Real-time measurements of ammonia, acidic trace gases and water-soluble inorganic aerosol species at a rural site in the Amazon Basin

[摘要] We measured the mixing ratios of ammonia (NH₃), nitric acid(HNO₃), nitrous acid (HONO), hydrochloric acid (HCl), sulfur dioxide(SO₂ and the corresponding water-soluble inorganic aerosol species,ammonium (NH₄⁺), nitrate (NO₃^-), nitrite(NO₂^-), chloride (Cl^- and sulfate (SO₄^2-), andtheir diel and seasonal variations at a pasture site in the Amazon Basin(Rondônia, Brazil). This study was conducted within the framework ofLBA-SMOCC (Large Scale Biosphere Atmosphere Experiment in Amazonia -Smoke Aerosols, Clouds, Rainfall and Climate: Aerosols from Biomass Burning Perturb Global and Regional Climate).Sampling was performed from 12September to 14 November 2002, extending from the dry season (extensivebiomass burning activity), through the transition period to the wet season(background conditions). Measurements were made continuously using awet-annular denuder (WAD) in combination with a Steam-Jet Aerosol Collector(SJAC) followed by suitable on-line analysis. A detailed description andverification of the inlet system for simultaneous sampling of soluble gasesand aerosol compounds is presented. Overall measurement uncertainties of theambient mixing ratios usually remained below 15%. The limit of detection(LOD) was determined for each single data point measured during the fieldexperiment. Median LOD values (3σ-definition) were ≤0.015ppb foracidic trace gases and aerosol anions and ≤0.118ppb for NH₃ and aerosolNH₄⁺. Mixing ratios of acidic trace gases remained below 1ppbthroughout the measurement period, while NH₃ levels were an order ofmagnitude higher. Accordingly, mixing ratios of NH₄⁺ exceededthose of other inorganic aerosol contributors by a factor of 4 to 10. Duringthe wet season, mixing ratios decreased by nearly a factor of 3 for allcompounds compared to those observed when intensive biomass burning tookplace. Additionally, N-containing gas and aerosol species featuredpronounced diel variations. This is attributed to strong relative humidityand temperature variations between day and night as well as to changingphotochemistry and stability conditions of the planetary boundary layer.HONO exhibited a characteristic diel cycle with high mixing ratios atnighttime and was not completely depleted by photolysis during daylighthours.

[学科分类] 大气科学