AIRMAP Bulk Aerosol Measurements: Regional NO3- events Related to Seasonal Climate Anomalies

This study examined the seasonal cycles and regional scale meteorological controls on the chemical properties of bulk aerosols at Mount Washington (1910 m), the highest peak in the northeastern United States. The site is operated by AIRMAP, a UNH air quality and climate program, and part of an atmospheric observing network located in New Hampshire, USA. An analysis of air mass transport to the site was conducted using backward trajectories. Twenty-four-hour bulk aerosol samples collected from 1999 to 2004 were paired with three daily trajectories, one at the beginning, middle, and end of the sampling period. Aerosol-trajectory pairs were then sorted with respect to the concentration of aerosol chemical constituents. Periods of high and low concentrations were identified for each species along with their seasonal trends.

The primary aerosol anion was sulfate (SO₄^2-), with ammonium (NH₄⁺) as the principal cation. The concentrations of NH₄⁺ and SO₄^2- peaked during the summer months, and the lowest concentrations occurred during the winter months. Sulfur dioxide oxidation rates are typically an order of magnitude lower in winter than in summer, in addition a larger component of the wind distribution at this site is northwesterly during winter as compared to summer. This seasonality may also reflect that Mount Washington is in the continental boundary layer during summer, but above the mixed layer and under the influence of the free troposphere during winter. In wintertime this may decouple the site from the surface, preventing direct reception of surface pollution. The summit is also covered in rime ice for a large portion of winter, and the low residence time of aerosols under icing conditions could depress winter concentrations and enhance observed seasonal differences.

The highest seasonal mean concentrations of both ions during the entire study period occurred in summer 2002, 26 nmolm^-3 and 43 nmolm^-3, for SO₄^2- and NH₄⁺ respectively. The lowest seasonal mean concentrations of SO₄^2- and NH₄⁺ occurred during winter 2001, 1.4 nmolm^-3 and 2.5 nmolm^-3 respectively. The seasonal relationship between NH₄⁺ and SO₄^2- showed slopes ranging from 1.3 in summer to 2.1 in winter. This indicates that during warmer months a mixture of (NH₄)₂SO₄ and NH₄HSO₄ is present, while it is mainly the latter in winter. This initial analysis shows that both high (> 95th percentile) SO₄^2- and NH₄⁺ ion concentrations were generally associated with westerly and southwesterly transport, and low (< 5th percentile) SO₄^2- and NH₄⁺ ion concentrations were generally associated with northwesterly transport.