Budget of NOy species measured at a coastal site

Concentrations of total NOy as well as the major individual species comprising NOy, i.e. NO, NO2, PAN, HNO3, HONO and aerosol nitrate, have been measured in three campaigns at the Weybourne Atmospheric Observatory on the north Norfolk coast in Eastern England. The sum of the individual NOy species (∑NOyi) accounted for a high proportion (typically 96%) of total NOy in the two Winter campaigns carried out, but considerably less (about 79%) in a Summer campaign. It was found that the apparent deficit in ∑NOyi in the summer campaign samples correlated closely with the concentration of NHx (sum of gaseous ammonia and particulate ammonium) and it appears that excessive cleaning of the converter led to a sensitivity to these species. Subtraction of the NHx interference leads to a high degree of closure, close to 100% in morning and nighttime samples and 95% in afternoon samples. Thus, unlike many published studies, there is no substantial “missing” NOy and the small deficit in afternoon samples is probably explicable in terms of unmeasured photochemically generated organic nitrates. The magnitude of the ratio ∑NOyi/NOy appeared largely independent of the NOy concentration. The NOx/NOy ratio tended to increase with increased NOy in the full dataset consistent with lesser oxidation of NOx in the more polluted airmasses. This facet was not obvious, however, in a subset of marine airmass samples. On a short timescale, however, the NOx/NOy ratio can exhibit a strong diurnal cycle. An attempt to simulate this cycle of NOx/NOy by means of a numerical model was fairly successful, though there was some lowering of the midpoint of the oscillation towards the end of the run. It was also demonstrated that if the rate of diurnally averaged nitric acid deposition were to match its rate of formation from NO2, a relatively constant NOx/NOy ratio in time-averaged samples could be explained without invoking atmospheric conversion of HNO3 to NO2. Nevertheless, the latter remains a possible explanation for the relative constancy of the NOx/NOy ratio.