ISSUES RELATED TO NO2 NAAQS MODELING Presented to North Texas Chapter of the Air & Waste Management Association By James Red Providence Engineering and Environmental November 17, 2015 1
OUTLINE Overview Chemistry Back to Permitting EPA s 3 Tier Approach Culpability 2
OVERVIEW The NO2 NAAQS was originally promulgated in 1971 as 53 ppb (100 ug/m3), annual average. EPA did not change the standard during reviews in 1985 or 1996, affirming the 1971 standard. But then came 2010. 3
OVERVIEW January 22, 2010, US EPA announced1 a new 1-hour NO2 NAAQS of 100 ppb (188 mg/m3) (Level) The NO2 standard is the 3-year average of the 98th percentile (8 th highest) of the annual distribution of daily maximum 1-hour concentrations. (Form) [Significant Impact Level (SIL) of 4 ppb (7.5 mg/m 3 )] 5-year average for modeling
OVERVIEW Since promulgation of the new standard, there have been no areas (Texas or nationwide) that have been designated as nonattainment for it. (Yea!) DFW current design value = 49 ppb 5
OVERVIEW So, the monitoring data looks good, but what if I want to build a new facility that emits NOx? Won t that affect ambient concentrations of NOx (and therefore, NO2)? I must obtain a construction permit from TCEQ and demonstrate that my new or increased emissions will not cause or contribute to an exceedance of any NAAQS Since I can t measure my impacts before construction, dispersion modeling is used 6
CHEMISTRY Although the NAAQS is based on NO2, emission estimates and calculations are typically based on total oxides of nitrogen (NOx) NOx = NO (nitric oxide) + NO2 (nitrogen dioxide) So, where does NOx come from? 7
CHEMISTRY Combustion CH 4 + 2(O 2 + 3.76N 2 ) -> CO 2 + 2H 2 O + 7.52N 2 (0% EA) CH 4 + 1.25 x 2(O 2 + 3.76 N 2 ) -> CO 2 + 2H 2 O + 0.5O 2 + 9.4N 2 (25% EA) NO & NO 2 Formation: N 2 + O 2 = 2NO ½ N 2 + O 2 = NO 2 8
CHEMISTRY Basic NOx cycle: NO 2 + uv NO + O - O 2 + O - O 3 NO + O 3 NO 2 + O 2 Or 1 mole of O 3 converts 1 mole of NO to 1 mole of NO 2 9
CHEMISTRY NO 2 + uv NO + O - (photolysis) Occurs much further downwind Neglected in NO2 permit modeling Conversion of NO to NO2 by ozone more close in to source Much more important 1 mole of O 3 converts 1 mole of NO to 1 mole of NO 2 More on this in a moment 10
BACK TO PERMITTING. Easy as 1-2-3 (or is it?) 1. Model (using AERMOD) proposed new or modified NOx sources at proposed maximum hourly rates 2. If maximum concentration < SIL (4 ppb or 7.5 ug/m3), exercise is complete 3. If > SIL, determine background from modeled external background sources and ambient monitoring data and compare cumulative/total impact to NAAQS 11
BACK TO PERMITTING. If my impact exceeds the SIL, what do I do next? Define how far out I exceed the SIL Request a point source emissions inventory from the State (Air Permits Allowable Database) that includes all sources within that area Model my sources, plus the retrieved sources, over all receptors where I modeled over the SIL Is that all? Nope 12
BACK TO PERMITTING. NAAQS is a total ambient measure, so what about non-point sources of NOX, like on-road mobile sources, non-road mobile sources, area sources, un-inventoried point sources, and biogenic sources? Obtain ambient monitoring data and add to the modeled impacts. 13
BACK TO PERMITTING. Easy enough if you have a monitor nearby to your location (within ~10 km) (but might double count impact of modeled sources) For example, urban areas like DFW typically have a good network of monitors: 14
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BACK TO PERMITTING. However, in many instances, a remote or regional monitoring site must be used and justified. That is, how is the proposed NOx source site similar to the monitoring site: Similar terrain? Similar climatology? Similar nearby emissions? County-wide down to 10 km radius This may take a little time. 16
NO2 3-TIER APPROACH Because of stringency of new 1-hour NO2 NAAQS, refinements to the modeling approach are needed Recall that emissions are typically NOx, but standard is NO2 EPA s Guideline on Air Quality Models lists a 3-tiered approach for refining NO2 impacts 17
NO2 3-TIER APPROACH Tier 1: Assume that all of the NOX is NO2 (extremely conservative) Tier 2: Use an ambient ratio of NO2/NOX (EPA current default for 1-hour NO2 is 0.8. Better, but still conservative) Tier 3: Refined screening techniques that make use of the NO + O 3 NO 2 + O 2 relationship 18
NO2 3-TIER APPROACH Tier 2 Ambient Ratio Method (ARM) Ties model predictions to actual observations Although 0.8 is national default, alternative ratio may be used if documented/defensible *NEW* EPA currently proposing to replace ARM with ARM2 (stay tuned!) 19
NO2 3-TIER APPROACH ARM2 (from API) A variable NO2/NOx ambient ratio or conversion curve was derived from the AQS monitoring data. This approach is less conservative than a fixed ratio. Because of the large number of data points, the data was sorted into bins from 20 to 600 ppb NOx each bin was 20 ppb wide. A reasonable upper bound ratio was estimated for each bin using the 98th percentile. The tail ends of the curve were limited to a maximum 0.9 and minimum 0.2 ratio. (but EPA proposal is for a minimum of 0.5) 20
NO2 3-TIER APPROACH ARM2 EPA proposed min API min 21
NO2 3-TIER APPROACH Tier 3 Refined screening techniques Ozone Limiting Method (OLM) Plume Volume Molar Ratio Method (PVMRM) Currently require approval from permitting authority (State or EPA) but EPA proposing to make these regulatory options (stay tuned!) 22
NO2 3-TIER APPROACH Tier 3 This approach assumes that ambient concentrations of ozone can convert NO to NO2 This does not account for downwind photolysis of NO2 in the plume Inputs required are: NO2/NOX in-stack ratio (initial condition) Equilibrium ratio (default is 0.9) Ozone data 23
NO2 3-TIER APPROACH Tier 3 In-stack ratio how much of the NOX in the emitted plume is already NO2, and how much of what is left can be converted from NO to NO2 Back in the day, we assumed it was 0.1 (10 percent), but EPA s national default is now 0.5 If you have measured or other test data for your particular source, you may use it 24
NO2 3-TIER APPROACH Tier 3 EPA has begun a project to compile in-stack ratios for various types of combustion sources, based on validated stack test data: http://www3.epa.gov/scram001/no2_isr_database.htm However, it s not anywhere near being a complete compendium only about 2,300 entries at this point. 25
NO2 3-TIER APPROACH Tier 3 Ozone data Can be input to AERMOD in several temporal flavors (but all based on measured ozone data): Annual a single hourly value applied to each hour of the year Temporally varying: Season, Month, Day of Week, Hour/Day of Week, etc or Hourly file of ozone data (8760 or 8784 hours). Time period/year of ozone data has to match meteorological data 26
NO2 3-TIER APPROACH Tier 3 OLM or PVMRM? EPA has no preference for one over the other OLM uses the ozone concentration as being at the receptor. OLM calculates NO 2 based on all O 3 as being available to the NO from each source. PVMRM calculates the volume of the plume from the stack to the receptor and uses the amount of ozone within the plume. 27
NO2 3-TIER APPROACH OLM example: NO 2 /NO X = 0.4, [100]NO X, BG of [50]O 3 [100]NO X = [40]NO 2 + [60]NO [40]NO 2 + [60]NO + [50]O 3 = [40]NO 2 + [50]NO 2 + [10]NO + [50]O 2 28
NO2 3-TIER APPROACH Tier 3 PVMRM calculates the conversion of NO X to NO 2 based on the NO X moles emitted into the plume, and the amount of O3 moles contained within the volume of the plume between the source and receptor. EPA has proposed a modified version of PVMRM, called PVMRM2, to be included as the regulatory version in AERMOD (Stay tuned!) 29
CULPABILITY What if I ran my cumulative 1-hour NO2 modeling, and I have predicted exceedances of the NAAQS? Options can include scrubbing down the State-supplied inventory it will have errors in it But if that s been completely corrected, I can look at determining source culpability No cause or contribute to an exceedance of the NAAQS 30
CULPABILITY What does cause or contribute to an exceedance mean? Can t exceed the standard all by myself or Can t exceed the standard at a receptor and a time period when/where I exceed the SIL There can be predicted exceedances of the standard due to the background sources, but if you re below the SIL in those cases, you do not contribute 31
CULPABILITY There is a way within AERMOD to determine if your impacts are contributing to the NAAQS exceedance. Output option called MAXDCONT However, only programmed to work for 1-hour NO2, 1-hour SO2, and 24-hour PM2.5 Requires setting up Source Groups at least ALL and your sources Goes through user-defined ranks of outputs and or a threshold (NAAQS) to assess your contribution to ALL s concentration 32
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CULPABILITY MAXDCONT caveat: Even though you may show no significance at the NAAQS rank (98 th percentile = 8 th high), you must follow MAXDCONT ranks down until no more exceedances show up AND continue to show that your sources were not significant! 34
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Any questions? 36
PRESENTED BY: James Red Providence Engineering and Environmental jamesred@providenceeng.com (512) 770-8862 (828) 628-0634 37