SURFACE REACTIONS OF DEPOSITED NO₂ IN THE MUSEUM ENVIRONMENT

Peter Brimblecombe, Michele Raychaudhuri, and Derek Bowden

University of East Anglia

ABSTRACT

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

Probably fast:
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?

NITRITE REACTIONS
Initially we have investigated the behaviour of nitrite (NO₂) in materials

1. constructional materials
2. ageing plaster
3. breeze-block
4. concrete tile
5. wool

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

E-mail: P.Brimblecombe@uea.ac.uk