Urban air quality

L4

Which factors determine the dispersion and transformation of air pollutants?

• for the dispersion of gases:

  • wind speed

  • urban configuration

  • geometry

• for the transformation of air pollutants for example the formation of ozone:

  • global radiation

  • temperature

What are the MAPs in the urban environment?

• major anthropogenically emitted/ produced substances, e.g:

  • SO2

  • NOX = NO + NO2

  • CO

  • aerosols or PM:

    • coarse: PM₁₀ (D ≤ 10 μm)

    • fine: PM_2.5 (D ≤ 2.5 μm)

    • fine: PM_0.1 (D ≤ 0.1 μm)

  • VOCs

  • Ozone

    • main ingredient of photochemical smog

    • 2ndary air pollutant formed from NOx and VOCs:

      • VOCs + NOx →^hv O₃

What are the HAPs in the urban environment?

• trace compounds in the atmosphere with high toxicity, e.g.:

  • benzene

  • formaldehyde

  • polyaromatic hydrocarbons (PAHs)

  • perchloroethylene

  • methylene chloride

  • asbestos

  • toxic heavy metals, e.g. mercury, cadmium, chromium, lead

• causes serious health effects, such as cancer, birth defects

What are the differences between MAPs and HAPs?

• HAPs are present in the atmosphere in much smaller concentrations than MAPs

• HAPs appear more localized, typically in urban areas or near industries

NO emitted by cars is converted rapidly to NO₂. Which chemical reaction is responsible for this?

NO + O₃ → NO₂ + O₂

What are the complex mixtures of organic/carbonaceous PM?

• Elemental carbon (EC) or black carbon (BC)

  • emitted directly (combustion)

  • structure similar to impure graphite

• Organic carbon (OC)

  • emitted directly (primary OC) incomplete fossil-fuel combustion

  • indirectly (secondary OC) condensation of VOCs (e.g. by-products of photochemical reaction (e.g. PAH))

• Soot:

  • Both EC and (primary) OC

What are the health impacts of major air pollutants?

What are the toxicological effects and importance of particle size for public health?

• toxicological effects:

  • inflammations (response immune system)

  • pulmonary effects

  • cardiovascular effects

• importance of particle size:

  • PM_0.1 > PM_2.5 > PM₁₀

  • because penetrates into lungs

How do particles penetrate into the lungs?

• large particles deposit in upper airways

• small particles deposit inside the lungs

Why are ultrafine and fine particles predominantly of anthropogenic origin?

• ultrafine particle are formed by chemical reactions or are condensed from hot vapor, for e.g. from diesel exhaust, and they coagulate into fine particles

• therefore the particles are predominantly of anthropogenic origin

Why is controlling the concentration of SO2 easer than ozone concentration?

• Ozone is a 2ndary pollutant,

  • hence it can only be regulated via the primary pollutants,

  • which are VOCs and NOx, emitted by various anthropogenic and natural sources

• SO2 is a primary pollutant,

  • can be regulated directly by emissions controls at coal power plants and on ships engines

Give 3 reasons why this guideline can be questioned.

the current air quality guideline for PM is based on the mass concentration of particles with diameter = or < than 10 μm (PM₁₀)

1. the effects on the respiratory system are caused by the fine and ultra-fine particles which are able to penetrate deeply into the lungs.

   1. although they dominate by far in number, their mass is negligible

2. no threshold value for the impact of particles on human health has been found

3. the chemical composition of particles is not well known, and health impacts may be due to other pollutants adsorbed on them

   1. typically heavy metals or less volatile organic compounds

What are the health impacts of ozone?

• adverse, structural, functional and biochemical alterations in the respiratory tract

• increases in daily mortality and levels in populations

• chronic effects: small airway function, asthma development

Explain why in many cases large industries and power plants in the vicinity of large cities contribute rarely to the local ground level air pollutant concentrations inside the urban area?

• usually large industries and power plants have tall chimneys

• air pollution emitted from a high release height will in many cases be transported out of the urban area before dispersed down to ground level,

• depending on size of the urban domain

What is actual exposure in cities equal to?

actual exposure = street + city + region + long distance contribution

Why would the urban heat island effect further worsen the air quality in a city?

• at low wind speeds the UGI effect can give rise to a circulation where the pollution is trapped

• an increase in temperature may lead to an increase in surface ozone concentrations

• during night-time urban air pollution may get worse due to development of a strong inversion across the urban canopy

What could be reason of two streets in different countries showing similar concentrations despite one having higher traffic in the streets?

• the street with higher traffic may have high wind speeds and neutral conditions

• the other may have low wind speeds and (near) stable conditions.

What ozone pollution result from?

• VOCs

• NOx

• in combination with sunlight

• weather & climate play an important role

Define aspect ratio of a street and the impact of aspect ratio on air quality at street level?

• defined as the ratio between mean building height and street width (H/W)

• has an important influence on the canyon vortex

  • on the dispersion and concentrations of air pollutants at street levels

• with higher H/W, less pollutants will be dispersed out of the street canyon

  • so surface pollution levels will tend to be higher

What is the influence of the UBL at daytime and nighttime?

• Daytime:

  • BL bulges upward due to city heat and building roughness

  • solar heating creates strong vertical mixing

  • air pollutants accumulate over the city and get swept downwind

• Nightime:

  • the UHI effect lifts the inversion layer up and over the city,

  • trapping pollutants tightly inside the UCL

  • hight nighttime pollution concentrations

What are the flow patterns within the UCL?

• average wind speeds in cities are lower

• large differences at building/street level

• impact on dispersion and concentrations of air pollutants

Explain this figure which shows a diurnal pattern for ground-level ozone concentration in areas that experience photochemical smog.

• morning peak of NO at 6-7

  • largely due to emissions from vehicles during the morning rush of traffic

• hydrocarbon peak at 9

  • temporal pattern similar to NO, except that their concentration peaks slightly later

  • emitted from vehicles and refineries

• peak of NO2 at 7-9

  • in bright sunlight, the NO is photochemically oxidized with help of reactive hydrocarbons to NO2

  • resulting in a decrease of NO concentration and increase of NO2

• broad peak of O3 concentration at 12-15:

  • photochemical reactions involving NO2 produce O atoms

  • which react with O2 to form O3

  • resulting in a net decrease in NO2 concentration and an increase in O3 concentration

• as the day proceeds, the various gases decrease in concentration as they are diluted by fresh air masses blowing in from the Pacific Ocean or are consumed by photochemical reactions

Why could ozone levels be higher in the weekends in cities that during the working week?

• during the weekends there is less traffic and NO emissions in the cities

• hence, less O3 is destroyed by the reaction with NO

In the ozone transverse concentration profile, explain the concentration profiles at the points 1,2,3, and 4.

during daytime*

1. close to emission sources, NO reduces ozone by titration (NO + O3 → NO2 + O2)

2. Net formation some distance downwind of the sources:

   1. in bright sunlight, the NO is photochemically oxidized with help of reactive hydrocarbons to NO2, resulting in a decrease in NO conc. and a peak of NO2

   2. photochemical reactions involving NO2 produce O atoms which react with O2 to form O3

   3. these result in a net decrease in NO2 concentration and an increase in O3 concentration

3. NOx becomes limiting

4. Loss of ozone due to deposition

O3 mixing ratios are high and NO mixing ratio is low in SB, reverse is true in central LA. Why?

• central LA is source region

  • heavy traffic and industry, pumping out primary pollutants like NO and VOCs

  • because NO is freshly emitted in high amounts, immediately titrates local O3

  • so NO high, O3 low

• SB is receptor region

  • sea breeze transports the primary pollutants towards the mountains

  • sunlight drives photochemical reactions (VOCs + NOx +hv → O3)

  • secondary pollutants like O3 and PAN has fully formed

Why is it difficult to compare the air quality between different cities?

How AQI mitigate this problem?

• difficult to compare the air quality between different cities:

  • differences in monitoring practices

  • differences in the mix of pollutants

  • differences in effects of local climate on pollutants

• AQI helps:

  • addressing the challenge of comparing cities,

  • by measuring AQ relative of health standards

  • so that lacking measurements of one pollutant does not prevent the calculation of AQI

Why does the ambient outdoor AQ not adequately describe the actual exposure of people to air pollutants?

the levels that an individual is exposed to, may vary drastically during the day depending on their activity pattern.