Nitrogen oxides measurements in an Amazon site and enhancements associated with a cold front

Abstract. An intensive atmospheric chemistry study was carried out in a pristine Amazonian forest site (Balbina), Amazonas state, Brazil during the 2001 wet season, as part of the LBA/CLAIRE 2001 (The Large Scale Biosphere Atmosphere Experiment in Amazonia/Cooperative LBA Airborne Regional Experiment) field campaign. Measurements of nitrogen oxide (NO), nitrogen dioxide (NO₂) and ozone (O₃) were performed simultaneously with aerosol particles and black carbon concentrations and meteorological parameters observations. Very low trace gases and aerosol concentrations are typically observed at this pristine tropical site. During the measurement period, there was a three-day episode of enhancement of NO₂ and black carbon concentration. NO₂ concentration reached a maximum value of 4 ppbv, which corresponds to three times the background concentration observed for this site. Black carbon concentration increased from the approximated 100 ng/m³ average value to a 200 ng/m³ maximum during the same period. Biomass burning spots were detected southward, between latitudes 15 to 10° S, 5–6 days before this episode from GOES-8 WF_ABBA (Wildfire Automated Biomass Burning Algorithm). An atmospheric numerical simulation of the whole measurement period was carried out using the RAMS model coupled to a biomass burning emission and transport model. The simulation results pictured a smoke transport event from Central Brazil associated to an approach of a mid-latitude cold front, reinforcing the hypothesis of biomass burning products being long-range transported from the South by the cold front and crossing the Equator. This transport event shows how the pristine atmosphere pattern in Amazonia is impacted by biomass burning emissions from sites very far away.