Lightning-produced NOx over Brazil during TROCCINOX: airborne measurements in tropical and subtropical thunderstorms and theimportance of mesoscale convective systems

[摘要] During the TROCCINOX field experiments in February–March 2004 and February2005, airborne in situ measurements of NO, NO_y, CO, and O₃ mixingratios and the J(NO₂) photolysis rate were carried out in the anviloutflow of thunderstorms over southern Brazil. Both tropical and subtropicalthunderstorms were investigated, depending on the location of the SouthAtlantic convergence zone. Tropical air masses were discriminated fromsubtropical ones according to the higher equivalent potential temperature(Θ_e) in the lower and mid troposphere, the higher CO mixingratio in the mid troposphere, and the lower wind velocity in the upper tropospherewithin the Bolivian High (north of the subtropical jet stream). During thunderstorm anvil penetrations,typically at 20–40 km horizontal scales, NO_x mixing ratios were distinctly enhanced and the absolute mixing ratiosvaried between 0.2–1.6 nmol mol⁻¹ on average. This enhancement was mainlyattributed to NO_x production by lightning and partly due to upwardtransport from the NO_x-richer boundary layer. In addition, CO mixingratios were occasionally enhanced, indicating upward transport from theboundary layer. For the first time, the composition of the anvil outflowfrom a large, long-lived mesoscale convective system (MCS) advected fromnorthern Argentina and Uruguay was investigated in more detail. Over ahorizontal scale of about 400 km, NO_x, CO and O₃ absolute mixing ratioswere significantly enhanced in these air masses in the range of 0.6–1.1,110–140 and 60–70 nmol mol⁻¹, respectively. Analyses from trace gascorrelations and a Lagrangian particle dispersion model indicate thatpolluted air masses, probably from the Buenos Aires urban area and frombiomass burning regions, were uplifted by the MCS. Ozone was distinctlyenhanced in the aged MCS outflow, due to photochemical production andentrainment of O₃-rich air masses from the upper troposphere – lowerstratosphere region. The aged MCS outflow was transported to the north,ascended and circulated, driven by the Bolivian High over the Amazon basin.In the observed case, the O₃-rich MCS outflow remained over thecontinent and did not contribute to the South Atlantic ozone maximum.

[学科分类] 大气科学