AtmacphericEnvimnmcnr, Vol. 17, No. 1. pp. 191.192, 1983 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE Printed in Great Britain. 0004-6981/83/01019142503.00/0 Pergamon Press Ltd. zyxwvutsrqp DRY DEPOSITION OF SO2 and NOx in winter L. Granat and C. Johansson Department of Meteorology, University of Stockholm, Arrhenius Laboratroy, S-106 91 Stockholm, Sweden (First received 18 May 1982 and received for publication 8 October 1982) Abstract - The deposition velocity of SO2, mated to be less than 0.03 cm s-l. NO and NO2 to a coniferous forest in winter is esti- Deposition velocity to a snow cover depends on the tempera- ture. INTRODUCTION The deposition of some trace gases to vegetation is being studied at this Institute and we here report results obtained from measurements on conifers and snow during winter. The results constitute one further piece of evidence that the rate of deposition to vegetation is much lower during winter than during Sumner. We think that it is important to report these findings since such low deposition rates are usually not considered in atmospheric budget calculations and long-range transport models, probably due to lack of data (see Sehmel, 1980). EQUIPMENT AND MATERIALS Branches of approximately 1 dm2 projected leaf area (LA11 were enclosed in an 8 e chamber made of FEP teflon film. Ambient air was pumped through the chamber at 7 e min -1 and the con- centration of several trace gases measured before and after passage of the chamber in which the air was stirred with a fan driven by an external motor to minimize boundary layer resist- ance. In other experiments, a certain amount of snow was transferred into the chamber or to an 8 e glass container and the rate of uptake was determined in the same way as above. (This method was considered to be more reliable than to use a chamber with open bottom placed direct- ly on the snow on the ground.) The following gases were measured: SO2 (flame photometer), NO (chemiluminescence), NO, (heated molybdenum converter), water vapour (dew point), CO2 (diffe- rential infrared absorption) and ozone (chemiluminescence). We believe that the difference in concentration between NOx and NO essentially is a measure of NO2 during this time of the year. All instruments were installed in a thermostated mobile van and connected to the chamber (which was located outdoors) through insulated and heated teflon tubes. The measurements were perfor- med 15 km SE of Stockholm at Alta in a rural environment. To facilitate the measurements, the absorption measurements for SO2 and NO2 were made at concentrations somewhat higher than ordi- nary ambient levels by adding gases from either permeation tubes (N02, SO21 or from pressure container (NO). Typical concentrations were: S02: 3-7 ppb, N02: 6-30 ppb and NO: 0.5-2 ppb compared to rural ambient levels in south central Sweden of 0.5-2 ppb both for SO2 and NOx. Measurements were made on several branches and various portions of snow - in each case typical- ly for 24 h - during the period January 21 to February 3 1982. The chamber temperature was l-3'C above ambient level and varied between +3'C and -16'C during the measurement period. Air temperatures in December and January were mostly below zero and ranged from approximately +5'C to -25'C. RESULTS 1. Vegetation No measurable concentration difference between inlet and outlet of the chamber could be ob- served with spruce or pine (less than about 2% in addition to some wall losses) for the gases SO2' NO, N02, water vapour and CO2 at temperatures ranging from +3 to -6'C. We conclude that 191