Coupled evolution of BrO_x‐ClO_x‐HO_x‐NO_x chemistry during bromine‐catalyzed ozone depletion events in the arctic boundary layer

Abstract

[1] Extensive chemical characterization of ozone (O 3 ) depletion events in the Arctic boundary layer during the TOPSE aircraft mission in March-May 2000 enables analysis of the coupled chemical evolution of bromine (BrO x ), chlorine (ClO x ), hydrogen oxide (HO x ) and nitrogen oxide (NO x ) radicals during these events. We project the TOPSE observations onto an O 3 chemical coordinate to construct a chronology of radical chemistry during O 3 depletion events, and we compare this chronology to results from a photochemical model simulation. Comparison of observed trends in ethyne (oxidized by Br) and ethane (oxidized by Cl) indicates that ClO x chemistry is only active during the early stage of O 3 depletion (O 3 > 10 ppbv). We attribute this result to the suppression of BrCl regeneration as O 3 decreases. Formaldehyde and peroxy radical concentrations decline by factors of 4 and 2 respectively during O 3 depletion and we explain both trends on the basis of the reaction of CH 2 O with Br. Observed NO x concentrations decline abruptly in the early stages of O 3 depletion and recover as O 3 drops below 10 ppbv. We attribute the initial decline to BrNO 3 hydrolysis in aerosol, and the subsequent recovery to suppression of BrNO 3 formation as O 3 drops. Under halogen-free conditions we find that HNO 4 heterogeneous chemistry could provide a major NO x sink not included in standard models. Halogen radical chemistry in the model can produce under realistic conditions an oscillatory system with a period of 3 days, which we believe is the fastest oscillation ever reported for a chemical system in the atmosphere.

Signature of Arctic surface ozone depletion events in the isotope anomaly (Δ<sup>17</sup>O) of atmospheric nitrate

“…Particulate bromide originates from the conversion of BrO x (≡ Br + BrO) and is indicative of the intensity of the halogen-mediated ODE chemistry. However, as evidenced by Lehrer et al (1997) and Evans et al (2003), inferring BrO mixing ratio from Br − is only possible for limited periods of times.…”

“…Therefore the HO x budget relies only on modeling calculations, which do not always include up-to-date chemical mechanisms and processes. It appears that the five dominant sources of HO x are the photolysis of HCHO, ozone, HOBr and HONO, and the reaction between HCHO and Br (Evans et al, 2003;Lehrer et al, 2004). HCHO and HONO are emitted by the seasonal snowpack (Sumner and Shepson, 1999;Zhou et al, 2001).…”

“…Overall, there is a large degree of interconversion between the HO x family and BrO x , mainly through HOBr. This species acts as a short-lived reservoir species for both families (Evans et al, 2003). NO x sources in the Arctic include global NO x sources (long range transport (in the form of PAN) from the industrialized mid-latitude countries, lightning, soil emissions) but also a significant local component, namely snowpack emissions (Beine et al, 2002).…”

“…Sea-salt aerosols (Fan and Jacob, 1992), but also the seasonal snowpack (Tang and McConnell, 1996;Sander et al, 1997;Michalowski et al, 2000) should be the predominant sources of bromine in the atmosphere.…”

An analysis of fast photochemistry over high northern latitudes during spring and summer using in-situ observations from ARCTAS and TOPSE

“…The catalytic cycling of these halogen radicals has been implicated as the cause of local scale near-complete destruction of surface O 3 and further, they influence the cycling and photochemistry of HO x , NO x , and O 3 . (e.g., Barrie et al, 1988;Evans et al, 2003;McElroy et al, 1999;von Glasgow et al, 2004). Studies stemming from the 2000 TOPSE aircraft campaign (Tropospheric Ozone Production about the Spring Equinox) highlight the importance of photochemistry at high latitudes, suggesting that gross photochemical O 3 formation is equal to or greater than the source from long range transport throughout the spring in the free troposphere, and is greater than transport sources at surface altitudes after March (Stroud et al, 2004;Emmons et al, 2003).…”