Atmospheric chemistry Day 4 Air pollution Regional ozone formation

Regional air quality – ozone formation Ozone is a greenhouse gas. It affects human health, plant growth and materials Ozone is a secondary pollutant and is not directly emitted. Emission of VOCs and NOx, coupled with sunlight leads to the formation of photochemical smog. Major component is ozone. Also aerosols, nitrates … Need to understand chemical mechanism for formation in order to develop strategies and legislation for reduction of ozone concentrations. The European limit values are linked to these aims Is it better to control NOx or VOCs – or both?

Chemical mechanism Initiation: OH formed from ozone photolysis at a rate P OH (= 2k 3 [H 2 O]J 1 [O 3 ]/{k 2 [M] + k 3 [H 2 O]} ) Propagation OH + RH (+O 2 ) → RO 2 + H 2 O(R4) RO 2 + NO → RO + NO 2 (R5) RO + O 2 → R’CHO + HO 2 (R6) HO 2 + NO → OH + NO 2 (R7) Termination HO 2 + HO 2 → H 2 O 2 (R8) OH + NO 2 + M → HNO 3 + M(R9) Ozone formation O 3 is formed by NO 2 photolysis with a rate equal to the sum of the rates of reactions 5 and 7 (= v 5 + v 7 )

NOx and VOC control of ozone formation Under polluted conditions, chain propagation is fast, so v 4 = v 5 = v 6 = v 7 P O3 = v 5 + v 7 = 2v 7 = 2k 7 [HO 2 ][NO]A Also v 4 = v 7  [OH] = k 7 [HO 2 ][NO]/{k 4 [RH]}B Steady state for radicals: rate of termination = rate of initiation, ie P OH = v 8 + v 9 1.Low NOx: v 8 >> v 9 P OH = 2k 8 [HO 2 ] 2 ; [HO 2 ] =  (P OH /2k 8 ) Sub in A: P O3 = 2k 7 [NO]  (P OH /2k 8 ). ( P O3  [NO], independent [RH]NOx limited) 2. High NO x : v 8 << v 9 [OH] = P OH /(k 9 [NO 2 ][M] Sub in B: [HO 2 ] = P OH k 4 [RH]/{k 7 k 9 {NO][NO 2 ][M] Sub in A: P O3 = 2k 4 [RH]/{k 9 [NO 2 ][M] ( P O3  [NO 2 ] -1 ; [RH])VOC limited)

DEPENDENCE OF OZONE PRODUCTION ON NO x AND HYDROCARBONS HO x family OH RO 2 RO HO 2 HNO 3 H2O2H2O2 O3O3 O3O3 O3O3 P HOx “NO x - saturated” or “hydrocarbon-limited” regime “NO x -limited” regime RH NO O2O2 NO 2

OZONE CONCENTRATIONS vs. NO x AND VOC EMISSIONS Air pollution model calculation for a typical urban airshed NO x - saturated NO x -limited Ridge

Can we determine the relative contributions of different VOCs to ozone formation? Master chemical mechanism (MCM) Constructed by University of Leeds, in collaboration with Imperial College and UK Met Office Explicit mechanism, based on a protocol which describes the chemistry. Includes reactions of OH, NO 3 and O 3 and photolysis. For development protocol see: M.E.Jenkin et al. Atmos. Env., 1997, 31, 81. Describes the oxidation of 123 VOCs, based on the UK emissions inventory. The MCM is set up to provide input directly to the FACSIMILE integrator. It can be accessed via the web: ( The MCM is used by Department of the Environment Food and Rural Affairs (DEFRA) to help develop its air quality strategy.

Master chemical mechanism (MCM) A specific, explicit implementation (

Navigational Features: Extract Use Mark List as primary species Choose output format - HTML - FACSIMILE - FORTRAN - XML - KPP

Navigational Features: Extract Listing

Navigational Features: Source information

Mechanism testing using chamber experiments

Developing and testing the MCM using chamber experiments Double outdoor chambers at Valencia, Spain. Carry out experiments under atmospheric conditions, but under defined conditions. Heavily instrumented. Measure NOx, O 3, VOCs, oxygenates, CO, particles, radicals (OH, HO 2 ) vs time. Applications: –Biogenics – pinenes –aromatics

Photo-oxidation of  -pinene / NO X : gas-phase simulation [  -pinene] 0 = 97 ppb; [NO] 0 = 9.7 ppb; [NO 2 ] 0 = 0.85 ppb Jenkin – OSOA project

Comparison of MCM3.1 to Toluene Chamber Experiment (27/09/01) Also possible to measure radicals OH, HO 2. Provides A sensitive test of the mechanisms The discrepancies show that there are significant deficiencies in the mechanism especially related to radical formation C. Bloss et al Atmospheric Chemistry & Physics, 2005, 5, 623 – 639.

Photochemical ozone creation potentials (POCPs) Is there a way in which we can quantify the differential impact of different VOCs on ozone formation? The UK DEFRA uses POCPs to assess differences between VOCs and hence to develop policy. The method is based on the use of a photochemical trajectory model (PTM), in which the chemical evolution of an air parcel is followed as it travels, under anticyclonic conditions, from central Europe to the UK, over a period of 5 days. Details: –air parcel extends from surface to top of boundary layer. It is 10kmx10km (horizontal dimensions) and has a height,h, of 300 m at h, rising to 1300m at 14.00h; maintained at 1300 m till early evening, then 300 m again. –Rate equation: dC i /dt = S i –L i (C i )-v i C i /h - w i C i /h -{w v (C i -C i 0 )/h}

POCP II Emissions (VOCs and NOx) estimates utilise 3 emissions inventories, UN ECE EMEP; EC CORINAIR and UKNAEI. These give total VOC emissions, which are speciated into 135 organic compounds + methane, using the UK emissions inventory. The master chemical mechanism is used to describe the chemistry and photochemistry. The coupled differential equations are integrated using the FACSIMILE integrator. Most concentrations are set initially to zero, except for NO, NO 2, SO 2, CO, methane, HCHO, ozone and hydrogen. The air parcel is carried on a straight line trajectory at 4 m s -1

Calculation of POCP values: ‘Photochemical Trajectory Model (PTM)’

POCP III( see Derwent et al, Atmos Environment, 1996, 30, ) The POCP is calculated by incrementing the emissions of each of the VOCs in turn by 4.7 kg km -2 across the entire domain. (corresponds to an increase in total VOC emissions of 4%) The ozone formed over the 5 day trajectory is increased as a result and by different amounts for each VOC. The POCP of the ith VOC is given by: POCP i = 100x(ozone increment with the ith VOC) (ozone increment with C 2 H 4 ) Examples (ethene = 100): methane = 3; ethane = 14, propane = 41, butane = 60 isoprene = 118 benzene = 33; toluene = 77; m-xylene = 109; 1,2,4 TMB = 130

MCM v3 POCP values

Global budget for ozone (Tg O 3 yr -1 ) Chemical production3000 – 5000 HO 2 + NO70% CH 3 O 2 + NO20% RO 2 + NO10% Transport from stratosphere400 – 1100 Chemical loss3000 – 4200 O 1 D + H 2 O40% HO 2 + O 3 40% OH + O 3 10% others10% Dry deposition

GLOBAL BUDGET OF TROPOSPHERIC OZONE – recent calculations O3O3 O2O2 h O3O3 OHHO 2 h, H 2 O Deposition NO H2O2H2O2 CO, VOC NO 2 h STRATOSPHERE TROPOSPHERE 8-18 km Chem prod in troposphere 4920 Chem loss in troposphere 4230 Transport from stratosphere 475 Deposition 1165 GEOS-CHEM model budget terms, Tg O 3 yr -1

HCHO JULY 1996 (molec cm -2 ) Biogenic Biomass Burning Quantifying emissions of natural VOCs using HCHO column observations from space Paul I. Palmer GOME

HCHO columns – July 1996 [10 16 molec cm -2 ] GEOS-CHEM HCHO GOME HCHO [10 12 atoms C cm -2 s -1 ] GEIA isoprene emissions BIOGENIC ISOPRENE IS THE MAIN SOURCE OF HCHO IN U.S. IN SUMMER GOME footprint 320X40 km 2

Cumulative HCHO yield per C atom from isoprene oxidation. ([O 3 ] = 40 ppb, [CO] = 100 ppb, [isoprene] = 1ppb. CO, NO x, O 3 held constant.) Full MCM mechanism. Final yield increased from GEOS-CHEM by 16% for high NOx, 65% low NOx

HCHO formation from  pinene acetone, which has a long atmospheric lifetime, is an intermediate in HCHO formation Decay of  pinene

Relating HCHO Columns to VOC Emissions (Palmer) VOC source Distance downwind  HCHO Isoprene  -pinene propane 100 km VOC HCHO hours OH hours h, OH Ultimate Yield Y (per C) Approx. Time to Y isoprene ~ hrs  pinene ~ days  pinene ~ days MBO~ days Master Chemical Mechanism