural sites including Kinston, Asheville, GRSM1, LIGO1, SHRO1, SWAN1, BFT142, CND125, COW137, PNF126, SPD111, and VPI120, because of higher BVOCs emissions at these sites.In December, the rates of OH-AVOCs are higher than those of OH-BVOCs at all sites.O 3 is produced through the photolysis of NO 2 followed by the reaction of atomic oxygen (O) with molecular oxygen (O 2 ).Most NO 2 come from the conversion of NO by HO 2 and RO 2 radicals.As shown in Figure 6, the amount of NO 2 produced by the reactions involving HO 2 is similar to that involving RO 2 at all sites in August, with slightly higher production rates from the NO + RO 2 reaction at the rural sites than at the urban sites.It is, however, higher than that by the reactions involving RO 2 in December.This indicates that VOCs contribute to O 3 formation similarly to NO x in August, but less than that of NO x due to a VOC-limited O 3 chemistry in December.The production rate of HNO 3 due to the reaction of OH with NO 2 dominates over that due to the nighttime reactions of VOCs with NO 3 radicals in both months, with much higher reaction rates of OH + NO 2 (by up to a factor of 42.6) at urban sites than other sites in August and more uniform reaction rates of OH + NO 2 (within a factor of 3) at all sites in December.

Integrated Reaction Rates (IRR)
The ratio of the production rates of H 2 O 2 and HNO 3 (PH 2 O 2 /PHNO 3 ) is a useful indicator for O 3 photochemistry that is calculated in IRRs.The threshold value of PH 2 O 2 /PHNO 3 is 0.2, values below which indicate a VOC-limited O chemistry and at or above which 3 indi cate a NO x -limited chemistry [14,15].The ratio of the mixing ratios of H 2 O 2 and HNO 3 (H 2 O 2 /HNO 3 ) and NO y mixing ratios in the afternoon have also been frequently used as photochemical indicators, with a range of threshold values suggested by several studies accounting for differences in meteorological and chemical conditions for measurements or model configurations such as horizontal grid resolutions and airsheds used in modeling studies.For example, the threshold value proposed for H 2 O 2 /HNO 3 was 0.2 by Sillman et al. [16], Tonnesen and Dennis [17], and Hammer et al. [18], 0.4 by Sillman [14], 0.8 -1.2 by Lu and Chang [19], and 2.4 by Zhang et al. [3].The values of H 2 O 2 /HNO 3 below these threshold values indicate a VOC-limited O 3 chemistry, otherwise a NO x -limited O 3 chemistry.The threshold value proposed for NO y was 3 -5 ppb by Lu and Chang [19], 5 ppb by Zhang et al. [3], 10 -25 ppb by Milford et al. [20] and 20 the most robust one in both summer and winter months [3].It is therefore selected as a benchmark to determine the robustness of H 2 O 2 /HNO 3 and NO y as a photochemical indicator for O 3 chemistry in this work.n of 36-km and showed values of PH 2 O 2 /PHNO 3 of 0.4 -2.4 over urban and suburban areas and higher values over the remaining areas in the simulation domain used in this study.Despite a different year (i.e., 2002) and emissions, the use of a much higher horizontal grid resolution of 4-km in this work shows a VOC-limited chemistry in urban and suburban areas that is not shown in the simulation at 36-km in Zhang et al. [3], demonstrating the benefit of the fine-scale modeling.The values PH 2 O 2 /PHNO 3 are below 0.2 in December in nearly the whole domain, indicating a VOC-limited O 3 chemistry, which is consistent with the O 3 chemical regime in December 2001 obtained by Zhang et al. [3].The comparison of this work and Zhang et al. [3] indicates that the predictions of PH 2 O 2 / PHNO 3 are highly sensitive to the horizontal grid resolution in summer but insensitive to it in winter.For H 2 O 2 /HNO 3 , all values are above 0.2 and nearly all values are above 2.4 in August, indicating a NO x -limited chemistry that is consistent with that based on PH 2 O 2 / PHNO 3 .In December, using a threshold value of 2.4 will indicate a VOC-limited O 3 chemistry in most of the domain except for an area in the eastern NC in the Coastal Plain region, which is consistent with that based on PH 2 O 2 /PHNO 3 .For NO y , in August, the threshold value of 10 ppb gives similar VOC-limited O 3 chemistry over urban and suburban areas and NO x -limited O 3 chemistry over remaining areas as compared to that indicated by PH 2 O 2 /PHNO 3 .In December, the lower the threshold value is, the more consistency can be obtained for areas with the VOC-limited

Summary
The process analysis is performed at a horizontal grid spacing of 4-km over an area in the southeastern US that is centered over NC for August and December, 2002.Emissions provide a dominant source for primary pollutants such as NH 3 , NO, and SO 2 , and an important source for some secondary pollutants such as NO 2 at all sites in August and December.While transport acts as a major sink for these pollutants, it provides a major source for HNO 3    for some species such as NO 2 and HNO 3 but a sink for other species such as O 3 at urban and suburban sites and NO and SO at all sites.The roles of these processes in August, transport provides a dominant sink for PM 2.5 and most of its composition except for and SOA at most STN sites, and it acts as a source secondary in- SO  and 3 NO  , and SOA at the IMPROVE and CASTNET sites.In December, transport provides a sink for all PM sp STN sites but ecies at most a source for most PM species at the IMPROVE and CASTNET sites.Dry deposition is an important sink for all PM species in both months.The fate of NH x is dominated by that of NH 3 , whereas the fate of TNO 3 is dominated by that of HNO 3 .
The total O x production and loss, and the total OH reacted are much higher in August than in December, particularly at most STN sites, indicating a higher oxidation capacity in August than in December and at urban and suburban sites than at rural sites.The est O x production and loss occur in urban and suburban areas in the Piedmont and Mountain regions t more uniformly throughout the simulation domain in December.The amount of OH reacted with major gases is much higher over urban and suburban areas in August and over the southern portion of the domain in December.Significant amounts of OH are consumed by BVOCs in the rural and remote areas and a combination of AVOCs and BVOCs in urban and subareas ar high s bu eas in both months.The rates of O tes a similar The 3-D model simulations are conducted for August and December of 2002 at a 4-km horizontal grid spacing over a domain that covers nearly the entire state of NC, and a portion of several adjacent states including South Carolina (SC), Georgia (GA), Tennessee (TN), West Virginia (WV), and Virginia (VA), as shown in Figure 1.This area consists of three well-developed physiographic divisions from east to west: the Coastal Plain, the Pidemont, and the Mountains.The complex topography and weather patterns as well as a combination of industrial, agricultural, traffic, and biogenic emissions make this area one of the most complex and representative airsheds in the US.The model input files for initial and boundary conditions (ICs and BCs) and meteorology at a 4-km horizontal grid spacing are developed based on the MM5/CMAQ model simulations at a 12-km horizontal grid spacing obtained from the Visibility Improvement State and Tribal Association of the Southeast's (VISTAS) 2002 modeling program (http://www.vista-sesarm.org.asp).For consistency, the model configurations and options for physics and chemistry for the MM5/CMAQ simulations at 4-km in this work are set to be the same as those used in the 2002 base year VISTAS Phase II modeling study at 12-km.The vertical resolution includes 19 layers from surface to the tropopause (~15 km) with ~38 m for the first layer height.The emission inventories for gaseous and PM species are based on the VISTAS 2002 emissions.As described in the work of Wu et al.

4 NHFigure 2 .
Figure 2. The monthly-mean process contributions to the surface mixing ratios of NH 3 , NO, NO 2 , HNO 3 , SO 2 , and O 3 in −1 ppb•hr during August and December 2002.

Figure 3 .
Figure 3.The monthly-mean process contributions to the surface concentrations of PM 2.5 , , EC, POA, SOA, and OIN in μg•m −3 •hr −1 during August and December 2002.

3 NO
 is reduced by transport at all STN sites and additionally by dry deposition at tains as t and CASTNET sites (except for SPD111) and additionally by dry deposition at three mountain sites (i.e., LIGO1, SHRO1, and PNF126).The gain and loss of 3 NO  in December show a strong correlation with those of 4 NH  at all STN sites, indicating the formation of x = NH 3 + 4 NH  ) and total nitrate (TNO 3 = HNO 3 + 3 NO  ) in the surface layer.The fate of NH x is dominated by that of NH

Figure 5
Figure5shows the spatial distributions of the mean production and loss of O x , the total hyd

Figure 4 .Figure 5 .
Figure 4.The monthly-mean process contributions to the surface concentrations of NH x in ppb•hr −1 and TNO 3 in μg•m −3 •hr −1 during August and December 2002.

Figure 6 .
Figure 6.The monthly-mean production and loss rates of O x , the rate of OH reacted with AVOCs and BVOCs, the NO 2 production rates by the reaction of HO 2 and RO 2 , and the HNO 3 production rates by the reactions of OH + NO 2 and NO 3 + VOCs at seven STN sites: Kinston, Asheville, Hickory, Fayetteville, Winston-Salem, Charlotte, and Raleigh; four IMPROVE sites: GRSM1, LIGO1, SHRO1, and SWAN1; and six CASTNET sites: BFT142, CND125, COW137, PNF126, SPD111, and VPI120 in August and December 2002.

Figure 7
shows simulated spatial distributions of three photochemical chemical indicators: PH 2 O 2 /PHNO 3 , H 2 O 2 / HNO 3 , and NO y mixing ratios in the afternoon (noontime-6 pm) in both months.In August, the values of PH 2 O 2 /PHNO 3 over most areas are above 0.2, indicating a NO x -limited O 3 chemistry.Those over urban and suburban areas are below 0.2, indicating a VOC-limited O 3 chemistry.Zhang et al. [3] performed a 1-year process analysis in 2001 using the PA tool in CMAQ over the continental US at a horizontal grid resolutio -

Figure 7 .Figure 8
Figure 7. Simulated spatial distributions of monthly-mean PH 2 O 2 /PHNO 3 , H 2 O 2 /HNO 3 , and afternoon (noon-6 pm) NO y mixin os in August and December 2002.winter,and a greater adjustment (e.g., adjusting the threshold value of H 2 O 2 /HNO 3 from 0.2 to 11 (instead of suggested 2.4 by Zhang et al.[3]) and that of NO y from 20ndicating O 3 chemistry regimes in winter.
and O 3 at most sites.Dry deposition is an important sink f Simulated PH 2 O 2 /PHNO 3 Observed and Simulated NO y

Figure 8 .
Figure 8.The temporal variations of simulated ratios of hourly production rates of H 2 O 2 and HNO 3 (PH 2 O 2 /PHNO 3 ) and observed and simulated afternoon (noon-6 pm) NO y mixing ratios at four sites in NC in August 2002.The solid and dash lines indicate the original and adjusted threshold values, respectively.

[ 1 ]
J.-C. C. Jang, H. E. Jeffries and S. Tonnesen, "Sensitivity esolution-II.Detailed Process istry," Atmospheric Environ-H-BVOCs are higher than those of OH-AVOCs at all rural sites in August because of higher BVOCs emissions but the opposite occurs in December.The amount of NO 2 produced by the reactions involving HO is similar to 2 that involving RO 2 at all sites in August but higher than that by the reactions involving RO 2 in December.The production rate of HNO 3 due to the reaction of OH with NO 2 dominates over that due to the nighttime reactions of VOCs with NO 3 radicals in both months.The values of PH 2 O 2 /PHNO 3 indicate a NO x -limited O 3 chemistry over most areas in August and a VOC-limited O 3 chemistry over all areas in December, which is consistent with previous studies [e.g., 3,9].They also indicate a VOC-limited O 3 chemistry in urban and suburban areas in the simulation domain in August that is not found in previous model simulations at a coarser grid resolution.The values of PH 2 O 2 /PHNO 3 are highly sensitive to the horizontal grid resolution in summer but insensitive to it in winter.H 2 O 2 /HNO 3 with a threshold value of 2.4 can indicate O 3 chemistry regimes that are overall consistent with those based on PH 2 O 2 /PHNO 3 over most of areas in both months.Simulated afternoon NO with a threshold value of 10 ppb indica y O 3 chemistry regime to that indicated by PH 2 O 2 /PHNO 3 in August.Its threshold value in December may need to be adjusted to be below 5 ppb to make it a more robust photochemical indicator.The O 3 chemistry regimes indicated by PH 2 O 2 /PHNO 3 at several sites are consistent with those indicated by observed afternoon NO y values at these sites when a threshold value of 10 ppb or lower is used in August.When the simulated NO y values deviate significantly from observed NO y values, they may not be as robust as PH 2 O 2 /PHNO 3 to indicate the O 3 chemistry regime.