The oxidizing capacity of the troposphere is controlled, to a large extent, by the abundance of hydroxyl radical (OH). The global mean concentration of OH, [OH]GLOBAL, inferred from measurements of methyl chloroform, has remained relatively constant during the past several decades, despite rising levels of CH4 that should have led to a steady decline. Here we examine other factors that may have affected [OH]GLOBAL, such as the overhead burden of stratospheric O3 and tropospheric H2O, using global OH fields from the GEOS-CHEM Chemistry-Climate Model. Our analysis suggests these factors may have contributed a positive trend to [OH]GLOBAL large enough to counter the decrease due to CH4

Canty, T.

Duncan, Bryan

Lang, Chang

Liang, Qing

Nicely, Julie M.

Oman, Luke David

Salawitch, R.J.

Stolarski, Richard S.

Waugh, Darryn

NASA Technical Reports Server

	  	  
 
1.	  Scien)ﬁc	  mo)va)on	  for	  studying	  tropospheric	  OH	  	  
Sensitivity of OHTROP to climate 
change 
 
• 	  IPCC	  (2001)	  predicted	  future	  OH	  would	  fall	  due	  to	  rising	  CH4	  
• 	  Best	  knowledge	  of	  global	  tropospheric	  OH	  (OHTROP)	  comes	  	  
	  	  	  from	  analysis	  of	  CH3CCl3	  observaGons	  
• 	  The	  CH4	  lifeGme	  inferred	  from	  OHTROP	  based	  on	  CH3CCl3	  is	  8.9	  	  
	  	  	  years;	  yet	  many	  sources	  use	  a	  CH4	  lifeGme	  of	  ~12	  years,	  which	  	  
	  	  	  was	  found	  by	  modeling	  studies	  described	  in	  IPCC	  (2001)	  
• 	  The	  12	  year	  CH4	  lifeGme	  is	  central	  to	  subsequent	  IPCC	  reports	  	  
	  	  	  and	  is	  called	  the	  “perturbaGon	  lifeGme”	  
• 	  Our	  work	  is	  moGvated	  	  by	  understanding:	  
	  	  	  	  	  	  	  	  –	  what	  factors	  other	  than	  rising	  CH4	  will	  aﬀect	  OHTROP	  
	  	  	  	  	  	  	  	  –	  veracity	  of	  noGon	  that	  future	  OHTROP	  will	  decline	  
Importance of OHTROP 
 
• 	  OH	  serves	  as	  the	  main	  sink	  for	  many	  species:	  CO,	  CH4,	  	  
	  	  	  CH3CCl3,	  all	  HCFCs	  and	  HFCs,	  biogenic	  VOCs	  (isoprene	  and	  	  
	  	  	  terpene),	  anthropogenic	  VOCs	  (formaldehyde,	  benzene,	  	  
	  	  	  toluene,	  etc.)	  
• 	  HOx	  is	  integral	  to	  O3	  formaGon	  and	  destrucGon	  pathways;	  	  
	  	  	  alone,	  HOx	  depletes	  O3,	  but	  with	  NOx	  and	  VOCs,	  HOx	  creates	  O3	  
Methane Oxidation and OH 
NOx	  emissions	  from	  industry,	  
power	  generaGon,	  traﬃc,	  
domesGc	  heaGng,	  and	  biomass	  
burning	  used	  as	  input	  for	  prior	  
CCM	  calculaGons,	  for	  years	  2000	  
(38.0	  Tg	  N	  /	  yr	  total)	  and	  2030	  
(67.6	  Tg	  N	  /	  yr	  total).	  	  From	  
Eyring	  et	  al.,	  ACP,	  2007.	  
RCP	  CH4	  
Scenarios	  
IPCC	  
(2001)	  
*	  
• 	  Under	  low	  NOx,	  the	  oxidaGon	  of	  CH4	  destroys	  HOx	  	  
• 	  Under	  high	  NOx,	  the	  oxidaGon	  of	  CH4	  produces	  HOx	  	  
• 	  Loss	  of	  methylperoxy	  radical	  CH3O2	  with	  NO	  or	  HO2	  is	  cri)cal	  
• 	  Future	  levels	  of	  CH4	  and	  NOx	  are	  highly	  uncertain	  
	  	  	  	  	  	  	  	  –	  CH4	  between	  now	  and	  2100	  varies	  dramaGcally	  among	  	  
	  	  	  	  	  	  	  	  	  	  	  	  RCP	  scenarios	  
	  	  	  	  	  	  	  	  –	  NOx	  between	  now	  and	  2100	  will	  depend	  on	  whether	  the	  
	  	  	  	  	  	  	  	  	  	  	  	  developing	  world	  implements	  selecGve	  catalyGc	  reducGon	  
	  	  	  	  	  	  	  	  	  	  	  	  on	  coal	  power	  plants	  and	  catalyGc	  converters	  on	  cars	  
• 	  Future	  OHTROP	  will	  also	  depend	  on	  overhead	  column	  O3,	  	  
	  	  	  local	  humidity,	  and	  biogenic	  emission	  of	  VOCs,	  etc.	  
• 	  Given	  this	  complexity,	  we	  expect	  CCMs	  to	  project	  	  
	  	  	  a	  wide	  range	  of	  values	  for	  OHTROP	  during	  the	  rest	  
	  	  	  of	  this	  century	  
	  
	  	  
      Current Conclusions from OHTROP Analysis 
 
• Eﬀect	  of	  CH4	  on	  OHTROP	  is	  taken	  from	  IPCC	  (2001),	  SecGon	  4.2.1.1,	  which	  states	  that	  “the	  feed-­‐	  
back	  of	  CH4	  on	  tropospheric	  OH”	  found	  using	  contemporary	  chemical	  transport	  models	  is	  −0.32%	  
for	  every	  1%	  increase	  in	  CH4	  (red	  line,	  middle	  panel	  of	  ﬁgure	  to	  right)	  
• Primary	  eﬀect	  of	  overhead	  O3	  is	  rise	  in	  OHTROP	  following	  the	  1991	  erupGon	  of	  Mount	  
Pinatubo	  (green	  dashed	  line)	  due	  to	  enhanced	  removal	  of	  stratospheric	  O3	  by	  volcanic	  aerosol	  
	  
• Rising	  H2O	  from	  MERRA	  and	  AIRS	  increases	  OHTROP	  (orange	  lines)	  by	  an	  amount	  comparable	  
to	  the	  decrease	  expected	  from	  rising	  CH4	  
	  
• Overall	  expected	  change	  in	  OHTROP	  (bofom	  panel)	  shows	  higher	  level	  of	  interannual	  variability	  
prior	  to	  ~1999	  and	  lower	  variability	  thereager	  
             Previous Work on OHTROP Trends 
 
• Montzka	  et	  al.,	  2011	  found	  that	  OHTROP	  does	  not	  vary	  interannually	  (from	  1997	  to	  present)	  
• Prior	  studies	  by	  Prinn	  et	  al.,	  2001	  and	  Bousquet	  et	  al.,	  2005	  suggest	  large	  interannual	  variability	  in	  
OHTROP	  (1985-­‐2000)	  
	  
• We	  suggest	  the	  OHTROP	  behavior	  in	  all	  three	  studies	  may	  be	  physically	  possible,	  based	  on	  our	  
preliminary	  results	  
 
3.	  Preliminary	  results	  for	  expected	  changes	  in	  OHTROP	  
EvaluaGng	  Eﬀects	  of	  H2O	  and	  Overhead	  O3	  on	  
Global	  Mean	  Tropospheric	  OH	  ConcentraGon	  
Julie	  M.	  Nicely1,	  R.	  J.	  Salawitch1,	  T.	  Canty1,	  C.	  Lang2,	  B.	  N.	  Duncan3,	  Q.	  Liang3,	  L.	  D.	  Oman3,	  R.	  S.	  Stolarski2,	  D.	  W.	  Waugh2	  
1University	  of	  Maryland,	  College	  Park,	  Md;	  2Johns	  Hopkins	  University,	  Bal)more,	  Md;	  3NASA	  Goddard	  Space	  Flight	  Center,	  Greenbelt,	  Md	  
	  	  
 
4.	  Future	  plans	  for	  reﬁning	  es)mates	  of	  ΔOHTROP	  
Improve Estimate of 
d(OHTROP) / d(H2O)  
•  Reaction rates from recent runs of GEOS CCM 
are archived for reactions such as: 
 
H2O + O(1D) à 2OH 
 
•  Using these reaction rates we will determine 
the proportion of OH that is produced via 
reaction with H2O 
 
•  The determined scaling factor would be used to 
calculate a new ΔOHTROP based on the H2O 
trends 
 
•  Estimate time- and pressure-varying values of 
d(OHTROP) / d(H2O) 
 
•  Evaluate discrepancies between MERRA and 
AIRS H2O trends 
 
Evaluate CH4/OH Feedback 
 
•  We will use a box model (details below) to probe 
relationship between CH4 and OHTROP and its 
dependence on NOx  
 
 
Propagate Uncertainties 
 
•  Calculate uncertainties in AIRS and MERRA 
H2O and NASA O3 product 
 
•  Evaluate standard deviation in average fraction 
of OH production occurring via H2O + O(1D) 
 
•  Estimate uncertainty in the box model 
evaluation of d(OHTROP) / d(CH4) 
 
Box Model	  
We	  will	  use	  the	  GSFC	  Combined	  Stratosphere-­‐
Troposphere	  (COMBO)	  box	  model	  provided	  by	  
Chang	  Lang	  (JHU):	  
• 	  GMI	  chemical	  mechanism	  
• 	  118	  species,	  321	  thermal	  reacGons,	  and	  81	  	  
	  	  	  photolysis	  reacGons	  
• 	  5	  modules:	  
	  	  	  	  	  	  –Aerosol	  opGcal	  depth	  &	  surface	  area	  
	  	  	  	  	  	  –Photolysis	  scheme	  
	  	  	  	  	  	  –Thermal	  reacGons	  scheme	  
	  	  	  	  	  	  –DiﬀerenGal	  eqn	  solver	  
	  	  	  	  	  	  –Input-­‐output	  
• 	  Fast-­‐JX	  photolysis	  &	  SMVGEAR	  II	  solver	  
Assessing OHTROP in CCMs 
	  
•  Through	  our	  involvement	  with	  the	  IGAC	  /	  SPARC	  
Chemistry-­‐Climate	  Model	  IniGaGve,	  we	  have	  
requested:	  
1.  hourly,	  instantaneous	  output	  from	  parGcipaGng	  
CCMs	  1	  day/season,	  1	  year/decade	  
2.  archival	  of	  all	  species,	  reacGon	  rates,	  J-­‐values,	  
and	  physical	  parameters	  relevant	  to	  OH	  
chemistry	  
3.  this	  output	  for	  both	  the	  REF-­‐C1	  (hindcast)	  and	  
REF-­‐C2	  (future)	  runs	  
•  We	  plan	  to	  assess	  the	  causes	  of	  diﬀerences	  
between	  OH	  in	  the	  CCMs	  
	  
•  Use	  of	  the	  box	  model	  enables	  us	  to	  disGnguish	  
between	  OH	  diﬀerences	  due	  to	  chemical	  
mechanism	  and	  those	  due	  to	  diﬀerences	  in	  OH	  
precursors	  
	  
•  We	  can	  also	  use	  this	  output	  to	  predict	  future	  trends	  
in	  OHTROP,	  based	  on	  CH4,	  H2O,	  and	  overhead	  O3	  from	  
the	  future	  CCM	  runs	  
s
Base Case OH Conditions 
 
• Initial modern-day conditions for OH are taken 
from a time-slice run of GEOS CCM using 2005  
emissions 
 
• Monthly mean mixing ratios of OH and related  
species are provided on a 144 longitude, 91  
latitude, 72 pressure level grid 
 
• Calculated changes in OH due to H2O and  
overhead O3 are applied to initial OH field 
	  	  
 
 
 
 
 
 
 
 
 
MERRA & AIRS H2O	  
• Speciﬁc	  humidity	  ﬁles	  from	  the	  NASA	  Modern-­‐Era	  	  
RetrospecGve	  analysis	  for	  Research	  and	  ApplicaGons	  
(MERRA)	  starGng	  prior	  to	  1985,	  and	  
	  
• H2O	  mixing	  raGo	  ﬁles	  from	  the	  NASA	  Atmospheric	  
Infrared	  Sounder	  (AIRS)	  starGng	  in	  2002	  were	  used	  to	  
establish	  trends	  in	  H2O	  by	  laGtude	  
	  
• OHTROP	  is	  assumed	  to	  follow	  the	  square	  root	  of	  the	  
change	  in	  H2O	  using	  a	  steady-­‐state	  assumpGon	  
	  
• We	  plan	  to	  reﬁne	  the	  eﬀect	  of	  changing	  H2O	  on	  
OHTROP	  by	  examining	  reacGon	  rates	  from	  archived	  	  
runs	  of	  GEOS	  CCM	  
	  
Overhead O3 Observations 
 
• Total	  column	  O3	  trends	  were	  obtained	  from	  the	  
NASA	  merged	  O3	  data	  set,	  consisGng	  of	  
measurements	  from	  SBUV,	  TOMS,	  and	  Aura	  OMI	  
instruments	  
	  
• We	  then	  use	  our	  photolysis	  code	  to	  esGmate	  the	  
impact	  on	  J(O3)	  à	  O(1D)	  of	  decreasing	  iniGal	  GEOS	  
CCM	  overhead	  O3	  columns	  by	  amount	  suggested	  by	  
the	  NASA	  product	  
	  
• OHTROP	  is	  assumed	  to	  change	  by	  the	  square	  root	  of	  
J(O3)	  à	  O(1D)	  
 
2.	  Methods	  for	  es)ma)ng	  changes	  in	  OHTROP	  
https://ntrs.nasa.gov/search.jsp?R=20130014068 2019-08-29T15:29:36+00:00Z


Evaluating Effects of H2O and overhead O3 on Global Mean Tropospheric OH Concentration

Evaluating Effects of H2O and overhead O3 on Global Mean Tropospheric OH Concentration

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