San Gabriel Mountains: photo credit Alex Cameron The Challenge of Modeling the Chemistry of Ozone
Greg Yarwood [email protected]
Health Effects Institute Annual ConferenceHEI 2017 May 2, 5/2/172017 Topics
Chemical life cycle of nitrogen oxide (NOx) emission
Chemical controls over NOx removal and potential for NOx recycling
NOx influence on distributions of ozone concentration Organic nitrate (ON) removal by condensed-phase hydrolysis Nitrate radical + terpene reactions
HEI 2017 5/2/17 Chemical life cycle of NOx emission
HONO Organic Nitrate
NO ON SOA
O3 PAN aerosol NO2
NOx recycling NO3 HNO3 pNO3 ON hydrolysis NO3 + terpene N2O5
HEI 2017 5/2/17 Daytime NOx chemical removal: Urban/rural differences
Ozone (ppb) Removing NOx stops it from participating in ozone formation
Urban (higher NOx): three pathways all active
Rural (low NOx): organic nitrate (ON) pathway becomes dominant
NO to PAN (ppb/h) NO to ON (ppb/h) NOx (ppb) NOx to HNO3 (ppb/h) x x
HEI 2017 5/2/17 NOx recycling: Distributing NOx to rural areas
Chemical reactions of ON, HNO3 and PAN can release NOx: termed “NOx recycling”
Uncertain magnitude of NOx recycling from ON: • Extent of ON partitioning to aerosol • Rate of ON hydrolysis within aerosol
NOx recycling from PAN mediated by PAN decomposition: T1/2 of 40 mins @ 298K vs. 40 hours at 273K
PAN to NO (ppb/h) 0.02 ON to NOx (ppb/h) x 0.4 NOx to ON (ppb/h) 0.4 NOx to PAN (ppb/h)
0.1
HEI 2017 5/2/17 Modeling H4MDA8 Ozone variation with NOx and VOC
This research funded by the American • Nationwide photochemical modeling Petroleum Institute (API) (WRF/CAMx) • Quantify emission reductions from 2006 to attain various potential National Ambient Air Quality Standards (NAAQS) • Novel application of ozone sensitivity (DDM) allows continuous and independent variation of NOx & VOC emission
H4MDA8 O (ppb) 3
• Provides long (annual, ozone season) simulated ozone time-series at
adjustable NOx & VOC emission level
H4MDA8 = annual fourth highest maximum daily 8-hour average HEI 2017 5/2/17 Reducing NOx changes the ozone distribution
Emissions
“Downtown” • Emission reductions truncate the high tail of maximum daily 8-hour (MDA8) and hourly ozone • Emission reductions also can truncate a low tail of suppressed ozone
• Especially “downtown”
• More pronounced for hourly than MDA8 ozone
“Downwind” • More impact for full year than ozone season (not shown) • As anthropogenic emissions -> zero the distribution of background ozone appears
HEI 2017 5/2/17 Ozone exposure (ppb hours) recalcitrant to reducing emissions
annual
ozone season
% 2006 anthropogenic emissions % 2006 anthropogenic emissions
HEI 2017 5/2/17 ON formation and examples
NO +
RO2 + NO2 Precursor Organic Nitrate
n-octane
1-butene
toluene
From the Master Chemical Mechanism (MCM) version 3.3.1
HEI 2017 5/2/17 Organic nitrates removed by condensed-phase hydrolysis alpha-pinene derived nitrate (APN) • acid-catalyzed mechanism • nitric acid formed • rapid for APN at relevant pH • consistent with findings from field studies in south eastern J. D. Rindelaub et al., 2016 United States APN • slower for small ON molecules
HEI 2017 5/2/17 Nitrate radical + terpenes: Remove NOx and form secondary organic aerosol (SOA)
SOAS - Alabama
B. R. Ayres et al., 2015
• Terpenes dominate NO3 radical loss at night and are important during the day • Inferred molar yield of aerosol-phase monoterpene ON of 23–44 % • ON comprised 30–45% of the total reactive
nitrogent (NOy) budget during Southern Oxidant and Aerosol Study (SOAS)
HEI 2017 5/2/17 Thank You
Reminders:
• NOx recycling: Distributing NOx to rural areas
• Ozone exposure recalcitrant to reducing NOx emissions
• Organic nitrates removed by condensed-phase hydrolysis
• Nitrate radical + terpenes: Removing NOx and forming SOA
HEI 2017 5/2/17