March 24, 2004EAS 4/88031 EAS 4/8803: Experimental Methods in AQ Week 11: Air Quality Management (AQM) Clean Air Act (History, Objectives, NAAQS) Emissions and Atmospheric Trends (Links) Principal Measurement Techniques (NOx, CO, SO 2 ) Measurement of CO (Exp 5) NDIR Method (Interferences, Stability, DL, Precision, Accuracy) Controlling O 3 and PM 2.5 Principal Measurement Techniques (O 3, PM 2.5 ) Atmospheric Transport & Photochemistry (NOx vs VOC, SOA) Ambient Measurements and Trends (World, USA, GA) Measurement of O 3 (Exp 6) UV Absorption (Interferences, Stability, DL, Precision, Accuracy) March 24, 2004EAS 4/88032 T apered E lement O scillating M icrobalance If PM of mass m deposit on piezoelectric quartz crystal, frequency changes by f = K q Q t c m with sensitivity K q, aerosol mass flow Q, time t, and PM mass concentration c m March 24, 2004EAS 4/88033 TEOM Method If PM of mass m deposit on piezoelectric quartz crystal, frequency changes by f = K q Q t c m with sensitivity K q, aerosol mass flow Q, time t, and PM mass concentration c m March 24, 2004EAS 4/88034 TEOM Setup and Operation Reducing H 2 O Interference Inclusion of Nafion dryer using TEOMs exhaust (low p, dry) as sheath flow. Filter housing T-controlled at 50 o C. March 24, 2004EAS 4/88035 Assessing Accuracy of PM 2.5 Mass Measurements Comparison of dry TEOM averages with dehydrated Teflon samples Williams Tower is ~20 km west of LaPorte, which is close to Ship Channel 254 m agl6 m agl March 24, 2004EAS 4/88036 High Resolution vs Integrated [PM 2.5 ] at LaPorte and Williams Tower Large [PM 2.5 ] transients (spikes) at both sites: Chemistry or transport? Transients (changes in [PM 2.5 ]) larger at WT, esp. at night. Averages of integrated samplers (8-24h) are very similar and follow a regional trend. March 24, 2004EAS 4/88037 Adding Photochemistry (O 3 ) LP max [O 3 ] on 08/30 is more than twice WT-[O 3 ], which seems to follow a rising tide. Fast P(O 3 ) at LP (P(O 3 ) from precursor mix and closer sources. Trend to higher WT-[PM 2.5 ] mostly at night, similar to vertical gradients at Hendersonville, but note 20 km WT-LP distance! March 24, 2004EAS 4/88039 Vertical Gradients of PM 2.5 Direct emissions and/or secondary formation of fine PM aloft. Free Troposphere Source for PM 2.5 ! During SOS99, 16 June - 22 July 1999, measurements near Nashville, TN, between 4 and 42 m agl showed positive vertical gradients for % of all daytime, and % of all nighttime samples of PM 2.5 mass, SO 4 =, NO 3 -, and NH 4 + !! March 24, 2004EAS 4/ Vertical Gradients of PM 2.5 Free Troposphere Source for PM 2.5 ! BL Dynamics Important Influence on Ground- Based AQ Monitoring !! March 24, 2004EAS 4/ Vertical Wind Profile: Advection Horizontal Transport Near logarithmic increase of WS and uniform WD within well-mixed BL. Clockwise rotation with height near BL top to merge with more geostrophic winds. Nighttime separation of layers with different wind speeds and directions. March 24, 2004EAS 4/ PM 2.5 Wind Roses: Seasonal Differences Across GA Indications for Regional Advective Transport? Period MAY-OCT NOV-APR Aug99 March 24, 2004EAS 4/ Similarity to Daytime O 3 Period MAY-OCT NOV-APR Aug99 March 24, 2004EAS 4/ Summertime PM 2.5 Max(O 3 ) Relationship Tighter correlation in July Downwind Griffin site offset to higher PM 2.5 mass. August 99 in Atlanta was hotter, dryer, more polluted with O 3 -precursor species. March 24, 2004EAS 4/ Seasonal & Regional Comparison of PM 2.5 Composition Summer Months Regional Difference: Higher OM/OC and OC/EC at more rural site! Seasonal Difference: Lower OM/OC and (higher) OC/EC in winter. More SOA in August 99? More oxygenated POCs away from Atlanta? Winter Months March 24, 2004EAS 4/ Atlanta JST Griffin downwind Elevated regional O 3 background reflected in regressions intercept: higher in Aug 99! At JST higher intercept and slope during Aug 99 (OPE= 4 vs 3): more efficient P(O 3 ). OPE in air mass arriving at Griffin is likely larger given by upper and lower limits. Lower limit assumes 1 st order loss of HNO 3 due to surface deposition at k 0.22 h -1. Air mass transitions from VOC-limited to NOx-limited regime due to Biogenic HC. High photochemical activity P(O 3 ) allows for high P(SOA): rural/urban gradient. Photochemical Activity Source Receptor Considerations: O 3 /NOz as OPE March 24, 2004EAS 4/ Photochemical Processes Leading to O 3 and PM SOA NOz An Assessment of Tropospheric Ozone Pollution, A North American Perspective, NARSTO, National Acad. Press, 2000. March 24, 2004EAS 4/ Ozone Isopleths Area of effective VOC control (most often highly populated areas) Volatile Organic Compounds (VOC) Nitrogen Oxides (NO x ) Constant [O 3 ] Low [O 3 ] High [O 3 ] NOx control effective (areas with high biogenics) March 24, 2004EAS 4/ SOA & O 3 Formation and Transport PM, SO 2, NO x Emissions VOC Emissions Wind Deposition Rainout O 3, HNO 3 PM NO hvhv RO 2 /HO 2 RO,OH NO 2 O2O2 O3O3 HNO 3 OH Fine PM, SOA March 24, 2004EAS 4/ P lanetary B oundary L ayer Dynamics Comparison of PBL and Free Troposphere Characteristics PropertyPBLFT TurbulenceNear continuous over Z i.Convective clouds; sporadic in thin layers extending horizontally. FrictionLarge drag & energy dissipation.Small viscous dissipation. DispersionRapid in vertical & horizontal.Small molecular diffusion; rapid horizontally by mean wind. WindsWS log profile in surface layer.Nearly geostrophic. Vertical TransportMainly turbulence.Mean wind, cumulus-scale. Thickness100 3000 m, f (time/space).8 18 km, less variable. Diurnal oscillations over land.Slow time variations. PBL strongly influenced by Earths surface, responding to surface forcings within min March 24, 2004EAS 4/ Turbulence in PBL Assuming an air parcel rises or sinks adiabatically, i.e. no energy is supplied nor removed, it expands and cools as it reaches lower ambient pressure aloft, or compresses and warms as it reaches higher pressure below. If the ambient vertical temperature profile (lapse rate) is less steep, the air parcel will continue to rise or fall once in (vertical) motion. Superadiabatic T profile (unstable layer) March 24, 2004EAS 4/ Consequences for Dispersion/Dilution Weakly instable to neutral layer: Dispersion driven by advection (horizontal WS). Highly instable layer: Dispersion driven by thermal looping (vertical & horizontal). March 24, 2004EAS 4/ Effects of Terrain (Friction) March 24, 2004EAS 4/ Temperature Inversion Assuming an air parcel rises or sinks adiabatically, i.e. no energy is supplied nor removed, it expands and cools as it reaches lower ambient pressure aloft, or compresses and warms as it reaches higher pressure below. If the ambient vertical temperature profile (lapse rate) is steeper, the air parcel will return to its original position. Subadiabatic T profile (stable layer) March 24, 2004EAS 4/ Inversion Types and Formation Elevated Surface Subsidence inversion: Large scale sinking of cold (but warming) air meets rising cooling air (thermals) under regional high pressure conditions. Frontal inversion: Warm moist air from S glides over cold dry air from N. Radiational inversion: Radiative heat loss at night from the Earths ground into space according to T g 4. March 24, 2004EAS 4/ Typical PBL Evolution in Summer Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic, 666 pp. March 24, 2004EAS 4/ Potential Temperature ( ) Profiles is T an air parcel at P and T would have if it were at P s (conserved for adiabatic motions, i.e., d /dt = 0). Afternoon After sunset Before sunrise After sunrise Before noon Noon March 24, 2004EAS 4/ PBL Winter vs Summer March 24, 2004EAS 4/ Seasonal Differences in Diurnal Cycles of PM 2.5 Midday minimum due to BL mixing seems compensated by SOA in summer. PM 2.5 sources near Columbus drive nighttime averages in winter 2001/02. Summer stagnation with high O 3 also leads to high PM 2.5 (e.g. 2000). Annual PM 2.5 NAAQS (15 g m -3 ) sensitive to: - SOA formed under regional stagnation in summer; - Primary PM 2.5 from local sources at night in winter. WinterSummer March 24, 2004EAS 4/ PM 2.5 Exceedances at Columbus in Oct-Nov 2001 March 24, 2004EAS 4/ PM 2.5 at Columbus in Oct-Dec 2001 Critical parameters driving [PM 2.5 ]: size of burn, distance and plume trajectory atmospheric divergence (horizontal wind speed) {vertical} boundary layer stability (T difference) BL mixing depth at night (BLH night )