Soil emission of nitric oxide may be a significant source of NOx in rural a
reas. Agricultural practices may enhance these emissions by addition of nit
rogen fertilizers,A system that enables continuous measurement of NO fluxes
from agricultural surfaces using the flux-gradient method was developed. H
ourly differences in NO concentrations in air sampled at two intake heights
(0.6 and 1 m) were determined using a chemiluminescence analyzer. Eddy dif
fusivities were determined using wind profiles (cup anemometers), and stabi
lity corrections calculated using a 5 cm path sonic anemometer. Fast switch
ing of sampling between air intake heights (every 30 s) and determination o
f concentration values at a frequency of 2 Hz minimized the errors due to f
luctuations in background concentration. Low travel times for air samples i
n the tubing (similar to 8 s) were estimated to result in small errors in f
lux values (<0.5 ng N m(-2) s(-1)) due to chemical reactions. The overall r
esolution of the system was estimated as similar to 1 ng N m(-2) s(-1). NO
fluxes from a bare soil were measured quasi-continuously from January to Ju
ne 1995 at Elora, Canada, comprising a total of 1833 hourly values. Daily N
O fluxes before nitrogen fertilization were small, increasing after nitroge
n fertilizer was added (>10 ng N m(-2) s(-1)) Monthly NO fluxes estimated w
ere similar to those observed in previous studies. The designed system coul
d be easily modified to measure NOx and NO fluxes by using an additional ch
emiluminescence analyzer. The system also could be adapted to measure fluxe
s sequentially from various plots, enabling testing of agricultural practic
es on NO emissions.