Measurements of atmospheric HONO vertical distribution and temporal evolution in Madrid (Spain) using the MAX-DOAS technique.

Nitrous acid (HONO) stands as one of the main species in tropospheric chemistry, primarily in polluted, urban regions. Due to its fast photodissociation, it is considered as one the main sources of the hydroxyl radical (OH), the most relevant oxidant in the atmosphere. Therefore, the evaluation of HONO concentration profiles and their temporal evolution is important for urban atmospheric chemistry. In this study, we report a year-round measurement of HONO vertical concentration profiles, as well as their diurnal and seasonal evolution during 2016 in Madrid. Making use of the Multi-AXis Differential Absorption Spectroscopy (MAX-DOAS) technique in addition to inversion algorithms, we retrieved the aerosol extinction and trace gas concentrations. Our results show HONO maximum values of 3.5-4 ppbv in the early morning and late afternoon, and minima around noon, when the lifetime of HONO against photolysis is shortest. On average, there is a pronounced HONO concentration gradient across different seasons, being higher during the autumn and winter months. Finally, we estimate and discuss the production rate of OH radicals from HONO photolysis, along with its variability throughout the year.

Application of a short term air quality action plan in Madrid (Spain) under a high-pollution episode - Part I: Diagnostic and analysis from observations.

Exceedances of NO2 hourly limit value (200 µg·m-3) imply the need to implement short term action plans to avoid adverse effects on human health in urban areas. The Madrid City Council applied the stage 3 of the NO2 protocol during a high-pollution episode under stable meteorological conditions on December 2016 for the first time. This included road traffic access restrictions to the city centre (50% of conventional private vehicles based on plate numbers). In this contribution we analyse different meteorological and air quality observations, including non-standard parameters (such as number of ultrafine particles and remote sensing techniques MAXDOAS) for a better understanding of the effectivity of short-term emission abatement measures under real conditions and to identify options to improve the NO2 protocol in the future. According to our results, the inversion base height computed from vertical temperature soundings is a meaningful index to anticipate very unfavourable conditions and trigger the actions included in the protocol. The analysis of the concentration levels of the main pollutants from the Madrid air quality monitoring network indicate that only stage 3 of the protocol had a significant effect on NO2 maximum concentrations. The restrictions applied may have prevented NO2 concentrations to further increase in the city centre (up to 15%) although pollution levels in the city outskirts, outside the area directly affected by the traffic restrictions, remained unchanged or may have been slightly increased. Nonetheless, further studies are needed to estimate more precisely the effect of the measures taken and to assess potential trade-offs. Our results suggest that emissions play an important role also under very strong stability conditions although drastic measures are needed to achieve a significant impact. This highlights the importance of an appropriate timing for short-term actions and the need of permanent abatement measures related to air quality plans and policies.