Chemistry, transport and dry deposition of trace gases in the boundary layer over the tropical Atlantic Ocean and the Guyanas during the GABRIEL field campaign

[摘要] We present a comparison of different Lagrangian and chemical boxmodel calculations with measurement data obtained during the GABRIEL campaignover the tropical Atlantic Ocean and the Amazon rainforest in the Guyanas, October 2005.Lagrangian modelling of boundary layer (BL) air constrained by measurements is used to derive a horizontal gradient(≈5.6 pmol/mol km⁻¹) of CO from the ocean to therainforest (east to west). This is significantly smaller than that derivedfrom the measurements (16–48 pmol/mol km⁻¹), indicating that photochemicalproduction from organic precursors alone cannot explain the observed strong gradient.It appears that HCHO is overestimated by the Lagrangian and chemical box models,which include dry deposition but not exchange with the free troposphere (FT).The relatively short lifetime of HCHO implies substantial BL-FTexchange. The mixing-in of FT air affected by African and South Americanbiomass burning at an estimated rate of 0.12 h⁻¹ increases the COand decreases the HCHO mixing ratios, improving agreement withmeasurements. A mean deposition velocity of 1.35 cm/s for H₂O₂over the ocean as well as over the rainforest is deduced assuming BL-FT exchangeadequate to the results for CO. The measured increase of the organic peroxidesfrom the ocean to the rainforest (≈0.66 nmol/mol d⁻¹)is significantly overestimated by the Lagrangian model, even whenusing high values for the deposition velocity and the entrainment rate.Our results point at either heterogeneous loss of organic peroxides and/ortheir radical precursors, underestimated photodissociation or missing reaction paths of peroxy radicals notforming peroxides in isoprene chemistry. We calculate a mean integrateddaytime net ozone production (NOP) in the BL of (0.2±5.9) nmol/mol (ocean)and (2.4±2.1) nmol/mol (rainforest). The NOP strongly correlates with NOand has a positive tendency in the boundary layer over the rainforest.

[学科分类] 大气科学