Observations of total peroxy nitrates and total alkyl nitrates during the OP3 campaign: isoprene nitrate chemistry above a south-east Asian tropical rain forest

Measurements of total peroxy nitrates (ΣRO2NO2, ΣPNs), total alkyl nitrates (ΣRONO2, ΣANs) and nitrogen dioxide (NO2) were made above the surface of a Malaysian tropical rain forest in Borneo, using a laser-induced fluorescence instrument developed at the University of L'Aquila (Italy). This new instrument uses the direct excitation of NO2 at 532 nm in order to measure its concentrations detecting by the NO2 fluorescence at wavelengths longer than 610 nm. ΣPNs and ΣANs are indirectly measured after their thermal dissociation into NO2. Observations showed enhanced levels of NO2 during nighttime, an increase of ΣPNs during the afternoon and almost no evident diurnal cycle of ΣANs. The diurnal maximums of 200 pptv for ΣPNs and ΣANs are well below the peaks reported in other forest sites. A box model constrained with measured species, reproduces well the observed ΣPNs, but overestimates ΣANs concentrations. The reason of this model-observation discrepancy could be a wrong parameterization in the isoprene nitrates (INs) chemistry mechanism. Sensitivity tests show that: (1) reducing the yield of INs from the reaction of peroxy nitrates with NO to almost the lowest values reported in literature (5%), (2) reducing the INs recycling to 70% and (3) keeping the INs dry deposition at 4 cm s−1, improve the agreement between modelled and measured ΣANs of 20% on average. These results imply that in the tropical rain forest, even if ΣPNs and ΣANs concentrations are lower than those observed in other North American forests, the yield and dry deposition of INs are similar. Another comparable result is that in the INs oxidation its recycling dominates with only a 30% release of NO2, which has implications on tropospheric ozone production and aerosol budget.