Atmospheric chemistry

1. Atmospheric chemistry Day 4 Air pollution Regional ozone formation

2. 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?

3. Chemical mechanism • Initiation: OH formed from ozone photolysis at a rate POH (= 2k3[H2O]J1[O3]/{k2[M] + k3[H2O]} ) • Propagation OH + RH (+O2) → RO2 + H2O (R4) RO2 + NO → RO + NO2 (R5) RO + O2→ R’CHO + HO2 (R6) HO2 + NO → OH + NO2 (R7) • Termination HO2 + HO2→ H2O2 (R8) OH + NO2 + M → HNO3 + M (R9) • Ozone formation O3 is formed by NO2 photolysis with a rate equal to the sum of the rates of reactions 5 and 7 (= v5 + v7)

4. NOx and VOC control of ozone formation • Under polluted conditions, chain propagation is fast, so v4 = v5 = v6 =v7 • PO3 = v5 + v7 = 2v7 = 2k7[HO2][NO] A • Also v4 = v7 [OH] = k7[HO2][NO]/{k4[RH]} B • Steady state for radicals: rate of termination = rate of initiation, ie POH = v8 + v9 • Low NOx: v8 >> v9 POH =2k8[HO2]2; [HO2] = (POH/2k8) Sub in A: PO3 = 2k7[NO] (POH/2k8). ( PO3  [NO], independent [RH] NOx limited) 2.High NOx: v8 << v9 [OH] = POH/(k9[NO2][M] Sub in B: [HO2] = POHk4[RH]/{k7k9{NO][NO2][M] Sub in A: PO3 = 2k4[RH]/{k9[NO2][M] ( PO3  [NO2]-1; [RH]) VOC limited)

5. DEPENDENCE OF OZONE PRODUCTION ON NOx AND HYDROCARBONS O3 HOxfamily NO RO2 RO 5 RH O2 4 6 PHOx 7 O3 OH HO2 NO NO2 9 8 HNO3 O3 H2O2 “NOx- saturated” or “hydrocarbon-limited” regime “NOx-limited” regime

6. OZONE CONCENTRATIONS vs. NOx AND VOC EMISSIONSAir pollution model calculation for a typical urban airshed Ridge NOx-limited NOx- saturated

7. 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, NO3 and O3 and photolysis. For development protocol see: M.E.Jenkinet 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: (http://www.chem.leeds.ac.uk/Atmospheric/MCM/mcmproj.html) • The MCM is used by Department of the Environment Food and Rural Affairs (DEFRA) to help develop its air quality strategy.

8. Master chemical mechanism (MCM) A specific, explicit implementation(http://Mcm.leeds.ac.uk/MCM

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

10. Navigational Features: Extract Listing

11. Navigational Features: Source information

12. Mechanism testing using chamber experiments

13. 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, O3, VOCs, oxygenates, CO, particles, radicals (OH, HO2) vs time. • Applications: • Biogenics – pinenes • aromatics

14. Photo-oxidation of a-pinene / NOX: gas-phase simulation [a-pinene]0 = 97 ppb; [NO]0 = 9.7 ppb; [NO2]0 = 0.85 ppb Jenkin – OSOA project

15. Comparison of MCM3.1 to Toluene Chamber Experiment (27/09/01) Also possible to measure radicals OH, HO2. 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.

16. 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 06.00 h, rising to 1300m at 14.00h; maintained at 1300 m till early evening, then 300 m again. • Rate equation: dCi/dt = Si –Li(Ci )-viCi/h - wiCi/h -{wv(Ci-Ci0)/h}

17. 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, NO2, SO2, CO, methane, HCHO, ozone and hydrogen. • The air parcel is carried on a straight line trajectory at 4 m s-1

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

21. POCP III( see Derwent et al , Atmos Environment, 1996, 30, 181-199) • 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: POCPi = 100x(ozone increment with the ith VOC) (ozone increment with C2H4) • 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

22. MCM v3 POCP values

23. Global budget for ozone (Tg O3 yr-1) • Chemical production 3000 – 5000 HO2 + NO 70% CH3O2 + NO 20% RO2 + NO 10% • Transport from stratosphere 400 – 1100 • Chemical loss 3000 – 4200 O1D + H2O 40% HO2 + O3 40% OH + O3 10% others 10% • Dry deposition 500 - 1500

24. GLOBAL BUDGET OF TROPOSPHERIC OZONE – recent calculations GEOS-CHEM model budget terms, Tg O3 yr-1 O2 hn O3 STRATOSPHERE 8-18 km TROPOSPHERE hn NO2 NO O3 hn, H2O OH HO2 H2O2 Deposition CO, VOC

25. Quantifying emissions of natural VOCs using HCHO column observations from space 2.5 2 1.5 1 Biogenic 0.5 0 Biomass Burning -0.5 HCHO JULY 1996 (molec cm-2) GOME Paul I. Palmer

26. HCHO columns – July 1996 GOME HCHO GEOS-CHEM HCHO [1016molec cm-2] GEIA isoprene emissions [1012 atoms C cm-2 s-1] BIOGENIC ISOPRENE IS THE MAIN SOURCE OF HCHO IN U.S. IN SUMMER GOME footprint 320X40 km2

27. Cumulative HCHO yield per C atom from isoprene oxidation. ([O3] = 40 ppb, [CO] = 100 ppb, [isoprene] = 1ppb. CO, NOx, O3 held constant.) • Full MCM mechanism. • Final yield increased from GEOS-CHEM by 16% for high NOx, 65% low NOx

28. HCHO formation from a pineneacetone, which has a long atmospheric lifetime, is an intermediate in HCHO formation Decay of a pinene

29. hours WHCHO hours Isoprene HCHO h, OH OH a-pinene propane VOC 100 km Distance downwind VOC source Relating HCHO Columns to VOC Emissions (Palmer) Master Chemical Mechanism