Observations of Mixing Ratios and Fluxes of Reactive Nitrogen Oxides above a Mixed Hardwood Forest in Central Ontario during the Summer and Fall of 2011

Anthropogenic activities have significantly disrupted the global nitrogen cycle causing numerous environmental problems such as tropospheric ozone and aerosol production, acidification, eutrophication, and biodiversity loss. At the same time, accelerated nitrogen deposition may also be driving increased carbon uptake by the world's forests, but many uncertainties regarding atmospheric nitrogen chemistry in forested environments remain. The work presented here is part of a long-term interdisciplinary project to study carbon and nitrogen cycling at a mid-latitude mixed hardwood forest in central Ontario, where estimates of nitrogen deposition are among the highest in North America.

Measurements of NO_x (= NO + NO₂) and NO_y (= NO_x + NO₃ + N₂O₅ + HNO₃ + HONO + volatile p-NO₃^- + organic nitrates) were made above the forest canopy from a 30 m tower by a two-channel chemiluminescence analyzer (Air Quality Design, Inc.), co-located with open path eddy covariance instrumentation for flux measurements. Conversion of NO₂ to NO is achieved using a blue LED converter, and NO_y to NO by a heated molybdenum converter. Sampling losses are minimized by locating the converters in an inlet case mounted at the top of the tower, connected to the rest of the instrument by 40 m of tubing. Detection limits at 1 Hz were between 10-20 ppt for NO_x and NO_y during the campaign. Ozone was also measured by a UV photometer (Thermo Scientific).

Dramatic early morning pulses of NO were observed that increased NO mixing ratios from around the detection limit at night to 250 ppt on average during the day. These increases were almost always related to photolytic losses of NO₂. Likewise, NO₂ levels (on average around 450 ppt throughout the campaign) closely followed photolytic conditions, building up during the day time only when skies were cloudy. Occasional pulses of NO_x in the early morning were also observed, which could be associated with soil-level emissions, but may also be related to long-range advection as they recurred on days with similar wind conditions.

NO_x oxidation products (NO_z = NO_y - NO_x) during the campaign were usually lowest late at night (on average around 450 ppt) and highest in the late afternoon (on average around 790 ppt), again suggesting mostly photochemical sources. As expected, afternoon O₃ was linearly correlated with NO_z, with a slope of 19 ppb O₃ per ppb of NO_x oxidation. Often, increases in NO_x oxidation products during the day could not be easily accounted for by photochemical NO₂ oxidation alone, without an advective or local source. Ozone levels throughout the period showed the typical diurnal evolution with a minimum around 7 am (on average 22.8 ppb) and a maximum around 4 pm (on average 31.7 ppb).

NO_x and NO_y fluxes were also measured intermittently throughout the campaign. Here we focus on the NO_y flux measurements. Eddy covariance measurements of NO_y and vertical wind were averaged over 30 minutes, and corrected for time response and high frequency noise by applying appropriate filters to the CO₂ covariances measured by a co-located open path instrument. The direction and magnitude of NO_y fluxes were correlated with NO_y mixing ratios, with deposition highest at high mixing ratios, and near zero fluxes or emissions at low mixing ratios. The emission fluxes organized even more tightly around NO₂ concentrations, with few emission fluxes measured when NO₂ was above 400 ppt. It is unclear if this may also be due to other co-variables especially given the diurnal behaviour of NO₂. Emission and deposition fluxes each have strong diurnal patterns when treated separately, peaking during the middle of the day. Further investigation will attempt to separate the effect of instrumental difficulties in measuring fluxes during the night which could conceivably contribute to this diurnal pattern. Nevertheless, NO_y mixing ratios also follow a diurnal pattern, and Particular cases where NO_y mixing ratios were affected by emission fluxes vs. deposition fluxes will be explored. These will be discussed in terms of the chemistry and transport  governing NO_y composition during those observations.