EFFECTS OF NITROGEN DIOXIDE
Traditionally this gas has been associated with the degradation of dyes, fabrics and other organic materials, although some recent work has argued in favour of its adjunct, nitric acid playing a greater role in damaging items indoors.
INDOOR CHEMISTRY of NO2
Nitrogen dioxide chemistry involve three potential processes in indoor air.
1. Production of nitric acid
2. Rapid equilibration with ozone and nitric oxide (NO)
3. Surface chemistry indoors where it generates nitrous acid (HONO)
1. NITRIC ACID PRODUCTION
This is the product of oxidation
NO2 + OH -> HNO3
NO2 + O3 -> NO3 + O2
NO3 + RH -> R + HNO3
Normally NO3 is a night time process in the outdoor air, but indoors much lower light intensities allow it to be active.
2. INDOOR OZONE CHEMISTRY
INDOOR/OUTDOOR NO2 ratios change in the summer to become higher. This is thought to be due to reactions with indoor ozone in large buildings with high air change rates and low absorbace surfaces
NO2 + O3 -> NO2 + O2
3. NITROUS ACID EMISSIONS
HONO + hv -> OH + NO
OH + RH -> R. + H2O
O2 + R. + M -> ROO. + M
ROO + NO -> RO + NO2
REACTIONS AT THE SURFACE
H2O + 2NO2(ads)-> HNO3(ads) + HONO(ads)
In alkaline conditions the HNO3, and HONO will convert to the anions: NO3 and NO2 . What happens to NO2?
Initially we have investigated the behaviour of nitrite (NO2) in materials
1. constructional materials
2. ageing plaster
4. concrete tile
NO2 was present in fresh plaster and doesn't oxidise or get lost...
Some penetration from external air into Breeze blocks, but little change at depth..., suggesting that some NO2 was inherent in the material.
Little evidence that NO2 deposited on wool is released over time or of nitrite oxidation. However nitrite present in wool appeared to increase over time.
Especially at elevated temperatures, possibly as a result of degradation of protein or release of bound nitrite?
DEPOSITION VELOCITY [Vd]
Fundamental parameter to describe indoor removal rate and describing absorption of NO2 from air...
NO2 DEPOSITION VELOCITY
Measurements of NO2 onto concrete floor tiles show a strong RH dependence reminding us of the important role of water...
HYPOTHESES ABOUT NITRITE
Ubiquitous in porous non-acidic materials
Originates in situ and via deposition
Long lived and strongly bound (unexpected)...?
Threat to conservation needs modification... textiles and HNO3
Re-emission as HONO may not predominate in total flux terms
Author to whom correspondence may be addressed:
Prof. Peter Brimblecombe
School of Environmntal Sciences
University of Est Anglia
Norwich NR4 7TJ UK
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© December 04th, 2001