SAPRC07T Implementation within the CMAQ model. Bill Hutzell and Deborah Luecken (AMDB/AMAD) 2009 CMAS Conference October 20, 2009 9/10/2018

Why bother with SAPRC Mechanisms? It provides an alternative approach for approximating photochemical reactions. Lumped species based on emissions and reactivity More easily expanded or de-lump, using their documentation Historically, SAPRC mechanisms have been more comprehensively evaluated. Evaluation used chamber studies Peer reviewed Many regulatory, i.e., CARB, and research groups prefer SAPRC mechanism.

What does the upgrade provide? Updated reaction kinetics and branches VOC species and reactions represents more compounds New operators addressing completion within VOC and peroxy reactions with NO radicals Alters production of radicals, peroxides and other oxygenated compounds Improved chemistry of aromatics and organic acids Optional chemistry of chlorine compounds Possibly better mechanism for SOA precursors

What version of SAPRC07? SAPRC07B BASE or first version, released in 2007 CS07A and CS07B Simplified versions of the BASE Mimics CB05 in # of species and reactions SAPRC07A Retro-version of the BASE We are using SAPRC07T Actually using the “B” variation

Why use SAPRC07T? Explicitly represents several hazardous air pollutants. Such as acetaldehyde, 1,3-butadiene, acrolein, and xylene isomers Includes organic acids needed to better represent chemistry within cloud droplets. Specifically peroxyacetic and higher organic acids Adds compounds to chlorine chemistry enables chlorine production from sea salt aerosols Specifically CLNOy compounds used in the Knipping and Dabdub (2003) mechanism

What is the price? Using current data structures in CMAQ photochemical mechanisms. Mechanism would contain over 600 reactions Algebraic manipulation can reduce the number for a compressed version Hence, the “B” variation Approximately 330 reactions Code development need to implement “B” version of SAPRC07T To describe and calculate new rate expressions used

Simulations January and July 2002 MM5 meteorology Two nested domains with 14 layers 36X36 km2 over the continental United States 12X12 km2 over the eastern United States 2002 NEI version 3.0 Simulations conducted CMAQ version 4.7 Using either SAPRC99TX3 and SAPRC07T mechanism

How do SAPRC07T predictions compare with its predecessor? SAPR07T averages tend to have higher values at lower concentration. Overall, SAPRC07T predictions have a smaller range of concentrations at a site. Averages determined from hourly observations at monitoring sites located in 12X12 km2 domain

O3, 8-Hour Averages: SAPRC07TB-SAPRC99TX3 However, SAPRC07T has slight more ozone at some rural location while predicting less ozone in urban areas.

Comparing regulatory standards between mechanisms Regulatory measures show similar patterns. Largest differences occur in urban (high NOx areas) with SAPRC99TX3 higher.

Bulk Comparison to Observations The Taylor diagram indicates that the SAPRC99TX3 and SAPRC07T have similar precisions over time and space at matching observations. The left-hand plot shows that the mechanism have similar error ozone and benzene predictions but formaldehyde and acetaldehyde error might increase for SAPRC07T.

Closer Look at Sites observations: Ozone Distributions Both mechanisms produce almost identical distributions of hourly ozone predictions and fail to match the observed frequency of ozone concentrations at the lower range.

Hourly Ozone : Comparison to observations SAPRC99TX3 SAPRC07T For individual sites, the mechanism produce almost identical plots for accuracy and precision measures. Errors increase for the lower range of concentration and predictions at given sites tend to under predict standard deviations in their observed ozones.

Ozone versus Location: Normalized Mean Bias , July 2002: SAPRC99TX3 Ozone versus Location: Normalized Mean Bias , July 2002: SAPRC99TX3 SAPRC07T The NMB versus location does not show much difference between mechanism. Errors increase in rural areas and southern portion of the domain. Does this suggest failures in reactions involving biogenic compounds or in low NOx chemistry.

Ozone, July 2002: Normalized Predicted s SAPRC99TX3 SAPRC07T However, the mechanism show some difference in the predicted range of concentrations.

Zoom: On region with differences SAPRC99TX3 SAPRC07T Note that the area around Lake Michigan show difference in predicted standard deviation. Does this difference show an interplay between being up and down wind of the high NOx regions such as Chicago.

Why do both mechanisms fail to simulate the observed variability? Boundary Conditions For 36X36 km2 grid, BC’s for Ozone set to 35 ppb Observed minimum 20 ppb lower Photolysis rates Temperature and Pressure effect generally neglected in their s and f values  2/3 of rates observed to show these dependencies Reaction schemes within mechanisms HOx cycle: isoprene scheme needs updates NOx cycle: several reactions uncertain and controversial such as NO2* and NO2-OH reactions Note that the items under the last bulletin are ongoing development activities in AMAD such as Ying’s and Sarwar’s research.

Why use SAPRC07T instead of SAPRC99TX3? Attempts to correct SAPRC99TX3’s shortcomings Products from aromatic oxidation VOC oxidation and peroxy reactions under low NOX Chlorine chemistry for urban environments More alkenes are explictly represented in radical cycles HAPs such as Xylene isomers and Toulene Biogenic compounds such as Alpha-Pinene Better potential pathways for Secondary Aerosol Production (SOA) Via higher organic peroxides Prior HAPs participate in ozone chemistry Truer to the Multi-Pollutant Concept than current version 1,3-Butadiene and Acrolein

Implications for HAPs: Higher Acrolein Predictions over Eastern States.

SAPR07T has photochemical sources besides 1,3-Butadiene Specifically, Explicit and Lumped Alkenes Species

Further Work For July 2002 simulations Analysis for individual as to setting Examine NOy cycling Analyze predictions from January 2002 simulations Sensitivity Tests Detailed investigation using IRR Temperature and Pressure effects on photolysis rates Emission reduction scenarios

Conclusions Implementation of SAPRC07T within CMAQ model possible Step require revised how model describes and solved a photochemical mechanism SAPRC07T gives similar ozone prediction to SAPRC99TX3 Some variability is lost in ozone predictions Both old and new version compared similar with observations SAPRC07T has slightly higher errors for aldehydes Still recommend moving to SAPRC07T corresponds current science in other photochemical mechanisms such as RACM2 Better suited future air quality applications such as SOA and Multi-Pollutant assessments

Slides with supplementary information

Comparing regulatory standards between mechanisms Largest differences occur in urban (high NOx areas) with SAPRC99TX3 higher.

What development is needed for CMAQ implementation? Need new expression for reaction rate constants Not true constants Sums of products, i.e., true rate constant times species concentrations CMAQ mechanism files require a modified pre-processor programs Additional data block contains new expressions Solvers require modified routines that use the new rate expressions