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. A multi-axis differential optical absorption spectroscopy (MAX-DOAS)instrument was deployed in May and June 2016 at a monitoring station(37.18 ∘  N, 114.36 ∘  E) in the suburban area of Xingtai,which is one of the most polluted cities in the North China Plain (NCP),during the Atmosphere-Aerosol-Boundary Layer-Cloud (A 2 BC) experiment and Air chemistry Research In Asia(ARIAs) joint experiments to derive tropospheric vertical profiles of NO2 , SO2 , HONO, HCHO, CHOCHO and aerosols. Aerosol opticaldepths derived from MAX-DOAS were found to be consistent with collocatedsun-photometer measurements. Also the derived near-surface aerosol extinctionand HCHO mixing ratio agree well with the coincident visibility meter and insitu HCHO measurements, with mean HCHO near-surface mixing ratios of ∼3.5  ppb. Underestimations of MAX-DOAS results compared to in situmeasurements of NO2 ( ∼60  %) and SO2 ( ∼20  %) are found expectedly due to vertical and horizontal inhomogeneityof trace gases. Vertical profiles of aerosols and NO2 and SO2 are reasonably consistent with those measured by a collocated Raman lidar andaircraft spirals over the station. The deviations can be attributed todifferences in sensitivity as a function of altitude and substantialhorizontal gradients of pollutants. Aerosols, HCHO and CHOCHO profilestypically extended to higher altitudes (with 75 % integrated columnlocated below ∼1.4  km) than NO2 , SO2 and HONO did(with 75 % integrated column below ∼0.5  km) under pollutedconditions. Lifted layers were systematically observed for all species(except HONO), indicating accumulation, secondary formation or long-rangetransport of the pollutants at higher altitudes. Maximum values routinelyoccurred in the morning for NO2 , SO2 and HONO but occurred ataround noon for aerosols, HCHO and CHOCHO, mainly dominated byphotochemistry, characteristic upslope–downslope circulation and planetaryboundary layer (PBL) dynamics. Significant day-to-day variations are foundfor all species due to the effect of regional transport and changes insynoptic pattern analysed with the backward propagation approach based onHYSPLIT trajectories. Low pollution was often observed for air masses fromthe north-west (behind cold fronts), and high pollution was observed from thesouthern areas such as industrialized Wu'an. The contribution of regionaltransport for the pollutants measured at the site during the observationperiod was estimated to be about 20 % to 30 % for trace gases andabout 50 % for aerosols. In addition, agricultural burning eventsimpacted the day-to-day variations in HCHO, CHOCHO and aerosols. It needs tobe noted that although several MAX-DOAS measurements of trace gases andaerosols in the NCP area have been reported in previous studies, this studyis the first work to derive a comprehensive set of vertical profiles of NO2 , SO2 , HONO, HCHO, CHOCHO and aerosols from measurementsof one MAX-DOAS instrument. Also, so far, the validation of MAX-DOAS profileresults by comparison with various surface in situ measurements as well asprofile measurements from lidar and aircraft is scarce. Moreover, thebackward propagation approach for characterizing the contributions ofregional transport of pollutants from different regions was applied to theMAX-DOAS results of trace gases and aerosols for the first time.

atmospheric chemistry and physics

Vertical profiles of NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, HONO, HCHO, CHOCHO and aerosols derived from MAX-DOAS measurements at a rural site in the central western North China Plain and their relation to emission sources and effects of regional transport