Agricultural and Forest Meteorology 105 2000 427–445
Surfaceatmosphere exchange and chemical interaction of gases and aerosols over oilseed rape
Eiko Nemitz
a,b,∗
, Mark A. Sutton
a
, G. Paul Wyers
c
, René P. Otjes
c
, Jan K. Schjoerring
d
, Martin W. Gallagher
b
, Judith Parrington
e
, David Fowler
a
, Thomas W. Choularton
b
a
Centre for Ecology and Hydrology CEH, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, Scotland EH26 0QB, UK
b
Department of Physics, UMIST, Sackville Street, PO Box 88, Manchester M60 1QD, UK
c
Netherlands Energy Foundation ECN, Petten ZG1755, Netherlands
d
The Royal Veterinary and Agricultural University RVAU, Plant Nutrition Laboratory and Centre for Ecology and Environment, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark
e
Centre for Ecology and Hydrology CEH, Merlewood Research Station, Grange-over-Sands, Cumbria LA11 6JU, UK Received 1 February 1999; received in revised form 19 May 2000; accepted 20 June 2000
Abstract
Measurements of NH
3
, HCl, HNO
3
and HNO
2
gas as well as NH
4 +
, NO
3 −
, Cl
−
and SO
4 2−
aerosol are used to investigate their surface exchange fluxes and the potential for gas–particle interactions at a clean coastal Scottish site. Mean concentrations
of HNO
3
and HCl were small at 0.68 and 0.32 mg m
− 3
, respectively. At relative humidities h 85 measured gas concen- tration products K
m
were smaller than the predicted dissociation constants K
e
, suggesting potential for aerosol evaporation, but at high h, K
e
of NH
4
Cl was exceeded at the mean canopy height. Above the canopy, small aerosol concentrations resulted in estimated chemical time-scales of 3 min. Thus, chemical reactions should not have affected NH
3
flux measurements by aerodynamic gradient methods AGMs, except for very low turbulence when AGM is not applicable. Within the canopy,
however, the diffusive transport provided enough time for NH
4
Cl to be generated. This was substantiated by measurements of NH
4 +
emission and high Cl
−
aerosol concentrations within the canopy. Micrometeorological measurements above the canopy indicated that gaseous Cl compounds were emitted for most of the
time, and this was supported by the sourcesink distributions of gaseous and aerosol Cl compounds calculated from in-canopy profiles as well as high apoplastic Cl
−
concentrations. Although emission of CH
3
Cl has been reported for other Brassica species, an unrealistically large emission would be necessary to cause the observed above-canopy gradients. Emission of HCl
liberated from unidentified water pools of high Cl
−
or leaf surface reactions is a more likely source of gaseous Cl compounds. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Gas-to-particle conversion; Ammonia; Ammonium; Brassica napus; Aerosol deposition; Chloromethane; Inverse Lagrangian technique; HCl emission
∗
Corresponding author. Tel.: +44-131-445-4343; fax: +44-131-445-3943.
E-mail address: enceh.ac.uk E. Nemitz.
1. Introduction
Since the deposition of atmospheric ammo- nia NH
3
can potentially contribute to ecosystem
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 2 0 7 - 0
428 E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445
eutrophication and soil acidification, many studies have been undertaken to quantify and parameterize
the net surfaceatmosphere exchange e.g. Sutton et al., 1993a, 1997. The construction of fast response
sensors for NH
3
, which would allow the flux to be derived directly by eddy-correlation, is difficult, and
their application is generally limited to high air con- centrations. Relaxed eddy accumulation systems for
NH
3
are currently under development e.g. Zhu et al., 2000, and these may soon enable slow response mon-
itors to be applied to measure the vertical NH
3
flux at a single height. In the meantime, NH
3
fluxes can only be inferred from vertical gradients e.g. aerodynamic
gradient method, AGM. These gradients can be mea- sured with automated slow response monitors, such
as continuous wet denuders e.g. AMANDA, Wyers et al., 1993 or batch sampling methods, including
filter-packs e.g. Allen et al., 1989 and dry denuders e.g. Ferm, 1979.
The AGM is only applicable if the measured flux is constant over the height range of the measure-
ments, a pre-requisite that is violated as soon as chemical conversion processes occur. As the major
gaseous base in the atmosphere, NH
3
takes part in a variety of neutralization reactions and in particu-
lar it reacts with sulphuric acid H
2
SO
4
, nitric acid HNO
3
and hydrochloric acid HCl to form the associated ammonium NH
4 +
aerosols. While the surface vapour pressures over ammonium sulphates
are negligible, NH
4
NO
3
and NH
4
Cl may re-evaporate once the vapour phase concentrations drop below
the value in equilibrium with the aerosol phase. Whereas the particle formation process is known as
gas-to-particle conversion GTPC, the evaporation of volatile NH
4 +
aerosol is here termed particle-to-gas conversion PTGC, and both processes together are
referred to as gas–particle interconversion GPIC. Owing to differences in the exchange rates of the
different species, these reactions can result in devi- ation from normal log-linear concentration profiles
and lead to fluxes that change with height. Several models have been developed for the implementation
of ’modified gradient techniques’ to infer the sur- face flux of chemically reactive species from profile
measurements e.g. Brost et al., 1988; Kramm and Dlugi, 1994; Nemitz et al., 1996; Van Oss et al.,
1998. Due to large uncertainties in the reaction rate coefficients Kramm and Dlugi, 1994 or chemi-
cal time-scales Wexler and Seinfeld, 1990, these models are not yet applicable on a routine basis. In
addition, model results have rarely been compared with measurements. Nevertheless, initial modelling
results have shown that reactions could theoretically change NH
3
fluxes by as much as 40 Kramm and Dlugi, 1994 or even lead to flux reversal Van
Oss et al., 1998. Measurements of NH
3
fluxes should therefore be accompanied by investigations
into the potential for effects of reactions on the gradients.
A field campaign carried out in June 1995 as part of the EC ‘EXAMINE’ project Sutton et al., 2000a
near North Berwick, Scotland, was focused on in- vestigations into the processes governing the NH
3
exchange with oilseed rape. At the same time the fluxes of atmospheric acids and aerosols were mea-
sured, with the idea to provide a dataset for rigorous assessment of new and existing models for ‘modi-
fied gradient techniques’. However, the clean Scottish measurement site and the advection of clean polar
air resulted in low concentrations close to the de- tection limits and restricts the applicability of these
data for detailed model validations. Nevertheless, the data provide information of the exchange of
HCl, HNO
3
and aerosols and permit general con- clusions on the importance of GTPCPTGC at clean
measurement sites. The objectives of this paper are to
1. calculate concentrations and, where available,
fluxes of acidic gases HCl, HNO
3
and HNO
2
and aerosol species NH
4 +
, NO
3 −
, Cl
−
and SO
4 2−
, with emphasis on the interpretation of the emis-
sions of gaseous Cl compounds observed at this site;
2. calculate sourcesink distributions of Cl
−
and NH
4 +
aerosol from within-canopy concentration profiles using the inverse Lagrangian technique by
Raupach 1989; 3. investigate the potential of gas–particle intercon-
version by assessing the gas–aerosol equilibria NH
3
–HNO
3
–NH
4
NO
3
and NH
3
–HCl–NH
4
Cl, and by estimating time-scales for chemical inter-
conversions; 4. quantify of the importance of GPIC on gradient
measurements of NH
3
at clean sites above and within vegetation canopies based on the results of
1–3.
E. Nemitz et al. Agricultural and Forest Meteorology 105 2000 427–445 429
2. Method and theory
