[1] The nitrate radical, NO 3 , and dinitrogen pentoxide, N 2 O 5 , are two important components of nitrogen oxides that occur predominantly at night in the lower troposphere. Because a large fraction of NO 2 reacts to form NO 3 and N 2 O 5 during the course of a night, their fate is an important determining factor to the overall fate of NO x (=NO and NO 2

[1] Volatile organic compounds (VOCs) and some of their oxidants (O 3 , NO 3 ) were measured on board the National Oceanic and Atmospheric Administration research ship R/V Ronald H. Brown along the coast of New England, downwind of New York, Boston, and Portsmouth and large forested areas in New Hampshire, Maine, and Massachusetts in July and August 2002. The diurnal variations of isoprene, monoterpenes, and aromatics were mainly dependent on their emissions and the abundance of the oxidants OH and NO 3 . Elevated mixing ratios of short-lived VOCs were only encountered at the ship, which was about 1-6 hours downwind of the sources, when the concentrations of the oxidants were low. For the biogenic compounds this was generally the case during morning and evening hours, when the lifetime of the biogenics was long because of low OH and NO 3 concentrations. Most anthropogenic VOCs do not react with NO 3 , and therefore their mixing ratios remained elevated during the night. The products of isoprene oxidation, methyl vinyl ketone, methacrolein, and peroxymethacrylic nitric anhydride (MPAN) were, on average, more abundant than isoprene itself. Only during the transition periods from day to night, when oxidation rates were at a minimum, could isoprene exceed its products. The loss of the biogenic VOCs was dominated by reactions with NO 3 , whereas the loss of anthropogenics came mostly from reactions with OH. The oxygenated VOCs are the major contributor to the OH loss, except in close vicinity of emission sources. The total loss of biogenic compounds during the night was so effective that after one night of transport they were in most cases completely reacted away, whereas the mixing ratios of the anthropogenic compounds remained high during the night. The pool of reactive hydrocarbons at sunrise was thus typically dominated by anthropogenic VOCs.

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[1] Formaldehyde (CH 2 O) measurements from two independent instruments are compared with photochemical box model calculations. The measurements were made on the National Oceanic and Atmospheric Administration P-3 aircraft as part of the 1997 North Atlantic Regional Experiment (NARE 97). The data set considered here consists of air masses sampled between 0 and 8 km over the North Atlantic Ocean which do not show recent influence from emissions or transport. These air masses therefore should be in photochemical steady state with respect to CH 2 O when constrained by the other P-3 measurements, and methane oxidation was expected to be the predominant source of CH 2 O in these air masses. For this data set both instruments measured identical CH 2 O concentrations to within 40 parts per trillion by volume (pptv) on average over the 0 -800 pptv range, although differences larger than the combined 2s total uncertainty estimates were observed between the two instruments in 11% of the data. Both instruments produced higher CH 2 O concentrations than the model in more than 90% of this data set, with a median measured-modeled [CH 2 O] difference of 0.13 or 0.18 ppbv (depending on the instrument), or about a factor of 2. Such large differences cannot be accounted for by varying model input parameters within their respective uncertainty ranges. After examining the possible reasons for the model-measurement discrepancy, we conclude that there are probably one or more additional unknown sources of CH 2 O in the North Atlantic troposphere.