A foliar rinse study of the dry deposition of nitric acid to a coniferous forest

Abstract

The foliar rinse method was used to study the deposition of nitric acid to a coniferous forest at the NOPEX site (NOrthern hemisphere climate Processes land-surface EXperiment) in Sweden, (60°05′N, 17°30′E) in July and August 1994. Ambient concentrations of HNO3, NH3, NO3(p) and NH4(p) were measured concurrently in and above the canopy with rotating wet annular denuders and Teflon+nylon filterpacks. It was demonstrated that different branches of the same species take up different amounts of nitrate per needle area, when hung freely and side by side in the field. Branches used in a deposition study must be ‘calibrated’ against each other. A large difference in the flux to branches of different species was also found. The flux (per total leaf area) to branches of Scots pine (Pinus sylvestris) was found to be 1.7±0.3 times greater than the flux to Norway spruce (Picea abies) under identical conditions. At the NOPEX site, the relative flux to the upper interior, middle exterior, and middle interior parts of the crown was found to be 70, 40, and 30%, respectively, of the flux to the upper exterior parts of the crown. Most the deposited nitrate was estimated to come from HNO3, as particulate nitrate accounted for only about 16% of the total atmospheric nitrate and did not show any concentration gradient from inside the forest to above. The deposition of ammonia could not be evaluated from the foliar rinse, but a comparison of the NH3 and NH4(p) concentration data suggests that its daytime deposition velocity was similar to that of HNO3, although there was large scatter in the data. Estimates of the daytime deposition velocity of HNO3 and two of the nighttime estimates were in relatively good agreement with those obtained from the inferential method using the big leaf model. Three of the nighttime values were a factor 2–3 higher. The reason(s) for the discrepancy are not known, but could mean that inferential techniques can, under some nighttime conditions, seriously underestimate the flux of nitrate to the forest ecosystem.