Reactive halogen species play a key role in regulating the oxidative capacity of the atmospheric boundary layer, yet their behavior in complex coastal environments affected by substantial anthropogenic pollution remains poorly constrained by observations. To address this issue, continuous multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were conducted at a coastal site in Haiyan, Hangzhou Bay (120.96°E, 30.54°N), from 1 May to 15 June 2024, using a self-developed two-dimensional MAX-DOAS instrument. Bromine monoxide (BrO), NO₂, HCHO, O₃, and aerosols were simultaneously observed. Spectral fitting was performed with the QDOAS software using dedicated wavelength windows for BrO (346-359 nm), HCHO (336.5-359 nm), O₃ (320-340 nm), and NO₂/O₄ (338-370 nm). Aerosol extinction (AE) profiles, aerosol optical depth (AOD), and the vertical profiles and vertical column densities of BrO, NO₂ and HCHO were retrieved with the SCIATRAN radiative transfer model and the PriAM optimal-estimation algorithm. Quality control comprised wavelength calibration against mercury-lamp spectra, dark-current and electronic-offset correction, root-mean-square residual filtering of spectral fits, fitting-error weighting in the optimal estimation, AOD thresholding (AOD > 2), and interquartile-range outlier screening, leaving 26 valid observation days. The retrievals show that BrO, aerosols and ozone precursors were predominantly concentrated within 0-1 km. During the relatively clean period (21-25 May), BrO remained low with weak temporal variability, and its average volume mixing ratio (VMR) below 2 km was 5.15 ppt, comparable to the marine boundary-layer background of 2-10 ppt. During the relatively polluted period (11-15 June), the 0-2 km BrO VMR rose to 6.69 ppt and reached a daily-mean maximum of 9.74 ppt on 11 June (AE = 1.13 km^-1), coinciding with elevated NO₂, HCHO and AOD under weak southwesterly winds (wind direction ~210°, wind speed < 3 m/s); the vertical stratification was more pronounced than in the clean period. Over the whole campaign, the differential slant column densities (dSCDs) of BrO and O₃ were moderately anticorrelated (r= -0.46, p < 0.001), consistent with brominemediated ozone depletion, although NO titration, air-mass transport, background O₃ variability, aerosol heterogeneous reactions and boundary-layer photochemistry may also contribute. BrO and NO₂ remained positively correlated throughout 0-2 km, most strongly at 0.4-1.6 km (r ≈ 0.58-0.90), suggesting shared sources or coupled reaction pathways. A stratified analysis based on near-surface AE terciles showed that the mean BrO concentration in the high-AE group was about 3.24 times that in the low-AE group, with a BrO-AE Pearson correlation of r= 0.84 (p < 0.001), indicating that aerosol loading co-varied consistently with BrO within each period. Weighted potential source contribution function (WPSCF) analyses of BrO, AE, NO₂ and HCHO indicate that, during the clean period, the high-WPSCF regions were concentrated over the near-shore and marine areas east of the site, pointing to a predominantly marine origin of the associated air masses. During the pollution period, the high-WPSCF regions extended southwestward from Hangzhou Bay toward the Yangtze River Delta hinterland, especially above 500 m, consistent with the enhanced inland transport identified by the trajectory cluster analysis. The spatial overlap among the BrO, AE and NO₂ WPSCF distributions across the 200-2000 m layers supports the interpretation that the pollution-period BrO enhancement was jointly modulated by transported aerosols and NO₂ together with near-surface environments favorable for heterogeneous bromine activation. These findings highlight the synergistic influence of marine and anthropogenic emissions on coastal BrO chemistry under the East Asian monsoon climate, and provide observational constraints for understanding reactive halogen chemistry in industrialized coastal environments, with implications for regional atmospheric oxidative capacity and photochemical pollution control.